air.pdf

I. AIR 1 NATURE OF ATMOSPHERIC POLLUTION 1.1 PHYSICAL STRUCTURE AND COMPOSITION OF THE TROPOSPHERE AND STRATOSPHERE The atmosphere serves as the medium into which air pollutants are relea...

I. AIR 1 NATURE OF ATMOSPHERIC POLLUTION 1.1 PHYSICAL STRUCTURE AND COMPOSITION OF THE TROPOSPHERE AND STRATOSPHERE The atmosphere serves as the medium into which air pollutants are released. The knowledge on how it functions is essential in understanding the fate and transport of air pollutants. Temperature, Pressure, Density, Spatial and Temporal Relationships The structure of our atmosphere is often defined in terms of the variations in temperature, pressure, and density with height. Pressure and density are controlled by the concentrations of gases in the air. They drop exponentially with height as the air thins. The temperature structure is controlled by the vertical distribution of gases that absorb UV and thermal-IR radiation. Its gradient defines the layers of our atmosphere – troposphere, stratosphere, mesosphere, and thermosphere. The troposphere and stratosphere are more relevant to air pollution. The troposphere extends to about 8-16 km from the poles to the equator above ground. It can be This copy is issued to Lau Chin Tung broadly divided into background troposphere and boundary layer (extending to ~0.5-3 km above ground) which is most relevant as it is where constant air movement occurs. The stratosphere, which protects us from harmful UV radiation, is also relevant in that ozone depletion is caused by human emissions (e.g., chlorofluorocarbons (CFCs)). Radiation In virtually all atmospheric processes, the energy expended is originally derived from solar radiation. The earth absorbs short-wave solar radiation and emits longer wavelength terrestrial radiation. Clouds, water vapor, and carbon dioxide (CO2) absorb thermal IR radiation, which causes the atmosphere to warm. The air around the earth is not heated directly by solar radiation but by terrestrial radiation and convective heating from the ground warmed by the solar radiation, thereby causing a general reduction of temperature from ground to higher altitude. The differential heating of the earth's surface results in different weather patterns. 1.2 NATURAL COMPOSITION OF THE ATMOSPHERE Typical Concentrations of Common Species in the Natural Background Our atmosphere is a mixture of gases and particulates. Some gases (e.g., nitrogen (N2; ~78%), oxygen (O2; ~21%) and other inert gases (e.g., argon (Ar; ~0.93%), helium (He; ~5 ppm)) in the bottom 10 km are well mixed because of their low loss rates. Others are variable gases with concentrations change in time and spaces. Water vapour, CO2, and methane (CH4) are the most abundant varying gases (about 4-5%, 400 ppm, and 1,850 ppm, respectively). The concentrations of other varying gases are generally low in natural background but are high in urban cities and polluted environment. Typical Concentrations of Common Pollutants in Polluted Environments In 2022, the hourly concentrations of sulphur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), ozone (O3), respirable suspended particulates (RSP, PM10), and fine suspended particulates (FSP, PM2.5) in Hong Kong could be as high as the values in Table I-1 on the next page. In general, the concentrations of PM10, PM2.5, SO2, NO2 in Hong Kong are decreasing but O3 is still on an upward trend. Also, although CO has the highest mass concentration, its health concern is no higher than others. V.2024.05.10 Page 2 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR Table I-1: Hourly concentration of air pollutants in 2022 Air Pollutant Concentration (μg/m3) Sulphur dioxide (SO2) 59 Nitrogen dioxide (NO2) 290 Carbon monoxide (CO) 2,390 nighupae 裝 hmmlutants Ozone (O3) 363 Respirable suspended particulates (RSP, PM10) 150 Fine suspended particulates (FSP, PM2.5) 118 1.3 AIR POLLUTANTS Definitions & characteristics (physical & chemical) This copy is issued to Lau Chin Tung Air pollutants are airborne substances that could threaten the health of people and animals, and harm vegetation, structures, and the environment. They can be gaseous air pollutants and particulates and are classified as primary and secondary. Primary air pollutants (e.g., CO, SO2, NOx, VOCs, particulate matters) are those which are emitted to the air directly from the sources. Secondary air pollutants are those formed in the atmosphere by chemical reactions (e.g., acid-base reaction, oxidation, photochemical reaction) among primary air pollutants known as “precursors”. Examples include sulphates, NO2, O3, peroxyacetyl nitrate, etc. Not being directly emitted in the exhaust gas streams, secondary air pollutants are usually more difficult to control. Particulates Particulates are a complex mixture of solid particles and aerosols (liquid particles) of a wide range of chemical, size, and physical characteristics. Particulates may also carry other pollutants dissolved in or adsorbed to their surface. Some common particulates include road dust, soot, oily fumes, nitrates, sulphates, organic aerosols, metallic compounds, dioxins, polycyclic biphenyls, pesticides, asbestos (e.g., chrysotile, crocidolite, actinolite, tremolite, anthophyllite), etc. Particulates can also be viable (e.g., bacteria, spores, pollens) and they are also called bioaerosols. Particles with aerodynamic diameters of about 75 µm or less can suspend in air for prolonged periods and are called total suspended particulates. Those with 10 µm or less in diameter, which are known as respirable suspended particulates or PM10, are sufficiently small that they can penetrate to the thoracic region. The fine fraction with 2.5 µm or less is referred as fine suspended particulate or PM2.5. They may even deposit in the smaller conducting airways and alveoli. Other than primary particulates, secondary particulates may be formed in the air and they can make up quite a significant percentage of PM2.5. Ammonium sulphates, nitrates, and secondary organic aerosol (SOA) are their major components. Gaseous pollutants Hundreds of different gas-phase substances are emitted to the atmosphere from both natural and anthropogenic sources. Common gaseous air pollutants include CO and CO2, sulphur oxides (SO2 and SO3), nitrogen oxides (N2O, NO and NO2), O3 and photochemical oxidants, NH3, CFC, methane, volatile organic compounds (VOCs, which are organic chemicals with high vapour pressures at ambient temperature, such as benzene, toluene, ethylene, formaldehyde, etc. and many of them are precursors of photochemical smog). Most gaseous pollutants, other than CFC and other stable air pollutants, are likely degraded chemically before reaching the stratosphere. V.2024.05.10 Page 3 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR Toxic air pollutants Toxic air pollutants (TAP) (or hazardous air pollutants (HAP)) are those air pollutants that are known or suspected to cause serious and irreversible adverse health or environmental effects. In the USA, 187 HAPs have been designated for protection of public health. Among them, 30 TAPs are identified as posing the greatest potential health threat in urban areas. They include a) Metals (As, Be, V, Cr, Pb, Mn, Hg and Ni) and their compounds b) Organics: acetaldehyde, acrolein, acrylonitrile, benzene, 1,3-butadiene, chloroform, dioxins, 1,3- dichloropropene, ethylene dichloride, ethylene oxide, formaldehyde, hydrazine, methylene chloride, propylene dichloride, quinoline, 1,1,2,2-tetrachloroethane, tetrachloroethylene, trichloroethylene, vinyl chloride and persistent organics (i.e., dioxins, polycyclic aromatic hydrocarbons (PAH) or polycyclic organic matter (POM), and hexachlorobenzene) Threshold and non-threshold pollutants The pollutant level below which no ill effects are observed is called the threshold level. The typical example of air pollutants with a threshold is CO. For these air pollutants, an exposure level, which is known as “no observed adverse effect level (NOAEL)” that at which there are no statistical increases in the frequency or severity of adverse effects can be identified. Other air pollutants (e.g., PM2.5, PM10, This copy is issued to Lau Chin Tung O3), however, are found to have no or very small health thresholds. A risk assessment approach is taken for determining the tolerable or acceptable levels: a) Risk ≦10-6: acceptable b) Risk between 10-4 and 10-6: best effort, with the consideration of cost, technical feasibility and other factors, should be taken to reduce the risk c) Risk > 10-4: not acceptable Radionuclides Radioactivity is a form of radiation that is emitted when a radioactive element decays emitting energy (γ-ray, α-, β-particles) in an attempt to gain nucleus stability. The use of nuclear energy, including power generation, nuclear fuel processing, medical uses, nuclear weapons, can be the sources of radioactive pollution. Releases of radioactivity (e.g., radionuclides include I-131, Sr-90, Pu239) during major disasters (e.g., fallouts of the Three Miles Island, Chernobyl, and Fukushima reactors) are major concerns. A more important radioactive pollution is the elevated indoor levels of radioactive radon (Rn-222) gas. It is a colourless, odourless noble gas produced from the radioactive decay of radium, which is naturally found in common minerals, such as granites. Its progenies are radioactive particles with long half-lives and may stay in the lung and respiratory system for even more prolonged period. Due to the extensive use of granites as building materials, Hong Kong’s indoor radon levels could be high, especially in poorly ventilated premises. Biological contaminants The common biological contaminants include pollens, moulds, bacteria, or viruses that adversely affect health. They are a major concern indoors, but the spread of biological contaminants can also be severe in poorly ventilated tall building clusters, e.g., SARS and COVID-19 virus episodes. Odour Odour is our perception of smell. It is induced by inhaling airborne volatile organic or inorganic materials. Most common odorous substances include hydrogen sulphide (H2S; rotten egg smell), O3 (fishy smell), NH3 (sharp pungent smell), carbon disulphide (CS2; ether like smell), products of decomposed proteins, phenols, and some petroleum hydrocarbons. The concentration of odour is determined by olfactometry, i.e., by human noses (and in some cases, by chemical methods and electronic nose) with the odour threshold taken as 1 odour unit (ou). V.2024.05.10 Page 4 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR 1.4 PHYSICAL AND CHEMICAL POLLUTANT PROCESSES 1.4.1 Transport, dispersion, dilution, transformation, scavenging, and atmospheric lifetimes The dispersion of air pollutants is the result of transport of the air pollutants away from the sources by air motion or wind and dilution or mixing by diffusion due to atmospheric turbulence and concentration difference. There are different scales of air motion which affect air pollutant transport and dispersion in different manners: a) Global and synoptic scales: wind patterns are set up due to differential heating of the earth's surface by solar radiation (e.g., at the equator and polar regions), rotation of the earth and the difference between the heat capacities of land and ocean masses b) Mesoscale: wind patterns develop because of the regional or local topography (mountain ranges, water bodies, deserts, forestation, etc.) c) Microscale: wind patterns are created by local temperature and turbulence conditions This copy is issued to Lau Chin Tung During dispersion and dilution, the air pollutants may be removed from the atmosphere. Chemical reaction, in particular, oxidation, is a prime removal mechanism. The important naturally occurring scavenger is the hydroxyl radical, which is formed by reaction of water with free oxygen atoms created by the photochemical breakdown of tropospheric ozone. It oxidizes many air pollutants to products (e.g., sulphuric acid, nitric acid) that can be brought down from air by precipitation. Also, humidity and rainfall (or precipitation) play important roles in removing air pollutants, in particular, the particulate matters, by (i) “in-cloud” process: in which particulates in the air act as nuclei for the condensation of rain droplets and when the droplets become heavy enough, they will precipitate; and (ii) collision with raindrops that dissolves or bring the pollutants to the ground. The place where the air pollutant is removed from atmosphere is called the “sink”. Sinks include the soil, vegetation, structures, and water bodies, particularly the oceans, as well as the substances that react and remove the air pollutant from the air (e.g., NO for O3). The time it takes for half of the quantity of pollutant emitting from a source to disappear into its various sinks is referred to as the “half-life”. Most pollutants have a short half-life (from hours to days), but CFCs and other chemically stable compounds can have half-lives from tens of years to over 100 years. 1.4.2 Meteorological effects: Influence of solar radiation and wind fields, lapse rate and stability conditions The concentrations of air pollutants are affected by winds, temperatures, vertical temperature profiles, turbulence, solar radiation, clouds, rainfall, and relative humidity. These meteorological parameters are influenced by both large- and small-scale weather systems in which the solar radiation and local topography play important roles. Large scale weather systems are controlled by vast regions of high and low pressure. The differential heating of earth’s surface by solar radiation and unequal absorption creates a dynamic system with differences in barometric pressure. Air flows counter clockwise and vertically upward in low pressures systems (cyclones) in the north hemisphere and, in the opposite in high pressure systems (anticyclones). The rising air in low pressure systems, together with the associated cloudy skies, fast surface winds, and low penetrations of solar radiation help disperse air pollutants. On the other hand, the sinking air, slow surface winds, cloud free skies, and high penetrations of solar radiation associated with high pressure systems favour the accumulation of air pollutants. V.2024.05.10 Page 5 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR Wind is the most significant meteorological element that controls day to day variation of air pollution over a city. It is characterised by both wind speed (velocity) and direction. It affects the dispersion in the way that the: a) Air pollutant concentration is inversely proportional to the wind speed b) Mechanical turbulence, which is created by wind, increases with wind speed c) Plume rise of the emissions decrease with wind speed In general, wind speed increases with height above the ground level (u(z) = u(z0)(z/z0)p ; p an exponent varying with the surface roughness and stability from about 0.1 (open country) to about 0.4 (urban area)), as frictional drag, which depends on surface roughness decreases. Wind direction depends on the prevailing air flow in the general global circulation, cyclonic and anticyclonic flows associated with migrating low pressure and high-pressure systems, monsoons, and local topography. Wind direction and its variability have significant effects on air quality. The irregular air movements over the main current of wind flow are called turbulent eddies. They are produced by: a) Mechanical turbulence, which is induced by wind moving over or around surfaces or structures This copy is issued to Lau Chin Tung and increases with wind speed and surface roughness b) Thermal turbulence, which is larger than mechanical eddies and resulted from the heating and or cooling of air and the convection of heat near the earth’s surface. Turbulence, other than the building downwash effect, enhances atmospheric mixing and helps air pollution dispersion Vertical air motion is equally important in air pollutant dispersion as it affects the height of the air layer where dispersion can take place and the amount of air available for air pollutant mixing. The change in temperature with elevation is key parameter that affects the vertical dispersion. Under ideal condition, the rate of temperature decrease with height (lapse rate; dT/dz) equals to the adiabatic lapse rate (Γa) of 9.8oC/km. The actual rate of temperature change with elevation is the ambient lapse rate or environmental lapse rate. If the lapse rate is greater than Γa, i.e., superadiabatic, air becomes buoyant and such condition is characterised as unstable atmospheric condition. This is usually common during daytime conditions when the sun heats the ground surface. Because of the greater degree of turbulence, it provides a good dispersion condition for air pollutants. On the contrary, stable atmospheric conditions which is unfavourable for dispersion occur if the lapse rate is less than Γa, i.e., subadiabatic. Neutral atmospheric condition occurs if the lapse rate equals to Γa. In extreme cases, the lapse rates increase with height and such atmospheric conditions are temperature inversion which is most susceptible to high and serious air pollution due to the suppression of the vertical mixing of air pollutants. Temperature inversions include the following: a) Radiation inversion: Resulting from the radiative cooling air near the ground surface during calm and cloud-free nights. Air pollution episodes in Hong Kong during wintertime (e.g., 19.1.2003, 31.12.2003) could be associated with radiation inversion. b) Subsidence inversion: Occurs within high-pressure system over large geographical areas. The descending air compresses and warms up an air layer aloft. In Hong Kong, subsidence inversions may also be formed as a result of air subsidence associated with tropical storms several km away (e.g., near the Taiwan Strait). The air pollution episode of Hong Kong in 2.11.2003 is an example. V.2024.05.10 Page 6 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR c) Frontal inversion: The cold, dense air of the cold front forces air in warm area to rise. Based on meteorological conditions and time of day, the atmospheric stability can be grouped into 6 categories, namely, A (very unstable), B (unstable), C (slightly unstable), D (neutral), E (stable), and F (very stable). The different categories and their relevant parameters are shown in Table I-2 below. Table I-2: Atmospheric stability categories Day Night Wind speed at Incoming solar radiation Thin overcast 10 m/s Strong Moderate Slight ½ low cloud ⅜ cloud 6 B D D D D The point at which the air parcel cooling at the dry adiabatic lapse rate intersects the ambient temperature profile represents the mixing height (MH), which is the height of the vertical volume of air above the earth’s surface where relatively vigorously mixing and pollutant dispersion occurs. MH varies both diurnally and seasonally, usually greater during afternoon or in summertime. It is also affected by topography and macro-scale air circulations, e.g., high pressure systems. The plumes from point sources (stacks) rise buoyantly because they are hotter than the surrounding air and that they exit upward with a vertical velocity. Plume rise, therefore, depends on the difference of stack and ambient temperature (Ts – Ta) and exit velocity (vs), and stack downwash (downdraught) occurs if vs < 1.5 x wind velocity. The vertical and horizontal air motions also influence the behaviour of plumes, such as: a) Looping plume, which occurs in highly unstable conditions associated with strong turbulences and rapid overturning of air b) π Fanning plume, which occurs in stable conditions which discourage vertical motion了 without prohibiting[horizontal motionv c) Coning plume, which is characteristic of neutral or slightly stable conditions d) e) Lofting plume, which occurs when the plume is above anCinversion 吧⼭ Fumigation, which occurs when the plume is released just under an inversion3 layer and serious air pollution situation can develop Topography also influences air pollutant levels. For example, the area near the seashore may be affected by sea land breezes, which are resulted from the differential heating and cooling air over land and sea surface. The circulation sea land wind patterns may allow air pollutants to be recirculated and accumulated. In Hong Kong, sea land breezes are significant to the low-level wind convergence in early afternoon in western part of the territory in autumn and winter. The air pollution episode in 14.2.2004 is an example. The other topographical effect includes the valley mountain breezes resulting from the differential heating and cooling of air on the mountain slopes. Valleys are more susceptible to air pollutant accumulation and for this reason, Hong Kong has been requiring the use of gaseous fuel only in the Sha Tin area. V.2024.05.10 Page 7 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR Urbanization is another factor that could influence the local meteorology and air pollution levels. In urban cities, concrete buildings absorb heat during the day and radiate it at night, creating a heat island, which sets up a self-contained circulation for the air pollutants. The urban heat island effect causes an increase in the temperature gradient between the land and sea surface, resulting in a strengthening of the sea land breeze circulation in the Hong Kong and PRD region. Urban cities are often suffering from street canyon effects. Street canyon is a relatively narrow street with tall, continuous buildings on both sides. Those with aspect ratios (height to width) greater than 2 are deep canyons in which air pollutants are likely be trapped and recirculated. It is a reason why Des Voeux Road and Queens Road Central in Hong Kong have significantly higher air pollutant levels than the nearby Connaught Road which has about 5-10 times more traffic. 1.4.3 Spatial and temporal variation of air pollutant concentrations Meteorological factors affect the spatial and temporal variation of air pollution. In general, as the 散開 ⼭ primary air pollutants (e.g., SO2, CO) disperse, a diminishing concentration gradient with increasing distance to the source develops. In contrast, the formation of secondary pollutants is a large-scale phenomenon, and these air pollutants have quite uniform spatial distributions. For PM, the spatial variability depends also on the size fraction. For fine particulates, e.g., PM2.5, the spatial variability is This copy is issued to Lau Chin Tung generally small although the spatial variation can be significant for specific components such as elemental carbon from diesel vehicles. NO2 generally exhibits some spatial variability owing to local sources such as vehicles. For O3, clean rural areas usually have significantly higher concentration than urban centres, in particular, roadsides, owing to the scavenging reaction with NO from vehicles. 清除反應 As regard to temporal variation, the most significant cycle that influences on variability of air pollutant ⽇ concentrations is the diurnal variation of the atmospheric stability and other relevant meteorological氣象 factors. The other cycle is the seasonal cycle associated with difference in climate and weather over the year. In general, Hong Kong’s air pollutant concentrations are heavily influenced by monsoons which are seasonal variations in atmospheric circulation associated with a difference in thermal capacity between the land and ocean. Other than the occasional O3 and photochemical air pollution E A L N C episodes on days with sunny, hot, and stagnant days, the southwesterly monsoons with less air pollutants help to disperse the local air pollutants in summer. On the other hand, the frequent and 9 S H 1 copious rainfall also helps to remove ambient pollutants. In winter, the northeasterly monsoons bring a cold, dry, and polluted air from the north to Hong Kong. The variation in meteorological conditions is unique for each location. There is a need to understand the impacts arising from different meteorological conditions and the topographical effects of a specific location with respect to temporal and spatial variations for effective tackling of the air pollution problem. V.2024.05.10 Page 8 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR 1.5 LOCAL, REGIONAL AND GLOBAL AIR POLLUTION Air pollution may be grouped according to spatial scales. Local scale issues are usually caused by a few major emitters or a large number of small or medium emitters. Roadside air pollution, which is a major issue in Hong Kong, is a typical example. In urban scale air pollution, both the release of primary air pollutants and formation of secondary air pollutants are concerns. In addition to the “hot spots” of high concentrations in the vicinity of major sources, many large urban cities are experiencing the formation of O3 from photochemical reactions of oxides of nitrogen and VOCs emitted from vehicles and other sources. For regional scale, the main concerns include the blend of urban oxidant problems at the regional scale in region with cities in close proximity (e.g., Hong Kong and PRD cities) and the release of relatively slow-reacting primary air pollutants that undergo reactions and transformations during lengthy transport times (e.g., secondary particulates). Like many cities, Hong Kong is suffering from local, urban, and regional air pollution. 1.5.1 Photochemical air pollution Photochemical air pollution is a regional air pollution problem. Secondary air pollutants are not directly emitted from sources but produced in the atmosphere by the action of sunlight on NOx and This copy is issued to Lau Chin Tung volatile organic compounds (VOCs) with O2. In polluting atmosphere, the presence of VOC shifts the balance between O3 formation (e.g., photolysis of NO2) and destruction (e.g., reaction with NO) that gives rise to low background O3 concentrations. Oxidation of VOCs by OH produces an organic peroxy radical. It reacts with and oxidizes NO to form: (i) NO2, which goes on to undergo photochemical reactions (at λ < 430 nm) to produce O3, and (ii) RO radical, which may react with O2, thermally decompose, or isomerize to produce many different compounds, including carbonyl compound R’CHO (e.g., acetaldehyde). diduit onsume O 3 → NNO 是七 Accordingly, NO is converted to NO2 without consuming O3 and as a result, O3 accumulates because there is not enough NO to destroy it. Peroxy radicals may also react with NO2 when its concentration is not too high to produce peroxides and other oxygenated compounds. Among others, PAN, which is an eye irritant and lachrymator, is one of the common photochemical oxidants produced. O3 also reacts with other air pollutants, e.g., SO2, NOx, to produce elevated levels of fine particulates. Meteorological conditions that favour photochemical smog formation include: a) High temperature, low humidity b) Strong, prolonged sunshine c) High irradiation 照射 d) Low wind speed As such, the subsidence inversion caused by typhoons away from Hong Kong (e.g., over Taiwan Strait) provides a favourable condition for elevated O3 in Hong Kong. Other weather patterns such as those with high-pressure centres located to the north of Hong Kong are also susceptible to elevated O3. Because of the complex air chemistry involved, O3 concentrations usually peak in the afternoon when sunlight is strong in a downwind distance from where the precursors are generated. Rural areas usually have significant higher O3 than urban areas (in particular, roadsides) due to the generally absence of NO for O3 removal. The O3 formation is sensitive to the VOCs (ppm)/NOx (ppm) ratios and the plot of O3 levels with the concentrations of these two precursors is known as the ozone isopleth. At regions with low NOx (e.g., VOCs/NOx >> 8), O3 levels are relatively insensitive to the quantity of VOCs, hence, it is more effective to reduce O3 by reducing NOx. It is referred to as NOx-limited. Conversely, at the regions with high NOx (e.g., VOCs/NOx alkenes > aldehydes > alkanes). As Hong Kong is mainly in a VOCs-limited situation, in addition to concerted regional control efforts, controlling local VOCs or cutting deeply the local NOx emissions would help reduce the O3 concentrations. 1.5.2 Acid rain, long-range transportation Long range transportation Acid rain or acid deposition, which includes the deposition of dry and wet acidic aerosols or particles, is an important long-range transportation air pollution problem. It originates from the emission of SO2 and NOx from combustion which are oxidized in the air to form acids (e.g., sulphuric acid, nitric acid) and very fine sulphate and nitrate particles. The acidic aerosols can travel for a very long distance from the source and deposit on land as acidic substances. Acidic deposition, whether in dry or wet form, contributes to changes in the pH of soil and freshwater lakes. Adverse effects include: a) Poor forest health due to acidification of soil and killing of nutrient-producing microorganisms by This copy is issued to Lau Chin Tung acids b) Death of aquatic life (e.g., trout and bass) c) Leaching of mercury out of the soil, causing toxic levels to build up in the fish that may eventually be consumed by human beings d) Erosion of buildings, monuments and destroy paint finishes Other examples of long-range transportation of air pollutants include dust storms and transportation of fine particulates (e.g., the Asian dust storm). Details relating to these long-range transportation with global scale are presented in section 1.5.3. 1.5.3 Global scale pollutants Greenhouse gases and climate change Global warming refers to the ongoing rise in global average temperature (1.09oC higher in 2011-2020 than 1850-1900) near earth's surface since the Industrial Revolution. It is caused by human activities (in particular, fossil fuel burning, deforestation, industrial processes, and some agricultural practices) that increase concentrations of CO2 and other greenhouse gases (GHG) such as CH4, N2O, HFCs that absorb thermal-IR and heat up the atmosphere. In 2019, the annual averages of these GHG gases reached about 410 ppm, 1866 ppb and 332 ppb, 237 ppt HFC-134a equivalent, 109 ppt CF4 equivalent, 10 ppt and 2 ppt. respectively. Global warming changes of climate patterns, including longer and hotter heat waves, more frequent droughts, heavier rainfall, and more powerful hurricanes as well as other effects such as melting of polar ice caps and glaciers, early snowmelt, increase the risk of wildfires, rising sea levels and disruption of habitats such as coral reefs. The UN Framework Convention on Climate Change (UNFCCC) 1 , which entered into force in 1994, aims to stabilize GHG concentrations and prevent ⼈為 dangerous anthropogenic interference with the climate system. Its Paris Agreement 2 , which was adopted in 2015, is to strengthen the global response by keeping a global temperature rise this century well below 2oC (and 1.5oC, if possible) above pre-industrial levels. According to the Intergovernmental Panel on Climate Change (IPCC), limiting 1.5oC warming requires global GHG emissions to peak before 2025, and be reduced by 43% by 2030; at the same time, CH4 would also need to be reduced by about a third. 1 https://unfccc.int/ 2 https://unfccc.int/process-and-meetings/the-paris-agreement V.2024.05.10 Page 10 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR Climate change can also negatively impact local air quality. Atmospheric warming has the potential to increase ground-level ozone. Prolonged high temperatures due to climate warming could lead to drought conditions where forest fires, releasing carbon monoxide and particulates. In October 2021, the Government has released the “Hong Kong's Climate Action Plan 2050”, setting out the vision of "Zero-carbon Emissions‧Liveable City‧Sustainable Development", and outlining the strategies and targets for combating climate change and achieving carbon neutrality. The EPD has also compiled a set of nine sector-specific carbon management guidebooks for offices, schools, sports centres, swimming pools, public markets, healthcare facilities, fire stations, postal facilities, and community halls to serve as step-by-step references to build up the capability of carbon auditing and promote carbon management best practices for carbon reduction and management. 3 Ozone layer depletion Stratospheric O3 is in a dynamic equilibrium between the chemical processes of formation (photolysis of O2 by far-UV in upper layer) and destruction (photolysis of O3 by near-UV). It provides a UV shield to protect human life and biological processes on the earth’s surface. Chlorofluorocarbons (CFCs, which are used for air conditioning and refrigeration) and other ozone depleting substances (ODS, e.g., halons, HCFCs), could modify the equilibrium by releasing O3 consuming radicals (e.g., Cl) under This copy is issued to Lau Chin Tung shortwave UV radiation. Because they serve only as a catalyst in the chemical processes, it can destroy many O3 molecules. Since early 1980s, an “ozone hole" has been observed above the Antarctic where O3 layer could have shrunk to as low as 100 Dobson units (c.f., ~300 before 1980s) at the end of southern winters. The increase of UV radiation due to O3 layer depletion can result in serious adverse health effects such as cataracts, skin cancers and genetic mutations. The Vienna Convention 4 was adopted in 1985 to protect the O3 layer. Its Montreal Protocol (MP) 5 adopted in 1987 has made significant progress to ensure the replacement of ODS. In Hong Kong, all ODS had been banned from 2020. To provide a concerted effort on tackling global warming, the Parties to the MP adopted in 2016 the Kigali Amendment (KA) 6 to require progressive phasing down of the production and consumption of 18 types of hydrofluorocarbons (HFCs; e.g., HFC-134a, HFC-23), which have been used as substitutes for ODS, with global warming potentials (GWP) ranging from about 12 to 14,800 from 2024. Under the KA, Hong Kong needs to implement an import and export licensing system and import quota control for HFCs to progressively reduce their uses by 85% from the baseline level by 2036. To fulfil the KA requirements, the shift to the use of low-GWP refrigerants, e.g., R1233zd (GWP=1) and R514A (GWP=2) for water-cooled chillers, R600a (GWP=3) for household refrigerators, and R1234yf (GWP=4) for automotive air conditioners is expected. Persistent organic pollutants Persistent organic pollutants (POPs) pose major and increasing threats to human health and the environment. The observation of POPs in pristine Arctic regions (which are thousands of km from any known source) has prompted the adoption of the Stockholm Convention 7 to control both intentionally and unintentionally produced POPs, including aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, hexachlorobenzene, mirex, toxaphene, PCBs, dioxins, and furans). In 2013, the Minamata Convention 8 has also been adopted to control emissions of mercury (Hg) and its compounds. To ensure compliance with the Convention, the Mercury Control Ordinance (Cap 640) was put into operation in December 2021. 3 https://www.climateready.gov.hk/education_centre.php?section=guideline_reference_links&lang=2 4 https://ozone.unep.org/treaties/vienna-convention 5 https://ozone.unep.org/treaties/montreal-protocol 6 https://ozone.unep.org/sites/default/files/Handbooks/MP-Handbook-2020-English.pdf, p.916-926 7 http://www.pops.int/ 8 https://mercuryconvention.org/en V.2024.05.10 Page 11 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR 1.6 INDOOR AIR POLLUTION Indoor air quality (IAQ) is vitally important as people spend a large part of their time each day. Poor IAQ can lead to discomfort, ill health (e.g., headache, itchy eyes, respiratory difficulties, skin irritation, nausea, and fatigue), and in workplaces, absenteeism and lower productivity. Children, the elderly, and those with existing respiratory or heart disease are more susceptible to the effects of poor indoor air quality. It is also linked to the sick building syndrome and other building related illnesses. Indoor air pollution originates from both outdoor and indoor sources. Major indoor air pollutants and their indoor sources include: particulates (fuel combustion, cooking, tobacco smoking, etc), carbon monoxide (fuel combustion, tobacco smoking, etc), PAH (fuel combustion, cooking, tobacco smoking), nitrogen oxides (fuel combustion), VOCs (fuel combustion, consumer products, furnishing, paints, construction materials, cooking), aldehydes (furnishing, paints, construction materials, cooking), pesticides, asbestos (demolition of construction materials), moulds and biological air pollutants (damp materials/furnishing, occupants, pets, the bacterium Legionella pneumophila responsible for Legionnaires’ disease), and radon (granites, construction materials, soil under building). As a rough estimation, the steady state air pollutant concentration can be calculated by a simple mass balance equation: This copy is issued to Lau Chin Tung C = (Q × Ca + E) / (Q + k × V) Where Q = ventilation rate Ca = ambient (outdoor) concentration of the air pollutant E = rate of emission of the air pollutant V = Volume of the concerned indoor space k = rate of removal of the air pollutant Accordingly, the means to improve IAQ are: a) Prevention by source management such as source removal, banning of polluting activities (e.g., smoking,) and products (e.g., high VOCs paints and furnishing, UV disinfection) and treatment b) Exposure control by better scheduling of activities to reduce occupant exposures c) Reducing air pollutant levels by increasing ventilation (air exchange rate) and its effectiveness, air cleaning and purification Additionally, high efficiency air cleaners have been used to remove some of the indoor particulates and biological pollutants (e.g., bacteria and viruses). The 2003 Severe Acute Respiratory Syndrome (SARS) epidemic and the recent COVID-19 pandemic showed that coronavirus can be spread via airborne particles and droplets, especially in indoor environments. To reduce the chance of transmission, lowering the concentration of coronavirus by good ventilation/increase of air changes and filtration (e.g., by high efficiency particulate air (HEPA) filters) is essential. Physical distancing, wear masks, and avoidance of crowded indoor spaces also help avoid virus exposure. V.2024.05.10 Page 12 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR The World Health Organisation (WHO) has issued IAQ guidelines on selected chemicals 9, moulds 10, and household fuel combustion 11. To promote IAQ, the EPD has been implementing programmes including the IAQ certification scheme for offices and public places. The key features of the IAQ certification scheme are: a) A two-level IAQ objectives (Excellent Class and Good Class) is used as the benchmark to assess IAQ of premises. b) A voluntary and self-regulatory approach is adopted for annual certification. c) The premises owners or management need to employ an accredited IAQ Certificate Issuing Body (CIB) to assess IAQ of their premises against the IAQ objectives. d) CIB will issue an IAQ certificate for premises owners or management to register with the IAQ Information Centre if the IAQ objectives are complied with. The certificate will be displayed at prominent location together with the IAQ labels for the public information. e) The certificate is valid for 12 months. For renewal, a full set of parameters on IAQ objectives needs to be assessed once every 5 years, and for the 4 years in between, annual assessments of CO2, PM10 and mould are required to ascertain the compliance status. f) Certification is granted to a building as a unit, but premises owners or management may choose to certify only parts of the building. g) Premises owners or management are required to manage post-certification IAQ to ensure IAQ is This copy is issued to Lau Chin Tung maintained at the certified level. In 2019, the EPD has revised the IAQ objectives for Hong Kong as shown in Table I-3 below. Table I-3: 2019 IAQ objectives for Hong Kong Parameters Excellent Good Class Class Pollutant Averaging Unit Time Carbon dioxide (CO2) 8-hour mg/m3 1,440 1,800 Carbon monoxide (CO) 8-hour μg/m3 2,000 7,000 Respirable suspended particulates (PM10) 8-hour μg/m3 20 100 8-hour μg/m3 40 150 Nitrogen dioxide (NO2) 1-hour μg/m3 100 200 Ozone (O3) 8-hour μg/m3 50 120 8-hour μg/m3 30 100 Formaldehyde (HCHO) 30-min μg/m3 70 100 Total volatile organic compounds (TVOCs) 8-hour μg/m3 200 600 Radon (Rn) 8-hour Bq/m3 150 167 Airborne bacteria 8-hour cfu/m3 500 1,000 Mould Satisfy the criteria set out in the prescriptive checklist 9 WHO guidelines for indoor air quality: selected pollutants (https://www.who.int/publications/i/item/9789289002134) 10 WHO guidelines for indoor air quality: dampness and mould (https://www.who.int/publications/i/item/9789289041683) 11 WHO Guidelines for indoor air quality: Household fuel combustion (https://www.who.int/publications/i/item/9789241548885) V.2024.05.10 Page 13 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR The EPD has also published “Guidance Notes for the Management of Indoor Air Quality in Offices and Public Places” and other IAQ leaflets to help premises owners/management and the public to improve the IAQ in their premises. Similar to indoor environment, semi-confined spaces, such as covered public transport interchanges, multi-storey car parks, are often suffering from poor natural ventilation. Air pollutants would accumulate to high concentrations within these spaces if adequate mechanical ventilation is not provided. 天井 Some semi-confined spaces of high-rise buildings, e.g., the light wells or semi-open light wells, might also be susceptible to the accumulation of air pollutants. Example includes the spread of the SARS/Covid-19 viruses among flats of different floors sharing same light well/semi-open light well of a high-rise residential building as a result of the chimney effect and building downwash. To reduce the escape of the viruses from toilets, proper filling of the drainage U-traps with water is essential. This copy is issued to Lau Chin Tung V.2024.05.10 Page 14 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR 2 AIR POLLUTION SOURCES AND IMPACTS 2.1 ANTHROPOGENIC SOURCES OF AIR POLLUTION 難以提摸 Stationary, mobile, fugitive, and non-fugitive Almost all human activities generate air pollutant emissions. In cities and other densely populated urban areas, air pollution is mainly due to anthropogenic sources, which may be classified as: ⼈為 a) Stationary sources: Includes combustion processes (e.g., power plants and industrial boilers), chemical processes (e.g., petroleum refining and organic liquid storage), mineral processes (e.g., 璧 hot mix asphalt, cement manufacturing and concrete batching), metallurgical processes (e.g., iron and steel production), waste disposal facilities (e.g., municipal/sewage sludge/medical waste incinerators) and VOCs emitting processes (e.g., graphic printing, surface coating). There are also facllities other small but widespread sources, e.g., restaurants and food cooking which could be major local air pollution and nuisance concerns due to the excessive emissions of cooking fumes and odour. In Hong Kong, electricity generation is the most important stationary source, accounting for about 52% of SO2, 24% of NOx, 13% of PM10, and 11% of PM2.5 emissions in 2020. This copy is issued to Lau Chin Tung b) Mobile sources: Includes vehicles, trains, aircrafts, vessels, and non-road mobile machinery, etc. Emissions of NOx and PM from diesel engines are of main concern. In Hong Kong, vessels are the biggest emission sources, accounting for about 39% of SO2, 36% of NOx and 29% of PM10, 35% of PM2.5 and 37% of CO emissions in 2020. Vehicles are responsible for about 1% SO2, 19% of NOx, 10% of PM10, 11% of PM2.5 and 47% of CO emissions in the same year. The air pollutant emissions changes of Hong Kong from anthropogenic sources over the last three decades are presented in Table I-4 below. Table I-4: Change of air pollutants emissions in Hong Kong from anthropogenic sources in tonnes Air Pollutants 2000 2010 2020 Particulate matters RSP (PM10) 6090 6,310 2,930 FSP (PM2.5) 4,780 5,000 2,290 Sulphur dioxide 79,530 36,310 4,940 Nitrogen oxides 107,080 109,100 56,680 Non-Methane VOCs 30,830 32,000 21,910 Carbon monoxide 80,690 85,600 57,810 * Does not include emissions from hill fires Air pollution sources may also be grouped as point sources (i.e., stationary source with significant emissions), area sources (i.e., a number of small stationary, including domestic and commercial enlarge sources, and mobile sources that may have enormous effect collectively over an area). Air pollutants 排出⼝ may also be emitted from sources without well-defined exhausts, e.g., dust emissions from construction activities, paved and unpaved roads; VOCs leakage from vents, valves, flanges from oil depots; odour from landfills. They are referred to as fugitive sources. V.2024.05.10 Page 15 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR 2.2 NATURAL SOURCES OF AIR POLLUTION Volcanoes, wildfires, sea spray, vegetation, etc. Despite that our air pollution problem is mainly due to anthropogenic sources, most of the air pollutants are coming from natural sources. They are highly variable based on many factors including geography, season, and climatology. Examples of natural air pollution sources include: a) Dust storms (e.g., the dust storms from the Gobi Desert and northern China may transport particulates to other part of Asia, including Hong Kong which has experienced the worst dust storm incident on 21-23.3.2010) b) Vegetation, plants, and trees (e.g., isoprene, which is emitted by many tree species, especially as part of their protective measure and can contribute to O3 and secondary organic aerosol formation and pollens that induce hay fevers) c) Sea spray (e.g., sea salt aerosols) d) Wild forest fires (e.g., smoke, PM, VOCs from Australian and Californian forest fires) e) Volcano eruptions (e.g., ash, PM, acid mists, SO2, hydrogen sulphide and toxic gases) This copy is issued to Lau Chin Tung 2.3 RECEPTORS (HUMAN, ANIMAL, PLANT, MATERIALS, ATMOSPHERIC PROCESSES) A receptor/receiver is something which is adversely affected by polluted air, including human beings, animals, trees or plants, materials, and the environment as a whole. Some sub-populations are more sensitive to air pollution and they include children, the elderly, and individuals with existing illnesses (e.g., chronic obstructive pulmonary disease (COPD), asthma, pneumonia, cardiovascular disease, lung cancer). 2.4 SOURCE / RECEPTOR RELATIONSHIPS (SPATIAL & TEMPORAL) The concentration of an air pollutant is, in general, proportional to the strength of emissions. Other than O3 and secondary air pollutants which could have maximum concentrations farther away from sources, most primary air pollutants (e.g., CO, SO2, NOx) usually have higher concentrations at receptors close to the sources. In coastal urban cities like Hong Kong, motor vehicles and vessels are important sources of elevated pollutant concentrations. Receivers at busy roadsides and shorelines are having higher concentrations of associated air pollutants (i.e., NOx and PM10/PM2.5 for roadsides and SO2 and NOx for shoreline receptors). The spatial relationship between sources and sub- population of lower socioeconomic position often reflects the presence of environmental equity issues within the city. By virtue of the location of their place of residence, school, or workplace, they are often receiving higher air pollutant exposures. Temporally, the concentrations of air pollutants, other than O3 and other secondary air pollutants which may have different diurnal patterns (e.g., O3 usually peaks in the afternoon because the solar radiation intensity is the highest), follow generally the diurnal pattern of human activities and traffic. For examples, higher levels of NO2, PM2.5 and PM10 are usually observed in the morning and the evening rush hours when there are more traffic and human activities. The weekday-weekend patterns can also be associated with differences of emissions in weekdays and weekends. In addition, the V.2024.05.10 Page 16 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR difference of demands for heating/cooling over seasons also results in the seasonal changes of combustion related air pollutants, e.g., SO2, NOx. The long-term annual air quality trend often has greater association with the emission sources due to the averaging effect of the meteorological variation. It is useful in assessing the effectiveness of emission control measures on improving air quality at the affected receptors. 2.5 ADVERSE EFFECTS 2.5.1 Health effects (mortality, morbidity, respiratory illness, sub-clinical effects) Air pollution is the largest contributor to the burden of disease from the environment. The WHO estimated that ambient air pollution in 2019 was responsible for 4.2 million premature deaths. Exposure to air pollution, depending on the nature of the air pollutants (e.g., physical state, particle sizes, vapour pressure, and toxicity), susceptibility of the receptors (e.g., gender, age, and health status), and concentrations and exposure duration, can have significant impacts on human health. A This copy is issued to Lau Chin Tung broad range of adverse health effects ranging from death from respiratory diseases, stroke, heart disease, brain damage, eye irritation, to reduced quality of life, and including some irreversible changes in physiological function, have been identified to be associated with air pollution. The adverse effects can be short term (acute) and long term (chronic), resulting from short term exposure at high concentrations and long-term exposure at lower concentrations of air pollutants. The short-term adverse effects are expressed as: daily mortality, respiratory and cardiovascular hospital admissions, respiratory and cardiovascular emergency department visits, respiratory and cardiovascular primary care visits, use of respiratory and cardiovascular medications, days of restricted activity, work absenteeism, school absenteeism, acute symptoms (wheezing, coughing, phlegm production, respiratory infections), and physiological changes (e.g., lung function). Long term effects include mortality due to cardiovascular and respiratory disease, chronic respiratory disease incidence and prevalence (asthma, COPD, chronic pathological changes), chronic changes in physiologic functions, lung cancer, chronic cardiovascular disease, and intrauterine growth restriction (low birth weight at term, intrauterine growth retardation, small for gestational age). The relationship between air pollution and health is established by: a) Epidemiological studies – e.g., time series studies that estimate the influence of temporal (usually daily) variations in air pollutant concentrations on mortality or morbidity by statistical models, and cohort studies that estimates chronic effects by comparing people living in different geographical locations with different air pollution exposures. 吸入 b) Toxicological studies – effects of air pollution have been generated through inhalation studies, whereby human volunteers or animals are placed under controlled exposure conditions. preventlon c) Studies of effects of air pollution interventions. hizn Some of the effects associated with the more common air pollutants are: ⼀ ⑥ ⑳ a) CO – Reduces the oxygen carrying capacity of red blood cells. ⑧ Typical symptoms of exposure to low levels of CO include headache, dizziness, and tiredness. Higher concentration of CO can lead to impaired vision, disturbed coordination and eventually death. V.2024.05.10 Page 17 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  四主⼒ δ ” -興[ I. AIR b) NO2 – Irritates the mucosa of the eyes, nose, throat, and the lower respiratory tract. Exposure to low level of NO2 may cause increased bronchial reactivity and in those with asthma increased response to allergens. NO2 also aggravates existing chronic respiratory diseases. Long- term exposure to NO2 can lower a person's lung function and resistance to respiratory infections. It is responsible for the largest short-term health risks in Hong Kong and other urban areas with 修號四 @ heavy traffic. c) O3 – Irritates eyes and brings upper and lower respiratory symptoms to healthy people. It may also provoke asthmatic attacks in asthmatics. O3 can also increase a person's susceptibility to respiratory infection and aggravate pre-existing respiratory illnesses. It is responsible for the largest short-term health risks in areas with little traffic, e.g., rural areas. d) Particulates – Both PM10 and PM2.5 are respirable and pose adverse health effects. The latter are more dangerous since, when inhaled, they may get deeper into the lungs. Increases in particulates concentration are associated with increases in daily hospital admissions and premature deaths from respiratory and cardiovascular diseases. Persons with pre-existing cardiovascular and respiratory diseases are most susceptible. Diesel particulates and PM have been classified as carcinogenic to humans (Group 1) by IARC. They are responsible for the largest air pollution induced cancer risk. This copy is issued to Lau Chin Tung e) SO2 – Irritates eyes and nose. Inhalation of SO2 causes narrowing of the airways (bronchoconstriction), which people suffering from asthma and chronic respiratory diseases are more sensitive to than other people. f) Lead – Highly toxic and is known to damage the nervous system and kidney and interferes with the synthesis of haemoglobin. Children are more vulnerable to the effects of lead, which can result in learning disabilities and impaired neurobehavioural functioning. g) Toxic air pollutants – TAPs pose various serious and irreversible adverse health threats to humans. Many (e.g., diesel particulates, formaldehyde, benzene, 1,3-butadiene, benzo[a]pyrene and other PAHs, dioxins, compounds of Cd, Cr(VI) and Ni) are carcinogens or suspected carcinogens. Other major health concerns include mutagenic, teratogenic, neurotoxic effects. The World Health Organization have prioritised and selected the relevant health effects/outcomes for each of the air pollutants in developing its air quality guidelines. For example, all-cause and respiratory mortality have been selected for PM2.5, PM10, O3, NO2. In addition, cardiovascular and lung cancer mortality have also been considered for PM2.5 and PM10. 12 2.5.2 Ecological impacts, vegetation, and forest deterioration Effects on vegetation The air pollution effects on plants and vegetation may be visible or nonvisible. The visible effects include the destruction of the waxy coatings on leaves or deviations in the normal healthy appearance of leaves, including tissue collapse and loss of colour. These observable alterations in the plant are termed air pollution injury. Non-visual effects involve reduced plant growth, changes in the reproductive cycle, or alteration of physiological or biochemical changes. The economic or aesthetic loss due to interference with the intended use of a plant is termed air pollution damage. The major air pollutants which are phytotoxic to plants are O3, SO2, NO2, fluorides, and PAN. 12 World Health Organization, WHO global air quality guidelines. Particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide, p.36-45, 2021 V.2024.05.10 Page 18 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR Effects on animals A major concern is with deposition and accumulation of airborne contaminants on vegetation or forage that serves as feed for the animals. The major air pollutants of concern include heavy metals (e.g., As, Pb, Hg) from industrial air emissions, fluoride emissions from fertilizer processing facilities, and dioxins from incinerators. Effects on forests The pollutants most often involved are SO2 and hydrogen fluoride. Historically, the most harmful sources of pollution for forest ecosystems have been smelters and aluminium reduction plants. The impact of acid deposition on forests depends on the quantity of acidic components received by the forest system, the species present, and the soil composition (e.g., alkalinity). Materials corrosion The major effects of air pollutants on materials include: a) Metal: corrosion of the surface, loss of metal, tarnishing by SO2 and other acidic gases b) Building materials: discoloration, soiling of surfaces, leaching of metals, and affecting the integrity of the structures by SO2, acidic gases, sticky particulates This copy is issued to Lau Chin Tung c) Monuments: deterioration of stone or marble structures by SO2 and other acidic gases d) Paint: discoloration, softening of finishes by SO2, H2S, sticky particulates e) Leather: powdered surface, weakening by SO2 and other acidic gases f) Paper: embrittlement by SO2 and other acidic gases g) Textiles: loss of tensile strength, spotting by SO2 and other acidic gases h) Dyes: fading, yellow discoloration by NO2 and oxidants i) Rubber: cracking, weakening by O3 and oxidants j) Ceramics: change of surface appearance by acidic gases Lake acidification Acidic deposition, in either a wet or dry form, contributes to the changes of the acidity in soil, lake and fresh water. Much of the acidity is neutralized by dissolving and mobilizing minerals in the soil. In watersheds with low alkalinity soil, lakes and streams are susceptible to low pH and leaching of aluminium, calcium, magnesium, sodium, and potassium from the soil into surface waters. This combination has been found to be very toxic to some species of fish. When the pH drops to >5, many species of fish are no longer to reproduce and survive. Another area of concern of fresh water or lake acidification is reduced tree growth in forests. As acidic deposition moves through forest soil, the leaching process removes nutrients. If the soil base is thin or contains barely adequate amounts of nutrients to support a particular mix of species, the continued loss of a portion of the soil minerals may cause a reduction in future tree growth rates or a change in the types of trees able to survive in a given location. 2.5.3 Others Visibility Visibility is the maximum distance at which a dark object can be observed against a bright background. Visibility impairment, or reduced visibility, which is the degradation of the ability to perceive the environment, is the most recognizable effect of air pollution. It is caused by the light extinction in the atmosphere as a result of light scattering and absorption by water vapour (humidity) and gaseous air pollutants (e.g., NO2, O3) and particulates (PM2.5; those with 0.1-1 µm in size are most effective at scattering light). In Hong Kong, it has been shown that ammonium sulphate, organic C, and elemental C in the PM2.5 are the more important contributors to visibility degradation. V.2024.05.10 Page 19 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR Quantitatively, visibility impairment is often referred to visibility below 8 km when the relative humidity is below 95% and there is no fog, mist, or precipitation. Due to the previous air pollution control efforts, the number of hours of reduced visibility observed in Hong Kong had been improved to 401 hours in 2022 from the worst of 1,570 hours in 2004. Odour Some air pollutants are odorous. Nuisance is the primary effect and the factors relevant to perceived odour nuisance are: a) Its offensiveness b) Duration of exposure c) Frequency of occurrence d) Tolerance and expectation of the receptor Some odourous pollutants (e.g., H2S, O3) can cause adverse health effects and for others, even they are not causing direct damage to health, they may give rise to nausea, insomnia, and discomfort. Very strong odour can result in nasal irritation, and/or can trigger symptoms in individuals with asthma or other breathing problems. This copy is issued to Lau Chin Tung 2.6 HEALTH IMPACT ASSESSMENTS, ECONOMIC IMPACT ASSESSMENTS, AIR TOXICS AND RISK ASSESSMENTS Health impact and risk assessments are a combination of procedures, methods, and tools to characterise respectively the human health and environmental risks associated with exposure to pollution. The term ‘risk’ is defined as the probability of having an adverse health or environmental outcome (e.g., heart and lung diseases, or dying). The increase of risk is expressed as its relative risk (RR). It is defined as the ratio of the incidence of (or death from) a disease or environmental outcome, in a population group that is exposed to air pollution compared to the incidence in a group that is not exposed to air pollution. Excess risk (ER) is defined as RR -1 and is usually expressed as a percentage. Another measure of the importance is the ‘attributable fraction’, which is equal to (RR-1)/RR. Typically, these assessments evaluate the impact of existing level of pollution and/or the effects of an air quality improvement policy or programme. The assessment involves the estimation of the total number in the population who developed health effect (ΔH) associated with the air pollutant exposure. ΔH = H (RR – 1) = H (exp(β ΔC) – 1) where: H = baseline number in the population who developed the health effect (i.e., baseline incidence rate x population) RR = the relative risk of developed health effect associated with the air pollutant exposure β = dose response coefficient of the health effect per unit change of air pollutant concentration ΔC = change in air pollutant concentration The economic impact assessment of an air quality improvement policy or programme is a combination of procedures, methods, and tools which aims to evaluate its cost and the economic impact of and quantify the economic value of its benefits. In addition to the costs (including capital costs and operating costs of air pollution mitigation and other social costs), the other important element of the assessment is the quantification of the health impacts. V.2024.05.10 Page 20 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR In general, health costs can generally be taken as the sum of the costs of mortality and morbidity. For the former, there are two approaches in quantifying the cost of mortality: i. The value of statistical life (VSL), which is derived from aggregating individuals’ willingness to pay (WTP) to secure the reduction in the risk of premature death (e.g., US$7.9 M (in 2008 dollars) in USA) ii. The value of a statistical life-year (VSLY), which estimates the values for a year of statistical life rather than life (in practice, it is often derived by dividing the VSL by remaining life expectancy) The respective epidemiological parameters used in calculating the economic cost are excess premature deaths and year of lives lost (YLL), respectively. The costs of morbidity include: a) Resource costs – e.g., the direct medical and non-medical costs associated with treatment for the adverse health impact of air pollution, plus avertive expenditures) b) Opportunity costs – e.g., those associated with the indirect costs related to loss of productivity and/or leisure time owing to the health impact c) Dis-utility costs – i.e., those related to the pain, suffering, discomfort, and anxiety linked to the This copy is issued to Lau Chin Tung illness Among others, the excess hospital admission and years lived with disability (YLD) are the more common epidemiological parameters of morbidity used in the assessment. V.2024.05.10 Page 21 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR 3 AIR POLLUTION MODELLING 3.1 PURPOSES OF AIR QUALITY MODELLING Air quality modelling uses mathematical and numerical techniques to simulate the physical and chemical processes that affect air pollutants as they disperse and react in the atmosphere. These models are important to an air quality management system and have been widely used to: a) Simulate ambient pollution concentrations under different scenarios b) Determine the relative contributions from different sources c) Augment the air quality monitoring for assessing the air quality standard compliance status d) Support real-time air pollution forecasting as routine operation and/or during episodes e) Support environmental impact assessment (EIA), plant siting and regulatory requirements 3.2 LEVELS OF MODELLING EFFORT This copy is issued to Lau Chin Tung Air quality modelling may involve two levels of sophistication. The first level consists of the use of relatively simple screening models to provide conservative estimates of the air quality impact of the sources. The purpose is to eliminate the need of more detailed modelling for those sources that clearly will not cause significant impacts on air quality. The second level involves the use of refined models with more refined inputs and detailed treatment of physical and chemical atmospheric processes. They need to be use for proper air quality assessment, planning and compliance evaluation. Air quality models may also be grouped as regulatory models and research models. The former are accepted for regulatory air quality assessments to ensure consistency. The latter are useful for air quality forecast and other scientific applications given their better air quality prediction accuracy. 3.3 TYPES OF AIR POLLUTION MODELS 3.3.1 Box, Gaussian dispersion Box Model The box model is the simplest of the models by assuming the volume of atmospheric air is in the shape of a box and that the air pollutants inside the box are homogeneously distributed. The steady state concentration is given by: C = Q × L / (H × u) where: Q = emission rate (g/s) H = height of the box (mixing height) (m) L = distance over which the emission takes place (m) u = mean wind speed (m/s) V.2024.05.10 Page 22 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR mormstribution Gaussian Dispersion The Gaussian dispersion model provides a simple solution to the dispersion (transport and diffusion) equations for constant emissions under steady-state meteorological conditions. It assumes that: a) Atmospheric stability and all other meteorological parameters (e.g., wind speed and direction) are uniform and constant throughout the layer into which the pollutants is discharged b) Turbulent diffusion is a random activity and therefore the pollutant concentrations can be taken as normally distributed in a bell-shaped curve about the plume centre line c) Pollutant is released at an effective stack which is the sum of the physical stack height and plume rise due to its momentum and buoyancy d) The degree of dilution is inversely proportional to the wind speed e) Pollutant material reaching the ground level is reflected back into the atmosphere f) The pollutant is conservative, i.e., not undergoing any chemical reactions, transformation or decay It can be described by the following equation: 2 2 2 −1 𝑦𝑦 −1 𝑧𝑧−𝐻𝐻 −1 𝑧𝑧+𝐻𝐻 Q 𝐶𝐶(𝑥𝑥, 𝑦𝑦, 𝑧𝑧, 𝑡𝑡) = 𝑒𝑒 2 𝜎𝜎𝑦𝑦 𝑒𝑒 2 𝜎𝜎𝑧𝑧 + 𝑒𝑒 2 𝜎𝜎𝑧𝑧 2πu𝜎𝜎𝑦𝑦 𝜎𝜎𝑧𝑧 This copy is issued to Lau Chin Tung where: C(x,y,z) : pollutant concentration at point (x,y,z) u: wind speed (in the x "downwind" direction) σy, σz: the standard deviation of the concentration in the y and z direction Q is the emission strength H is the effective stack height The standard deviations are dependent on atmospheric stability and distance from the source. The equation can be spatially integrated to simulate the effects of line, area, and volume sources. Gaussian dispersion models commonly used by the modelling community are: (i) AERMOD developed by USEPA; (ii) CALINE-4 developed by the Department of Transportation of California, USA for modelling vehicular emission impacts; and (iii) ADMS developed by CERC in UK. 3.3.2 Photochemical Photochemical models simulate the changes of pollutant concentrations in the atmosphere over large spatial scales using a set of mathematical equations characterizing the chemical transformation (gas and aqueous phase and heterogeneous chemistry) and physical processes (horizontal and vertical advection and diffusion) and removal process (dry and wet deposition) in the atmosphere. Depending on the frame of reference, photochemical air quality models may take the form of a Lagrangian trajectory model that employs a moving frame of reference, or the Eulerian grid model that uses a fixed coordinate system with respect to the ground. Lagrangian models keep track of the movement of a large number of air parcels carried by the wind along trajectories. They simulate pollutant concentrations inside the air parcels at different locations at different times. Eulerian models allow air pollutants enter and leave each grid cell and simulates the concentrations at all locations as a function of time. Lagrangian models are computationally simple but the physical processes they can describe are somewhat incomplete. On the other hand, Eulerian models are more capable of accounting for topography, atmospheric thermal structure, physical processes, and reactive pollutants from many sources. Most of the current operational photochemical models have adopted the three- dimensional Eulerian grid modelling approach. V.2024.05.10 Page 23 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR The photochemical grid model simulates the atmosphere by dividing it into thousands of individual grid cells (e.g., 1 km x 1 km with thickness varying from less than 20 m near the ground to a few km at the higher levels of the atmosphere). Driven by a meteorological model that is similar to those used for weather forecasting, the winds that carry pollutants around the city are first accurately characterised. The model calculates concentrations of air pollutants, such as secondary particulate matter, secondary gaseous pollutants such as O3 and NO2, in each cell by simulating movement of air into and out of cells (advection and dispersion); mixing of air pollutants upward and downward among layers; injection of new emissions from sources such as point, area, mobile, and biogenic into each cell; and chemical reactions based on chemical equations, pollution precursors, and incoming solar radiation in each cell. The two photochemical models most used by the air quality modelling community are (i) Comprehensive Air Quality Model with Extensions (CAMx); and (ii) Community Multiscale Air Quality (CMAQ) Modelling System. 3.3.3 Physical, CPD models Physical Model This copy is issued to Lau Chin Tung Physical modelling involves the simulation of the physical process on a smaller scale in the laboratory by the use of wind tunnel or other fluid modelling facilities. Because of the complexity and high construction and operational cost, physical model will only be used for dealing with complex flow situation, such as building, terrain, or stack downwash conditions, plume impact on elevated terrain, diffusion in an urban environment or complex terrain. Computer Fluid Dynamics Model Computer fluid dynamics (CFD) modelling aims to solve the partial differential equations representing atmospheric dispersion phenomena by numerical integration techniques and provide wind fields at high grid resolution. CFD models are capable to deal with very complex building shapes and boundary conditions at fine grid down to meter scale. CFD codes are structured around numerical algorithms that can tackle fluid flow problems. Most commercial CFD packages contain three main elements: i. The pre-processor, which serves to input problem parameters, generate the grid of computational domain, select the physical and chemical phenomena that needed to be treated, define the fluid properties, and specify the appropriate boundary conditions ii. The solver, which first approximates numerically the unknown flow variables, then discretizes the governing flow equations using these approximations, and finally solves the resulting system of equations iii. The post-processor, which displays the grid and geometry of the domain, plots wind vectors and pollutant concentration contours and provide animation facilities for dynamic result display In Hong Kong, CFD is one of the tools being used for assessing of the effects on air ventilation from major development or redevelopment proposals in accordance with the Technical Guide for Air Ventilation Assessment for Developments in Hong Kong to prevent stagnant or slow air movements in streets and urban spaces. V.2024.05.10 Page 24 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR 3.3.4 Receptor models Receptor models are mathematical or statistical procedures for identifying and quantifying the sources of air pollutants at a receptor location. Unlike photochemical and dispersion air quality models, receptor models do not use pollutant emissions, meteorological data, and chemical transformation mechanisms to estimate the contribution of sources to receptor concentrations. Rather, they use observational data and rely on the different chemical signatures of gases and particles measured at source and receptor to both identify the presence of and to quantify the relative importance or contributions of the sources to receptor concentrations. Common receptor models include: a) Chemical Mass Balance (CMB) It uses source profiles and speciated ambient data to quantify source contributions. Contributions are quantified from chemically distinct source-types rather than from individual emitters. Sources with similiar chemical and physical properties cannot be distinguished from each other by CMB. b) Positive Matrix Factorization (PMF) The PMF technique is a form of factor analysis where the underlying co-variability of many variables (e.g., sample to sample variation in PM species) is described by a smaller set of factors This copy is issued to Lau Chin Tung (e.g., PM sources) to which the original variables are related. 3.4 EMISSION INVENTORY AND EMISSION MODELLING Emissions inventory is the estimates of the emissions from various pollution sources in a geographical area. The typical emission categories include:  Stationary source – (a) Point sources, i.e., those with well-defined emission exhaust and location with significant emissions; (b) Area sources: those stationary sources, including residential and fugitive emissions, that are too small or too spread out to be classified as a point source  Mobile Sources – (a) on road vehicles; (b) non-road mobile machinery; (c) marine; (d) aviation  Natural Sources – e.g., biogenic emissions It helps the regulatory authority to determine significant sources of air pollutants and to target regulatory actions. The emission trends over time, which reflect the effectiveness of the control actions, can be established with periodic updates of the emissions inventory. Emissions inventories are an essential input to mathematical models that estimate air quality. The effect on air quality of potential regulatory actions can be predicted by applying estimated emissions reductions to emissions inventory data in air quality models. In Hong Kong, the EPD compiles and reports the Hong Kong Air Pollutant Emission Inventory annually. Methods to determine emissions include continuous monitoring of emissions from a source and use of emissions factors. The latter is a representative value that attempts to relate the quantity of a pollutant emitted with an activity level (e.g., kg of particulate emissions per Mg of coal burned) associated with the emission of that pollutant. Useful compilations of emission factors include the USEPA’s AP-42 13. 13 https://www.epa.gov/air-emissions-factors-and-quantification/ap-42-compilation-air-emissions-factors V.2024.05.10 Page 25 Copyright @ 2024 Hong Kong Institute of Qualified Environmental Professionals Limited. All Rights Reserved.  I. AIR The general equation for emissions estimation is: E = A × EF × (1 – C × RE) where: E = emission estimate for the process A = activity level such as throughput (maximum continuous rating x load factor x activity hours) EF = emission factor assuming no control C = control efficiency RE = regulatory effectiveness, an adjustment to C to account for failures and uncertainties that affect the actual performance of control Emission inventories (e.g., Hong Kong’s Emission Inventory) are usually presented as the sum of the emissions within the same source category although additional information would also be available for major point sources. In air quality modelling applications, higher resolutions, both spatially and temporally are needed. Emission modelling techniques are used to disaggregate and resolve the emissions spatially, e.g., by geographical information such as population densities, land use and other data, and temporally, e.g., the activity pattern, traffic intensities (rush hours, weekends and working days, summer and winter driving patterns, etc). This copy is issued to Lau Chin Tung Among other sources, estimating the type and quantity of contaminants emitted from roadways is inherently complex because emissions vary according to many factors including: a) Travel Related Factors – e.g., the number of trips, distance travelled and driving mode (idling, cruising, acceleration and deceleration, traffic volume, traffic condition, speed, engine load of the vehicle b) Road Network Related Factors – e.g., the geometric design features of the roads such as grade, signalized intersections, freeway ramps, toll booths, weaving sections c) Vehicle Related Factors – e.g., engine types and sizes, horsepower, weight, and age of the vehicles The on-road vehicle emission model