Aerosols Lect-3 (Physical Properties) PDF

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aerosols air pollution physical properties environmental science

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This document provides an overview of aerosols, highlighting their physical properties, types (natural and anthropogenic), and their impact on the environment and air quality. It also touches upon the importance of studying them for climate, radiation, and human health. Key concepts like size, shape, and composition are discussed relating to aerosol behaviour.

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Lect-3 Aerosols (physical properties) Overview ❑Definition and types of aerosols ❑The importance of studying aerosols ❑Physical properties of aerosols ❑Measurement and detection of aerosols ❑Interaction of Aerosols with Biological Matter Chemical pollutions...

Lect-3 Aerosols (physical properties) Overview ❑Definition and types of aerosols ❑The importance of studying aerosols ❑Physical properties of aerosols ❑Measurement and detection of aerosols ❑Interaction of Aerosols with Biological Matter Chemical pollutions Chemical pollution refers to the contamination of our environment with chemicals that are not found naturally which can adversely affect human health, ecosystems, and the atmosphere. Examples of chemical pollution Air Pollution Water Pollution ⁠Soil Pollution ⁠Persistent Organic Pollutants (POPs) Heavy Metal Pollution Air pollution Effect of aerosols on the environment Aerosols are tiny particles suspended in the air. They can be solid, liquid, or a mixture of both. They are often not (or barely) visible to the human eye. Aerosols play a vital role in various atmospheric processes, including cloud formation, climate regulation, and air quality. Types of aerosols Aerosols can be classified based on their origin, composition, and size: Based on Origin Natural Aerosols (direct emission): Anthropogenic Aerosols (indirect emission): Produced by natural processes, such as: Resulting from human activities, including: - Burning of fossil fuels (e.g., vehicle - Sea-spray emissions, industrial processes) - Volcanic eruptions - Biomass burning (e.g., forest fires, - Dust storms agricultural burning) - Biological activities (e.g., pollen, bacteria) - Construction and mining activities Based on Composition Organic Aerosols: Contain carbon-based compounds, Inorganic Aerosols: Composed which can be further divided into: mainly of mineral particles, metals, - Primary organic aerosols (POA): Directly emitted and salts. Examples include: from sources (e.g., soot, plant material) - Sulfates - Secondary organic aerosols (SOA): Formed through - Nitrates chemical reactions in the atmosphere. - Sea salt Sea-spray aerosol, desert-sand dust, volcanic ash, and fly ash are sources of aerosols The importance of studying aerosols Aerosols can significantly alter respiratory health when inhaled. Biophysicists investigate how these aerosols interact with the lungs, airways, and other respiratory system components, which is critical for understanding the health consequences of air pollution, virus transmission, and other respiratory disorders. Aerosols affect the climate directly by scattering and absorbing the incoming solar radiation and outgoing infrared radiation in the atmosphere. Aerosols alter the formation and precipitation efficiency of liquid water, ice, and clouds. Aerosols are the primary cause of visibility degradation in polluted areas, and sometimes even lead to transportation accidents. Physical properties of aerosols The global impact of aerosols on the climate of the earth is difficult to quantify since there are no comprehensive and reliable measurements in most parts of the world. Aerosol properties are critical parameters to be investigated for understanding their influence on climate, radiation budget, air quality, and human health. Particles in the atmosphere are distinguished by their size, shape and composition. They can be directly emitted from sources and can be formed in the atmosphere by chemical reactions and physical processes. Once particles are formed, their properties can be modulated in space by atmospheric physical and chemical processes, such as condensation, evaporation, and coagulation. Eventually, the particles are removed from the atmosphere by wet or dry deposition, with such removal occurring minutes to weeks after their release or formation, and after traveling meters to thousands of kilometers. Size Aerosols can range from a few nanometers )nanoaerosols) to several micrometers in diameter. The size of a particle alters its behavior, including how long it remains suspended in the air and its ability to penetrate biological membranes (e.g., the respiratory system). Aerosol particles of the same size is called a monodisperse aerosol. Test aerosols carefully produced in the laboratory, are usually monodisperse. Atmospheric aerosols are polydisperse, with a wide range of particle sizes, because they are generated by several different mechanisms. Aerosol Modes Fine Mode: 1. Nucleation Mode: (5-100nm) Formed by condensation of hot vapors or gas-to-particle conversion. High number concentration, low mass contribution. Rapidly coagulate or condense, transitioning to the accumulation mode. 2. Accumulation Mode: (100- 2 μm) Grown from nucleation mode particles. Significant surface area and mass contribution. Long atmospheric lifetime (1-2 weeks). Involved in long-range transport. Coarse Mode: Large particles (>2 μm). Formed by mechanical processes (soil dust, sea spray, industrial dust). Efficiently removed by precipitation or deposition. Limited long-range transport. Demonstration Demonstration The following size ranges are defined Shape Aerosol particles can be spherical, irregular, or aggregate structures. The shape affects how they scatter light and how they interact with other particles and gases. Shape affects settling velocity and dispersion in the atmosphere. Size distribution Size distribution refers to the variation in the sizes of aerosol particles within a given sample of air and characterized by the number, mass or volume of particles. The different sources make particles of different sizes. The radius or diameter of aerosols characterizes the size of one particle and these particles aren't always perfectly round. They can have different shapes, and their size can vary a lot. Many physical properties of ambient aerosols can be estimated from their size distribution and chemical properties. Distribution of Aerosols as a Function of Number, Mass, and Volume Number Distribution (how many particles are of a certain size) It counts the number of particles in a specific size range smaller particles, even though numerous, may contribute little to the overall mass or volume of the aerosol. Mass Distribution: (how much the particles of different sizes weigh) It shows the distribution of the total mass of aerosols across different size ranges. Bigger particles weigh more. Volume Distribution: (how much space the particles of different sizes take up) It shows the distribution of the total volume occupied by aerosols across different size ranges. Large particles might take up more space even if there are fewer of them. Optical Properties Scattering: Aerosols scatter light, which affects visibility and climate. The scattering efficiency depends on size, shape, and composition. Absorption: Some aerosols absorb light, particularly in the ultraviolet and infrared regions, impacting climate and air quality. Single scattering albedo (SSA) Is a dimensionless quantity that represents the fraction of light scattered by a particle relative to the total amount of light that is either scattered or absorbed. It measures how aerosols are interacting with light. SSA = Scattered light / (Scattered light + Absorbed light) SSA values typically range from 0 to 1 SSA = 1: The particle scatters all incoming light and absorbs none. SSA = 0: The particle absorbs all incoming light and scatters none. SSA between 0 and 1: The particle scatters some of the light and absorbs others. The SSA of a particle depends on its size, shape, composition, and the wavelength of the incident light. Aerosol Optical Depth (AOD): AOD is a dimensionless parameter that indicates the (quantity) degree to which radiation transmission is inhibited by the absorption or scattering of sunlight through aerosols in the atmosphere. AOD can be briefly defined as the reduction of electromagnetic energy at a specific wavelength due to aerosols in the atmosphere. Density The density of aerosol particles varies depending on their composition (e.g., organic, inorganic, or biological. This property affects how long they can remain suspended in the air. Solubility Some aerosols are soluble in water, while others are not. Solubility affects how aerosols interact with moisture in the atmosphere and their potential to form clouds. Hygroscopicity This property refers to the ability of aerosol particles to absorb water vapor (moisture) from the atmosphere. Hygroscopic aerosols can grow in size and change their physical and chemical properties under humid conditions. Settling Velocity Settling velocity is the speed at which an aerosol falls through the air under the influence of gravity. Factors affecting the settling velocity o Density: A denser aerosol experiences a stronger downward pull from gravity. This increased gravitational force leads to a higher settling velocity, causing the aerosol to fall more rapidly. o Size: Larger particles generally settle faster than smaller ones, regardless of density. o Shape: Spherical particles settle faster than irregularly shaped ones due to less surface area exposed to air resistance.

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