20 Particles in the Atmosphere – Aerosols

Dr. Vijay Shridhar

Contents

1. Introduction
2. Sources of particles
3. Different types of aerosols
4. Particle size and distribution
5. Classification of particulate matter
6. Formation & growth of atmospheric particles
7. Chemical composition of atmospheric aerosols
8. Ions in atmosphere (secondary organic and inorganic compounds)
9. References

Introduction

Aerosol is defined as a dispersion of solid or liquid particles of microscopic size in gaseous medium with low settling velocities, such as smoke, fog or mist and behaves almost like gases. Atmospheric aerosols have size range from few nanometer to several micrometers in different shapes with wide range of chemical composition.

Aerosols are of central importance for atmospheric chemistry and physics, the biosphere, climate, and public health. The airborne solid and liquid particles in the microscopic –sub-microscopic size range in fluence the energy balance of the Earth, the hydrological cycle, atmospheric circulation, and the abundance of greenhouse and reactive trace gases.

Aerosol particles scatter and absorb solar and terrestrial radiation, they are involved in the formation of clouds and precipitation as cloud condensation and ice nuclei, and they affect the abundance and distribution of atmospheric trace gases by heterogeneous chemical reactions and other multiphase processes.

Sources of particles

The atmospheric aerosol has many sources such as gas to particle conversion, cloud to particle conversion, soil and deserts, oceans, biosphere, volcanoes, biomass burning, direct injection which can be clubbed in natural and anthropogenic categories. During its lifetime, the aerosol experiences many transformation processes and their transformed state categorized as secondary aerosols. Despite of all these transformation, the aerosol exhibits size distributions with very typical characteristics. Because of their particle nature, they are also called as particulate matter.

Natural sources of air pollution include volcanoes, sea spray, forest fires etc. The major anthropogenic sources are transportation, thermal power generation; refuse burning, industrial and domestic fuel burning and industrial processes. Contaminants from these sources may either be harmful or harmless depending on their residence time, concentration and duration of exposure and the susceptibility of the receptors. Although the mass concentration of aerosols arising from human activities contribute only 7-12% of the total aerosol budget, the health and non-health related effects of even this small fraction are causing serious concern throughout the world. The aerosol particles can strongly influence the lifetimes of various trace gas species, such as sulfur dioxide and ammonia, by providing a rapid removal mechanism.

Different types of Aerosols

These abundant particles in atmospheric condition are found as dust, fumes, fog, smoke, etc.

Dust: Solid particles formed by mechanical breakage of parent materials or crushing. These have irregular shapes and > 1 μm.

Fumes: Particles formed by condensation or chemical reaction. It also refers noxious vapor components. (usually < 1 μm )

Fog: Suspension of water droplets and they are effecting visibility < 1 km. A term loosely applied to visible aerosols in which the dispersed phase is liquid; usually, a dispersion of water

Mist: Suspension of droplets and they are effecting visibility > 1 km. (< 200 μm )

Smoke:  Visible  aerosol  from  incomplete  combustion.  Particles  may  be  solid  or  liquid. (usually < 1 μm )

Smog: Consisting of solid and liquid particles formed by the presence of sunlight on vapors. It is combination of smoke and fog.

Haze: A visibility – reducing aerosol.Haze is defined as the weather phenomenon featuring a high concentration of fine particles that leads to horizontal visibility below 10 km at a relative humidity (RH) less than 90%. Haze pollution is characterized by the elevated levels and high fractions of the secondary components of sulfates, nitrates and ammonium (SNA) in fine particles.

Particle size and Distribution

The size distribution of the atmospheric aerosol is one of its core physical parameters. It determines how the various properties like mass and number density, or optical scattering, are distributed over the particle size. For the atmospheric aerosols this size range covers more than five orders of magnitude, from one nanometer to several hundred micrometers. The aerosol size distribution varies from place to place with altitude and with time. Because of this size distribution, many integral parameters of the atmospheric aerosol, like the total mass, the optical extinction coefficient and the radioactive properties, have their centre of gravity focused in a certain size range only. And this size range determines the residence time and the atmospheric transport characteristics of that integral parameter. The behaviour of atmospheric particles and their effects on human health and the environment is strongly governed by a series of simple physical processes and particle properties. These differ for each particle according to its size, shape and chemical composition, with particle size being the most important. The main properties of aerosol particles in this context are the aerodynamic properties which determine the residence time of airborne particles.

Classification of Particulate Matter

A wide variety of suspended particles are found in a typical atmosphere. Size, chemical affinity for liquid water and light scattering & light absorbing properties all have the potential to affect public health and perception of pollution. Particulate matter can be classified in many categories on the basis of their origin, chemical composition and physical size.

(1) Size- Based Classification of Aerosols

Based on particle size, atmospheric particles can be classified in three major fraction.

1. Ultrafine particulate matter (uf PM) ranging from few nanometer to 0.1 micrometer. Aitken Nuclei, are those that are smaller than 0.2 micrometers in diameter.
2. Fine particulate matter from 0.1-2.5 micrometer
3. Coarser particulate matter in the size range >2.5 micrometer. The giant aerosols are all those larger than 2 micrometers in diameter.

In view of health perspective study these atmospheric particles can have classification in form of RSPM( respirable suspended particulate matter) or PM10 and PM2.5 describing particles lesser than 10 or 2.5 micrometer respectively. PM stands for particulate matter Based on number concentration and their frequency distribution, the PM10 typically has a bimodal distribution with maximum at approximately0.5μm (the accumulation mode) and between 5 and 10μm (the coarse mode). Intra-site analyses in urban areas show that PM2.5 is highly correlated with PM10, but not with PM10-2.5 and site-to-site correlations are high forPM2.5 but not for PM10-2.5. Coarse particles have alarger settling velocity and hence are more rapidly deposited.

Aerosols are generally distributed into three majormass/volume/surfacearea/number classes:

(i) Coarse Fraction: Mass and volume (and partially surface area) is dominated by the large particles named the coarse fraction of the aerosol

(ii)Fine Fraction: Number (and partially surface area) distributions are dominated by the fine fraction  of the particles, often defined as the particles not exceeding an equivalent diameter of ~2.5 μm.

(iii) Within the fine particles, one distinguishes a nucleation mode and an accumulationmode, the former dominating number distribution during aerosolnucleation, the latter dominating surface area distribution. The accumulation mode is formed from nucleation mode particles as they coagulate, a process driven by their very high number density.

Fine particulate fraction is defined to include particles with an aerodynamic diameter of ~ 0.1-2.5 μm. This fine-mode fraction contains a mixture of particles including carbonaceous material like soot (with possibly adsorbed reactive metals and organic compounds), and secondary aerosols like acid condensates, and sulphate & nitrate particles, and is derived directly or indirectly from combustion of fossil fuels used in power generation, industry and automobile engines. This fraction has the largest surface area and contributes also to the total particle mass concentration. The generally chosen lower cut-off of 2.5 μm is arbitrary.

Particles, which are larger than 2μm, are called “coarse particles”. They result from grinding activities and are dominated by material of geological origin. Coarse particles at the low end of the size range also occur when cloud and fog droplets form in a polluted environment, then dry out after having scavenged other particles and gases. Demolition of buildings and evaporation of sea spray also generate coarse particles. Pollen, mould spores and parts of plants and insects also are found in the coarse-particle-size range.

(2) Classification Based on Origin

On the basis of origin, particulate matter can be divided in two groups: Primary particulates and Secondary particulate matter

Primary particulates

Primary particulates are produced directly by the physical or chemical processes peculiar to a specified emitter, come from extremely varied sources, ranging from those in the industrial sector, such as gravel crushers and blast furnaces, to those in nature, such as forest fires and ocean spray. Even mechanically generated particles by the anthropogenic activities come in primary particulate matter. The chemical composition, of course, varies with the type of source.

Secondary particulates

Secondary particulates are the products of chemical and physical processes occurring in the atmosphere. Chemical reaction can initiate in the gas phase or between gases and already existing particles. They are a major source of the ubiquitous Aitken nuclei, or solid condensation centres, that take place in the atmosphere. They are also a prime component of urban smog. Secondary particles are formed by the atmospheric conversion of gases into particles. In one mechanism, a gas is converted into the vapour of a material with a low saturation vapour pressure.

Secondary particulates are in general composed of three types of chemical compounds: sulphates, hydrocarbons, and nitrates. Sulphates areubiquitous. The second major constituent of particulates is hydrocarbons, which react with oxidants (e.g. NO2, O3) in the atmosphere to produce peroxide radicals. Through a series of chain reactions, these radicals eventually form large organic molecules which condense to form droplets or solid particles. Primary sources of hydrocarbons in urban areas are automobile exhaust, power generating stations, and industrial effluents. In rural areas, a rich source is the vapour produced by various plants, which often contains a class of compounds known as terpenes.

Primary pollutants have the same physical and chemical properties in the atmosphere as they were introduced by the source, while the secondary pollutants may change in some way.

3. Chemical Composition Based Classification

The pollutants range from inorganic to organic and can be grouped broadly as follows:

1. Sulfur – containing compounds
2. Carbon- containing compounds
3. Nitrogen- containing compounds
4. Halogen -containing compounds

4. Source Based Classification

On the basis of sources aerosols are classified in tobiological, anthropogenic, oceanic, geochemical aerosols.There are many sources of trace components in the atmosphere, which can be divided into different categories, such as geochemical, biological and anthropogenic sources. Perhaps the largest geochemical sources are wind- blown dust and sea sprays, which put huge amounts of solid material into the atmosphere. If this dust is fine enough, it can spread over large areas of the globe and is important in redistributing material.

A significant fraction of the tropospheric aerosol is anthropogenic in origin. Tropospheric aerosols contain sulphate, ammonium, nitrate, sodium chloride, tracemetals, carbonaceous material, crustal elements and water. The carbonaceous fraction of the aerosols consists of both elemental and organic carbon. Elemental carbon, also called black carbon is emitted directly into the atmosphere, predominantly from combustion processes. Anthropogenic emissions leading to atmospheric aerosol have increased dramatically over the past century and have been implicated in human health effects, in acid deposition, and in perturbing the Earth’s radiation balance. As a result of variations in emission source strength and character, variations in atmospheric chemical reactions and variable removal efficiency by both wet and dry deposition and concentration of the aerosol dust veil over the globe is very inhomogeneous. The aerosol dust veil is predominately inorganic in origin and consists mainly of sulphate and nitrate salts, crustal dust and sea salt. These aerosol particles are generated through a variety of processes including anthropogenic activities.

Formation & growth of Atmospheric Particles

Condensation and evaporation of aerosols play an important role in particle formation. Formation of an aerosol initially requires a surface for condensation. This surface could be made up of vapor molecules, an ion or ionic cluster or small particle, termed a condensation nucleus. When condensation of a vapor takes placesolely on clusters of similar vapor molecules, it is called homogenous nucleation, while condensation on dissimilar molecules called heterogenous nucleation. Particles formed by the condensation of vapor molecules are generally spherical in shape, especially if they go through a liquid phase during condensation. Particles formed by breaking or grinding larger particles, termed attrition, are seldom spherical, except in the case where liquid droplets are broken up to form smaller liquid droplets. High aerosol concentrations individual particles coalesce to form larger chains or flocs made up of many particles through process of coagulation. Process brought about solely by the random motion and subsequent thermal coagulation or collision under force of turbulence or electricity.

Particles transformation and growth mechanisms (e.g., nucleation, condensation, and coagulation, Adapted from Wilson and Suh, 1997).

The terms nucleation mode and accumulation mode refer to the mechanical and chemical processes by which aerosol particles in those size ranges are usually produced. The smallest aerosols, in the nucleation mode, are principally produced by gas-to-particle conversion (GPC), which occurs in the atmosphere. Aerosols in the accumulation mode are generally produced by the coagulation of smaller particles and by the heterogeneous condensation of gas vapor onto existing aerosol particles.

While defined as airborne mixture of liquid or solid material and air, aerosols are commonly understood as the nongaseous component(s) of our atmosphere. They are a very complex, multiphase, multi-component system involving both inorganic and organic species.

The dominant inorganic components of aerosol are sulfate, nitrate, and ammonium, and chloride and sodium for sea spray (sea salt aerosol).Besides carbonate, carbon in aerosols exists in many forms: Elemental carbon (EC), black carbon (BC), or soot are terminologies used for a highly refractory fraction of carbon in aerosols that resembles graphite to different extents. While the origins of inorganic ions and carbon are well understood, the origins of organic carbon (OC) material in aerosols is not. OC may be primary or secondary organic aerosol (SOA) and consists of hundreds of different organic species. Organic carbon has the ability to act as cloud condensation nuclei (CCN). Emerging details about organic carbon are that (i) high amounts of OC in urban aerosols is due to anthropogenic primary emissions, such as long chain hydrocarbons from cleaners or decomposition products from fats during cooking; (ii) varying amounts of OC in rural aerosol are dominated by SOA from biogenic hydrocarbon emissions; (iii) water soluble OC (WSOC) increases with time due to chemical processing of aerosol OC, called aging, which further increases the complexity of the chemical functionality of OC, which could be described as an increase of the Oxygen: Carbon ratio of aerosol OC.

Coarse mode particles originate almost solely due in direct emissions of natural aerosols from soils (a process called saltation) creating“dust”, or the ocean (sea spray formed from ejecting bubbles).Anthropogenic processes creating dust are, for instance, agricultural activities, or grinding, cutting, and demolishing in connection to construction. Fine mode particles are due to gas to particle conversion, in which each other weakly attracting gas molecules, particularly sulfuric acid, form clusters that grow to several hundred molecules within a few hours. Once a few nanometers large, they become observable and exhibit macroscopic properties, such as hygroscopicity.

Chemical composition of Atmospheric Aerosols

The atmosphere is the smallest of the Earth’s geological reservoirs. It is this limited size that makes the atmosphere potentially so vulnerable to contamination that even the addition of a small amount of material can lead to significant changes in the way the atmosphere behaves (Andrews et al., 1996). Bulk composition of the atmosphere is quite similar all over the Earth because of high degree of mixing within the atmosphere. This mixing is driven in a horizontal sense by the rotation of the Earth. Vertical mixing is largely the product of heating of the surface of the Earth by incoming solar radiation. Atmosphere is a vehicle controlling the dispersal of trace metals. Its complex motion controls both the transport and dilution. The dispersal mechanism varies with time and space. It depends on the nature of underlying surface and on the local meteorological conditions.

Physical and chemical transformation of aerosols in the atmosphere helps in catalyzing other gases and in generating secondary aerosols viz., oxidation of SO2 by Fe, Mn and Vin water droplets. Particle size of aerosols play important role in transportation of particulate matter. Accumulation mode particle(0.1-1.0 μm) deposit slowly and therefore can be transported over long distances, having consequent effects in regions remote from the source. Deposition is the last stage in the pathway of the pollutants from their source to receptor. It contributes the atmospheric pollutants through both dry and wet mechanisms to both natural and managed ecosystems.

Dry deposition is aerodynamic transfer of atmospheric gases and particles to the earth’s surface by mechanisms. This is caused by diffusion to and adsorption on the surface. This process is important for large particles (particles above 2 μm) which are removed by gravitational settling and for very small particles (Aitken’s nuclei) and gases which are removed by diffusion processes (particles below 0.2 μm). The effectiveness of this type of process depends on both the meteorological parameters and the surface factors.

Wet deposition is an important, and highly variables and intermittent process in the pathway from surface to receptor. It involves rain out (in cloud scavenging) and wash out (scavenging below the cloud) mechanisms. Particles are scavenged by precipitation due to impaction, electrical effects, Brownian motion, and by serving as cloud condensation nuclei. At altitudes of 100m and above, the particles can be incorporated into the raindrops or snowflakes which then fall to the earth. This phenomenon is known as rainout of pollutants. Metal particulates emitted in air are ultimately washed out by rain, polluting land and water ways. Thus, air becomes a major route of contamination of the rest of the living environment.

Airborne particles are important carriers of metals, some of which possess toxic properties and commonly are present in excess of natural levels. Many trace metals may create adverse effects on environment and human health due to their toxicity and bioaccumulation in various environmental compartments. These trace metals effects depend on the concentration levels and their chemical forms in various compartments of the environment as well as on the chemical, physical and biological conditions of a given environmental compartment.

Trace metal fluxes can originate either from natural or anthropogenic sources. Natural sources are related primarily to the geological presence of trace metals in the crustal material and are brought to the air during various physical, chemical, biological and meteorological processes with no direct influence of humans. Release of trace metals during various industrial production processes and disposal of wastes is regarded as anthropogenic emissions. Many trace metals are ubiquitous in various raw materials, such as fossil fuels and metal ores as well as in industrial products. Some trace metals evaporates entirely or partially from raw materials during the high temperature production of industrial goods, combustion of fuels, and incineration of municipal and industrial wastes, entering the ambient air with exhaust gases. Release to other environmental compartments (e.g. spills to water bodies, landfills, sewage lagoons, holding ponds) may also result in volatilization and entrainment of several trace metals. While emitted to the atmosphere, trace metals are subject to transport with in air masses and migration through the ecosystem, which cause perturbations of their geochemical cycles not only on a local scale but also on regional and even global scales.

Deposition and transportation of trace metals in areas without any emission source, through their long-range transport may contribute to native concentration to exceed the maximum permissible values. The concentrations of chalcophiled Pb, Sb, Cd, Se, As, Zn and Mo increase markedly with decreasing particle size, while lithophiled Mn and Zr increase show little or no enrichment with decreasing particle size. One measure of the anthropogenic origin of atmospheric trace metals in a given receptor is the estimation of enrichment factors (EFs) of trace metals. It presents concentration increase of a given metal in the ambient aerosol, compared to its concentration in certain reference material, such as crustal rocks or soils.

Ions in Atmosphere (Secondary Organic and Inorganic Compounds)

Secondary organic aerosols (SOAs) in the atmosphere are produced through oxidation of volatile organic compounds (VOCs) followed by gas-particle partitioning of semi-volatile products. SOAs are a major contributor to fine particulate matter (PM) in urban and rural areas, and have given rise to several environmental and geophysical concerns.

Sulfate, nitrate and ammonium (SNA) are the dominant species in secondary inorganic aerosol, and are considered an important factor in regional haze formation. In general it was reported that aqueous inorganic seeds have no significant effect on SOA formation in aromatic photo oxidation systems. But particle-phase heterogeneous reactions of multifunctional carbonyl compounds can be catalyzed by the presence of strong aqueous acidic seeds (e.g. a composite seed aerosol of (NH4)2SO4 and H2SO4). The air is continuously fed with particulates, gases and vapours from anthropogenic sources. These include minerals, metals and chemicals like SO2,NOx, O3, HC and their halogenated compounds etc. Some of these toxic gases form secondary particulates (SO42-, NO3-) after going through physical and chemical changes. The rate of formation of these secondary aerosols and the thermodynamic equilibrium relationships among the aerosol species depends on the local gas-phase concentrations, and ambient meteorological conditions.

Sulfate (SO42-)

Generally, the sulfates are primarily produced through the gas-phase oxidation of SO2 by the OH radical followed by nucleation and condensational growth, or are produced by the heterogeneous uptake of SO2 on pre-existing particles or cloud droplets followed by being oxidized by O3, H2O2, or O2 catalyzed by Fe(III) and Mn(II).

Sulfate is considered as one of the most important secondary atmospheric aerosol which is closely related to the acidification of rain and degradation of visibility. The global background level of tropospheric sulphate are estimated to be 2 μg/m3. Sulphate generally comprises more than 90% of total aerosol sulfur. As such sulphates accounts up to 60% (by mass) of the secondary fine particulate burden in urban air and are generally found in smaller than 2 μm size range (predominant in the 0.1 to 1.0 μm range).Large fraction of sulfate in atmosphere occurs as ammonium sulphate. Sulphuric acid along with sulphate salts such as PbSO4 is found in most urban aerosols. The residence time of sulfate particulate in the atmosphere is about 4 to 5days on average and it can be transported to about 1000 to 3000 kms from the source. The term atmospheric sulfate describes a variety of sulfur compounds including ammonium sulphate, ammonium bisulphate, sulphuric acid, calciumsulphate, zinc ammonium sulfate, zinc sulfate and variety of other metallic sulfates. Sulfate speciation data also help in studying the several SO2 to SO4-2transformation mechanisms.

Nitrate (NO3-)

Nitrates are considered an important secondary atmospheric aerosol and are formed as end product of photochemical processes and the oxidation of NO2. It is closely related to the acidification of rain water and degradation of visibility. Nitrates account for 7 to 16% of fine particulates. Almost all are formed by secondary reactions in the atmosphere. Studies also indicate that most of the nitrate occurs in the respire able particles (2μmor smaller). A very small percentage of atmospheric nitrate is of natural origin(biological action and natural events lightening).

Ammonium (NH4+)

Ammonia (NH3) is the basic gas in the atmosphere and after N2 and N2O, is the most abundant nitrogen containing compound in the atmosphere. The significant sources of NH3 are animal waste, ammonification of humus followed by emission from soils, losses of NH3 based fertilizers from soils, and industrial emissions. The ammonium ion (NH4+) is an important component of the continental tropospheric aerosol. Because NH3 is readily adsorbed by surfaces such as water and soil, its residence time in the lower atmosphere is expected to be quite short, about 10days. Atmospheric concentrations of NH3 are quite variable, depending on proximity to a source rich region.

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