Urban Aerosols and Their Impacts

Chapter 1

Introduction to Urban Aerosols and Their Impacts Nancy A. Marley and Jeffrey S. Gaffney

Downloaded by 80.82.77.83 on January 18, 2018 | http://pubs.acs.org Publication Date: November 15, 2005 | doi: 10.1021/bk-2006-0919.ch001

Environmental Research Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439

Atmospheric aerosols, or particulate matter, may be solid or liquid, with effective diameters from ~0.002 to ~100 µm. They can be emitted in particulate form directly into the atmosphere (primary aerosols) or formed in the atmosphere by chemical reactions of gases (secondary aerosols). Aerosol sources include mechanical processes such as grinding or wind erosion, gas-to-particle conversion of gas-phase primary pollutants, and combustion processes which occur primarily in urban centers. The impact of aerosols on the environment, human health, and climate depends on their number concentration, mass, size, chemical composition, phase (i.e. liquid or solid), morphology, and surface properties. Outlined in this Chapter is a brief overview of the physical and chemical properties of atmospheric aerosols as they relate to their to potential impacts on the environment and human health.

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© 2006 University of Chicago

Gaffney and Marley; Urban Aerosols and Their Impacts ACS Symposium Series; American Chemical Society: Washington, DC, 2005.


3 Atmospheric aerosols are solid or liquid atmospheric particles whh diameters of approximately 0.002-100 |jm (7). Aerosols can be emitted in particulate form directly into the atmosphere {primary aerosols) or formed in the atmosphere by gas-to-particle conversion involving chemical reactions of gases that result in condensable species (secondary aerosols). Although primary aerosol sources can produce particles of all sizes, secondary aerosol sources produce particles in the nanometer size range ( 2 |am) also have relatively short atmospheric lifetimes, because they are removed rapidly by sedimentation. Particulate matter larger than 20|im can remain airborne for only a limited time and is therefore usually restricted to the immediate vicinity of the source (2). Atmospheric aerosols, therefore, can exist in a range of sizes spanning four orders of magnitude. Traditionally, the size ranges have been subdivided into three modes shown in Figure 1 (7). The smallest size range, composed of aerosols < 0.1 Jim, is called the Aitken range (or nuclei range). The existence of these aerosols was discovered in 1875 by Coulier (3), and were thoroughly studied for the first time by Aitken at the turn of the century (4). These ultrafine aerosols are produced by ambient-temperature gas-to-particle conversion and also by combustion processes generating hot gases that subsequently undergo condensation (1). Aerosols in the Aitken range have particle diffusion coefficients > 0.001 cm /sec (Figure 1), and therefore they can diffuse rapidly to the surfaces of other particles (5). In addition, these ultrafine aerosols can act as nuclei for the condensation of low-vapor-pressure gases causing them to grow rapidly into the next size range. Atmospheric aerosols in the coarse range, larger than approximately 2 |nm, are usually produced by mechanical processes such as grinding or wind erosion. Thus, because of the nature of their sources, they are composed predominately of minerals and inorganics such as sand and sea salt. Also included in this size range are larger bioaerosols such as spores, pollen, and bacteria. With settling velocities >0.01 cm/sec (see Figure 1), coarse aerosols are generally removed from the atmosphere fairly rapidly by sedimentation. However, the atmospheric transport of coarse aerosols can occur over relatively long distances by convective processes where fallout is balanced by reentrainment. Mineral dusts from western China have been detected in western North America (6) and Canada (7), and African mineral dusts have been detected in south central Florida (8). Aerosols in the intermediate size range from approximately 0.1-2 \im are known as the accumulation range. They are so named because the particle removal mechanisms of sedimentation and diffusion to surfaces are least effective in this range causing particles of this size to accumulate in the atmosphere. Aerosols in thisfinesize range typically arise from coagulation of 2



2

Diffusion Coefficient (cm /sec)

Settling Velocity (cm/sec)

Aerosol Mode

Electromagnetic Waves

Diameter (urn)

IO-

1

2

10-

Xrays

0.001 UV

0.1

4

io-

io1(H

101-2 7

10"

Bacteria

10

m

Hair

Pollen

Clouds & Fcfg

Fine Si nd

IR

100

Figure 1. Characteristics of atmospheric aerosols.

10-

Viruses

Fly Ash Sea Spray

MR

10

V find Blown D ust

Visible

Smoke

Combi stion Nuclei

>rbonBkck

Aitcen

0.01


«

1000 fl mm)

Rain

5 smaller particles in the Aitken range orfromcondensation of low-volatility gases (water vapor, organics, etc.) onto existing particles. Because of their sources, particles in the accumulation range typically contain organic compounds and soluble inorganics such as ammonium (NH4*), nitrate ( N 0 ) , and sulfate (S0 ~). Being too small to settle out of the atmosphere, they are removed relatively slowly, primarily by incorporation into clouds and subsequent rainout. The rate of cloud droplet formation from these aerosols depends on their chemical compositions and their hygroscopicity, with the more soluble species being removed faster. Alternately, fine aerosols can be removedfromthe atmosphere by dry deposition after being carried to surfaces by eddy diffusion (9). The properties of atmospheric aerosols that are important in determining their impacts to the environment and human health include their number concentration, mass, size, chemical composition, phase (liquid or solid), morphology, and surface properties. Currently, the U.S. Environmental Protection Agency (EPA) has two standards for particulate matter (aerosols) that are based on mass loadings. These are particulate matter

Urban Aerosols and Their Impacts

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