Air Pollution Gases and Aerosols

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Wide varieties of pollutants are emitted to the atmosphere each year by the tons. The Environmental Protection Agency (EPA) has chosen to classify them in two broad categories based on their respective origination points.

1. Primary Pollutants are those classified as being emitted directly from sources and not undergoing any chemical or physical transformation. An example would be the carbon monoxide emitted through the automobile exhaust. Such gases are called non-reactive since they generally do not interact with other gases nor are they altered by solar radiation.

2. Secondary Pollutants are those that are formed in the atmosphere as a result of chemical reactions among primary pollutants and other species, including radiation that may be present in the atmosphere. One of the most notable of these chemiluminescent reactions is that which leads to the formation of tropospheric ozone. Other reactions can lead to the formations of particulate matter (nitrates, sulfates), acidic droplets, salt particles, and hydrocarbons.

Pollutants can also be broken into two other categories according to their physical properties: (1) gases such as carbon monoxide (CO), O3, or SO2, and (2) particulate matter, such as sulfates, nitrates, black carbon, and the heavier hydrocarbons. Particulates, also commonly known as aerosols, include both solid and liquid particles that become airborne. This includes solids that accumulate water and become difficult to assign optical properties to, as will be shown later. Some of the more common air pollution sources are shown below in Table 11.1.

Table 11.1 Sources of air pollution

Anthropogenic

Natural

Activity

Source

Pollutant

Source/Activity

Pollutant

Transportation Autos, buses

CO, NO*, PM*

Erosion

Windblown PM

Industrial

Mining, pulp and paper mills

PM, H2S, CO2

Decay

NH3, methane, H2S

Construction

Paving, painting

VOCs**

Forest fires

PM, VOCs, NOX, etc

Open burning

Leaves, Slash burning, fireplaces, etc.

VOCs, particulate matter

Soil processes

N2O, CO2, NH3

Waste

Household waste disposal

CO2, methane, H2S

Evaporation

Sea salts

Power

Coal and oil fired

Hg, PM

Volcanoes

SO2, CO, Fine PM

generation

plants

hydrocarbons,

Biogenic emissions

VOC's, terpenes, isoprenes, pollen

* PM = Particulate Matter

** VOCs = Volatile organic compounds

* PM = Particulate Matter

** VOCs = Volatile organic compounds

3. Gases. In this chapter it will be proposed that due to the short path length of the UV through the polluted portion of the atmosphere and the normally very low concentrations (parts per billion, ppb) of the gaseous pollutants, there will be little to no effect of these gases on the UV reaching the surface. The dry atmosphere is primarily composed of nitrogen (N2), O2, and several inert noble gases—argon (Ar), neon (Ne), krypton (Kr), and xenon (Xe). Their relative concentrations have remained essentially fixed over time (Arya, 1999). The concentrations of water vapor (H2O) are observed to be highly variable, both in time and space. It is one of the most important ingredients of weather and climate in the lower atmosphere, but is not normally considered an air pollutant. Carbon dioxide (CO2) also has a relatively high concentration, mostly due to emissions as part of the natural carbon cycle. However, the increasing ambient levels of CO2, at the current rate of 0.5% per year, are clearly due to anthropogenic emissions from fossil fuel burning, slash burning, and deforestation. Even though increasing levels of CO2 might be beneficial to agriculture and forestry, as some studies have suggested

(Booker, et al., 2007), the potential consequences of climate warming due to increasing levels of CO2, methane (CH4), and other greenhouse gases are considered serious enough to put these gases under the category of air pollutants. Most of the other species that are considered to be air pollutants have both natural as well as anthropogenic sources. Their concentrations in polluted atmospheres, such as over large urban and industrial complexes, are found to be several orders of magnitude higher than the "clean" atmospheric values normally measured in the parts per billion (ppb) range.

Tropospheric ozone has seasonal variations that depend on the sun's intensity, and most importantly, on the strength of the UV radiation. In this case, the UV, in conjunction with oxides of nitrogen and varying types of hydrocarbons, produces the tropospheric ozone. This ozone, in turn, absorbs one of its own creators; the UV radiation. Without the UV, there would be no ozone. Anthropogenic precursors, photochemically reacting in the troposphere, result in accumulations of tropospheric ozone in urban and industrialized regions. These reactions give rise to a diurnal ozone pattern in the more urban regions (Fig. 11.3). Tropospheric ozone is produced in more rural areas by naturally emitted precursors. It can also be transported from nearby urban areas.

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Figure 11.3 Peak ozone concentrations for 3 days in September, Rubidoux, CA

6 7 8 9 10 11 12 13 14 IS 16 17 18 19 20 21 22 23 24 Time of day (PST)

Figure 11.3 Peak ozone concentrations for 3 days in September, Rubidoux, CA

While there are a number of significant gaseous pollutants in the troposphere, only three absorb in the UV region of the spectrum; SO2, NO2, and O3 (Fig. 11.4). As shown in Fig. 11.4, the scattering cross-sections of SO2 and O3 are approximately the same magnitude up to about 305 nm, but that of NO2 is relatively flat in this region and actually surpasses O3 and SO2 above 315 nm. Nitrogen dioxide is a precursor for ozone. Its morning peaks can be shown in a daily plot of a polluted urban environment (Fig. 11.5) along with the three peaks of NO2, UV radiation, and O3 (Barnard et al., 2003). As the NO2 reacts with other pollutants and the incident UV radiation, it produces an afternoon peak of ozone that generally occurs around 14:00 -16:00 local time. Sulfur dioxide is not a pollutant found during the summer and generally occurs as a result of fossil fuel burning during the winter. Unless found in abnormally high concentrations (greater than 150 parts per billion), the contribution of SO2 and NO2 to UV absorption is minimal (less than 1%).

300 310 320 Wavelength (nm) Figure 11.4 Pollutant absorption cross-sections in the UV
So2 Absorption

Time of day (hour) * DUV=diffey weighted Brewer daily output Figure 11.5 Pollution and UV radiation peaks for July 8, 1997. Rubidoux,

4. Aerosols. Atmospheric aerosol concentrations provide a complex medium for light scattering and absorption (Ball and Robinson, 1982; Coakley et al., 1983; Charlson et al., 1991; Penner et al., 1992). Coming in all sizes, shapes, hydroscopic, hygroscopic, reflective, refractive, and absorbent, these aerosols create the mathematical soup (mixed and/or layered) through which the UV must pass to get to the earth's surface. They are also present at many altitudes. Volcanic eruptions, such as El Chichon in 1984 and Mt. Pinatubo in 1991, have sent significant amounts of material into the stratosphere (Stephens, 1995). These materials have reacted with, and depleted the stratospheric ozone. Volcanic gases have reacted at these altitudes to provide fine sulfate particulate matter that is highly reflective. Volcanic materials, gaseous and particulate, have provided the opportunity to assess the climate forcing (Hansen et al., 1997; Kiehl et al., 2000) of stratospheric aerosols (Lacis et al., 1992; Dutton et al., 1994).

Tropospheric aerosol concentrations have increased dramatically since the early 20th century. Anthropogenic emissions have led to increased concentrations over urban areas and have had a significant impact on the air quality east of the Mississippi River valley. Since aerosols have become both positively and negatively implicated in the area of climate change, studies have focused on their radiative properties in the visible and the infrared, but not significantly in the UV region (Flowers et al., 1969). Recently, UV measurements and one of aerosol's main constituents, black carbon (Im et al., 2001), have been implicated in the climate change arena in the southeastern part of the U.S. (Saxena and Menon, 1999). Visible range studies in the eastern United States (Liu et al., 1991) show that biologically active UV reaching the surface has decreased 5% -18% since the industrial revolution. Similar evidence of decreased surface UV-B irradiances attributable to aerosol attenuation has been reported by Seckmeyer and McKenzie (1992), Varotsos et al. (1995), Blumthaler et al. (1996), Mims (1996), and Estupinan et al. (1996). A 14% increase in attenuation of total UV-B irradiance was observed by Lorente et al. (1994), between the highest and lowest turbidity conditions over an urban site. Also observed was an important 27% enhancement of the diffuse UV-B component. For high sun conditions, approximately 50% of UV irradiance was diffuse, and a higher percentage with increasing zenith angle and turbidity. Diffuse radiation, especially for specific species, is very damaging. Frogs laying eggs in what was once thought to be a visibly shady area is now radiated with increased levels of UV, caused by increased aerosols creating a higher percentage of diffuse UV radiation (Blaustein and Wake, 1995; Mims, 1995). The effect of atmospheric aerosols on UV radiation is very complex. It would appear that as opposed to the known adverse effects, increased anthropogenic aerosols have an apparent beneficial effect of reducing surface UV radiation levels, particularly in the presence of increased tropospheric ozone (Bruhl and Crutzen, 1989; Liu et al., 1991).

Black carbon makes up a portion of the aerosol conglomerate in the atmosphere. Concentrations of black carbon in urban areas range up to 13.3 |ig/m3 (Wolff, 1981), while in the rural areas of western North Carolina, levels can be found in the 0.03 |ig/m3 range (Bahrmann and Saxena, 1998). It is created in almost all combustion processes, and since it is not degraded under normal atmospheric conditions, wet and dry depositions are its major sinks. Since it occurs primarily in submicron particles (Cadle and Mulawa, 1990), its life in the atmosphere varies from several days to several weeks. It plays an important role in the visibility of the atmosphere through its light extinction properties (Gundel et al., 1984; Cadle and Mulawa, 1990; Hansen and Rosen, 1990) as well as having catalytic properties that play a role in atmospheric chemistry (Goldberg, 1985). Automotive and diesel exhausts are the prime sources of black carbon.

Particles can also be hydroscopic (absorb water easily) and grow in size. These types frequently reflect radiation due to their outer shell of water, but may also absorb to reradiate at a longer wavelength. Nitrates and sulfates grow from gases in the atmosphere and generally become very good scatterers. Due to the nature of the particle sizes and their ability to retain water, scattering and absorption of light is very wavelength dependent.

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