The Impact of Aerosols on Solar Ultra Violet Radiation and Photochemical Smog

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  • Topic: Ozone, Particulate, Smog
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  • Published : January 31, 2013
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The Impact of Aerosols on Solar Ultraviolet Radiation and Photochemical Smog

R. R. Dickerson*, S. Kondragunta, G. Stenchikov, K. L. Civerolo, B. G. Doddridge, B. N. Holben ABSTRACT
Photochemical smog, or ground-level ozone, has been the most recalcitrant of air pollution problems, but reductions in emissions of sulfur and hydrocarbons may yield unanticipated benefits in air quality. While sulfate and some organic aerosol particles scatter solar radiation back into space and can cool Earth's surface, they also change the actinic flux of ultraviolet (UV) radiation. Observations and numerical models show that UV-scattering particles in the boundary layer accelerate photochemical reactions and smog production, but UV-absorbing aerosols such as mineral dust and soot inhibit smog production. Results could have major implications for the control of air pollution.

ABSTRACT
Photochemical smog, or ground-level ozone, has been the most recalcitrant of air pollution problems, but reductions in emissions of sulfur and hydrocarbons may yield unanticipated benefits in air quality. While sulfate and some organic aerosol particles scatter solar radiation back into space and can cool Earth's surface, they also change the actinic flux of ultraviolet (UV) radiation. Observations and numerical models show that UV-scattering particles in the boundary layer accelerate photochemical reactions and smog production, but UV-absorbing aerosols such as mineral dust and soot inhibit smog production. Results could have major implications for the control of air pollution.

More than 100 counties in the United States regularly violate the Environmental Protection Agency's (EPA) Ambient Air Quality Standard for ozone (O3) of 120 ppbv (parts per 109 by volume averaged over 1 hour) (1). This ozone results from the interaction of pollutant oxides of nitrogen and nonmethane hydrocarbons (NMHCs) with solar radiation, for example, via reactions (1) to (4), and is thus sometimes called photochemical or Los Angeles–type smog.

(1)
(2)
(3)
(4)

(1)
(2)
(3)
(4)

(Where CARB is carbonyl compounds, which can further break down to produce additional O3, and hν represents a quantum of light). The rate of production of smog depends on the concentrations of these pollutants and [for reactions such as (4)] on the intensity of solar near-UV (300 < λ < 400 nm) radiation (2, 3). On hot, smoggy summer days in many North American and European cities the cloud-free sky shows a milky white color—the result of particulate air pollution scattering solar radiation. The impact of these aerosols on Earth's radiative balance and on climate (aerosol radiative forcing) has been studied extensively (4). The amount of radiation available to drive these reactions depends on the solar zenith angle (θ), absorption and scattering by gases and particles, and the surface albedo (the fraction of light reflected from Earth's surface). In the atmosphere, ozone is usually the only important absorbing gas in the near-UV spectrum. Scattering of radiation by gases (Rayleigh scattering) redistributes much of the UV radiation; Bruehl and Crutzen (5) showed that Rayleigh scattering can increase the effective path length and relative importance of tropospheric ozone in shielding us from harmful UV radiation. Optically thick clouds reduce the radiation below them, but calculations by Madronich (6) showed that scattering by cloud droplets can actually increase the actinic flux and the rate of photochemical reactions in the upper parts of clouds. Theoretical calculations and observations of photolysis rates under clear skies agree well (2, 3, 6), but observations in clouds or aerosol layers are few (7). There have been estimates of the effects of aerosols on UV flux, but only now are simultaneous observations of...
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