As previously shown in this chapter, the main constituent of air pollution that affects the transmission of UV to the earth's surface is aerosols. While three gases exhibit strong UV absorption characteristics, their concentrations and path-lengths in the troposphere, and primarily the planetary boundary layer, are both insufficient to produce significantly reduced UV levels at the surface. The physical, chemical and optical properties were studied, and the results strongly indicate that for dryer conditions, the retrieved single scatter albedo was higher in the eastern part of the U.S., and that relative humidity played an important role in determining this level of single scatter albedo. Highly polluted air masses also exhibited larger aerosol optical depths and in turn, the lowest UV transmission.
The second study also showed a novel non-invasive remote sensing procedure. A Mie scattering code and a radiative transfer model were utilized in conjunction with measured parameters to retrieve reasonable values of aerosol single scatter albedo and asymmetry factor using transmission and size distribution measurements. The aerosol single scatter albedo was seen to vary substantially; a probable cause due to the concentrations of black carbon in the atmosphere.
There is an obvious need to initiate an investigation similar to the Wenny study that monitors both the radiation and the aerosol size and speciation at the top of a mountain and in a nearby valley. Recent advances in aerosol instrumentation allow researchers to not only perform real time monitoring of aerosol size distributions, but are also now able to determine aerosol speciation. These data, combined with the procedures presented here, can fully characterize certain aerosol classes within a study area that could be applied throughout the country. Sulfates and nitrates, both thought to be highly scattering media, can each be detected in real time with ion chromatography. An aethalometer can measure black carbon, a DMPS or similar instrument can measure aerosol size distributions, and a good relative humidity or dew point hygrometer, combined with either a Brewer spectroradiometer or a UV MFRSR, should be able to fully characterize the aerosol optical properties in the UV and relate them to the species of aerosols present in the ambient atmosphere.
A recent paper by Petters et al. 2003 describes a methodology that can be used with the UV MFRSR and coupled with a tropospheric UV radiative transfer model to produce values of the single scatter albedo. This study conducted at the same site as the Wenny study has slightly wider variations than previously reported values. The lower values of SSA could indicate that, at least at this site, preferential absorption of UV radiation by black carbon aerosols could be occurring. More SSA data taken in the UV spectrum will allow for better estimation of this parameter for UV radiative transfer modeling and will lessen the modeling error in determining surface UV irradiances.
All of a study's monitoring data must have its quality control factors of precision, accuracy, representativeness, completeness, and comparability known and documented prior to the beginning of the study. If EPA data are used, the state of the local agency that reports that data to EPA will have a complete set of such documents that will include a Quality Systems Implementation Plan and a systematic approach to provide external system and performance audits. The data quality must be maintained throughout the study. It is a waste of the investigator's time, and more importantly, money, to gather data that cannot be used, simply because its accuracy and precision were not documented. This is true for both the air pollution data and the radiation data.
One must keep in mind that there have only been two studies presented here to show the methodology employed in determining site specific UV transmissions to the surface. Each site will have its own variables, not the least of which will be the differing combinations of scattering and absorbing aerosols. There is a great need to characterize more sites and identify aerosol types according to their chemical species and by doing so, relate the species to the single scatter albedo and the aerosol optical depth. By doing this it will become possible to transfer the ground-based knowledge to satellite observation points so that predictions of surface UV can become a reality.
The reader is referred to the Indian Ocean Experiment (INDOEX), a 1999 international field campaign conducted to study how air pollution affects climate processes over the tropical Indian Ocean, and to the South Coast Ozone Study (SCOS'97) conducted in the Los Angeles Basin during the summer of 1997 where radiation and pollution measurements were taken at the surface and various altitudes. Numerous publications were produced from each of these studies.
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