Airborne Characterization of Regional Aerosol Origins and Optical Properties
Dickerson, Russell R
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The ubiquity of air pollution in the Mid-Atlantic and northeastern U.S. is a major concern for human and ecological health as well as the climate. Between February 2001 and February 2004, 160 flights on a light aircraft outfitted for atmospheric research were conducted over the region to characterize the factors that lead to severe air pollution episodes and how the pollutants impact the radiation budget. One salient discovery was that the chemistry and physics of multi-day haze and ozone episodes over the Mid-Atlantic and northeastern U.S. may be accurately represented by a two-reservoir system composed of the planetary boundary layer (PBL) and the lower free troposphere (LFT). Primary pollutants are typically emitted in the PBL, where they are subject to greater humidification effects and surface deposition. Pollutants in the LFT are not subject to direct deposition and photochemical processes are accelerated, which appears to play a larger role in particle growth processes than relative humidity. Measurements of a smoke plume from Canadian forest fires showed that the plume was separated from the underlying layer by a morning subsidence inversion. Absorption of solar radiation within the optically thick plume nearly equaled the total amount attenuated at the surface, creating a feedback loop that sustained the vertical stability and protracted the lifetime of the plume. Satellite reflectances were used to calculate the optical depth of the smoke plume with two sets of inputs; AERONET retrieved optical properties and optical properties measured aboard the aircraft. The optical depths calculated using the AERONET optical properties were the lowest, while retrievals using the in-situ values were 22-43% larger, due to greater absorption measured in-situ. Radiative forcings calculated with the in-situ optical properties matched surface and TOA measurements more closely than those calculated with the AERONET retrievals. Measurements made downwind of power plants during the North American electrical blackout showed reductions in SO2 (>90%), O3 (~50%), and light scattered by particles (~70%). These observations indicate the central role power plants play in regional air quality. Finally, statistical analyses of all flights point to the dominance of photochemical and meteorological processes in determining aerosol optical properties.