The Radiative E ects of Aerosols on Photochemical Smog

High concentrations of both ozone and aerosols are observed in the eastern United States during stagnant weather conditions associated with transport from the W or NW; they show similar spatial and temporal patterns. We discuss a causal mechanism that may contribute to this correlation-the radiative eeects of aerosols on photolysis rates. We measured j(NO 2), the rate coeecient for nitrogen dioxide photolysis, and column aerosol optical depths at NASA/Goddard Space Flight Center in Greenbelt, MD (39.01 N and 76.87 W) during the smog seasons of 1995 and 1997. Direct measurements and radiative transfer model calculations show that particles can reduce surface j(NO 2) by 5-60%, depending on solar zenith angle and aerosol loading. Although particle scattering by dense aerosol loading on smoggy days decreases near-surface photolysis rates, it increases the integrated boundary layer photolysis rates by up to 20% and leads to accelerated photochemical smog formation in urban areas. Dickerson et al. 1997] showed that a range of aerosol eeects on ozone formation, in both magnitude and sign, are possible depending on aerosol column loading, single scattering albedo, and vertical distribution. Here, we expand on those ndings and present new observations, and results from model simulations. For the episode of July 13-15, 1995, relative to an aerosol-free atmosphere, model predicted ozone production in the boundary layer increased by up to 50 ppb for purely scattering particles (the theoretical maximum eeect), and by up to 23 ppb for our best estimate of mean aerosol optical properties. The same aerosols when placed above the boundary layer were calculated to decrease boundary layer ozone production by up to 30 ppb, and strongly absorbing aerosols produce similar reductions regardless of their vertical distribution. Particles that absorb strongly in the UVB but not in the UVA accelerate ozone production over a broader area, but less intensely in urban plumes. The combined weight of these studies supports joint control strategies to comply with the EPA's new ozone and ne particulate matter standards. 3