The Effect of Spectral Optical Properties of Tropospheric Aerosols on Photolysis Rates

Atmospheric aerosols can scatter and absorb ultraviolet (UV) radiation and hence affect the actinic flux and photolysis rates that drive the chemistry of the atmosphere. In this study we used the data collected during the ACE-2 and ACE-Asia field experiments to explore the impact of atmospheric aerosols on photochemistry in the marine conditions. The focus is on the role of spectral optical properties of aerosols in controlling photolysis rates under different aerosol loading (clean marine, polluted marine, dust outbreaks, and dust/pollution outbreaks). Since measurements of aerosol optical properties in the UV region are extremely limited, we used the data of size-resolved aerosol composition to reconstruct the spectral optical properties and their variation with height and relative humidity required for calculations of the actinic flux. For each aerosol mixture, optical properties were calculated using the spectral refractive indices from the Library of Atmospheric Aerosol Refractive Indices (LAARI) and Mie code. The aerosol models were incorporated into the TUV radiative transfer code developed at the National Center of Atmospheric Research (NCAR) to compute the profile of photolysis rates. Such an approach involves several critical assumptions that result in the uncertainty of spectral optical properties (especially, the single scattering albedo) and hence actinic fluxes. The analysis of the relative importance of the assumptions made in optics modeling and associated uncertainties in calculated photolysis rates will be presented. The diurnal variability of photolysis rates in the presence of different types of tropospheric aerosols and implications for the ozone production will be addressed.