Effects of aerosol UV extinction on the formation of ozone and secondary particulate matter

A radiative transfer model based on the Delta-Eddington approximation is coupled into the framework of a three-dimensional air quality model that represents airborne particles as a source-oriented external mixture. Model simulations performed for Southern California on 25 September 1996 show that ultra-violet (UV) irradiance calculated by the model is in good agreement with measured ground level UV at central Los Angeles. Ozone performance statistics show that the use of the fully coupled radiative transfer calculation within the air quality models leads to more realistic predictions in regions with significant concentration gradients of airborne particulate matter and absorbing gases. The new air quality model with fully coupled radiative transfer calculations predicts lower ozone concentrations (10-22% reduction) and lower PM2.5 concentrations (2.3-8.5% reduction) relative to air quality models that use decoupled radiative transfer calculations or surface UV observations from sparse measurement networks. The greatest ozone reduction predicted by the fully coupled model occurs in polluted regions where photolysis rates are greatly reduced. The feedback effect on ozone concentrations decreased when VOC emissions increased but was insensitive to temperature. The greatest PM2.5 reduction occurs in regions with high gas-phase ammonia and particle-phase ammonium nitrate concentrations. The representation of airborne particulate matter as an internal mixture vs. source-oriented external mixture and homogenous particle vs. core-and-shell configuration did not have a significant effect on the relationship between UV feedback and secondary pollutant concentrations. (C) 2003 Elsevier Ltd. All rights reserved.