Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 5 2009 10.5194/acpd-9-18985-2009 http://www.atmos-chem-phys-discuss.net/9/18985/2009/ http://www.atmos-chem-phys-discuss.net/9/18985/2009/acpd-9-18985-2009.html http://www.atmos-chem-phys-discuss.net/9/18985/2009/acpd-9-18985-2009.pdf 18985 19008 2009-09-11 Impacts of electronically photo-excited NO₂ on air pollution control strategies in the South Coast Air Basin of California J. J. Ensberg M. Carreras-Sospedra D. Dabdub ddabdub@uci.edu Department of Mechanical and Aerospace Engineering, University of California at Irvine, Irvine, California, USA now at: The California Institute of Technology, Pasadena, California, USA A new path for hydroxyl radical formation via photo-excitation of nitrogen dioxide (NO₂) and the reaction of photo-excited NO₂ with water is evaluated using the UCI-CIT model for the South Coast Air Basin of California (SoCAB). Two separate studies predict different reaction rates, which differ by nearly an order of magnitude, for the reaction of photo-excited NO₂ with water. Impacts of this new chemical mechanism on ozone and particulate matter formation, while utilizing both reaction rates, are quantified by simulating a two-day summer episode. In addition, sensitivity simulations are conducted to evaluate the uncertainty in the rate of reaction of photo-excited NO₂ with water reported in the literature. Results indicate that the addition of photo-excited NO₂ chemistry increases peak 1-h average ozone concentrations by up to 20.6%. Also, the new chemistry leads to moderate increases in particulate matter concentrations of up to 2.9%. <br><br> The importance of this new chemistry is then evaluated in the context of pollution control strategies. A series of simulations are conducted to generate isopleths for ozone and particulate matter concentrations, varying baseline nitrogen oxides (NO_x) and volatile organic compounds emissions. Results show that including NO₂ photo-excitation increases the sensitivity of ozone concentration to changes in NO_x emissions. Namely, increasing NO_x when NO₂ photo-excitation is included, while utilizing the higher reaction rate, leads to an increase in ozone concentration of up to 38.7% higher than a case without photo-excited NO₂. Ozone and particulate matter control strategies rely heavily on the variation of NO_x and VOC emissions and the addition of the new chemical mechanism increases peak ozone and 24-h average PM concentrations in all locations under all NO_x and VOC scaling factors while utilizing both reaction rates. 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