ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-10-1171-2010 Impacts of electronically photo-excited NO<sub>2</sub> on air pollution in the South Coast Air Basin of California Ensberg J. J. 1 2 Carreras-Sospedra M. 1 Dabdub D. 1 Department of Mechanical and Aerospace Engineering, University of California at Irvine, Irvine, California, USA now at: the California Institute of Technology, Pasadena, California, USA 03 02 2010 10 3 1171 1181 Copyright: © 2010 J. J. Ensberg et al. 2010 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/10/1171/2010/acp-10-1171-2010.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/10/1171/2010/acp-10-1171-2010.pdf

A new path for hydroxyl radical formation via photo-excitation of nitrogen dioxide (NO<sub>2</sub>) and the reaction of photo-excited NO<sub>2</sub> 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 NO<sub>2</sub> with water. Impacts of this new chemical mechanism on ozone and particulate matter formation, while utilizing both reaction rates, are quantified by simulating two summer episodes. First, sensitivity simulations are conducted to evaluate the uncertainty in the rate of reaction of photo-excited NO<sub>2</sub> with water reported in the literature. Results indicate that the addition of photo-excited NO<sub>2</sub> chemistry increases peak 8-h average ozone and particulate matter concentrations. <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<sub>x</sub>) and volatile organic compounds (VOC) emissions. Isopleths are obtained using 1987 emissions, to represent past conditions, and 2005, to represent current conditions in the SoCAB. Results show that the sensitivity of modeled pollutant control strategies due to photoexcitation decreases with the decrease in baseline emissions from 1987 to 2005. Results show that including NO<sub>2</sub> photo-excitation, increases the sensitivity of ozone concentration with respect to changes in NO<sub>x</sub> emissions for both years. In particular, decreasing NO<sub>x</sub> emissions in 2005 when NO<sub>2</sub> photo-excitation is included, while utilizing the higher reaction rate, leads to ozone relative reduction factors that are 15% lower than in a case without photo-excited NO<sub>2</sub>. This implies that photoexcitation increases the effectiveness in reducing ozone through NO<sub>x</sub> emissions reductions alone, which has implications for the assessment of future emission control strategies. However, there is still disagreement with respect to the reaction rate constant for the formation of OH. Therefore, further studies are required to reduce the uncertainty in the reaction rate constant before this new mechanism is fully implemented in regulatory applications.

 References Blanchard, C. L. and Tanenbaum, S. J.: Differences between weekday and weekend air pollutant levels in southern California, J. Air Waste Manage., 53, 816–828, 2003. CARB, California Air Resources Board: CEPAM: 2009 Almanac – Standard Emissions Tool, <a href="http://www.arb.ca.gov/app/emsinv/fcemssumcat2009.php">http://www.arb.ca.gov/app/emsinv/fcemssumcat2009.php</a>, last access: January 2010, 2009a. CARB, California Air Resources Board: Database: California Air Quality Data – Selected Data Available for Download, <a href="http://www.arb.ca.gov/aqd/aqdcd/aqdcddld.htm">http://www.arb.ca.gov/aqd/aqdcd/aqdcddld.htm</a>, last access: January 2010, 2009b. Carr, S., Heard, D. E., and Blitz, M. A.: Comment on "Atmospheric Hydroxyl Radical Production from Electronically Excited NO<sub>2</sub> and H<sub>2</sub>O, Science, 324(5925), https://doi.org/10.1126/science.1166669, 2009. Carreras-Sospedra, M., Dabdub, D., Rodriguez, M. and Brouwer, J.: Air Quality Modeling in the South Coast Air Basin of California: What do the numbers really mean?, Air Waste Manage. Assoc., 56, 1184–1195, 2006. Carter, W. P. L.: Programs and Files Implementing the SAPRC-99 Mechanism and its Associated Emissions Processing Procedures for Models-3 and Other Regional Models, available at: <a href="http://www.cert.ucr.edu/&nbsp;carter/SAPRC99/s99files.htm">http://www.cert.ucr.edu/&nbsp;carter/SAPRC99/s99files.htm</a>. last accessed: December 2009, 2000. Chinkin, L. R., Coe, D. L., Funk, T., Hafner, H., Roberts, P., Ryan, P., and Lawson, D.: Weekday versus weekend activity patterns for ozone precursor emissions in California's south coast air basin, J. Air Waste Manage., 53, 829–843, 2003. Chock, D. P., Chang, T. Y., Winkler, S. L., and Nance, B. L.: The impact of an 8 h air quality standard on ROG and NO<sub>x</sub> controls in Southern Californian, Atmos. Environ., 33, 2471–2485, 1999. Crowley, J. N. and Carl, S. A.: OH Formation in the Photoexcitation of NO<sub>2</sub> beyond the Dissociation Threshold in the Presence of Water Vapor, J. Phys. Chem., 101, 4178–4184, 1997. Finlayson-Pitts, B. J. and Pitts Jr., J. N.: Chemistry of the Upper and Lower Atmosphere, Academic Press, San Diego, California, USA, 95–96, 2000a. Finlayson-Pitts, B. J., and Pitts Jr., J. N.: Chemistry of the Upper and Lower Atmosphere, Academic Press, San Diego, California, USA, 548–549, 2000. Fujita, E. M., Stockwell, W. R., Campbell, D. E., Keislar, R. E., and Lawson, D. R.: Evolution of the magnitude and spatial extent of the weekend ozone effect in California's south coast air basin,1981–2000, J. Air Waste Manage., 53, 802–815, 2003. Griffin, R. J., Dabdub, D., and Seinfeld, J. H.: Secondary organic aerosol 1. Atmospheric chemical mechanism for production of molecular constituents, J. Geophys. Res., 107, 4332–4358, 2002. Harley, R. A., Russell, A. G.,McRae, G. J., Cass, G. R., and Seinfeld, J. H.: Photochemical Modeling of the Southern California Air Quality Study, Environ. Sci. Technol., 27, 378–388, 1993. Hogrefe, C., Civerolo, K. L., Hao, W., Ku, J. Y., Zalewsky, E. E., and Sistla, G.: Rethinking the assessment of photochemical modeling systems in air quality planning applications, J. Air Waste Manage. Assoc., 58, 1086–1099, 2008. Li, S., Matthews, J., and Sinha, A.: Atmospheric Hydroxyl Radical Production from Electronically Excited NO<sub>2</sub> and H<sub>2</sub>O, Science, 319, 1657–1660, 2008. Meng, Z., Dabdub, D., and Seinfeld, J. H.: Chemical Coupling Between Atmospheric Ozone and Particulate Matter, Science, 277, 116–119, 1997. Milford, J. B., Gao, D., Sillman, S., Blossey, P., and Russell, A. G.: Total Reactive Nitrogen (NO<i><sup>Y</sup></i>) as an Indicator of the Sensitivity of Ozone to Reductions in Hydrocarbon and NOx Emissions, Journal of Geophysical Research, 99D, 3533–3542, 1994. Nguyen, K. and Dabdub, D.: NO<sub>x</sub> and VOC control and its effect on the formation of aerosols, Aerosol Sci. Technol., 36, 560–572, 2002. Qin, Y., Tonnesen, G. S., and Wang, Z.: Weekend/weekday differences of ozone, NO<sub>x</sub>, CO, VOCs, PM<sub>10</sub> and the light scatter during ozone season in southern California, Atmos. Environ., 38, 3069–3087, 2004. Rodriguez, M. A., Brouwer, J., Samuelsen, G. S., and Dabdub D.: Air quality impacts of distributed power generation in the South Coast Air Basin of California 2: Model uncertainty and sensitivity analysis, Atmos. Environ., 41, 5618–5635, 2007. Rodriguez, M. and Dabdub, D.: Monte Carlo uncertainty and sensitivity analysis of the CACM chemical mechanism, J. Geophys. Res., 108(D15), 4443, https://doi.org/10.1029/2002JD003281, 2003. Russell, A. and Dennis, R.: NARSTO critical review of photochemical models and modeling, Atmos. Environ., 34, 2283–2324, 2000. Sarwar, G., Pinder, R. W., Appel, K. W., Mathur, R., and Carlton, A. G.: Examination of the impact of photoexcited NO2 chemistry on regional air quality, Atmos. Environ., 43, 6383–6387, 2009. South Coast Air Quality Management District of California (SCAQMD): Final 2007 Air Quality Management Plan, available at: <a href="http://www.aqmd.gov/aqmp/07aqmp/index.html">http://www.aqmd.gov/aqmp/07aqmp/index.html</a>, last access: December 2009, June 2007. Wennberg, P. O. and Dabdub, D.: Atmospheric chemistry - Rethinking ozone production. Science, 319, 1624–1625, 2008. Winner, D. A., Cass, G. R., and Harley, R. A.: Effect of alternative boundary conditions on predicted ozone control strategy performance: a case study in the Los Angeles area, Atmos. Environ., 29, 3451–3464, 1995. Zeldin, M. D., Bregman, L. D., and Horie, Y.: A Meteorological and air quality assessment of the representativeness of the 1987 SCAQS Intensive Days, final report to the South Coast Air Quality Management District, 1990.