1. Examination of the impact of recent laboratory evidence of photoexcited NO 2 chemistry on simulated summer-time regional air quality Golam Sarwar, Robert Pinder, Wyat Appel, and Rohit Mathur National Exposure Research Laboratory, U.S. Environmental Protection Agency Research Triangle Park, NC 27711, USA 1. Introduction Li et al. (2008) recently suggested that excited nitrogen dioxide, produced from nitrogen dioxide (NO 2 ) by visible sunlight, can react with water vapor to form hydroxyl radical (OH) and nitrous acid (HONO) (reactions 1-3). Wennberg and Dabdub (2008) incorporated the chemistry into an air quality model and reported that it increased ozone (O 3 ) by up to 55 ppbv for Los Angeles in Fall of 1987. Current US emissions are significantly different than those in 1987. This study examines the impact of the excited NO 2 chemistry on current day air quality in the US. NO 2 + h  (  420 nm) NO 2 * (1) NO 2 * + M NO 2 + M (2) NO 2 * + H 2 O OH + HONO (3) 2. Methodology The Community Multiscale Air Quality (CMAQ) modeling system (version 4.7) (Byun and Schere, 2006) with the 2005 version of the Carbon Bond mechanism was used. The 2001 and 2002 US National Emissions Inventories were used in the study. The Biogenic Emissions Inventory System (version 3.13) was used to prepare biogenic emissions for the study (Schwede et al., 2005). Two different modeling domains were used: one for the eastern US and the other for the western US. Two simulations were performed for July 2001 for eastern US domain and two other simulations were performed for July 2002 for the western US domain. One simulation included the CB05 chemical mechanism and the other simulation included the CB05 mechanism augmented with the excited NO 2 chemistry. The formation of excited NO 2 by visible light was calculated using absorption cross- section data from Rothman et al. (2003). 3. Results The largest differences in hourly predicted O 3 between the two model versions during the month are shown in Figure 1. The inclusion of the excited NO 2 chemistry enhanced O 3 by more than 1 ppbv in several urban areas in the eastern US: Houston, New York/New Jersey, Chicago, and New Orleans. It increased O 3 by more than 1 ppbv in five urban areas in the western US: Los Angeles, San Francisco, Portland, Seattle, and Phoenix. The largest increases in hourly predicted O 3 were 6 ppbv in Houston, 17 ppbv in New York/New Jersey, 14 ppbv in Chicago, 11 ppbv in Los Angeles, 4 ppbv in San Francisco, 7 ppbv in Seattle, 7 ppbv in Portland, and 5 ppbv in Phoenix. A comparison of predicted maximum 8-hr average O 3 with observed data from Air Quality System (AQS) for Houston, New York/New Jersey, and Los Angeles is shown in Figure 2. The difference in predicted maximum 8-hr average O 3 between the two simulations was small, resulting in a small change in the model bias and error for predicted O 3. For example, VOC levels of up to about 1,500 ppbC and 3,200 ppbC had been measured in Summer and Fall of 1987 in Los Angeles (Fujita et al., 1992). Thus, the impacts of excited NO 2 on O 3 under current US emissions conditions are perhaps much less than those suggested by Wennberg and Dabdub (2008). Additionally, we used a horizontal grid resolution of 12-km. Wennberg and Dabdub did not report the grid resolution used in their study. Differences in horizontal grid resolution and different modeling time period may have also affected the results. 4. Summary Excited NO 2 chemistry increases O 3 in several urban areas in the US for current emissions; however these increases are small. Ambient VOC and NOx levels in Los Angeles in 1987 were much greater than the current levels; thus the impact of excited NO 2 chemistry on O 3 were greater. The inclusion of the excited NO 2 chemistry in air quality models can potentially increase O 3 by a large margin in areas with high NO x and VOC conditions. 5. References Byun, D., Schere, K. L., 2006. Review of the governing equations, computational algorithms, and other components of the Models-3 Community Multiscale Air Quality (CMAQ) modeling system. Applied Mechanics Reviews, 59, 51–77. Fujita, E. M., B. E. Croes, C. L. Bennett, D. R. Lawson, F. W. Lurmann, and H. H. Main, 1992. Comparison of emission inventory and ambient concentration ratios of CO, NMHC, and NO x in California's South Coast Air Basin, J. Air Waste Manage. Assoc., 42, 264-276. Li, S.L., J. Matthews, and A. Sinha, 2008. Atmospheric hydroxyl radical production from electronically excited NO 2 and H 2 O, Science, 319, 1657-1660. Schwede, D., Pouliot, G., and Pierce, T., 2005. Changes to the biogenic emissions inventory system version 3 (BEIS3), 4th Annual CMAS Models-3 Users' Conference, September 26-28, 2005, UNC- Chapel Hill, NC. Available at http://www.cmascenter.org/html/2005_conference/abstracts/2_7.pdf http://www.cmascenter.org/html/2005_conference/abstracts/2_7.pdf Rothman, L.S., et al., 2003. The HITRAN Molecular Spectroscopic Database: Edition of 2003, Journal of Quantitative Spectroscopy & Radiative Transfer, 82, 5. Wennberg, P.O. and D. Dabdub, 2008. Rethinking ozone production, Science, 319, 1624-1625. To better understand the impact of the excited NO 2, box model simulations were performed with and without the excited NO 2 chemistry by using different initial NO X and VOC conditions. Isopleths of the increases in average predicted O 3 are shown in Figure 3. The increases in O 3 are large when both NO x and VOC are high. Increases in O 3 at other VOC and NO x conditions are small. While NO x levels in CMAQ simulations were high, VOC levels were generally low. The highest hourly VOC levels for Los Angeles were about 300-400 ppbC, for Houston 200-400 ppbC, and for New York/New Jersey 500-700 ppbC. VOC levels at other times were even lower. Thus, increases in O 3 with the excited NO 2 chemistry in CMAQ were generally small. The impact of the excited NO 2 chemistry on O 3 in Los Angeles is less than that reported by Wennberg and Dabdub (2008). We performed a model simulation for 2002 while Wennberg and Dabdub performed a simulation for 1987. VOC and NO x levels in Los Angeles were greater in 1987 than in 2002. Figure 1. Predicted largest increases in O 3 due to excited NO 2 chemistry (a) eastern US (b) western US Figure 2. CMAQ prediction vs. AQS maximum 8-hr O 3 for a) Houston b) New York/New Jersey c) Los Angeles Figure 3. Increase in average O 3 (ppbv) due to the excited NO 2 chemistry