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WRF-Chem simulation of NOx and O3 in the L.A. basin during CalNex-2010

Abstract

NOx emissions and O3 chemistry in the Los Angeles (L.A.) Basin during the CalNex-2010 field campaign (May–June 2010) have been evaluated by analyzing O3 and NOy (NO, NO2, HNO3, PAN) observations using a regional air quality model (WRF-Chem). Model simulations were conducted at 4-km spatial resolution over the basin using the Carbon-Bond Mechanism version Z (CBM-Z) and NOx emissions reduced by 24% relative to 2005 National Emissions Inventory (NEI’05), according to recent state emission statistics (BASE_NOx scenario). In addition, a 22–26% NOx emission reduction from weekday to weekend was applied. WRF-Chem reproduced the observed diurnal cycle and day-to-day variations in surface O3, Ox, HNO3 and HCHO (correlation r2 = 0.57 − 0.63; pairs of data n > 400; confidence value p < 0.01) at the CalNex supersite at Caltech but consistently overestimated surface NO and NO2. A 45% reduction of NOx emissions relative to NEI’05 (LOW_NOx scenario), as suggested by the OMI-NO2 column trend in California over the same period, improved the agreement of modeled NO2, NOx, and O3 with observations on weekdays. Three-dimensional distributions of daytime O3 and NOy were compared with five daytime NOAA WP-3D flights (three on weekdays and two on weekends) to study the Weekend-to-Weekday (WE-to-WD) effects by using the LOW_NOx scenario. Aircraft data showed a 17.3 ppb O3 increase and a 54% NOy reduction in the boundary layer on weekends relative to weekdays, while modeled WE-to-WD differences were much smaller, with a 2.9 ppb O3 increase and 16% NOy reduction only. Model results on weekends underestimated O3 by 23% and overestimated NOy and HNO3 by 40% and 27%, respectively, which may indicate that weekend NOx emissions (45% reduction relative to NEI’05 with a 22–26% reduction on weekends compared to weekdays) were still overestimated in the model. Comparisons of PAN to HNO3 ratios also indicated that the enhanced photochemistry on weekends was not well represented in the model. Although modeled weekday O3 was close to the observations in the boundary layer, modeled PAN and HNO3 were overestimated by 30% and 22%, respectively, and modeled NOy was underestimated by 24% on weekdays. Interpreted as emission ratios, the slopes of volatile organic compound (VOC) species versus CO concentrations indicated that speciated VOC emissions in the model were not accurately represented, impacting the photochemistry in the model. These findings argue for the need to improve our understanding of VOC emissions and their photochemical processing in the model.

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