WRF-Chem simulation of NOx and O3 in the L.A. basin during CalNex-2010
Introduction
The Los Angeles (L.A.) basin has been ranked as the most polluted megacity in the U.S. with respect to O3 (The American Lung Association, 2012). High O3 in the L.A. basin results from the extremely high emission of O3 precursors, nitrogen oxides NOx = NO + NO2 and volatile organic compounds (VOC). Over the past two decades (1985–2005), the total emissions of VOC and NOx in the South Coast Air Basin (SCAB) have been reduced significantly (California Air Resources Board, CARB, 2012c; data available at http://www.arb.ca.gov/app/emsinv/emssumcat.php). Although decades of emission reductions have been effective at mitigating O3 levels, many monitoring locations in the SCAB are still not in attainment of the National Ambient Air Quality Standard (NAAQS) (www.aqmd.gov).
Regional air quality models are useful tools to study this air pollution problem. Previous modeling studies have revealed that the photochemical pollution in the basin is exacerbated by the fact that the high emission intensity area is surrounded by mountains, which tends to trap the air pollution as it is blown eastward by the prevailing westerly winds (e.g. Lu and Turco, 1995, Lu and Turco, 1996). However, the application of regional models to the L.A. basin is challenging due to the combined effects of complex terrain-meteorology and high emissions in the basin. The challenges and the evaluations of the terrain-meteorology simulations using the WRF-Chem model for the L.A. basin have been described in an accompanying paper, Chen et al. (2013). The primary goal of the current paper is to evaluate the model capability to simulate the photochemistry in the basin.
Another motivation for revisiting O3 pollution in the basin is the need to understand the effect of NOx emissions, which changes in terms of the total amounts, major sources and temporal variation. Many studies have observed the recent decrease of NOx emissions in the U.S. and most of them show evidence that the decrease is due to the implementation of NOx control technology in power plants (e.g. Kim et al., 2006, Kim et al., 2009). Fuel-consumption-based calculations and ambient measurements have also been used to estimate exhaust NOx emissions from mobile sources (e.g. Parrish, 2006, Harley et al., 2005). A 9–10% per year reduction trend was found during the 2007–2009 period (McDonald et al., 2012) in the L.A. basin, where mobile sources account for ∼80% of total NOx emissions. In the study of Russell et al. (2010), OMI satellite data and ground surface observational data were used to constrain NOx emissions in the California region. The observed decrease of NO2 vertical columns in Los Angeles and the surrounding cities was 9% per year for 2005–2008. Brioude et al. (2011) presented top-down estimates of anthropogenic NOx surface fluxes and found that NOx emissions were 32 ± 10% lower in the L.A. county relative to NEI’05 for 2010. However, statistics from the California Air Resources Board (CARB) show a much slower reduction. CARB reports a decrease in total NOx from 2005 to 2010 of around 24% (CARB, 2012c; data available at http://www.arb.ca.gov/app/emsinv/emssumcat.php). In addition to the uncertainties of total NOx emission reductions, observations also showed a large Weekend-to-Weekday (WE-to-WD) NOx emission reduction, which leads to higher O3 on weekends (Pollack et al., 2012, Russell et al., 2010).
In this work, we apply the WRF-Chem model to simulate O3 and NOy species for selected days in May–June 2010 in the L.A. Basin during the CalNex-2010 intensive field experiment, focusing on three weekday daytime flights of the NOAA WP-3D research aircraft on May 4, 14 and 19, and two weekend daytime flights on May 16 and June 20. We conducted simulations at 4-km spatial resolution over the L.A. Basin using the Carbon-Bond Mechanism version Z (CBM-Z). In an accompanying paper (Chen et al., 2013), we described the model configurations and evaluations of meteorological and carbon monoxide (CO) predictions. This study evaluates chemical predictions resulting from two adjusted NOx emission scenarios. The monitoring database includes CARB stations, surface observations on the campus of the California Institute for Technology (Caltech), and the aircraft data. Chemical species evaluated in our study include ozone (O3), NOx (= NO + NO2), formaldehyde (HCHO), nitric acid (HNO3) and peroxyacetyl nitrate (PAN).
In this manuscript, we validate the model chemical predictions through comparisons of the modeled surface O3, NOy, NO, NO2, Ox, HCHO and HNO3 mixing ratios with those measured at the Caltech site. The NOx emission inventory is evaluated by comparing the simulated O3 and NOx to measurements at 20 CARB surface sites. More definitive conclusions regarding the total NOx emissions, model capabilities and Weekend-to-Weekday (WE-to-WD) effects are obtained by comparing the model against daytime aircraft observations on weekdays and weekends. The WRF-Chem model deficiencies are discussed in the context of these comparisons.
Section snippets
WRF-Chem description
The Weather Research and Forecasting (WRF) model coupled with online chemistry (WRF-Chem) version 3.1.1 was used in this study (http://ruc.noaa.gov/wrf/WG11/; Grell et al., 2005, Fast et al., 2006). Three nested domains with 36-, 12- and 4- km horizontal resolution cover the regions of the western United States, California and the L.A. basin, respectively (Fig. 1 in Chen et al., 2013). The topography in the 4-km domain is shown in Fig. 1a. There are 30 vertical layers extending from the surface
Comparisons at surface sites
We evaluated WRF-Chem simulations against surface observations at the Caltech super site and CARB sites. The measurements at the Caltech super site were compared to simulations with the baseline emission scenario (BASE_NOx). CARB observations were compared with model results using two emission scenarios BASE_NOx and LOW_NOx. Modeled and observed hourly concentrations, model bias, standard deviations (S.D.), root mean square error (RMSE) and correlations r2 are listed in Tables 2 and 3. To
Summary and conclusions
A regional dynamical/chemical model (WRF-Chem) was used to simulate and analyze O3, NOx emissions, and NOx oxidation productions in the L.A. basin during the CalNex-2010 campaign in May and June 2010. A series of 4 km WRF-Chem model simulations using the NEI’05 derived emissions inventories (two scenarios with NOx emissions reduced by 24% and 45% relative to NEI’05) were compared with in-situ aircraft measurements and surface observations to evaluate the NOx emission inventory, model
Acknowledgments
The authors would like to thank Caltech for providing the site and CARB for the infrastructure at the site. The author is grateful to Stu McKeen and Si-wan Kim for providing the emission inventory and helping set up the model. Thanks are also given to Ken Aikin at NOAA for providing the WP-3D meteorological data. This study was funded by the NOAA Global Atmospheric Composition and Climate Program.
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- 1
Now at: National Center for Atmospheric Research, Boulder, Colorado, USA.
- 2
Now at: Department of Mechanical Engineering, University of Colorado Boulder, Boulder, Colorado, USA.