Abstract
Abstract. Air pollution is of concern in many parts of California and is impacted by both local emissions and also by pollution inflow from the North Pacific Ocean. In this study, we use the regional chemical transport model WRF-Chem V3.2 together with the global Model for OZone and Related Chemical Tracers to examine the CO budget over California. We include model CO tracers for different emission sources in the models, which allow estimation of the relative importance of local sources versus pollution inflow on the distribution of CO at the surface and in the free troposphere. The focus of our study is on the 15 June–15 July 2008 time period, which coincides with the aircraft deployment of the NASA Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission over California. Model simulations are evaluated using these aircraft observations as well as satellite retrievals and surface observations of CO. Evaluation results show that the model overall predicts the observed CO fields well, but points towards an underestimate of CO from the fires in Northern California, which had a strong influence during the study period, and towards a slight overestimate of CO from pollution inflow and local anthropogenic sources. The analysis of the CO budget over California reveals that inflow of CO explains on average 99 ± 11 ppbV of surface CO during the study period, compared to 61 ± 95 ppbV for local anthropogenic direct emissions of CO and 84 ± 194 ppbV for fires. In the free troposphere, the average CO contributions are estimated as 96 ± 7 ppbV for CO inflow, 8 ± 9 ppbV for CO from local anthropogenic sources and 18 ± 13 ppbV for CO from fires. Accounting for the low bias in the CO fire emission inventory, the fire impact during the study period might have been up to a factor 4 higher than the given estimates.
Highlights
Carbon monoxide (CO) is the third most abundant carbonbased trace gas in the atmosphere, after carbon dioxide and methane and plays a key role in the production of tropospheric ozone and the budget of the hydroxyl radical
In addition to satellite and aircraft observations, we evaluate the model through comparison with data collected from surface monitoring sites throughout California from State and Local Air Monitoring Stations (SLAMS) and National Air Monitoring Stations (NAMS) networks
The underestimate in the CO fire emissions is a limitation of the model simulations, but by having the information for CO tracers available and given that the CO chemistry is linear to first order, we can use the COfire tracer to simulate how the modeled CO fields would change with a four-fold time increase in fire CO emissions
Summary
Carbon monoxide (CO) is the third most abundant carbonbased trace gas in the atmosphere, after carbon dioxide and methane and plays a key role in the production of tropospheric ozone and the budget of the hydroxyl radical. Its global distribution at the surface reflects the location of large emission sources but with a tropospheric lifetime on the order of weeks to months, CO is a useful tracer for atmospheric pollution transport (Staudt et al, 2001; Liang et al, 2004; Yashiro et al, 2009). Pfister et al.: CO source contribution analysis for California during ARCTAS-CARB. In this study we use the regional Weather and Research Forecasting Model with Chemistry (WRF-Chem V3.2) together with the global Model for OZone and related Chemical Tracers (MOZART-4) to examine CO contributions over California (CA) and assess the CO budget. Huang et al (2010) used the regional STEM tracer model with CO tracers in support of the analysis of the effects of transported background ozone on California during ARCTAS-CARB.
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