1Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA
2California Air Resources Board, Sacramento, CA, USA
3Chemistry and Dynamics Branch, NASA Langley Research Center, Hampton, VA, USA
4NASA Ames Research Center, Moffett Field, CA, USA
5Institute for Ion Physics & Applied Physics, University of Innsbruck, Innsbruck, Austria
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 Pacific. In this study, we use the regional chemical transport model WRF-Chem V3.2 to examine the CO budget over California. We include model CO tracers for different emission sources in the model, which allow estimating 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 53 ± 21% of surface CO during the study period, compared to 22 ± 18% for local anthropogenic sources and 18 ± 22% for fires. In the free troposphere, the average CO contributions are estimated as 78 ± 16% for CO inflow, 6 ± 4% for CO from local anthropogenic sources and 11 ± 13% for CO from fires.