Role of Asian plumes and forest fire smoke on background O3 in the Northeast Pacific/western U.S.

There is some evidence that background ozone in the Northeast Pacific/western U.S. is increasing [Jaffe et al., 2003], however the exact cause for this increase remains unclear. This is especially true given the multiple sources for tropospheric O3 in the region. Using ground or airborne datasets the traditional analysis method is to examine the entire dataset for correlations and relationships. However because of the multiple sources, the correlation of various tracers is often very weak in the entire dataset and important relationships are masked by atmospheric mixing and chemistry. An alternate approach is to segregate the dataset into portions with an identifiable influence from an individual source or group of sources. To do this segregation, we have tried various approaches including trajectories, chemical transport models and some combination of these with the observations. The enhancement ratios of various species (especially CO to ozone) then give specific information on the contribution of that source to the regional ozone budget. To investigate the composition of background O3 and aerosols in the Northeast Pacific/western U.S. our team recently established a new free tropospheric sampling site on the summit of Mt. Bachelor in central Oregon at 2.7 km above sea level. Because of its elevation Asian plumes and stratospheric O3 can be more readily identified. In addition, we have also identified enhancements due to emissions from forest fires in Alaska, Siberia and the western U.S. However even when selecting out one source type, the ratios still have a large deal of variability, likely reflecting variations in emissions and chemical processing during transport. For example, during the spring of 2004, we identified numerous episodes of enhanced CO, ozone, aerosols and elemental mercury associated with sources in Eurasia. During these episodes, we measured enhancement ratios for ozone to CO and mercury to CO of 0.18 +/- 0.20 ppbv/ppbv and 0.0037 +/- .0031 ng/m3/ppbv (n=10), respectively. By comparison, plumes from biomass burning emissions have much higher aerosol/CO enhancement ratios and lower Hg/CO ratios. Based on a comparison of this enhancement ratio with the source ratios, we propose that the Hg/CO ratio is our most specific indicator of the degree of Asian influence on background air in the western U.S.