Six ‘new’ episodes of trans-Pacific transport of air pollutants

Chemical measurements of CO, O3, non-methane hydrocarbons, aerosol chemistry, and aerosol scattering in air masses arriving at the west coast of North America demonstrate that a variety of chemical species can be transported across the Pacific from the Eurasian continent. In this paper, we analyze data from several ground sites in the Pacific Northwest and from aircraft observations in the region to identify six ‘new’ (i.e. previously unreported) episodes of trans-Pacific transport that occurred between 1993 and 2001. Of the six new episodes identified, one consisted of mineral dust combined with industrial emissions, while the other five appeared to consist of predominantly industrial emissions. In addition to gas and aerosol measurements, we calculated the Ångström exponent, an indicator of the aerosol size distribution, to help identify the aerosol characteristics in 4 of the cases considered. Combining these data with previous reports, we find that there is a high degree of variability in these trans-Pacific episodes. In four episodes there were significant O3 enhancement, with mixing ratios sometimes exceeding 80 ppbv. However, O3 was only enhanced in episodes that were transported in the free troposphere and in the absence of mineral dust. In other words, transport in the boundary layer or transport of industrial emissions with mineral dust seems to preclude any significant O3 enhancement. Clarifying such patterns increases our understanding of not only trans-Pacific transport of air pollutants, but also intercontinental transport in general.