The episodic nature of air pollution transport from Asia to North America

We employ the Geophysical Fluid Dynamics Laboratory (GFDL) global chemistry transport model (GCTM) address the episodic nature of trans-Pacific pollut ion. The strongest Asian CO episodes over North America (NA), occurring mos t frequently between February and May, are often associated with disturbanc es that entrain pollution over eastern Asia and amplify over the western Pa cific Ocean. Using 55 ppb of Asian CO as a criterion for major events, we f ind that during a typical year three to five Asian pollution events analogo us to those observed by Jaffe et al. [1999] are expected in the boundary la yer all along the U.S. West Coast between February and May. Tn contrast to CO, Asia currently has a small impact on the magnitude and variability of b ackground ozone arriving over NA from the west. Direct and indirect Asian c ontributions to episodic O-3 events over the western United States are gene rally in the 3 - 10 pphv range. The two largest total O-3 events (>60 pphv) , while having trajectories which pass over Asia, show negligible impact fr om Asian emissions. However, this may change. A future emission scenario, i n which Asian NOx emissions increase by a factor of 4 from those in 1990 pr oduces late spring ozone episodes at the surface of California with Asian c ontributions reaching 40 ppb. Such episodic contributions are certain to ex acerbate local NA pollution events, especially in elevated areas more frequ ently exposed to free tropospheric and more heavily Asian -influenced air.