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 pollution. The strongest Asian CO episodes over North America (NA), occurring most frequently between February and May, are often associated with disturbances that entrain pollution over eastern Asia and amplify over the western Pacific Ocean. Using 55 ppb of Asian CO as a criterion for major events, we find that during a typical year three to five Asian pollution events analogous to those observed by Jaffe et al. [1999] are expected in the boundary layer 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 background ozone arriving over NA from the west. Direct and indirect Asian contributions to episodic O-3 events over the western United States are generally 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 from Asian emissions. However, this may change. A future emission scenario, in which Asian NOx emissions increase by a factor of 4 from those in 1990 produces late spring ozone episodes at the surface of California with Asian contributions reaching 40 ppb. Such episodic contributions are certain to exacerbate local NA pollution events, especially in elevated areas more frequently exposed to free tropospheric and more heavily Asian -influenced air.