Late-spring increase of trans-Pacific pollution transport in the upper troposhere

The observations during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment show large enhancements of NO_x, PAN, O₃, CO, CFCs, and Halon-1211 in the upper troposphere over North America in late spring. Analysis of these observations and model results indicate that the enhancements are most likely driven by a surge of trans-Pacific pollutant transport in late spring. The rapid seasonal transition is particularly striking for upper tropospheric NO_x, resulting in large increases in photochemical oxidation and O₃ production during the period. The transition is later in season than that of low-altitude trans-Pacific transport, which peaks in March and April. The current generation of global chemical transport models clearly underestimates this long-range transport of pollutants, implying an underestimation in the model-projected impact on regional air quality over North America (through subsidence).