The observations during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment show large enhancements of NOx, 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 NOx, 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).