RELATIONSHIP BETWEEN BACK TRAJECTORIES AND TROPOSPHERIC TRACE GAS CONCENTRATIONS IN RURAL VIRGINIA

The day-to-day variations in mixing ratios of trace gases carbon monox ide (CO), ozone (O3), and total reactive nitrogen (NO(y)) at rural She nandoah National Park, VA, are explained to a large extent by the circ ulation of the air prior to arrival. Using 48 h back trajectories, we analyzed the variability of air quality. In one approach, we separated the highest 2% and lowest 32% of NO(y) mixing ratio, and compared com posite sets of back trajectories for each group. In the ''dirty'' grou p, the air passed over the industrialized Midwest more frequently than in the ''clean'' group (51 vs 23%). When air from the industrialized Midwest was clean, its travel had often been interrupted by convective clouds that mix pollutants vertically and wash out soluble species. R adar summaries show that 80% of the clean trajectories from the northw est passed through areas of convection. In a second approach, we clust ered daily trajectories into groups with similar circulation patterns. The cleanest air was associated with rapid westerly flow and strong s ubsidence, or with origins in the less-populated areas to the south an d southeast of the Park. The highest levels of photochemical smog were not associated with stagnation and local emissions, or even transport from the nearby Washington metropolitan area. The dirtiest days resul ted from slow air circulation and transport from the industrialized Mi dwest (as seen from the first approach).