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

The day-to-day variations in mixing ratios of trace gases carbon monoxide (CO), ozone (O3), and total reactive nitrogen (NO(y)) at rural Shenandoah National Park, VA, are explained to a large extent by the circulation 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 composite sets of back trajectories for each group. In the ''dirty'' group, 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. Radar summaries show that 80% of the clean trajectories from the northwest passed through areas of convection. In a second approach, we clustered daily trajectories into groups with similar circulation patterns. The cleanest air was associated with rapid westerly flow and strong subsidence, or with origins in the less-populated areas to the south and 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 resulted from slow air circulation and transport from the industrialized Midwest (as seen from the first approach).