A synthesis of data and analyses identified eight separate wildfire events (five in boreal Canada and three in the western United States) that impacted the Pittsburgh Supersite, as well as Toronto, during June and July 2002. These data also revealed a larger structure in the nature of pollution episodes in Pittsburgh characterized by alternating periods of stagnation and cleansing. Stagnation resulted in significant and sustained increases in pollutant concentrations that were eventually flushed by clean air transported from Canada. Stagnation events were further characterized by decreases in wind speed, increases in ambient temperature, and enhanced atmospheric processing. Reverse trends were observed during the "clean" periods. Wildfire emissions consistently arrived immediately prior to the stagnation events and tended to be trapped and recirculated by the associated surface high pressure system, prolonging their impact. These receptor site dynamics were correlated to the structure and propagation of transient extratropical synoptic-scale waves, which were characterized by eastward propagating transcontinental trough-ridge-trough configurations. Temporal patterns observed in the Pittsburgh data (a repeating sequence of long-range, transport followed by stagnation and then flushing) were in phase with alternating synoptic structures of the propagating waves. Northwesterly subsiding flow between the ridge and trough transported wildfire emissions from the injection height above the fires down into the Pittsburgh and Toronto air sheds. As the wave propagated, the surface high associated with the upper-level ridge passed Pittsburgh, initiating a stagnation event. The cold front associated with the approaching trough flushed the stagnant air but reset the sequence of events as emissions from new wildfires were again transported behind the front. The sequence consistently repeated throughout the study period with a frequency directly correlated to the frequency of extratropical wave formation. Stagnation severity was related to the speed and maturity of the synoptic disturbances while wildfire impact severity corresponded to fire size.
ASJC Scopus subject areas
- Atmospheric Science
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science