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Event Evidence Analysis
During the period June 23-29, 2003 a major PM2.5 episode occurred over much of the eastern US. This is a complicated episode, since it involves smoke from major distant fires in Canada, as well as regional scale stagnation occurring about the same time. In their petition, the states of South Carolina, Georgia and Tennessee have stated that a significant fraction of the PM2.5 during this period was attributable to smoke from large forest fires in southeast Canada and nearby sources. In the analysis below, the episode is discussed in its entirety, June 23-29, 2003. Also, the same description is applied to the South Carolina as well as to the Chattanooga petitions.
The chemical data for June 23, 2003 show moderate SmokeBioMass concentrations (~10 ug/m3). There is no evidence of high smoke emission sources, since the concentrations are quite uniformly distributed throughout the eastern US. On June 26, 2003 the SmokeBioMass concentration pattern was also uniform but much higher over the eastern seaboard (>15 ug/m3) and tapered off toward the West. The high SmokeBioMass concentration spike in Alabama is indicative of a local smoke source, superimposed on a high regional SmokeBioMass pattern. For June 29, 2003 the SmokeBioMass concentration is again reduced to 10 ug/m3 or below, throughout the eastern US.
The Eastern US sulfate shows similar pattern as for SmokeBioMass: low regional concentrations on June 23, 2003, high values on June 26 and again lower concentrations on June 29, 2003. Since sulfate is explicitly from anthropogenic sources and the emission rate is steady, sulfate is a good indicator for air mass stagnation over the source region. These chemical data provide strong indication that during the period June 23-29, 2003 there was a major stagnation episode that resulted in the accumulation of PM2.5 from various sources in the E. US region.
It is also instructive to examine the SmokeBioMass fraction of PM2.5 in the Chattanooga-South Carolina sub-region. On June 26, 2003 episode day, the SmokeBioMass fraction averaged over the 14 chemical sites in the Chattanooga-Greenville region was about 30%. At the Chattanooga STN site on June 26,2003 the SmokeBioMass fraction was below the seasonal average of 34%. This low SmokeBioMass fraction indicated that the high PM2.5 concentration was not caused by exceptionally high SmokeBioMass Rather, the high PM2.5 was caused by the large-scale stagnation in the region which increased the concentration of all PM species.
Fire and Transport:
During the June 23-29, 2003 period there was clearly a major smoke event over southern Canada. The satellite AOT for June 23, 2003 clearly delineates several smoke palls drifting from Ontario toward the East. The absorbing aerosol index provided by the TOMS satellite sensor shows that the high AOT regions in Canada and Maine are indeed due to the light-absorbing smoke. However, the superposition of surface PM2.5 on AOT also shows that the high AOT from smoke is not reflected in corresponding high PM2.5 at the surface (Lake Eire-Maine corridor). Evidently, that smoke plume on June 23, 2003 was mostly in elevated layers passing over the northeastern US.
The complexity of the three-dimensional transport flow field during the June 23-29, 2003 E. US episode precludes the delineation of the smoke transport pattern from Canada to the Southeast. During such anticyclonic stagnating conditions the upper air transport (where the smoke may reside) can be significantly different from the movement of surface air. Subsidence within high pressure systems is also a complicating factor. Finally, the ATAD trajectories used in this analysis simulate the air transport in an aggregated mixing layer, which makes it inappropriate for 3D air mass transport analysis.
During the June 23-29, 2003 episode, particularly on June 26, 2003, there was a 2000x1000 km size region of the Eastern US that was covered with high surface PM2.5 (>30 ug/m3). As discussed earlier, both the SmokeBioMass and SO4 were distributed rather uniformly throughout the E.US.
The satellite data indicate more patchy spatial pattern. Some of these AOT patches may be due to anthropogenic haze, while others are due to drifting smoke palls, but the elevation of these smoke layers is not discernable from the available data. Furthermore, since clouds prevent aerosol retrieval, the cloudy region of the southeastern US during the episode prevented the aerosol characterization through satellites.
Overall, the monitoring time series indicate to the rise and fall of the PM2.5 concentration during the episode. In fact, on June 26, 2003 the multi-site average PM2.5 in the Chattanooga-SC region exceeded 35 ug/m3, which is higher than any other day during the 2002-2003 period. Thus, the event was indeed a significant episode for the region. At the Chattanooga STN site the PM2.5 concentration peaked at 43 ug/m3 on June 26, 2003.
The aerosol episode during June 23-29, 2003 was an aerosol event that produced extreme PM2.5 concentrations throughout the Eastern US, including the Chattanooga-SC corridor. Based on the chemical and spatial-temporal data, the episode was caused primarily by anti-cyclonic stagnation covering much of the Eastern US. There is considerable evidence that smoke palls from Canadian forest fires were drifting over the Eastern US during this episode. However, there is no indication that the Canadian smoke palls were in fact transported to the southeastern US and descended to the ground to increase the surface concentration. In fact, the chemical mix of PM2.5 at Chattanooga during the peak of the episode was 34% SmokeBioMass, which is only slightly above the seasonal average at Chattanooga. However, this conclusion needs to be tempered by the immense complexity of emission and transport processes during stagnating conditions and long-range smoke transport.