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Event Evidence Analysis
During the period August 19-28 there was a significant PM2.5 episode (~40 ug/m3) over the TN-GA-SC region. The chemical signature at Chattanooga, TN indicates that the episode was dominated by sulfate. In fact, the SO4levels at both locations were higher (15-20 ug/m3) than any other time in the 2002-2005 period. On the other hand, the SmokeBioMass levels during the episode were modest, in the range of about 10 ug/m3. As a consequence, the SmokeBioMass fraction was 27-32%, which is less than or equal to the seasonal average of 32%. Thus, the episode was driven by SO4 accumulation, rather than by exceptional SmokeBioMass levels.
Fire and Transport:
During the August 19-28 period major forest fires raged over Idaho, Montana and adjacent Canada in the Northwest. Multi-state-size smoke palls were transported from the NW source region toward the East. On August 22, for example, the SeaWifs-derived AOT shows a 1000 km size smoke pall over the Upper Midwest as it is drifting toward the East. Conceivably, this sizeable smoke plume could have an impact on the surface PM2.5. However, both the PM2.5 mass and the chemical composition of the TN-GA-SC region indicates that the western smoke impact on the surface PM in undetectably small. Throughout this period the eastern US experienced a prolonged anticyclonic stagnation. Furthermore, during much of this period this is well evidenced by the spiral counterclockwise back trajectories.
During the period August 19-28 the PM2.5 mass concentration was high throughout much of the eastern US. During the chemical monitoring days, August 19, 22, 26 and 29 the spatial pattern of PM2.5 and sulfate were similar. This is consistent with the sulfate dominated speciation of PM2.5 mass.
On August 22, the combination of vertically integrated satellite AOT and surface-based PM2.5 mass concentration data indicate significantly different pattern. The intense smoke plume recorded in the satellite images over the Upper Midwest is not present in the surface PM2.5 data. In fact, the Upper Midwest on that day shows the lowest PM2.5 concentrations. This discrepancy leads to the inference that the Upper Midwest smoke pall was elevated and decoupled from the surface layer.
The time series at Chattanooga, TN and Greenville, SC indicate a major, persistent PM2.5 peak lasting about 10 days. This time series pattern is consistent with the PM accumulation during anticyclonic stagnation. During the peak of the episode at Chattanooga, TN, the SmokeBioMass was low (<4 ug/m3).
The period August 19-28 was a major stagnation period that caused PM2.5 accumulation, composed mostly of sulfate aerosols. During this period major forest fires in the Northwest US and adjacent Canada were emitting large smoke plumes that could be tracked during their transport toward the eastern US. However, there is no evidence that the western smoke has impacted the surface concentrations at Chattanooga, TN or Greenville, SC. The evidence suggests that the western smoke was transported in layers well elevated and decoupled from the surface. In fact, the SmokeBioMass fraction at Chattanooga, TN was below average throughout the episode.