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Event Evidence Analysis
The chemical speciation data for Greenville, SC show high sulfate concentrations (10-15 ug/m3), but low SmokeBioMass (3-5 ug/m3). As a consequence, the SmokeBioMass fraction of PM2.5 mass was about 12 % which is below a typical values. The absence of SmokeBioMass in the speciated PM2.5 precludes that the PM2.5 is influenced by exceptional smoke.
Fire and Transport:
During July 17-20, 2004 major smoke events occurred in different parts of North America. There is no direct evidence that the smoke plumes were transported to the surface over Chattanooga, TN and Greenville, SC. Rather, the air mass transport trajectories indicate slow moving, stagnating air that resided over the Ohio River Valley for several days. The highest PM2.5 concentrations were recorded on July 20, 2004. The five-day airmass histories for that day indicate fast air movement from western Canada toward the Ohio River Valley where it resided for 2-3 days. Subsequently, the air drifted slowly toward the Southeast, Greenville and Chattanooga. The airmasses arriving to Greenville and Chattanooga on July 23, 2004, resided over the Southeast throughout their 5-day history.
By July 20, elevated PM2.5 mass (>30 ug/m3) was observed over much of the eastern US. The spatial pattern also indicates elevated sulfate concentrations throughout the eastern US (10-15 ug/m3). The large regional scale sulfate episode included the Chattanooga -Greenville region and it was caused by the stagnating air over the Ohio River sulfur source region. On the same day, the concentration of SmokeBioMass was moderate (5-8 ug/m3) over the Chattanooga-Greenville region. Thus, the spatial data indicate a regional sulfate episode as the key contributor to the PM2.5 episode. Prior and after the July 20 peak the concentrations of PM2.5 and sulfate were considerably lower, while the SmokeBioMass remained roughly the same.
Throughout the July19-25 period, data from SeaWifs and TOMS satellites clearly indicate 500-1000km scale smoke palls drifting over different parts of eastern US. Evidently the sources of these smoke clouds were in the West (Canada, Alaska). However, there is no evidence that would indicate that distant smoke sources have impacted the surface PM2.5 concentrations in Chattanooga, TN and Greenville, SC. It is conceivable, that elevated smoke layers, separated from the surface, have drifted over these cities.
The PM2.5 time series data indicate a concentration spike (>30 mg/m3) on July 20 at both monitoring sites. The spike is also evident in the sulfate concentrations, but not observable for SmokeBioMass. The concentration spike is an indication of an episode. However, the high PM2.5 on July 20, was driven by sulfate not by high SmokeBioMass.
The period July 19-25, 2004, is characterized by stagnation and sulfate accumulation over the eastern US including Chattanooga, TN and Greenville, SC sites. As a consequence of the high PM2.5 and low SmokeBioMass concentrations, the SmokeBioMass fraction was only 27% of PM2.5 which is well below the 45% average. This precludes the possibility that the PM2.5 spike was due to an exceptional smoke event. While major smoke plumes from Alaskan and Canadian fires were drifting over the eastern US, there is no evidence that these have impacted the surface PM2.5 concentrations at Chattanooga and Greenville