3506B24 Final Report
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- Table 28 and Figure 45
Figure 44: Prescribed burning area (acres) and location (direction from OLC) with wind direction (in deg N), CO (ppbv), and OC mass fraction (per mil) measured at OLC (top); PM2.5 mass and composition (center); and speciated POC from HVS samples taken at OLC during the burn events in February. The numbers in the center panel represent the OM/OC ratio from the mass closure approach. Numbers in the bottom panel denote the size of the burn areas in acres. 6.6.2 Case Study of the Mid April 13-18 Event POC data from high-volume samples taken at the OLC site during the April 13 to 18 period are presented in comparison with the PCM results as well as the continuous meteorological and trace gas data. Prescribed burns were conducted on Fort Benning’s installation on all sampling days with roughly 530 to 1280 acres burned each day. April was characterized by periodic scattered showers and thunderstorms across the State. A particularly large system with persistent high winds moved through the region between April 7 and 10, causing PM2.5 and O3 values to drop equally across the State, followed by a dry period with mostly clear skies, convective flow (i.e. local winds with daytime highs and nighttime lows), and subsequent regional build-up in PM2.5 and daytime O3 maxima. Winds measured at OLC were generally weak (0.6 to 1.8 m/s with average 0.9 m/s) from the south and southwest, except on 3/24, where they blew from northerly directions at average 0.9 m/s. Hence advective daytime air mass flows indicated unlikely transport of prescribed burn emissions to the OLC site during the daytime flaming stages. However, nighttime conditions allowed for indirect influences similar to the ones described above, in that CO mixing ratios and OC fine PM mass fractions increased during nighttimes under weak easterly compnent flows. The biomass burning indicators Levoglucosan and resin acids were also highest during those nighttime periods, as shown in Table 28 and Figure 45. Similar to the February event analysis above, Table 28 summarizes the most important meteorological quantities and gas-phase concentrations, both from continuous CO, NO, NOy, and O3 measurements and from discrete PCM-denuder measurements (NH3, SO2, HNO3, HCl, acetic, formic and oxalic acids). It also includes PM2.5 mass and composition, important POC species, and again CO background mixing ratios and NO fractions of NOy, as an indicator of the freshness of potential traffic emissions. The first daytime sample on the 14th was taken under predominantly strong westerly flow, bringing to the OLC site regionally polluted and photochemically aged air masses as indicated by the high ozone and total fine PM mass concentrations, and furthermore supported by the relatively large OM/OC ratio of 1.9 in combination with an also very large OC/EC ratio of 25. It can be postulated that the increased atmospheric abundance of photo-oxidants like O3 represent atmopheric conditions of enhanced oxidative capacity, which on the other hand can lead to an increased abundance of more highly oxygenated POC, causing a higher OM/OC ratio, and more rapid formation of Secondary Organic Aerosol (SOA). Again, as seen in the February case, the wood burning indicators levoglucosan and resin acids were highest during the evening and nighttime periods immediately following the flaming stages of the active burns under weak southeasterly component air flow, whereas stronger daytime advective flows were from directions opposite to (upwind from) the burn locations, and more representative of regionally transported air masses. CO mixing ratios and OC fraction of fine PM mass showed increasing trends whenever the site was under the weaker nocturnal SE flows. But the OC/EC ratios were systematically elevated compared to earlier in the season and especially in February, again pointing to a more oxidative capacity of the atmosphere to form SOA region-wide. Table 28: Average meteorological quantities, gas-phase, PM2.5 mass and species concentrations, incl. major POC species from GC/MS during the burn events in mid April.
  
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