3506B24 Final Report
Download 11.49 Mb.
|
- Navigate this page:
- Table 27 and Figure 44
Figure 43: PM2.5 concentrations in ng m-3 of main POC groups in 17 samples representing background conditions, and potential influences from flaming and smoldering PB sources. 6.6.1 Case Study of the February 5-7 Event POC data from high-volume samples taken at the OLC site during the February 5 and 6 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 at two TA (K14 and K20) only on February 5th, burning a total of 937 acres at ~28 km to the E and ENE from OLC. Fuel from both TA was mainly pine straw with a lower understory in K14 due to its 2-year rotation, compared to K20, which was on a 3-year rotation. The prescribed burns were started shortly before noon that day, completed with respect to the active burning stage by 1600 EST, going into smoldering at around 1700 until next morning. The synoptic weather condition during this period was characterized by a low pressure system associated with an area of low clouds moving northeast out of AL into and through central GA with winds increasing from Feb 3rd to 4th. The dynamic front produced showers and elevated thunder storms that moved rapidly through northern GA. In the wake of this front, colder air moved into the region from the north, causing below normal temperatures across the State, as freezing temperatures were reached at OLC early morning of the 5th. A detailed analysis of the individual CO, NO and NOy peaks that occurred during the Feb 5-6 period was presented earlier, where it was found that the largest NO/NOy fractions occurred during calm mornings, when the lower atmosphere was poorly mixed and the OLC site was likely influenced by emissions from very local sources. These air masses did not always carry the signature of vehicular emissions only, leading to evidence that the site was not directly impacted by PB emissions during the daytime flaming period. Instead, it was postulated that under conditions of thermodynamic stability of the lower atmosphere, i.e. slow drainage flow under a nocturnal surface inversion may have carried air masses with “older” emissions from the prescribed burning. The highest average PM2.5 and CO levels came from SW directions with lowest average wind speeds at night. The PCM and POC data indicate a significantly higher PM2.5 mass loading of organic compounds during the early February burn period relative to the prior non-burn period at the end of January, while the sulfate mass fraction is clearly lower. Among the light organics, acetate seems to be particularly increased during the burn period, as is elemental carbon, levoglucosan, resin acid, and other higjer molecular acids of the POC samples, all indicators for biomass burning. This is an indication that the smoldering burn sites are potential sources, which continue to inject emissions into less mixed shallow inversion layers at night. Table 27 and Figure 44 also indicate that the strongest winds blew most frequently from NW during daytime, carrying the lowest average pollution levels as a result of intense mixing and dilution. Daytime winds from NNE carried on average the highest absolute NO and NOy concentrations, with NO being also among the highest fractions of NOy, pointing to nearby Columbus City as potential origin. Table 27 comprehensively summarizes and distills 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 adds PM2.5 mass and composition, incl. some speciated POC from the HVS analyzed via GC-MS. As explained earlier, the background CO mixing ratios are derived from the intercept of the linear CO/NOy regressions for each period. The NO/NOy fractions, i.e. the NO portions of NOy, are derived from linear regressions with forced intercept of -0.2 ppbv, allowing for a reasonable non-NO background NOy mixing ratio. Table 27: Average meteorological quantities, gas-phase, PM2.5 mass and species concentrations, incl. major POC species from GC/MS during the burn events in February.
The wood burning indicator levoglucosan was highest during the evening and nighttime periods immediately following the flaming stage of the active burn under weak SE air flow (which had been NNW during the prior flaming stage!), and again during the next day when air masses were advected strongly and directly from the still smoldering burn locations. These were the times when also OC accounted for the largest fraction of total PM2.5 mass (35 to 45%). A CMB analysis for source apportionment was applied to this case’s data set and will be presented later.   
Download 11.49 Mb. Share with your friends:
|