3506B24 Final Report
Download 11.49 Mb.
|
- Navigate this page:
- Figure 46
Figure 45: 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 mid April. 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. 6.6.3 Case Study of the Late April 28-30 Event Preliminary POC data from GC/MS have become available from HVS samples taken during the April 28 to May 1 period. These data have been incorporated into the relational data base and compared with the PCM results as well as the continuous meteorological and trace gas data in similar fashion to the previous two cases. On the first two sampling days, prescribed burns were conducted on Fort Benning’s installation burning 329 and 285 acres, respectively. On the 28th, both TA burned were S and ESE from OLC at 10 and 18 km, whereas on the 29th, 110 acres were burned on A20, only ~7 km to the ESE, and 175 acres further away (~17 km) and more to the north (ENE). With exception of the TA A20, which was a 1-year longleaf needle accumulation mixed with grass, the fuel of the other TA were 3 year rough with some logging slash and log decks. The flaming phases for the latter fuel kind lasted between 2 and 3 hours, whereas A-20 burned in about 1 hour. However, the A20 burn site was the closest to OLC, where the meteorological and ambient concentration measurements were made illustrated in Figure 46 and summarized in the following Table 29. During the last week of April 2003 Georgia and most of the SE-US was under the influence of high pressure over the eastern coast causing mostly clear skies, convective flow [local winds with daytime highs and nighttime lows], and subsequent regional build-up in PM2.5 and daytime O3 maxima. The discrete PM2.5 mass and composition samples were taken beginning 12 noon on the 28th for 24 hours, followed by two back-to-back 9 hour samples, one daytime 1200 to 2100 and one nighttime 2100 to 0600. The last and fourth sample was taken from noon to noon (24 hours) starting on the 30th, causing a non-sampling gap between 0600 and 1200 on the 30th. The daytime maximum O3 levels were increasing from 70 to 80 ppbv between April 28th and 29th, and dropped to 60 ppbv on the 30th. Winds were weak from southerly directions under clear skies and daytime maximum temperatures reaching 30 °C and nighttime minimum below 12 °C, indicating stable boundary layer conditions that are typical for nocturnal temperature inversions. For easy comparison with previous cases, the same parameters are tabulated and depicted as above, i.e. basic meteorological quantities and gas-phase concentrations (CO, NO, NOy, O3 averages, as well as NH3, SO2, HNO3, HCl, acetic, formic and oxalic acids from PCM denuders), plus PM2.5 mass and composition, important POC species, and again CO background mixing ratios and NO/NOy fractions. A significant change in [CO] and OC fraction occurred between midday and late evening of the 29th, when the wind shifted from strong westerlies to weak, almost calm easterlies, the direction of the burns. Consequently, the levels of CO, EC and OC significantly increased while the OC/EC ratio reached its minimum for this 3-day period. The wood burning indicator levoglucosan was also highest (115 ng/m3) during this 3rd sample period (taken at night between 2100 and 0600), when also OC accounted for the largest fraction of total PM2.5 mass (38%), and CO averaged the highest at almost 400 ppbv. Resin acids and the sum of PAHs, additional indicators for incomplete biomass combustion reached maximum 57 and 1.3 ng m-3, respectively, then as well. Table 29: Average meteorological quantities, gas-phase, PM2.5 mass and species concentrations, incl. major POC species from GC/MS during the burn events in late April.
  
Download 11.49 Mb. Share with your friends:
|