3506B24 Final Report
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- CO2/COx CO CO2 CH4
- Alkenes Aromatics Biog HC
- Table 18 . Table 18
- CO/CO2 CO CO2 CH4
- Figure 27
- Concentrations Absolute ppbC
Table 17: Averages and standard deviations of CO2/COx, and mixing ratios of CO, CO2, CH4, and VOC groups (enhanced above background) at the burn sites during flaming and smoldering.
As seen by the CO2/COx ratios, the flaming stages of the burns at both installations are combusting the bio-fuel significantly more efficient than during the smoldering stages. The higher temperatures that are typically achieved with more rigorous combustion causes a greater abundance of OH inside the flame regions, which in turn may be the reason for the significantly higher emissions of organic nitrates during the flaming stages at both installations. All species seem to be emitted at higher rates during the flaming stage. The higher biogenic HC average for the smoldering stage at Fort Gordon is driven exclusively by one burn conducted on April 15th, where the enhanced mixing ratio during the smoldering stage was 493 ppbv versus only 25 ppbv for the flaming stage, which also drives the average value down for the ratio of compounds emitted during flaming versus smoldering (FL/SM), as listed in Table 18. Table 18: Average ratios for compounds and VOC groups emitted during flaming vs. smoldering of same burn sites at Fort Gordon (3) and Fort Benning (4).
The detailed VOC emission profiles for the individual burns are listed in Tables A.xxx. of the Appendix. The following is an evaluation of the potential impact of the emitted VOC species on atmospheric ozone formation based on their reactivity with OH. Considering the important role that isoprene plays in both photo-chemical ozone and SOA formation mentioned earlier, it seems relevant to note, that from a total of 9 burns measured (5 at Fort Benning, 4 at Fort Gordon), more isoprene was emitted on average relative to the total VOC measured during the smoldering phases compared to during the flaming phases, which bears consequences for air quality, as prescribed burns smolder over significantly longer time periods. For example, ranking the measured VOC species emitted from the flaming sources at Fort Benning according to their moles-C mixing ratio (ppbC), puts isoprene into 12th place, and the other two biogenic compounds α- and ß-pinene into 9th and 14th highest species of all VOC, respectively. Considering the different reactivity that each VOC species has towards photo-chemical O3 production in the atmosphere, the propylene-equivalent method mentioned in the background section earlier, and as defined by Chameides et al.,[1992] was applied, which changed the ranking and relative importance of the VOC species as illustrated in Figure 27, and explicitly in Table 19. Most alkenes and all three biogenic hydrocarbons have increased in importance, so that isoprene, α-, and ß-pinene now rank 4th, 5th, and 8th respectively. Both concentration profiles, the absolute and reactivity-based, are very similar between the two military installations. The only significant differences appear for the two terpenes and i-pentane. The Fort Benning averages of these three VOC seem systematically higher, possibly due to the fact that the fuel contained typically less grass and more needle litter and woody material. The Figure also shows compounds that are potentially involved in SOA formation, e.g. the main aromatic species (benzene, toluene, and xylenes) are being emitted in relatively large quantities. The relationships for the smoldering emissions are added to Table 19 for completeness. 
Figure 27: Average ranking of VOC (in ppbC above background) emitted during the flaming phases of prescribed burns on Fort Benning/Gordon (red/orange), compared to the corresponding propylene-equivalent ranking (dark/light blue), indicating OH-reactivity towards O3 formation. Table 19: Same as Figure 27 with values for smoldering (SM) phases added.
6.2.2 Emission Estimates and Comparison with Mobile Sources In a previous study [Dookwah, 2003], emissions profile of the mobile sources in Muscogee and Richmond counties to which Forts Benning and Gordon belong, respectively, were developed. Daily county mobile VOC emissions were obtained from Georgia EPD, Air Protection Branch, which uses the US EPA approved Sparse Matrix Operator Kernel Emission (SMOKE) modeling system, which was created by the MCNC Environmental Modeling Center (EMC) to allow emissions data processing methods to integrate high-performance-computing (HPC) sparse-matrix algorithms. In the late 90s, SMOKE was redesigned and improved for use with EPA’s Models-3 Air Quality Modeling System [www.epa.gov/asmdnerl/models3/]. In 2002, SMOKE was enhanced to support driving the MOBILE6 model used to create on-road mobile emission factors and to support on-road and non-road mobile toxics inventories, resulting in SMOKE version 1.5. Most recently, in 2003 SMOKE version 2.0 was created to include all toxic inventories, including point and non-point (stationary sources reported at the county level) sources. SMOKE version 2.0 can process criteria gaseous pollutants such as carbon monoxide (CO), nitrogen oxides (NOx), volatile organic compounds (VOC), ammonia (NH3), sulfur dioxide (SO2), PM2.5 and PM10, and various toxic pollutants (see www.cep.unc.edu/empd/products/smoke/version2.1/manual.pdf for a copy of the detailed manual of version 2). County specific parameters such as road types, vehicle miles traveled (VMT), gritted emissions of pollutants (VOC, NOx, CO), and emission factors such as fuel volatility, fuel type, speeds, temperature, mode of vehicle operation, etc. are considered in this programming code. Emissions were given for 8 vehicle types and 12 road types giving a total of 96 categories, which were subsequently divided into gasoline operated and diesel operated vehicles. From these VOC mobile emissions data, a speciation profile [Sagebiel et al., 1996; Sigsby et al., 1987] for gasoline and diesel fuel was applied to determine the average daily emissions of the aromatic aerosol forming species (in kg/day) from these two fuel types. For comparison with the above mobile source emissions, the unit mass emissions from the prescribed burns were approximated under the assumption that the vertical uplift of the thermally buoyant smoke plume was vz = 20 cm s-1 during flaming, and 5 cm s-1 during the smoldering phase of each burn equally. These are conservative estimates, considering that a minimum summer-time isoprene emission rate of 2 mg C m-2 h-1 [Guenther et al, 1996], and the measured average ambient isoprene mixing ratio of 0.4 ppbv (see upwind values in Table 16) yield an average natural isoprene emission velocity of ~50 cm s-1. It was furthermore assumed that the rate of the flaming front was ~200 acres/h equally for each conducted burn, which scaled the duration of each burn tb linearly to the size of the area burned Ab. Hence the estimated amount of unit mass VOC emitted is EVOC = [VOC] * vz * Ab * tb. It is realized here, that this is a very crude estimate, offering future opportunities to determine a more accurate plume rising velocity. The smoldering phase was assumed to last uniformly 12 h, with the exception of Fort Benning’s TA A07, that had smoldered more than 24 h with samples taken on March 27th and 28th. Download 11.49 Mb. Share with your friends:
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