Emission factors and evolution of SO₂ measured from biomass burning in wild and agricultural fires

Abstract

Abstract. Fires emit sufficient sulfur to affect local and regional air quality and climate. This study analyzes SO2 emission factors and variability in smoke plumes from US wild and agricultural fires, and their relationship to sulfate and hydroxymethanesulfonate (HMS) formation. Observed SO2 emission factors for various fuel types show good agreement with the latest reviews of biomass burning emission factors, producing an emission factor range of 0.47–1.2 g SO2 kg-1 C in the emissions. These emission factors vary with geographic location in a way that suggests that deposition of coal burning emissions and application of sulfur-containing fertilizers likely play a role in the larger observed values, which are primarily associated with agricultural burning. A 0-D box model generally reproduces the observed trends of SO2 and total sulfate (inorganic + organic) in aging wildfire plumes. In many cases, modeled HMS is consistent with the observed organosulfur concentrations. However, a comparison of observed organosulfur and modeled HMS suggests that multiple organosulfur compounds are likely responsible for the observations, but that the chemistry of these compounds yield similar production and loss rates to that of HMS, resulting in good agreement with the modeled results. We provide suggestions for constraining the organosulfur compounds observed during these flights and we show that the chemistry of HMS can allow for organosulfur to act as a S(IV) reservoir under conditions of increased pH (>6) and liquid water content (>10-7 g m-3). This can facilitate long-range transport of sulfur emissions resulting in increased SO2 and eventually sulfate in transported smoke.

Daily Local-Level Estimates of Ambient Wildfire Smoke PM2.5 for the Contiguous US

“…To further confirm that our method of constructing smoke PM 2.5 from ground station anomalies is indeed picking up PM 2.5 from smoke and not from local time-varying sources of PM 2.5 unrelated to smoke, we apply our method to harmonized speciated data from Interagency Monitoring of Protected Visual Environments (IMPROVE) and Chemical Speciation Network (CSN) monitors. − If our approach is identifying smoke-sourced PM 2.5 and not PM 2.5 from other sources, then species most likely to be present in smoke PM 2.5 which includes organic carbon throughout the US and perhaps additionally sulfates from agricultural fires in the southeastern US , will increase on smoke days but other non-fire-associated species will not increase. This is indeed what we find: the share of anomalous PM 2.5 made up by organic carbon increases substantially, particularly on days that our method would predict are very high smoke days and particularly in settings where background sources of other PM 2.5 are low (such as in the western US) (Figure S6).…”

Parameterizations of US wildfire and prescribed fire emission ratios and emission factors based on FIREX-AQ aircraft measurements

“…Given the fast conversion of NO and HONO to NO2 and nitrate, and NH3 to particulate ammonium, we also include in Table 3 the conserved quantity of NOy, as well as NOx as NO, and NHx as NH3 + particulate ammonium. Emissions of SOx as SO2 that include the conversion of SO2 to particulate sulfate are discussed in Rickly et al (2022).…”

Emission Factors for Crop Residue and Prescribed Fires in the Eastern US During FIREX‐AQ

“…There was a significant positive correlation between SO 2 and MCE for crop residue and prescribed fires (Table S6 in Supporting Information S2, r = 0.46 to 0.72). The higher sulfur content of crop residue and grassland fuels (Hatch et al, 2015;Stockwell et al, 2014), combined with sulfur deposition and the use of sulfur-containing fertilizers (Rickly et al, 2022), are likely causes of the differences in SO 2 EF. Other sulfur-containing compounds, such as methanethiol (CH 3 SH, Figure 10b), were similarly emitted in greater amounts from crop residue and grassland fires than from prescribed fuels.…”