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

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 wildfires and agricultural fires, as well as 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.
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 yields similar production and loss rates as
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
organosulfur to act as an S(IV) reservoir under conditions of pH > 6 and liquid water content
>10−7 g sm−3. This can
facilitate long-range transport of sulfur emissions, resulting in increased
SO2 and eventually sulfate in transported smoke.
