Emissions and evolution of SO2 from biomass burning during FIREX-AQ

Abstract:

The chemical composition of biomass burning emissions has the capacity to significantly affect the health and wellbeing of populations and influence climate through severe haze events. Sulfur dioxide (SO₂), which is commonly released during biomass burning, has shown to directly and indirectly contribute to these issues. SO₂ directly effects respiratory function and plant growth and indirectly influences radiative forcing through the production of sulfate aerosols. However, the conversion mechanism of SO₂ to sulfate remains uncertain resulting in the underprediction of sulfate aerosol concentrations in atmospheric models by up to a factor of two (Wang et al., 2016; Shao et al., 2019; Wang et al., 2014). While this underprediction has only been reported for industrialized pollution due in part to aerosol dimming, photochemistry cannot explain the observed sulfate production in many polluted regions (Cheng et al., 2016; Shao et al., 2019). It is expected that a similar relationship would occur in biomass burning plumes; however, due to limited reports of SO₂ emissions during biomass burning events, large uncertainty remains in its chemical contribution to severe haze events. Of the measurements reported, a range in the SO₂ emission factor, 0.2 to 0.87 g SO₂/kg C has been observed from various environments with large standard deviations associated with each fuel type which has resulted in a combined SO₂ emission factor of 0.61 ± 0.64 g SO₂/kg C (Andreae, 2019). Additional measurements can help to reduce the uncertainty associated with these reported emission factors as well as allow for practical modeling estimates in order to understand the contribution of SO₂ to air quality and climate from biomass burning.

Here we report in situ measurements of SO₂ through laser induced fluorescence from 20 different fires on board the NASA DC-8 aircraft during FIREX-AQ. Analysis of this data has allowed for the calculation of SO₂ emission factors from varying environments showing a similar range of values as previously reported. A similar dependence on the type of fuel consumed and the type of combustion, flaming or smoldering, is observed allowing for suggested emission factors for modeling use. In addition, a mechanism of sulfur chemistry has been produced for inclusion in the F0AM 0-D box model for understanding the transition of SO₂ to sulfate and addresses the contribution of SO₂ emissions to the total aerosol and sulfur content.