Submicron Particle Composition and Acidity in Fire Plumes during FIREX-AQ aircraft study

Abstract:

During the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) aircraft study, the chemical composition of fire-emitted submicron particles was quantified with a High-Resolution Aerosol Mass Spectrometer (AMS). The western wildfire-emitted particles show homogeneous composition across the plumes and are overwhelmingly dominated by organic aerosol (OA). The eastern agricultural fires show larger variability in composition with a higher inorganic fraction, in particular Cl and K. Fast (1s) measurements of K in fire plumes, which show excellent correlation with SAGA filter measurements, allow a quantitative closure of the particle anion/cation balance. Although lab experiments suggest variable AMS relative ionization efficiency (RIE) of K for mixed K inorganic salts, field observations indicate a uniform RIE for fresh fire-emitted particles dominated by OA. SO₄ in some fresh biomass burning plumes (both wild and agricultural fires) had major contributions from organosulfur species (as quantified by two separate methods), vs. typically a few percent in regional background air. This is consistent with limited previous observations (DC3, FLAME-3). The AMS inorganic SO₄ agrees better with SAGA-MC SO₄, as expected from the ion chromatography detection of the latter instrument. The organosulfur appears to be dominantly a primary emission and was removed on a similar timescale as fresh primary biomass burning OA (BBOA). Ultrahigh-resolution analysis of FIREX-AQ filter samples is used to aid in the identification of the organosulfur species. Lastly, we use two thermodynamic models to estimate aerosol pH, an important lever on many particulate physical and chemical processes, based on AMS quantified K, inorganic-only SO₄, pNO₃ and collocated gas measurements (NH₃ and HNO₃). We also evaluate the impact of organic acids, amines, and nitrite. Preliminary results show that fresh western biomass burning particles had near-neutral pH (on average ~5-7), which was buffered by high levels of NH₃ and contrasts with far lower pH observed for continental (~1-4) and remote oceanic (~0) submicron particles. Regional background particles during FIREX-AQ show moderate acidity (pH~2-3), which indicates drastically different rates for H⁺-influenced processes inside and outside of the plumes.