EC-13. Submicron Particle Composition and Acidity in Fire Plumes during FIREX-AQ aircraft study

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 US wildfire particles show similar composition across the plumes and are overwhelmingly dominated by organic aerosol (OA). The agricultural fires sampled in the eastern US show larger variability in particle composition with a higher inorganic fraction, in particular Cl and K. Fast (up to 5Hz) measurements of K in fire plumes, which show excellent correlation with collocated IC filter measurements, allow a quantitative closure of the particle ion balance. Although lab experiments suggest variable AMS instrumental response of K for several K inorganic salts, field observations indicate a uniform response for fresh fire-emitted particles dominated by OA. AMS sulfate in some fresh biomass burning plumes had major contributions from organosulfur species, in contrast with typically a few percent in the regional background air. The AMS inorganic-only SO4 agrees better with IC sulfate, as expected. The organosulfur appears to be dominantly primary for a similar aging profile with a PMF primary factor and the evidence from lab studies (FLAME-III). Ultrahigh-resolution analysis of FIREX-AQ filter samples is used to aid in the identification of the organosulfur species that include both organosulfonates and organosulfates. Lastly, we use thermodynamic models to estimate aerosol pH, an important lever on many particulate physical and chemical processes, based on AMS-quantified K, inorganic-only SO4, NO3, and collocated gas-phase NH3 and HNO3 measurements. The gas-particle partitioning was reasonably reproduced by the model. We find that fresh western biomass burning submicron particles had near-neutral pH (on average ~6-7), which was buffered by high levels of NH3 and contrasts with regional background particles with moderate acidity (pH~2-3) and remote oceanic particles (pH~0).