Atmospheric Chemistry Program Seminar

Submicron Particle Composition and Acidity in Fire Plumes during FIREX-AQ aircraft study (1/2 seminar)
Hongyu Guo, Postdoctoral Fellow, Jimenez Group
And
Emissions, Partitioning, and Aging of Biomass Burning Organic Aerosol at FIREX-AQ (1/2 seminar)
Demetrios Pagonis, Postdoctoral Fellow, Jimenez Group
 
Guo:
"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 similar composition across the plumes and are overwhelmingly dominated by organic aerosol (OA). The eastern agricultural fires show larger variability in particle composition with a higher inorganic fraction, in particular Cl and K. Fast (1 or 5 Hz) 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 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). The fresh western biomass burning particles had near-neutral pH (on average ~6), 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~3), which indicates drastically different rates for H + -influenced processes inside and outside of the plumes."
Pagonis:
"The chemical composition of biomass burning organic aerosol (OA) evolves rapidly as it is transported downwind of a fire. Changes in composition are driven by dilution, evaporation, and secondary OA (SOA) production. Here we present airborne OA measurements from an extractive electrospray mass spectrometer (EESI) and an Aerodyne high-resolution Aerosol Mass Spectrometer (AMS) during FIREX-AQ. By combining the bulk OA composition measured by AMS with speciated OA composition measured by EESI, we quantify the relative contributions of SOA production and primary OA (POA) evaporation to the evolution of OA composition downwind of wildland fires. Evaporation rates of bulk OA and levoglucosan (a component of POA) are quantified through a combination of thermodenuder measurements and sampling smoke plumes at varying ambient temperatures. Rates of SOA production are quantified through measurements of individual SOA components by EESI, and through positive matrix factorization of AMS spectra. We observe SOA production rates exceeding 100 μg sm -3 h -1 (standard conditions 273 K, 1013 mbar) and simultaneously measure evaporation of POA components, with minimal change in dilution-corrected OA concentration; this provides direct evidence for balance in SOA production and POA evaporation rates in near-field aging of biomass burning OA."