Fast Airborne Extractive Electrospray Mass Spectrometry (EESI) Measurements of the Chemical Composition of Biomass Burning Organic Aerosol

Fast measurements of the chemical composition of organic aerosol (OA) at the molecular level are essential to furthering the understanding of the sources and evolution of ambient particulate matter. To that end, we carried out airborne in-situ extractive electrospray time-of-flight mass spectrometry (EESI) measurements of aerosol in a large set of wildland and agricultural fire smoke plumes during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign in summer 2019. We present the methodology that allowed for stable, quantitative measurements of targeted analytes up to altitudes of 7 km. Optimization of electrospray solvent, fine control of electrospray capillary position, pre- and post-flight calibrations, and tightly regulated inlet pressure all contributed to extending airborne EESI measurements to these altitudes.

The EESI was operated with both positive and negative ion polarity during the study, and we report 1-Hz aerosol concentrations of levoglucosan for EESI(+) and nitrocatechol for EESI(-). Campaign-averaged 1-second detection limits for each compound were 720 and 17 ng m-3 during low-altitude sampling. Intercomparison of EESI with an Aerodyne high-resolution Aerosol Mass Spectrometer (AMS) flown during FIREX-AQ shows the fast response time of EESI in concentrated aerosol plumes. Total EESI signal was well correlated with AMS OA for both EESI(+) and EESI(-) measurements, and we present bulk EESI OA sensitivities. We also compare EESI measurements of levoglucosan to a CHemical Analysis of aeRosol ONline Proton-Transfer Reaction Mass Spectrometer (CHARON PTR-MS) flown during FIREX-AQ, demonstrating quantitative agreement between the two instruments. We also compare compounds detected in-situ by EESI with offline electrospray ionization (ESI) of filter samples collected during FIREX-AQ, showing overlap in the detected spectra of the two techniques.

Positive matrix factorization (PMF) of EESI data shows that the chemical composition of biomass burning OA evolves as it is transported downwind, with production of some species and loss of others. This evolution occurs while dilution-corrected OA concentrations remain roughly constant, suggesting that there is a balance between processes that increase and reduce OA concentrations.