Direct Observation of Wintertime Secondary Formation of Sulfate in Ambient Aerosols in Fairbanks, Alaska

JINGQIU MAO, Kunal Bali, James Campbell, Ellis Robinson, Peter F. DeCarlo, Amna Ljaz, Brice Temime-Roussel, Barbara D'Anna, William Simpson, Athanasios Nenes, Rodney J. Weber, University of Alaska Fairbanks

     Abstract Number: 477
     Working Group: Urban Aerosols

Abstract
Sulfate comprises an average of 20% of the ambient PM2.5 mass during the winter months in Fairbanks, based on 24-hour average filter measurements. During the ALPACA 2022 field campaign (Jan 15th-Feb28th of 2022), we deployed two aerosol mass spectrometers (AMS) and one aerosol chemical speciation monitor (ACSM) at three urban sites, combined with Scanning Mobility Particle Sizers (SMPSs), to examine the evolution of aerosol composition and size distribution at a sub-hourly time scale. During an intense pollution episode (ambient temperature is between -25 and -35 °C), all three instruments (two AMS and one ACSM) exhibit a sharp increase in sulfate mass within a matter of hours, contributing to approximately half of the observed change in ambient PM2.5 mass concentration. The abrupt rise in sulfate mass concentration is concurrent with a substantial increase in particle size, growing from small particles (Dp < 100 nm) into larger particles (Dp > 100 nm). This increase in size suggests the secondary formation of sulfate onto pre-existing aerosols. We further investigate possible mechanisms and have ruled out the roles of cloud chemistry and transition metal ion conversion of SO2 to sulfate in PM2.5. The rapid formation of sulfate appears to coincide with the spikes of ambient nitrogen oxides concentration. Further investigation is underway to elucidate the intricate connections underlying this rapid sulfate formation.