Biomass Burning Emissions Are a Main Driver of the PM2.5 Pollution Problem in the San Joaquin Valley of California

JUSTIN TROUSDELL, Lijuan Li, Keming Pan, Qi Zhang, University of California, Davis

     Abstract Number: 150
     Working Group: Source Apportionment

Abstract
Fresno and other cities within the San Joaquin Valley face persistent air quality challenges like compliance with the 24-hr avg. national ambient air quality standard (NAAQS) for PM2.5. To investigate sources of PM2.5 to Fresno, non-refractory PM2.5 (NRPM2.5) data were collected at 10 min time resolution at the California Air Resources Board (CARB) Garland, Fresno air quality monitoring site between 10/16/2018 and 02/11/21 with a Time-of-Flight Aerosol Chemical Speciation Monitor (TOF-ACSM) equipped with a capture vaporizer (CV) and a PM2.5 lens. Previous studies have reported significant contributions from biomass burning organic aerosol (BBOA) in winter from residential wood burning (RWB) and increased size and frequency of wildfires (WF) in recent history. Furthermore, BBOA from prescribed burns (Rx) including agricultural burns may represent an as of yet underrated contribution. Therefore we performed source apportionment using a 4-day rolling window with a 6-hour timestep on the 26-month ToF-ACSM dataset within the ME-2 Source Finder (SoFi) software to investigate BB impacts on NRPM2.5. We estimated the background level for the fraction of ion signal at m/z 60(f60), a typical BB tracer, to be 0.06% for the capture vaporizer (CV) and found the measured f60 to be above background ~ 80% of the measurement period suggesting frequent BB influences. Periods of BB influence are categorized into one of the three BB types; WF, RWB, and Rx on a day-to-day basis using seasonality, CARB records of Rx burning permissions, backtrajectories and satellite products. Data segregation by BB type reveals characteristic differences in PM composition and loadings, diurnal behavior, and seasonality depending on the dominant BB type. Total BBOA contributes on average to NRPM2.5 loadings 29% during WF event days, 15% RWB, and 19% Rx. Additionally, a 12-day period of extremely poor air quality during the Camp Fire in November 2018 exacerbated by a regional, low-dispersion episode is investigated in more detail. Total BBOA represented ~ 55% of NRPM2.5 during this period.