Chemical Characterization and Source Apportionment of Winter Organic Aerosols in Fresno, California, Through Combined FIGAERO‑CIMS and SP-AMS Measurements
WENQING JIANG, Ashutosh Kumar Shukla, Douglas Worsnop, Anita Avery, Harald Stark, Manjula Canagaratna, Mitchell Alton, Qi Zhang, University of California, Davis
Abstract Number: 401
Working Group: Source Apportionment
Abstract
Fresno, California experiences some of the highest wintertime PM concentrations in the US, driven by intense emissions coupled with the region’s unique geographical and meteorological conditions that promote pollutant accumulation. To investigate the emission sources and formation pathways of PM in this region, we performed an intensive field campaign in winter 2023-2024 using a soot-particle aerosol mass spectrometer (SP-AMS) and a chemical ionization mass spectrometer (CIMS) coupled with a Filter Inlet for Gases and AEROsols (FIGAERO). Positive matrix factorization (PMF) was performed on the SP-AMS and CIMS datasets both separately and in combination (cPMF) to identify key sources and processes, leveraging SP-AMS’s quantitative capabilities and CIMS’s molecular-level resolution. The analysis resolved several primary sources, including biomass burning (BB), traffic, and cooking, alongside multiple secondary organic aerosol (SOA) types formed from pathways such as aqueous-phase chemistry in fog and aerosol water, as well as nighttime chemistry in the residual layer. BB emissions peaked during nighttime under low wind speed conditions, consistent with local residential wood burning for heating. Aqueous-phase SOA (aqSOA) factors were characterized by significant contributions from organic acids such as oxalic acid and malonic acid. One aqSOA factor showed a clear link with biomass burning, featuring enhanced signals of phenolic oligomers which are typical products of aqueous reactions involving BB-derived phenols. SOA factors associated with nocturnal residual layer chemistry showed strong morning peaks starting around 7 am, coinciding with rising nitrate concentration following boundary layer mixing. These residual layer SOA factors show enhanced signals of oxalic acid, malonic acid, C9H13NO2S, and a group of CHON species likely generated via dark NO3 radical reactions. This study highlights the importance of both primary emissions and secondary formation processes in contributing to PM pollution in Fresno.