UC Davis

Atmospheric particulate matter (PM) has important implications on air quality, global climate and human health. However, significant uncertainty remains in the chemical and physical transformations of PM during atmospheric transport. Ambient measurements are critical for understanding the sources and processing of the aerosols in order to inform models and public policy. In this work, high-resolution soot-particle aerosol mass spectrometry (SP-AMS) was used during three different field campaigns to obtain real-time measurements of aerosol composition and properties under a variety of different environments to better understand the atmospheric evolution of PM. The first study was conducted at the Mt. Bachelor Observatory (MBO), a mountaintop site in the Pacific Northwest, in order to explore the aging of biomass burning (BB) emissions and their effects on the remote troposphere. Despite low aerosol concentrations, oxidized biomass burning organic aerosol (BBOA) exhibiting extremely low volatility was detected throughout the study. Five wildfire plume events that had undergone between 10 hours and 10 days of atmospheric transport were sampled and a statistically significant loss of OA mass relative to CO was seen, indicating the removal of OA during long range transport. The results from this field study were also integrated with observations from the Biomass Burning Observation Project (BBOP) campaign, which characterized fresh wildfire emissions to provide a detailed depiction of the evolution of biomass burning organic aerosol over its atmospheric lifetime. The second study took place in Fresno, a highly polluted city in the San Joaquin Valley of California impacted by residential wood burning emissions. Here, the SP-AMS was operated in a configuration to selectively measure only black carbon (BC) containing soot particles. A persistent, multiday fog event was used as a case study to examine the aqueous-phase processing of the soot aerosol within fog droplets. Fog processing resulted in a substantial increase in BC coating thickness due to the accumulation of ammonium nitrate and secondary OA formed in the aqueous phase. Reactions occurring in cloud and fog droplets were found to play a key role on the amount and composition of coating material present on black carbon aerosol. Finally, a SP-AMS was operated during the Tracking Aerosol Convection Interactions Experiment (TRACER) in Houston, TX in order to explore the sources of soot particles and their effects on cloud properties. Both ensemble and single particle spectra were collected in order to directly characterize the black carbon mixing state. K-means clustering analysis of the single particle measurements revealed diverse and highly variable soot particle compositions with coating-to-mass ratios spanning between 0.1 and 100. The coating material was generally hygroscopic, including oxidized OA and sulfate, suggesting that these particles may act as effective CCN.