Mixing State and Aging of Biomass Burning Aerosols During the 2007 San Diego Wildfires

Biomass burning aerosols (BBA) significantly affect regional and global air quality, health and climate, yet their mixing state is not fully characterized. Specifically, aerosols from burning land dominated by chaparral shrubs, such as in Southern California, are less characterized than other BBA, although fires in this area have been increasing in frequency since 1980s. During the 2007 San Diego Wildfires the size-resolved chemistry of 100-400 nm single particles was determined in real-time with an ultra-fine aerosol time of flight mass spectrometer (UF-ATOFMS). BBA, identified by having a strong potassium peak and smaller carbonaceous markers present in the mass spectra, made up 84% of all particles measured between 10/22/07 and 11/1/07. Even though levoglucosan is known as a good biomass burning tracer, only 36% of all BBA in this study had this tracer present. Positive Matrix Factorization (PMF) analysis was utilized to group different BBA chemical markers, such as potassium salts, sulfate, ammonium, oxalate and levoglucosan. A spike in ammonium was observed with the increase in relative humidity on 10/25/07 and correlated with nitric acid and nitrate, indicating that the majority of ammonium was present as NH4NO3. The presence of different potassium salts were used to identify the age of BBA. K2Cl+, indicative of fresh BBA, was only seen at the beginning of the wildfires when the size mode of particles was ~<120 nm. K2NO3+ and K3SO4+ spiked at different times, with K2NO3+ peaking before K3SO4+. Particles with K3SO4+ had larger sizes than those with K2NO3+, thus K2NO3+ represents slightly aged whereas K3SO4+ represents moderately aged BBA. The largest BBA observed, and hence the most aged, were those characterized by the lack of potassium salts and the presence of secondary markers, such as sulfate and oxalate. In summary, we observed the evolution of BBA undergoing four distinct aging steps based on particle size and composition: slightly fresh, slightly aged, moderately aged and aged.