Formation and aging of biomass burning organic aerosols from wildfires in the Western U.S.
Authors
Qi Zhang — University of California, Davis
Shan Zhou — University of California, Davis
Sonya Collier — University of California
Daniel Jaffe — University of Washington
Timothy B Onasch — Aerodyne Research, Inc.
Larry Kleinman — Brookhaven National Laboratory
Arthur J Sedlacek — Brookhaven National Laboratory
John E Shilling — Pacific Northwest National Laboratory
Category
General topics – Aerosols
Description
Biomass burning (BB) is one of the largest sources of global aerosols and can significantly influence global climate. Aerosol composition in BB emissions is overwhelmingly organic and BB organic aerosol (i.e., BBOA) comprises both primary organic aerosol (POA) emitted directly and secondary organic aerosol (SOA) formed from gaseous precursors. The environmental impacts of BB emissions are strongly correlated with the chemical, optical, and microphysical properties of BBOA, which are, in turn, dependent in a complex manner on combustion and atmospheric aging processes. Understanding the emission, formation and aging of BBOA is thus important. Here we report the results from the DOE Biomass Burning Observation Project (BBOP) campaign in the western US in summer 2013, during which aerosols in wildfire smokes were characterized extensively from the peak of Mt. Bachelor (~ 2700 m a.s.l.) in Central Oregon and the G-1 airborne platform. A wide range of wildfire plumes were sampled, ranging from freshly emitted to days old. The average oxidation of BBOA evidently increased with plume age but the BBOA to CO ratio was nearly constant, indicating no net increase of BBOA mass in aged plumes. Chemical analysis identified three types of BBOA – a semivolatile BBOA-1 representing BB POA and two more oxidized BBOAs (BBOA-2 and BBOA-3) approximately corresponding to BB SOA. In particular, BBOA-3 was highly oxidized (O/C = 1.06), depleted of levoglucosan, and showing very low volatility. We also found that while BBOA emission is strongly influenced by combustion efficiency, BBOA enhancement conserved among fresh and aged plumes. This is an indication that regional enhancement of BBOA is driven mainly by burn condition and that measurements of emissions ratios at BB sources are applicable for estimating downwind BBOA contribution. However, since atmospheric aging leads to more oxidized BBOA despite no net increase of BBOA, the aging processes must be correctly represented to properly predict the climate effects of wildfires. Furthermore, we found that the chemical aging behaviors of ambient BBOA were relatively similar among different fires, but appeared to be significantly more oxidized than BBOA oxidized in the laboratory for similar extent of OH radical processing. These results highlight the important role that field observations play in understanding aerosol lifecycle processes and climate effect.
