Characterizing Atmospheric Processing of Biomass Burning Organic Aerosols from Forest Fires at the Mt. Bachelor Observatory during BBOP

 

Authors

Qi Zhang — University of California, Davis
Daniel Jaffe — University of Washington
Sonya Collier — University of California
Shan Zhou — University of California, Davis
Nicole Briggs — Gradient Corp
Jonathan Hee — University of Washington, Seattle

Category

Secondary Organic Aerosol

Description

The physical and chemical characteristics and atmospheric processing of organic aerosols from uncontrolled forest fires in Pacific Northwest were studied based on measurements made at the Mt. Bachelor Observatory (MBO; elevation = 2,763 m asl) in central Oregon in summer 2013 during the Biomass Burning Observation Project (BBOP) field campaign. Use an Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) coupled with a thermodenuder. Observations, we characterized the size resolved composition and volatility profiles of non-refractory submicron aerosol particles (NR-PM1) during periods affected by biomass burning (BB) pollution. Episodes of high concentrations of NR-PM1 (up to ~140 µg/m3) were observed, associated with elevated PM1 light scattering (up to ~ 600 Mm-1 at 550 nm) and gas phase CO (up to ~0.4 ppmv). The fraction of organic mass spectral signal at m/z = 60 (i.e., f60), which is an AMS tracer for biomass burning organic aerosol (BBOA), also showed large enhancement, with a maximum of ~ 2% during these periods. Organic aerosol (OA) dominated the PM composition in BB plumes (94.1% of the NR-PM1 mass) with an average concentration of 13.9 µg/m3. Three distinctive BBOA factors were identified by Positive Matrix Factorization (PMF): a fresh BBOA-I factor (O/C=0.27, f60 = 2.26%) that correlates well with ammonium nitrate; an intermediately oxidized BBOA-II (O/C=0.52, f60 = 1.05%), and a highly oxidized BBOA-III (O/C=0.95) with a low f60 (< 0.01%). During persistent BB plume events from fixed fire sources, fresh BBOA-I initially dominated the OA composition, but decreased as the more oxidized BBOA-II increased while BBOA-III remained unchanged. These events shed light on the chemical transformation of BBOA during atmospheric aging. The enhancement of different BBOA factors relative to CO sheds lights on secondary organic aerosol (SOA) formation processes in BB plumes.