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

Abstract:

This study investigates the physical and chemical characteristics and atmospheric processing of aerosols from uncontrolled forest fires across the Pacific Northwest. The measurements were made at the Mt. Bachelor Observatory (MBO) located at the summit of Mt. Bachelor in central Oregon (43.9794° N, 121.6885° W, 2,763 m asl) in summer 2013 during the DOE sponsored Biomass Burning Observation Project (BBOP) field campaign. We utilized an Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) coupled with a thermodenuder. Observations during periods affected by biomass burning (BB) pollution showed elevated non-refractory submicron aerosol (NR-PM₁) concentration up to 140 µg/m³. NR-PM₁ correlated well with PM light scattering (up to ~ 600 Mm^-1 at 550 nm) and gas phase CO (up to ~0.4 ppmv). The AMS BB tracer, f₆₀, i.e., fraction of organic signals at m/z = 60, was also enhanced with a maximum of ~ 2%. 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, H/C=1.52, f₆₀ = 2.26%) that correlates well with ammonium nitrate; an intermediately oxidized BBOA-II (O/C=0.52, H/C=1.47, f₆₀ = 1.05%), and a highly oxidized BBOA-III (O/C=0.95, H/C=1.02) with a low f60 (< 0.01%) and enhanced tracer ions for carboxylic acids (e.g., CHO₂⁺). 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 BB aerosol during atmospheric aging. We will examine the enhancement of different BBOA factors relative to CO to investigate secondary organic aerosol (SOA) formation processes in BB plumes.