Optical properties of biomass burning aerosol during the 2021 Oregon fire season: comparison between wild and prescribed fires

Abstract

The Mt. Bachelor Observatory (MBO, 2.76 km a.s.l.) was frequently impacted by biomass burning (BB) smoke in 2021, an extreme forest fire year in the state of Oregon. We used measurements of fine particulate matter (PM₁) and dry aerosol scattering and absorption coefficients (σ_scat and σ_abs) to determine intensive aerosol optical properties for 27 BB events observed at MBO from April to September 2021. Measurements of carbon monoxide (CO), carbon dioxide (CO₂), and total oxidized nitrogen (NO_y) were also made during the study period. Four BB events originated from prescribed fires (PFs) between April and May, and 23 events originated from wildfires (WFs) between June and September. On average, the ΔPM₁/ΔCO normalized enhancement ratio (NER) was higher for the PF events, which we propose is due to more efficient organic aerosol condensation during quick plume dilution at colder ambient temperatures. At the same time, the PF events exhibited significantly higher Δσ_abs/ΔCO NERs at 652 nm, indicating a larger black carbon component. We attribute this to more efficient combustion, as supported by higher modified combustion efficiency (MCE) as well as higher ΔNO_y/ΔCO NERs for the PF events. Median mass scattering efficiencies (MSE; σ_scat/PM₁) ranged from 3.3 to 7.4 m² g⁻¹ (at 530 nm) across all biomass burning events, with no significant difference between WF and PF events. We found MSE to be positively correlated with plume concentration (ΔPM₁) and negatively correlated with the scattering Ångström exponent, suggesting that fast coagulation in dense smoke drives size distributions towards larger particles with greater scattering efficiency.