Characterizing Aerosol Optical Properties and Black Carbon in Biomass Burning Plumes at the Mt. Bachelor Observatory

Biomass burning (BB) events representing a major source of trace gases and aerosol. The optical properties of BB emission are highly variable and can depend on fuel type, combustion conditions, and evolution during transport. The Mt. Bachelor Observatory (MBO, 2.7 km a.s.l.) mountain top research site in Central Oregon offers a unique location to explore the natural variably of BB plumes. We have observed a variety of BB events, from local prescribed burns to large regional wildfires to trans-Pacific transported Siberian wildfire smoke. Transport times range anywhere from 2 hours to 10 days.

From 2016-2018 we have observed over 80 individual plumes at MBO. We measured aerosol light absorption (σ_abs) at 3 wavelengths with a Tricolor Absorption Photometer (TAP), light scattering (σ_scat) at 3 wavelengths (TSI nephelometer), CO, CO₂, O₃, and PM1 mass. During two intensive campaigns we measured total aerosol size distribution (SMPS) and refractory black carbon (rBC) mass concentration and size distribution with a Single-Particle Soot Photometer (SP2).

The two main absorbing components of BB aerosol are black carbon (BC) and brown carbon (BrC), with BC absorbing uniformly across the UV-visible spectrum and BrC preferentially absorbing at shorter wavelengths. While the mass absorption cross-section (MAC) for BC is nominally an order-of-magnitude larger than for BrC, the positive forcing contributions by these two light absorbing species can be competitive during BB events due to large BrC-to-BC mass ratios in BB aerosol. In this study we investigate the total aerosol MAC, the combined BC and BrC absorption per unit mass BC.

We calculated MACs for BB events (MAC = σ_abs/rBC) and found they had large ranges, 8.62-34.5 m² g^-1 for 467 nm, 6.78-22.0 m² g^-1 for 467 nm, and 5.05-12.9 m² g^-1 for 467 nm. We investigated these large ranges by parameterizing MAC using absorption angstrom exponent (AAE, is an indicator of the proportion of brown carbon (BrC) in the aerosol). For all wavelengths MAC increases with AAE due to the additional absorption of BrC. As BrC preferentially absorbs at lower wavelengths, the increase in MAC with AAE is most pronounces at 467 nm. This parameterization provides a better understanding of total aerosol absorption per unit mass rBC in wildfire aerosol.

We also calculated enhancement ratios for all plumes and found Δσ_scat550/ΔCO ranged from 0.41 to 1.72 Mm^-1 ppbv^-1; Δσ_abs528/ΔCO ranged from 0.024 to 0.127 Mm^-1 ppbv^-1; and ΔrBC/ΔCO ranged from 2.40 to 17.6 ng m^-3 ppbv^-1.

Despite the various origins and transport times the single scattering albedo (SSA) for all plumes was between 0.92 and 0.98, and was not dependent on transport time, distance to the fire source, or modified combustion efficiency (MCE). MCE was not found to be correlated with the AAE or Δσ_scat550/ΔCO. This means that we did not find combustion conditions to influence BrC or total aerosol emissions.

Mean particle diameter (D_pm) of the events had a wide range of 61 nm to 222 nm. D_pm was found to correlate well (R² = 0.80) with event average σ_scat, meaning the higher the plume concentration, the larger diameter of the particles. This relationship was independent of any intensive aerosol optical properties.

As emissions from wildfires increase in the Western US, there is a need to further our understanding of BB aerosols. The wide range of ambient BB plumes observed at MBO fulfills this need and allows for the characterization of black carbon content and optical properties in BB aerosol observed in the Western US.