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We characterize the aerosol physical and optical properties of 13 transported biomass burning (BB) events. BB events included long-range influence from fires in Alaskan and Siberian boreal forests transported to Mt. Bachelor Observatory (MBO) in the free troposphere (FT) over 8–14+ d and regional wildfires in northern California and southwestern Oregon transported to MBO in the boundary layer (BL) over 10 h to 3 d. Intensive aerosol optical properties and normalized enhancement ratios for BB events were derived from measured aerosol light scattering coefficients (σscat), aerosol light-absorbing coefficients (σabs), fine particulate matter (PM1), and carbon monoxide (CO) measurements made from July to September 2019, with particle size distribution collected from August to September. The observations showed that the Siberian BB events had a lower scattering Ångström exponent (SAE), a higher mass scattering efficiency (MSE; Δσscat/ΔPM1), and a bimodal aerosol size distribution with a higher geometric mean diameter (Dg). We hypothesize that the larger particles and associated scattering properties were due to the transport of fine dust alongside smoke in addition to contributions from condensation of secondary aerosol, coagulation of smaller particles, and aqueous-phase processing during transport. Alaskan and Siberian boreal forest BB plumes were transported long distances in the FT and characterized by lower absorption Ångström exponent (AAE) values indicative of black carbon (BC) dominance in the radiative budget. Significantly elevated AAE values were only observed for BB events with d transport, which suggests strong production of brown carbon (BrC) in these plumes but limited radiative forcing impacts outside of the immediate region.

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 (σscat and σabs) 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 (NOy) 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 Δσabs/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 ΔNOy/ΔCO NERs for the PF events. Median mass scattering efficiencies (MSE; σscat/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.

Long et al. (2021) conducted a detailed study of possible interferences in measurements of surface O3 by UV spectroscopy, which measures the UV transmission in ambient and O3-scrubbed air. While we appreciate the careful work done in this analysis, there were several omissions, and in one case, the type of scrubber used was misidentified as manganese dioxide (MnO2) when in fact it was manganese chloride (MnCl2). This misidentification led to the erroneous conclusion that all UV-based O3 instruments employing solid-phase catalytic scrubbers exhibit significant positive artifacts, whereas previous research found this not to be the case when employing MnO2 scrubber types. While the Long et al. (2021) study, and our results, confirm the substantial bias in instruments employing an MnCl2 scrubber, a replication of the earlier work with an MnO2 scrubber type and no humidity correction is needed.

We characterize the aerosol physical and optical properties of 13 transported biomass burning (BB) events. BB events included long-range influence from fires in Alaskan and Siberian Boreal Forests transported to MBO in the free troposphere (FT) over 8–14+ days and regional wildfires in Northern California and Southwestern Oregon transported to MBO in the boundary layer (BL) over 10 h to 3 days. Intensive aerosol optical properties and normalized enhancement ratios for BB events were derived from measured aerosol light scattering coefficients (σscat), aerosol light absorbing coefficients (σabs), fine particulate matter (PM1), and carbon monoxide (CO) measurements made from July to September 2019, with particle size distribution collected from August to September. The observations showed that the Siberian BB events had lower scattering Ångström exponent (SAE), higher mass scattering efficiency (MSE; Δσscat/ΔPM1), and a bimodal aerosol size distribution with a higher geometric mean diameter (Dg). We hypothesize that the larger particles and associated scattering properties were due to the transport of fine dust alongside smoke, in addition to contributions from condensation of secondary aerosol, coagulation of smaller particles, and aqueous phase processing during transport. Alaskan and Siberian Boreal Forest BB plumes were transported long distances in the FT and characterized by lower absorption Ångström exponent (AAE) values indicative of black carbon (BC) dominance in the radiative budget. Significantly elevated AAE values were only observed for BB events with

Long et al (2021) conducted a detailed study of possible interferents in measurements of surface O3 by UV spectroscopy, which measures the UV transmission in ambient and O3 scrubbed air. While we appreciate the careful work done in this analysis, there were several omissions and, in one case, the type of scrubber used was mis-identified as manganese dioxide (MnO2), when in fact it was manganese chloride (MnCl2). This misidentification led to the erroneous conclusion that all UV-based O3 instruments employing solid-phase catalytic scrubbers exhibit significant positive artifacts, whereas previous research found this not to be the case when employing MnO2 scrubber types. While the Long study, and our results, confirm the substantial bias in instruments employing an MnCl2 scrubber, a replication of the earlier work with an MnO2 scrubber type and no humidity correction is needed.