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The Mt. Bachelor Observatory was frequently impacted by biomass burning smoke in 2021, an extreme forest fire year in the state of Oregon. 

Abstract. We characterize the aerosol physical and optical properties of 13 transported biomass burning (BB) events. BB events included long-rangeinfluence 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 scatteringproperties 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 duringtransport. Alaskan and Siberian boreal forest BB plumes were transported long distances in the FT and characterized by lower absorptionÅngström exponent (AAE) values indicative of black carbon (BC)dominance in the radiative budget. Significantly elevated AAE values wereonly observed for BB events with <1 d transport, which suggests strong production of brown carbon (BrC) in these plumes but limited radiative forcing impacts outside of the immediate region. 

Ground-level ozone (O3) is a key atmospheric gas that controls the oxidizing capacity of the atmosphere and has significant health and environmental implications. Due to ongoing reductions in the concentrations of O3 precursors, it is important to assess the variables influencing baseline O3 to inform pollution control strategies. This study uses a statistical model to characterize daily peak 8 h O3 concentrations at the Mount Bachelor Observatory (MBO), a rural mountaintop research station in central Oregon, from 2006–2020. The model was constrained by seven predictive variables: year, day-of-year, relative humidity (RH), aerosol scattering, carbon monoxide (CO), water vapor (WV) mixing ratio, and tropopause pressure. RH, aerosol scattering, CO, and WV mixing ratio were measured at MBO, and tropopause pressure was measured via satellite. For the full 15-year period, the model represents 61% of the variance in daily peak 8 h O3, and all predictive variables have a statistically significant (p < 0.05) impact on daily peak 8 h O3 concentrations. Our results show that daily peak 8 h O3 concentrations at MBO are well-predicted by the model, thereby providing insight into what affects baseline O3 levels at a rural site on the west coast of North America. 

Abstract. Long et al. (2021) conducted a detailed study of possible interferences inmeasurements 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. 

Abstract. Understanding the properties and life cycle processes of aerosol particles inregional air masses is crucial for constraining the climate impacts ofaerosols on a global scale. In this study, characteristics of aerosols in theboundary layer (BL) and free troposphere (FT) of a remote continental regionin the western US were studied using a high-resolution time-of-flight aerosolmass spectrometer (HR-AMS) deployed at the Mount Bachelor Observatory (MBO;2763 m a.s.l.) in central Oregon in summer 2013. In the absence of wildfireinfluence, the average (±1σ) concentration of non-refractorysubmicrometer particulate matter (NR-PM1) at MBO was 2.8 (±2.8)µg m−3 and 84 % of the mass was organic. The otherNR-PM1 components were sulfate (11 %), ammonium (2.8 %),and nitrate (0.9 %). The organic aerosol (OA) at MBO from these cleanperiods showed clear diurnal variations driven by the boundary layer dynamicswith significantly higher concentrations occurring during daytime, upslopeconditions. NR-PM1 contained a higher mass fraction of sulfate andwas frequently acidic when MBO resided in the FT. In addition, OA in the FTwas found to be highly oxidized (average O∕C of 1.17) with lowvolatility while OA in BL-influenced air masses was moderately oxidized(average O∕C of 0.67) and semivolatile. There are indications thatthe BL-influenced OA observed at MBO was more enriched in organonitrates andorganosulfur compounds (e.g., MSA) and appeared to be representative ofbiogenic secondary organic aerosol (SOA) originated in the BL. A summary ofthe chemical compositions of NR-PM1 measured at a number of otherhigh-altitude locations in the world is presented and similar contrastsbetween FT and BL aerosols were observed. The significant compositional andphysical differences observed between FT and BL aerosols may have importantimplications for understanding the climate effects of regional backgroundaerosols.