During the summer of 2012 and 2013, we measured carbon monoxide (CO), carbon dioxide (CO 2 ), ozone (O 3 ), nitrogen oxides (NO x ), reactive nitrogen (NO y ), peroxyacetyl nitrate (PAN), aerosol scattering (σ sp ) and absorption, elemental and organic carbon (EC and OC), and aerosol chemistry at the Mount Bachelor Observatory (2.8 km above sea level, Oregon, US). Here we analyze 23 of the individual plumes from regional wildfires to better understand production and loss of aerosols and gaseous species. We also developed a new method to calculate enhancement ratios and Modified Combustion Efficiency (MCE), which takes into account possible changes in background concentrations during transport. We compared this new method to existing methods for calculating enhancement ratios. The MCE values ranged from 0.79-0.98, ΔO 3 /ΔCO ranged from 0.01-0.07 ppbv ppbv -1 , Δσ sp /ΔCO ranged from 0.23-1.32 Mm -1 (at STP) ppbv -1 , ΔNO y /ΔCO ranged from 2.89-12.82 pptv ppbv -1 , and ΔPAN/ΔCO ranged from 1.46-6.25 pptv ppbv -1 . A comparison of three different methods to calculate enhancement ratios (ER) showed that the methods generally resulted in similar Δσ sp /ΔCO, ΔNO y /ΔCO, and ΔPAN/ΔCO; however, there was a significant bias between the methods when calculating ΔO 3 /ΔCO due to the small absolute enhancement of O 3 in the plumes. The ΔO 3 /ΔCO ERs calculated using two common methods were biased low (~20-30%) when compared to the new proposed method. Two pieces of evidence suggest moderate secondary particulate formation in many of the plumes studied: 1) mean observed ΔOC/ΔCO 2 was 0.028 g particulate-C gC -1 (as CO 2 )-27% higher than the midpoint of the biomass burning emission ratio range reported by a recent review-and 2) single scattering albedo (ω) was relatively constant at all MCE values, in contrast with results for fresh plumes. The observed NO x , PAN, and aerosol nitrate represented 6-48%, 25-57%, and 20-69% of the observed NO y in the aged plumes, respectively, and other species represented on average 11% of the observed NO y .

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In this paper, we examine biomass burning (BB) events at the Mt. Bachelor Observatory (MBO) during the summer of 2015. We explored the photochemical environment in these BB plumes, which remains poorly understood. Because we are interested in understanding the effect of aerosols only (as opposed to the combined effect of aerosols and clouds), we carefully selected three cloud‐free days in August and investigate the photochemistry in these plumes. At local midday (solar zenith angle (SZA) = 35°), j(NO2) values were slightly higher (0.2–1.8%) in the smoky days compared to the smoke‐free day, presumably due to enhanced scattering by the smoke aerosols. At higher SZA (70°), BB aerosols decrease j(NO2) by 14–21%. We also observe a greater decrease in the actinic flux at 310–350 nm, compared to 360–420 nm, presumably due to absorption in the UV by brown carbon. We compare our measurements with results from the Tropospheric Ultraviolet‐Visible v.5.2 model. As expected, we find a good agreement (to within 6%) during cloud‐free conditions. Finally, we use the extended Leighton relationship and a photochemical model (Aerosol Simulation Program v.2.1) to estimate midday HO2 and RO2 concentrations and ozone production rates (P(O3)) in the fire plumes. We observe that Leighton‐derived HO2 and RO2 values (49–185 pptv) and instantaneous P(O3) (2.0–3.6 ppbv/h) are higher than the results from the photochemical model.