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 .