Particulate Matter, Ozone, and Nitrogen Species in Aged Wildfire Plumes Observed at the Mount Bachelor Observatory

ABSTRACTDuring the summer of 2012 and 2013, we measured carbon monoxide (CO), carbon dioxide (CO2), ozone (O3), nitrogen oxides (NOx), reactive nitrogen (NOy), peroxyacetyl nitrate (PAN), aerosol scattering (σsp) 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, ΔO3/ΔCO ranged from 0.01–0.07 ppbv ppbv–1, Δσsp/ΔCO ranged from 0.23–1.32 Mm–1 (at STP) ppbv–1, ΔNOy/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 Δσsp/ΔCO, ΔNOy/ΔCO, and ΔPAN/ΔCO; however, there was a significant bias between the methods when calculating ΔO3/ΔCO due to the small absolute enhancement of O3 in the plumes. The ΔO3/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/ΔCO2 was 0.028 g particulate-C gC–1 (as CO2)—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 NOx, PAN, and aerosol nitrate represented 6–48%, 25–57%, and 20–69% of the observed NOy in the aged plumes, respectively, and other species represented on average 11% of the observed NOy.