Significant enhancements of nitrogen oxides, black carbon, and ozone in the North Atlantic lower free troposphere resulting from North American boreal wildfires

Abstract

Extensive wildfires burned in northern North America during summer 2004, releasing large amounts of trace gases and aerosols into the atmosphere. Emissions from these wildfires frequently impacted the PICO‐NARE station, a mountaintop site situated 6–15 days downwind from the fires in the Azores Islands. To assess the impacts of the boreal wildfire emissions on the levels of aerosol black carbon (BC), nitrogen oxides and O3 downwind from North America, we analyzed measurements of CO, BC, total reactive nitrogen oxides (NOy), NOx (NO + NO2) and O3 made from June to September 2004 in combination with MOZART chemical transport model simulations. Long‐range transport of boreal wildfire emissions resulted in large enhancements of CO, BC, NOy and NOx, with levels up to 250 ppbv, 665 ng m−3, 1100 pptv and 135 pptv, respectively. Enhancement ratios relative to CO were variable in the plumes sampled, most likely because of variations in wildfire emissions and removal processes during transport. Analyses of ΔBC/ΔCO, ΔNOy/ΔCO and ΔNOx/ΔCO ratios indicate that NOy and BC were on average efficiently exported in these plumes and suggest that decomposition of PAN to NOx was a significant source of NOx. High levels of NOx suggest continuing formation of O3 in these well‐aged plumes. O3 levels were also significantly enhanced in the plumes, reaching up to 75 ppbv. Analysis of ΔO3/ΔCO ratios showed distinct behaviors of O3 in the plumes, which varied from significant to lower O3 production. We identify several potential reasons for the complex effects of boreal wildfire emissions on O3 and conclude that this behavior needs to be explored further in the future. These observations demonstrate that boreal wildfire emissions significantly contributed to the NOx and O3 budgets in the central North Atlantic lower free troposphere during summer 2004 and imply large‐scale impacts on direct radiative forcing of the atmosphere and on tropospheric NOx and O3.