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

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 (NO_y), NO_x, (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, NO_y and NO_x 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 ΔABC/ΔCO,ΔNO_y/ΔCO and ΔNO_x/ΔCO ratios indicate that NO_y and BC were on average efficiently exported in these plumes and suggest that decomposition of PAN to NO_x, was a significant source of NO_x. High levels of NO_x 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 NO_x, 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 NO_x, and O₃.