Transport of North American anthropogenic and boreal wildfire emissions is a large source of nitrogen oxides over the North Atlantic region. To characterize the influence of transport of these emissions on nitrogen oxides levels over the central North Atlantic lower free troposphere (FT) and their further implications for hemispheric O3, we analyze measurements of NO x (NO + NO2), total reactive nitrogen oxides (NO y ), CO, and O3 made at the Pico Mountain station (38.47°N 28.40°W, 2.2 km above sea level) in the Azores archipelago from July 2002 to August 2005. Transport of pollution from North America causes significant enhancements of nitrogen oxides year-round. An analysis of the export of United States NO x emissions to the FT based on observed ΔNO y /ΔCO in the anthropogenic plumes indicates that more than 94% of the NO x emitted over the United States is lost within the continent and/or during export out of the United States boundary layer and to the Azores, consistent with previous studies. However, our observations indicate that about 30% of the NO y initially exported out of the United States boundary layer reach the Azores lower FT. NO x was also significantly enhanced in these plumes. Since the lifetime of NO x is shorter than the transport timescale of most events, PAN decomposition and potentially photolysis of HNO3 provide a supply of NO x over the central North Atlantic lower FT. Observed ΔO3/ΔNO y ratios and significant NO y levels remaining in the North American plumes suggest a potential for O3 formation well downwind from North America. Summertime boreal wildfires in North America are responsible for important shifts in the nitrogen oxides distributions toward higher levels, with medians of NO y (117–175 pptv) and NO x (9–30 pptv) greater in boreal wildfire plumes. These findings demonstrate the potential hemispheric scale impact that boreal wildfire and North American anthropogenic events have on background NO x and NO y levels and on the tropospheric O3 budget.
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