Large-scale impacts of anthropogenic pollution and boreal wildfires on the nitrogen oxides over the central North Atlantic region

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 O-3, we analyze measurements of NOx (NO + NO2), total reactive nitrogen oxides (NOy), CO, and O-3 made at the Pico Mountain station (38.47 degrees N 28.40 degrees 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 NOx emissions to the FT based on observed Delta NOy/Delta CO in the anthropogenic plumes indicates that more than 94% of the NOx 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 NOy initially exported out of the United States boundary layer reach the Azores lower FT. NOx was also significantly enhanced in these plumes. Since the lifetime of NOx is shorter than the transport timescale of most events, PAN decomposition and potentially photolysis of HNO3 provide a supply of NOx over the central North Atlantic lower FT. Observed Delta O-3/Delta NOy ratios and significant NOy levels remaining in the North American plumes suggest a potential for O-3 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 NOy (117-175 pptv) and NOx (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 NOx and NOy levels and on the tropospheric O-3 budget.