Seasonal variation of nitrogen oxides in the central North Atlantic lower free troposphere

Abstract

Measurements of NO, NO2, and NOy (total reactive nitrogen oxides) made at the Pico Mountain station, 38.47°N, 28.40°W, 2.2 km above sea level, from July 2002 to August 2005 are used to characterize the seasonal and diurnal variations of nitrogen oxides in the background lower free troposphere (FT) over the central North Atlantic Ocean. These observations reveal a well‐defined seasonal cycle of nitrogen oxides (NOx = NO + NO2 and NOy), with higher mixing ratios during the summertime. Observed NOx and NOy levels are consistent with long‐range transport of emissions, with significant removal en route to the measurement site. Larger summertime nitrogen oxides levels are attributed to boreal wildfire emissions and more efficient export and transport of NOy from eastern North America during that season. In addition, measurements of NOx and NOy obtained during in‐cloud and cloud‐free conditions are used to estimate PAN and HNO3 mixing ratios and examine the partitioning of the reactive nitrogen species. These estimates indicate that reactive nitrogen over the central North Atlantic lower FT largely exists in the form of PAN and HNO3 (∼80–90% of NOy) year‐round. The composition of NOy shifts from dominance of PAN in winter‐spring to dominance of HNO3 in summer‐fall, as a result of changes in temperature and photochemistry over the region. A further comparison of the nitrogen oxides measurements with results from the global chemical transport model GEOS‐Chem finds that simulated nitrogen oxides are significantly larger than the observations.