Transport of ozone precursors from the Arctic troposphere to the North Atlantic region

The importance of Arctic outflow events to the budgets of nitrogen oxides and hydrocarbons in the North Atlantic region is estimated using a climatology of isentropic airflow trajectories, in combination with current understanding of the levels of these compounds in the Arctic troposphere. We first review available measurements of nonmethane hydrocarbons (NMHCs), total reactive oxidized nitrogen (NOy), and major NOy species in the Arctic troposphere to develop best estimate average vertical profiles during January-May outflow events. Measurements of these compounds in the winter-spring Arctic are generally consistent. Average levels during March are greater than or equal to 500 parts per 10(12) by volume (NOy) and similar to 20 parts per billion carbon (NMHC). Current evidence for a significant vertical gradient above the boundary layer is weak, although additional measurements are needed. Secondly, the flow patterns and frequency of Arctic outflow events which reach the North Atlantic region south of 50 degrees-55 degrees N are investigated using an 11-year climatology of isentropic forward trajectories originating at 70 degrees N in the months of January-May. The dominant route of trajectories reaching the temperate North Atlantic originates north of Canada at 2-6 km altitude and continues southward along a semipermanent trough located near the East Coast of North America. Trajectories reaching the temperate North Atlantic originated in this region on similar to 70% Of the days analyzed. Significant subsidence occurs during the southward flow, resulting in warming conducive to photochemical processing of the Arctic pollutants. Based on these analyses, the southward fluxes of NOy and NMHCs out of the Arctic in events which reach the North Atlantic south of 50 degrees N total 7.3 GgN/month NOy and 250 GgC/month NMHC during March. These values are biased low as they include only those trajectories originating below 6 km and exclude trajectories which pass over the United States or southeastern Canada. The calculated NOy flux during May is lower but may be underestimated due to uncertainty in conditions in the Arctic free troposphere in that month. The May flux of NMHCs is larger than that in March as a result of a more frequent occurrence of outflow events. These fluxes impact air parcels which are not affected by direct transport from source regions and appear to be seasonally significant relative to other sources of ozone precursors to the North Atlantic troposphere. If a significant fraction of the peroxyacetyl nitrate and alkyl nitrates which comprise most of the advected NOy decomposes over the North Atlantic, the transport of anthropogenic pollutants through the Arctic may play a significant role in the ozone budget of the North Atlantic troposphere.