Synoptic‐scale transport of reactive nitrogen over the western Pacific in spring

Abstract

Pathways of synoptic‐scale uplifted transport of pollutants from East Asia and their effects on chemical distributions of NOy species are investigated based on a subset of the aircraft data obtained during the NASA Transport and Chemical Evolution over the Pacific (TRACE‐P) experiment, conducted in February–April 2001. Meteorological and chemical analyses indicate that 73% of the uplifted transport was associated with warm conveyor belts (WCBs) and convective outflow (COF), which transported air masses strongly impacted by biomass burning over Southeast Asia. The rest (27%) of the uplifted air masses originated over coastal regions of northeast China, where fossil fuel combustion was a dominant source of pollutants. Both WCB associated with a midlatitude cyclone and COF associated with a stationary front over southeast China are examined in detail for the April 4 case. During the TRACE‐P period, low NOx(= NO + NO2)/NOy ratios in the WCB and COF indicate that a significant part of the NOx was oxidized to nitric acid (HNO3) and peroxyacetyl nitrate (PAN) during transport. Low HNO3/NOy ratios in the WCB and COF airstreams indicate that a large amount of HNO3 was removed during transport on timescales within 1–3 days. PAN was found to be the dominant form of NOy in air masses transported by the WCB and COF, likely due to the production of PAN in regions of biomass burning and industrial emissions, as well as due to the rapid removal of HNO3 during transport. For emissions that were transported to the free troposphere by WCBs and COF, about 10–20% of the NOy remained after transport to the free troposphere, and 30% of the NOy surviving in the boundary layer in limited cases. The results indicate that the WCB and COF provide both an efficient sink for HNO3 and an efficient mechanism for the transport of PAN from the boundary layer to the free troposphere over the western Pacific.