Export of anthropogenic reactive nitrogen and sulfur compounds from the East Asia region in spring

Abstract

Measurements of gaseous and particulate reactive nitrogen and sulfur species, as well as other chemical species, were made using the P‐3B and DC‐8 aircraft over the western Pacific during the NASA Transport and Chemical Evolution over the Pacific (TRACE‐P) experiment, conducted between February and April 2001. These measurements provide a good opportunity to study the extent to which anthropogenic NOx and SO2 emitted over the East Asian countries remain as NOy and SOx (=SO2 + nssSO42−) in the form of gas or fine particles when an air mass is transported into the western Pacific region. In this paper a method to estimate transport efficiencies, ε(NOy) and ε(SOx), in an air mass that has experienced multiple injection, mixing, and loss processes is described. In this analysis, CO and CO2 are used as passive tracers of transport, and the emission inventories of CO, CO2, NOx, and SO2 over the East Asia region are used. Results from the P‐3B presented in this study indicate that 20–40% and 15% of NOx emitted over the northeastern part of China remained as NOy over the western Pacific in the boundary layer (BL) and free troposphere (FT), respectively. In the FT, PAN is found to have been the dominant form of NOy, while only 0.5% of emitted NOx remained as NOx. The transport efficiency of SOx is estimated to have been 25–45% and 15–20% in the BL and FT, respectively. Median values of the nssSO42−/SOx ratio are 0.4–0.6 both in the BL and FT, however large variability is found in the FT. These results are generally consistent with those derived using DC‐8 data. The results obtained in this study indicate that more than half of NOy and SOx were lost over the continent and that the vertical transport from the BL to FT further reduced their amounts by a factor of 2, likely due to wet removal. Budgets of NOy and SOx were also studied for air masses, which we sampled during TRACE‐P and the flux out from the continent in these cases is estimated to be 20% of the emissions. Flux in the BL and FT is found to have a similar contribution.