Profiles and partitioning of reactive nitrogen over the Pacific Ocean in winter and early spring

Abstract

Measurements of NO, NOy, PAN, HNO3, O3, CO, CH4, nonmethane hydrocarbons (NMHCs), and H2O were made over the Pacific Ocean in February and March during the Pacific Exploratory Mission West B (PEM-West B). NOx was calculated from NO using a photochemical model. These data were classified according to six air mass categories: western Pacific maritime, tropical, tropical convective, western Pacific continental, high latitude, and stratospheric. It has been found that the mixing ratios of many of the observed species and partitioning of NOy varied significantly depending on altitude, air mass, and season. These variations have been interpreted in terms of chemical and transport processes. In the maritime air below 7 km, the mixing ratios of calculated NOx ((NOx)mc), PAN, and NOy were lower than those in the continental air masses. The lowest values of PAN, HNO3, NOy, and O3 were observed in the tropical region below 5 km. In the continental air, NOy, PAN, CO, and NMHC levels below 4 km were much higher than those obtained in September and October during PEM-West A, due to rapid transport of these species from anthropogenic sources on the continent to the Pacific Ocean by the westerly winds which dominated in early winter and spring. Reduced photochemistry during winter also contributed to the higher values of CO and NMHCs. Above 7 km the values of these species were lower during PEM-West B, possibly due to much weaker convective activity in early spring. In spite of the weaker vertical transport, the median values of (NOx)mc and the (NOx)mc/C3H8 ratio at 10 km were 110-140 parts per trillion by volume (pptv) and 2.9-3.4 pptv/pptv, respectively, in the continental and maritime air masses, indicating the importance of in situ NOx production in the upper troposphere. In the continental air the PAN/NOy and HNO3/NOy, ratios ranged between 0.2 and 0.5, showing clear anticorrelation. The PAN/NOy ratio was also anticorrelated with the temperature. In the high-latitude air between 1 and 7 km the PAN/NOy ratio was 0.5-0.8, and the (NOx)mc/NOy ratio was less than 0.05. Temperature and the concentrations of OH and NMHCs are considered to have strongly influenced the partitioning of NOy at middle and high latitudes. Generally, the sum of (NOx)mc, PAN, and HNO3 constituted 90 ±10% of the observed NOy from the boundary layer up to 7 km in all types of air masses. This finding improves our basic understanding on the chemistry and budget of the reactive nitrogen.