Reactive nitrogen and ozone over the western Pacific: Distribution, partitioning, and sources

Measurements of important reactive nitrogen species (NO, NO 2 , HNO 3 , PAN, PPN, NO 3 − , NO y ), C 1 to C 6 hydrocarbons, O 3 , chemical tracers (C 2 Cl 4 , CO), and meteorological parameters were made in the troposphere (0 to 12 km) over the western Pacific (0°–50°N) during the Pacific Exploratory Mission‐West A campaign (September–October 1991). Under clean conditions, mixing ratios of NO, NO 2 , NO y , and O 3 increased with altitude and showed a distinct latitudinal gradient. PAN showed a midtropospheric maximum, while nitric acid mixing ratios were generally highest near the surface. Measured NO y concentrations were significantly greater than the sum of individually measured nitrogen species (mainly NO x , PAN, and HNO 3 ), suggesting that a large fraction of reactive nitrogen present in the atmosphere is made up of hitherto unknown species. This shortfall was larger in the tropics (≈65%) compared to midlatitudes (≈40%) and was minimal in air masses with high HNO 3 mixing ratios (>100 ppt). A global three‐dimensional photochemical model has been used to compare observations with predictions and to assess the significance of major sources. It is possible that the tropical lightning source is much greater than commonly assumed, and both lightning source and its distribution remain a major area of uncertainty in the budgets of NO y and NO x . A large disagreement between measurement and theory exists in the atmospheric distribution of HNO 3 . It appears that surface‐based anthropogenic emissions provide nearly 65% of the global atmospheric NO y reservoir. Relatively constant NO x /NO y ratios imply that NO y and NO x are in chemical equilibrium and the NO y reservoir may be an important in situ source of atmospheric NO x . Data are interpreted to suggest that only about 20% of the upper tropospheric (7–12 km) NO x is directly attributable to its surface NO x source, and free tropospheric sources are dominant. In situ release of NO x from the NO y reservoir, lightning, direct transport of surface NO x , aircraft emissions, and small stratospheric input collectively maintain the NO x balance in the atmosphere. It is shown that atmospheric ratios of reactive nitrogen and sulfur species, along with trajectory analysis, can be used to pinpoint the source of Asian continental outflow. Compared to rural atmospheres over North America, air masses over the Pacific are highly efficient in net O 3 production. Sources of tropospheric NO x cannot yet be accurately defined due to shortcomings in measurements and theory.