REACTIVE NITROGEN AND OZONE OVER THE WESTERN PACIFIC - DISTRIBUTION, PARTITIONING, AND SOURCES

Measurements of important reactive nitrogen species (NO, NO2, HNO3, PA N, PPN, NO3-, NOy), C-1 to C-6 hydrocarbons, O-3, chemical tracers (C2 Cl4, CO), and meteorological parameters were made in the troposphere ( 0 to 12 km) over the western Pacific (0 degrees-50 degrees N) during t he Pacific Exploratory Mission-West A campaign (September-October 1991 ). Under clean conditions, mixing ratios of NO, NO2, NOy, and O-3 incr eased with altitude and showed a distinct latitudinal gradient. PAN sh owed a midtropospheric maximum, while nitric acid mixing ratios were g enerally highest near the surface. Measured NOy concentrations were si gnificantly greater than the sum of individually measured nitrogen spe cies (mainly NOx, PAN, and HNO3), suggesting that a large fraction of reactive nitrogen present in the atmosphere is made up of hitherto unk nown species. This shortfall was larger in the tropics (approximate to 65%) compared to midlatitudes (approximate to 40%) and was minimal in air masses with high HNO3 mixing ratios (>100 ppt). A global three-di mensional photochemical model has been used to compare observations wi th predictions and to assess the significance of major sources. It is possible that the tropical lightning source is much greater than commo nly assumed, and both lightning source and its distribution remain a m ajor area of uncertainty in the budgets of NOy and NOx. A large disagr eement between measurement and theory exists in the atmospheric distri bution of HNO3. It appears that surface-based anthropogenic emissions provide nearly 65% of the global atmospheric NOy reservoir. Relatively constant NOx/NOy ratios imply that NOy and NOx are in chemical equili brium and the NOy reservoir may be an important in situ source of atmo spheric NOx. Data are interpreted to suggest that only about 20% of th e upper tropospheric (7-12 km) NOx is directly attributable to its sur face NOx source, and free tropospheric sources are dominant. In situ r elease of NOx from the NOy reservoir, lightning, direct transport of s urface NOx, aircraft emissions, and small stratospheric input collecti vely maintain the NOx balance in the atmosphere. It is shown that atmo spheric ratios of reactive nitrogen and sulfur species, along with tra jectory analysis, can be used to pinpoint the source of Asian continen tal outflow. Compared to rural atmospheres over North America, air mas ses over the Pacific are highly efficient in net O-3 production. Sourc es of tropospheric NOx cannot yet be accurately defined due to shortco mings in measurements and theory.