PHOTOCHEMICAL TRAJECTORY MODELING STUDIES OF THE NORTH-ATLANTIC REGION DURING AUGUST 1993

A Lagrangian photochemical trajectory model has been used to assess th e factors affecting O-3 production during transport of polluted air ma sses across the North Atlantic Ocean. Sensitivity studies have been pe rformed along idealized trajectories, and it is found that the potenti al impact of North American emission sources is maximized by transport of air at high altitudes, in drier conditions and in conditions where mixing of the air with background air masses is relatively limited. M easurements taken from the NCAR King Air aircraft as part of the North Atlantic Regional Experiment (NARE) August 1993 intensive have been u sed to initialize forward trajectories, calculated using European Cent re for Medium-Range Weather Forecasting analyzed wind fields, from eas tern North America to assess O-3 production over the Atlantic during t his period. The effects of dilution of a polluted air parcel with air from the upper troposphere have also been studied, and the contributio n of photochemical O-3 production to the air mass composition is found to be smaller than that of dilution, particularly for long trajectori es and for conditions where dilution is relatively rapid or involves a ir from the stratosphere. Measurements taken from the Meteorological R esearch Flight Hercules aircraft over the eastern Atlantic as part of the Oxidizing Capacity of the Tropospheric Atmosphere campaign have be en examined in the light of these studies. A backward trajectory analy sis has been performed from one of the vertical profiles taken off the coast of Portugal on August 31, 1993, to assess the origin of the dif ferent air masses intercepted. While the lower levels are characterist ic of air from the European boundary layer advected over the ocean, th e upper levels show strong evidence for anthropogenic influence from N orth American sources, with elevated levels of O-3, NOy, CO, and aeros ol. Although it cannot be concluded that this air mass definitely orig inated from over North America, the measured concentrations are shown to be consistent with those for an air mass from this source region ex periencing some mixing with air masses in the upper troposphere.