CHEMICAL AND PHYSICAL-PROPERTIES OF PLUMES OF ANTHROPOGENIC POLLUTANTS TRANSPORTED OVER THE NORTH-ATLANTIC DURING THE NORTH-ATLANTIC REGIONAL EXPERIMENT

Plumes of photochemical pollutants transported from the industrialized regions of the northeast United States and Canada were sampled over t he North Atlantic Ocean at distances up to 1000 km from the coast. The plumes were found in well defined layers up to 1 km thick and were us ually isolated from the surface by a low altitude inversion. Plume com position was consistent with the occurrence of extensive photochemical processing during transit from source regions as indicated by high O- 3 concentrations (O-3 maximum similar to 150 parts per billion by volu me (ppbv)), generally high fractional conversion (>85%) of NOx to its oxidation products, and high peroxide concentrations (median 3.6 ppbv; maximum 11 ppbv). These observations are in accord with processing ti mes estimated from back trajectory analysis. CO and O-3 Concentrations were well correlated (r(2) = 0.64) with a slope (0.26) similar to pre vious measurements in photochemically aged air. Good correlations were also observed between CO and accumulation mode particle number densit ies (r(2) = 0.64), and CO and NOy (r(2) = 0.67). O-3 was found to depe nd nonlinearly on the NOx oxidation product concentration. At low valu es of (NOy-NOx), the slope (14) was within the range of values measure d previously in photochemically aged air masses, at higher concentrati ons the slope was much lower (4.6). The low slope at high concentratio ns is attributed to minimization of losses of NOx oxidation products i n spatially well-defined plumes during transport. A strong linear corr elation (r(2) = 0.73) was found between O-3, and the concentration of radical sink species as represented by the quantity ((NOy-NOx) + 2H(2) O(2)).