The impact of U.S. continental outflow on ozone and aerosol distributions over the western Atlantic

Abstract

Aircraft measurements of selected trace gas species, aerosols, and meteorological parameters were performed in the lower troposphere off the U.S. east coast during August and September 1989 as part of the NASA Global Tropospheric Experiment (GTE) Chemical Instrumentation Test and Evaluation (CITE 3) expedition. In this paper, we examine these data to assess the impact of continental outflow on western Atlantic O 3 and small aerosol budgets. Results show that mixed layer (ML) O 3 concentrations and small aerosol number densities (Np) were enhanced by factors of 3 and 6, respectively, within air masses of predominantly continental origin compared with clean maritime background air. These enhancements exhibited a marked altitude dependence, declining rapidly above the ML to the point where only slight to moderate differences in O 3 and Np, respectively, were notable above 2.4 km. Within continentally influenced ML's, both O 3 and Np were correlated with CO, exhibiting linear regression slopes averaging 0.4 ppbv(O 3 )/ppbv(CO) for O 3 and 7.7 (particles cm −3 )/ppbv(CO) for Np and indicating a primarily anthropogenic origin for the observed enhancement of these species. Comparisons between profiles in continental and background maritime air masses suggest that photochemical production below 1.4‐km altitude adds over 10% to western Atlantic tropospheric column O 3 abundance in continental outflow regimes. For aerosols, eastward advection of low‐level continental air contributes an average net flux of 2.8 metric tons of submicron (accumulation mode) particles per kilometer of shoreline per day to the western Atlantic troposphere.