Three-dimensional global modeling studies of the transport and photochemistry over the North Atlantic Ocean

A three-dimensional, global model is used to characterize the troposphere over the North Atlantic Ocean. In August, tropospheric ozone appears to be transported from North America across the Atlantic Ocean in ''events,'' one of which is presented. The North Atlantic Ocean may be characterized by three regions (western, central, and eastern) with distinct photochemical and transport signatures. In the western Atlantic, emissions are fresh and O-3 and CO, O-3 and NOy, and O-3 and (NOy - NOx) show a strong positive correlation. As air travels eastward across the Atlantic Ocean, it photochemically ages, selected species are deposited, and the degree of correlation tends to decrease. Ozone is photochemically removed more rapidly than CO, and the slope Delta O-3/Delta CO tends to decrease with time and distance. Conversely, the slope Delta O-3/Delta(NOy-NOx) tends to increase with time and distance, because HNO3 and PAN tend to be removed more quickly than O-3 A region within the central Atlantic (35 degrees-45 degrees N, 35 degrees-50 degrees W) is much less affected by episodic transport from North America than the three regions above. Concentrations and correlations of species are compared with observations from North Atlantic Regional Experiment and the eastern United States. Differences between simulated concentrations and measurements may be due to a combination of effects that include boundary layer phenomena, fog events, the scale of the model, and uncertainties in emission inventories.