MODEL STUDIES OF THE METEOROLOGY AND CHEMICAL-COMPOSITION OF THE TROPOSPHERE OVER THE NORTH-ATLANTIC DURING AUGUST 18-30, 1993

A mesoscale chemistry transport model driven by meteorological data fr om a numerical weather prediction model is used to calculate ozone, ca rbon monoxide, oxides of nitrogen, and other chemical species over the North Atlantic for a 13-days period (August 18-30, 1993). The model h as a circumpolar grid so that the boundary condition problems are mini mized, and the influence of North American emissions on the chemical c omposition of the troposphere over the North Atlantic and Europe is ca lculated. During the first part of the period there is a zonal flow ac ross the North Atlantic in the free troposphere; later, there is a str ong north-south as well as vertical component in the advection field. The variability in the concentrations of ozone in the free troposphere is mainly caused by dynamical processes, while the chemical modificat ion is small over an integration time of less than two weeks. A contin ental plume off the North American continent extending 2000 km or more into the North Atlantic is identified toward the end of the calculati on period. There is then a maximum in the concentration of ozone aroun d 1 km above the sea surface, with a much lower concentration in the m arine boundary layer close to the ocean surface. Measurements from the U.K. Meteorological Office Hercules C-130 in the free troposphere off the Atlantic Provinces, across the Atlantic Ocean, and around the Azo res together with ozone soundings from the Azores, Bermuda, and Icelan d were used for model comparison. The calculations indicate that in th e free troposphere the initial conditions as well as the upper boundar y conditions are important for ozone distribution. In the upper tropos phere the net change in the chemical formation rate of ozone due to a change in the NOx concentration is quite independent of the absolute v alue of the ozone concentration itself and, consequently, the choice o f boundary conditions for ozone is not so important in this context, I n the lower troposphere the change in the net chemical formation rate of ozone, which follows from a change in the concentration of NOx, sho ws a marked dependence on the concentration of ozone.