A model investigation of tropospheric ozone chemical tendencies in long-range transported pollution plumes

The impact of continental outflow on the ozone chemical tendencies (i.e., production and loss rates) is quantified in the North Atlantic and northwest Pacific regions using the GEOS-Chem and the MOZECH global models of chemistry and transport. The ozone tendencies simulated by these global models are compared to box model simulations constrained by observations in different regions and seasons. The two global models generally capture the seasonal and regional variations of the ozone tendencies. The largest discrepancies between the ozone tendencies from the box model and those from the global models are found in the lower troposphere of the eastern North Atlantic during the ACSOE campaign and are attributed to differences between chemical schemes and too strong NOx concentrations in the global models. The background and plume (i.e., impacted by continental outflow) environments are differentiated over the oceanic areas using criteria based on simulated daily mean CO concentrations. The ozone tendencies in the plume environment differ from that in the background over the entire column in North Atlantic and northwest Pacific at all seasons. According to the models, net ozone production is enhanced by 2 to 6 ppbv/day in the boundary layer and by 1 to 3 ppbv/ day in the upper troposphere, whereas the effect of pollution ranges from -1 ppbv/day to +1 ppbv/day in the middle troposphere (3-7 km), depending on the model used. The different responses of the two models are determined by differences in the water vapor distributions relative to those of pollutants in the plumes. In particular, GEOS-Chem tends to transport pollution in a drier sector of cyclones than MOZECH.