Trans-Pacific transport of ozone, CO and aerosols

Since 1997, we have made observations of gases and aerosols along the west coast of the U.S. using ground and airborne platforms. These have been interpreted with trajectories, as well as the GEOS-Chem global chemical transport model, to quantify the influence of upwind sources of ozone, CO and aerosols. From these data we have identified a number of specific transport events that bring ozone and aerosols to North America from sources on the Eurasian continent. Generally, the transport is most commonly seen in the free troposphere, but mixing brings these materials down into the boundary layer. For ozone, we have identified a number of transport episodes with enhanced mixing ratios, but only in the free troposphere. In these events, the ozone is associated with CO and aerosols, and ozone mixing ratios can reach 100 ppb. The correlation with CO and aerosol scatter indicates a pollution source, however the high ratio of ozone to CO suggests an additional contribution from other ozone sources. The GEOS-Chem model indicates that in some of these events a mixture of pollution and stratospheric sources are responsible. In the boundary layer or when an airmass contains high levels of mineral dust, we have not been able to identify any ozone enhancements from long range transport during events, even when significant levels of CO are present. This is likely due to heterogeneous chemistry, a shorter ozone lifetime or reduced production in the boundary layer.