Using Satellite Data to Aid Quantification and Attribution of Background Ozone Changes in the Western US

We examine changes in tropospheric ozone in the Western United States over the past decade utilizing the GEOS-Chem chemistry-transport model and satellite data. Our results extend earlier work showing that increases in NO_x emissions from East Asia prevented ~1/2 of the reduction in mid-tropospheric ozone that should have occurred over the Western US from 2005-2010. Our new model simulations assimilate stratospheric ozone from the Microwave Limb Sounder (MLS) and changes in NO_x emissions are prescribed based on NO₂ measurements from the Ozone Monitoring Instrument (OMI) satellite instrument. Middle tropospheric ozone measurements using retrievals that combine radiances from the NASA Atmospheric Infrared Sounder (AIRS) and OMI as well as surface ozone measurements from existing networks are used to evaluate the model. Sensitivity simulations and adjoint sensitivity analysis are used to attribute the observed changes in ozone over the Western US to local and non-local sources and to variability in stratosphere-to-troposphere transport of ozone.