Impact from international and interstate transport on O3 exceedances in Yuma, Arizona

Yuma County in Arizona is a new ozone nonattainment area according to the most recent Environmental Protection Agency (EPA) National Ambient Ari Quality Standards (NAAQS) of ozone established in 2015. However, its local emissions are much smaller than many other non-attainment areas. We investigated the sources of ozone in Yuma using sensitivity analysis with the GEOS-Chem adjoint model, back trajectories from the Stochastic Time-Inverted Lagrangian Transport (STILT) model, and meteorological data from the NASA GMAO and the Yuma airport. We attribute ozone on exceedance (above 70 ppb) days in Yuma to wind from the southwest, suggesting international transport from northern Mexico and interstate transport from southern California. Ozone exceedances in Yuma seldom occur during the North America Monsoon season, when the temperature is the highest but wind directions switch more towards the south than in preceding months. Adjoint sensitivity tests show that reductions of anthropogenic and soil NO_x emissions in northern Mexico are the most effective ways to reduce ozone concentrations in Yuma during exceedance days. However, top-down estimates show that NO_x emissions have been increasing over the entire country of Mexico and contribute to a 1.4% increase in summertime ozone in Yuma from 2006 to 2016; in comparison, a decrease of NO_x emissions in the US and meteorological factors have lead to an overall reduction (by 0.7% - 5.7% and 11.3 – 18.2%, respectively) of summer mean and annual MDA8 ozone in Yuma. The concentrations of volatile organic compounds (VOCs), another important group of ozone precursors, have increased in southern Arizona, southern California and northern Mexico due to the rising temperatures. To better control ozone concentrations at border cities like Yuma, international and inter-state collaborations are required.