Using WRF-STILT to Determine the Relative Contributions of U.S. and Mexican Emissions to High-Ozone Events in El Paso, Texas

Over the past few years, the U.S. Environmental Protection Agency (EPA) has set increasingly stringent National Ambient Air Quality Standards for ozone (O₃), and the design value for El Paso in recent years has been close to the latest ozone standard. As El Paso is on the border between the US and Mexico and directly borders Ciudad Juárez, the air quality in El Paso is influenced by emissions from both the US and Mexico. Therefore, it is useful for regulatory purposes to separate days where El Paso had high O₃ primarily due to Mexican emissions of O₃ precursors from days where the high O₃ is primarily due to US emissions. We used the Stochastic Time-Inverted Lagrangian Transport (STILT) model driven by high resolution ( 1km ) Weather Research and Forecasting (WRF) simulations to identify upwind source regions O₃ precursors. The WRF-STILT framework calculates emissions “footprints” for each measurement that can be combined with bottom-up emission inventories to estimate the relative impacts of sources from different geographic regions. The WRF-STILT footprints were calculated for the 10 highest MDA8 O₃ values in the El Paso area for each year from 2012 -2018. These WRF-STILT footprints were then applied to the bottom-up estimates of NOx and VOC emissions from the Emissions Database for Global Atmospheric Research (EDGAR) to determine the relative influence of US and Mexican sources of NOx and VOCs on high O₃ days in El Paso.