Contributions of North American emissions and global transport to ozone in the US and Texas

Ground-level ozone (O3) is associated with adverse effects on human health and welfare and is formed from precursor emissions of nitrogen oxides (NOx), volatile organic compounds (VOCs) and carbon monoxide (CO). Tropospheric O3 has multiple sources including exchange from the stratosphere and in-situ formation from both natural and anthropogenic precursor emissions. Furthermore, the tropospheric lifetime of O3 is sufficiently long so as to permit transport between continents and around the globe. This study used a global chemistry and transport model (GEOS-Chem) for 2012 to compare contributions of anthropogenic emissions within North America (NA) and from the rest of the world (ROW) to O3 in the US. The relative magnitudes of the NA and ROW contributions can indicate whether trends in world emissions have bearing upon the design of strategies to manage the NA contribution to ground-level O3. The ROW contribution tended to be higher in the western than eastern US due to prevailing winds transporting O3 from Eurasia in combination with effects of mountainous terrain of the western US. In contrast, the NA contribution tended to be larger in the eastern than western US. The ROW contribution tended to peak in the spring whereas the NA contribution tended to peak in the summer and autumn. We looked at three cities in Texas finding that cities in eastern Texas (Houston and Dallas) followed the general trends just described suggesting that changes in anthropogenic emissions outside NA should have little bearing on developing local strategies to manage O3 in Houston and Dallas. For El Paso, in western Texas, the ROW contribution could be a substantial fraction of the NA contribution and additional study is needed using higher resolution models to better resolve influences of complex terrain.