Explanations for the Spring peak in Ground-level Ozone in the Southwestern United States

Abstract

Many remote locations in the Northern Hemisphere have a spring peak, rather than a summer peak, in ground-level ozone concentrations, and the principal cause is presumed to be stratosphere-troposphere exchange (i.e., stratospheric intrusions). Grand Canyon National Park (GCNP) in northern Arizona also has a spring peak, and the purpose of this study is to explore the impact of stratospheric intrusions and another process synoptic-scale pollutant transport on ground-level ozone levels at GCNP from 1996 to 2000. The primary methods involve the stratification of days to identify stratospheric-intrusion days and the compositing of days to assess the impact of pollutant transport on ground-level ozone concentrations. Results indicate that stratospheric intrusions contributed little to the ozone budget at GCNP. In fact, atmospheric pollution originating in southern California was the likely cause of the May peak in ozone. The transported pollution also appeared to be responsible for high ozone days during all spring months. Tracer-based research (i.e., beryllium-7 and methylchloroform) at multiple locales in the southwestern United States is needed to fully confirm the weak impact of stratospheric intrusions and the strong contribution of ozone and its precursors originating in southern California.