Disentangling the Drivers of the Summertime Ozone-Temperature Relationship Over the United States

Abstract

Summertime surface-level ozone (O3) is known to vary with temperature, but the relative roles of different processes responsible for causing the O3-temperature relationship are not well quantified. In this study we use simulations of NASA's Global Modeling Initiative chemical transport model to isolate and assess the relative impact of atmospheric transport, chemistry, and emissions on large-scale O3 variability, events, and and the covariance of O3 with temperature. Using observations from the Clean Air Status and Trends Network in the contiguous United States, we show that the Global Modeling Initiative chemical transport model reproduces the spatiotemporal variability of O3 and its relationship with temperature during the summer. We use the change in O3 given a change in temperature (dO3/dT) along with other metrics to understand differences between our simulations. In regions with moderate to strong positive correlations between temperature and O3 such as the northeast, Great Lakes, and Great Plains, temperature's association with transport yields a majority of the total O3-temperature relationship (∼60%), while temperature-dependent chemistry and anthropogenic NO emissions play smaller roles (∼30% and ∼10%, respectively). There are regions, however, with insignificant correlations between temperature and O3, and our findings suggest that transport is still an important driver of O3 variability in these regions, albeit not correlated with temperature. Transport is not directly dependent on temperature but rather is linked through an indirect association, and it is therefore important to understand the exact mechanisms that link transport to O3 and how these mechanisms will change in a warming world.