A proposed physical mechanism for ozone-meteorology correlations using land–atmosphere coupling regimes

Abstract

Correlations between surface ozone and meteorological variables exhibit a north-south gradient over the Eastern United States (US), with the ozone–temperature correlation weakening and the ozone–humidity correlation transitioning from positive to negative south of 37°N. Using 17 years of hourly August ozone, nitrogen oxide, and isoprene measurements from the Environmental Protection Agency's Air Quality System and Photochemical Assessment Measurement Stations and hourly meteorological fields from the North American Land Data Assimilation System (Phase 2), we propose that the north-south transition and widely observed ozone–humidity correlation results from a shift in the soil moisture-atmosphere coupling regime. Due to soil water limitations over the Southeast, evapotranspiration and specific humidity increase following precipitation events, and this coincides with reductions in temperature and ozone precursors. Therefore, the negative ozone–humidity correlation in the Southeast is likely a manifestation of several meteorological factors directly influencing ozone production. Surface drying, as defined by the evaporative fraction, provides a better predictor of O3 than temperature, specific humidity, or radiation for the Southeast due to its ability to retain prior precipitation information and reflect same-day atmospheric conditions relevant to O3 production. Behavior of surface fluxes and coupling may be particularly relevant for prediction of seasonal and future O3 air quality, and further investigation into the links between land–atmosphere coupling and O3 is necessary.