The mechanisms and meteorological drivers of the ozone–temperature relationship

Abstract. Surface ozone (O 3 ) pollution levels are strongly correlated with daytime surface temperatures, especially in highly polluted regions. This correlation is nonlinear and occurs through a variety of temperature dependent mechanisms related to O 3 precursor emissions, lifetimes, and reaction rates, making the reproduction of temperature sensitivities – and the projection of associated human health risks – a complex problem. Here we explore the summertime O 3 –temperature relationship in the United States and Europe using the chemical transport model GEOS-Chem. We remove the temperature dependence of several mechanisms most frequently cited as causes of the O 3 –temperature climate penalty , including: PAN decomposition, soil NO x emissions, biogenic VOC emissions, and dry deposition. We quantify the contribution of each mechanism to the overall correlation between O 3 and temperature both individually and collectively. Through this analysis we find that the thermal decomposition of PAN can explain, on average, 20 % of the overall O 3 –temperature correlation in the United States. The effect is weaker in Europe, explaining 9 % of the overall O 3 –temperature relationship. The temperature dependence of biogenic emissions contributes 3 % and 9 % of the total O 3 –temperature correlation in the United States and Europe on average, while temperature dependent deposition (6 % and 1 %) and soil NO x emissions (10 % and 7 %) also contribute. Even considered collectively these mechanisms explain less than 46 % of the modeled O 3 –temperature correlation in the United States and 36 % in Europe. We use commonality analysis to demonstrate that covariance with other meteorological phenomena such as stagnancy and humidity can explain the bulk of the remainder of the O 3 –temperature correlation. Thus, we demonstrate that the statistical correlation between O 3 and temperature alone may greatly overestimate the direct impacts of temperature on O 3 , with implications for the interpretation of policy-relevant metrics such as climate penalty .