The two main tropospheric oxidants, ozone and OH radical, are critical drivers of photochemical processes that determine the lifetime of most air pollutants and methane. Current models, however, cannot explain the observed ozone trend and the inferred variability in global mean OH over the past few decades, which limits our capability to estimate ozone radiative forcing and policyrelevant ozone background. The failure of models to capture OH variability also makes it challenging to interpret trends in methane, whose main sink is OH. This project aims to explore the processes controlling long-term changes of tropospheric ozone over 1980-2018. Our proposal plans to examine the seasonal, intra- and interannual changes of the existing decades-long observational records of ozone, nonmethane volatile organic compounds (VOCs), and nitrate wet deposition fluxes. Comprehensive data analysis of the long-term observations will then be used to constrain the processes of the GEOS-Chem chemical transport model to address the following three sets of questions: 1. Can observations of nitrate wet deposition fluxes be used to derive the long-term trend of anthropogenic NOx emission over the U.S. and Europe? And, could NOx be a key driver for the observed ozone changes in the lower troposphere since the 1980s? 2. Do the seasonal cycles and interannual variabilities of non-methane VOCs such as aromatics and acetylene reflect not only the change of anthropogenic VOC emissions, but also provide a measure of seasonal and regional OH variability? 3. Has the long-term change of the tropospheric ozone budget over the last four decades been controlled by variations in anthropogenic emissions (NOx and non-methane VOCs), as well as stratosphere-troposphere exchange? We will examine the roles of anthropogenic VOCs and NOx emission trends and ozone variability in the lowermost stratosphere in driving tropospheric ozone change over the last four decades. With improved knowledge of the controlling factors, we will then quantify the radiative forcing from tropospheric ozone trends since the 1980s and also revisit previous estimates of the forcing due to anthropogenic ozone changes since the preindustrial era. Finally, we will also examine the implications of our results for global mean OH. This proposal targets the NOAA AC4 Call for Proposals in FY19, and aims to explain the trends and variabilities in tropospheric ozone and OH radical using existing long-term observational records of ozone, non-methane VOCs, and nitrate wet deposition. This work will also improve our understanding of the factors controlling tropospheric oxidation capacity, and represents a critical step toward better prediction of tropospheric chemistry and air quality in the future under a climate change regime.