The impact of emissions scenarios and climate change over China on ozone air quality in China and the United States

China still suffers from extremely poor air quality, and despite modest emissions reductions of air pollutants in recent years, the outlook for future air quality in China is uncertain. We explore the impact of two disparate emissions scenarios and climate change separately on surface ozone (O₃) concentrations over China and the United States via long-range transport in the near-term future. We use a suite of simulations performed with the NOAA Geophysical Fluid Dynamics Laboratory AM3 global chemistry-climate model, and the ECLIPSE v5a (Evaluating the Climate and Air Quality Impacts of Short-Lived Pollutants) CLE (current legislation) and MFR (maximum feasible reduction) scenarios for estimates of future emissions over China. To estimate the impact of climate change in isolation on Chinese air quality, we hold emissions of air pollutants including O₃ precursors fixed at 2015 levels but allow global sea surface temperatures and sea ice cover to change according to decadal averages for the years 2026-2035 and 2046-2055 from a three-member ensemble of GFDL-CM3 simulations under the RCP8.5 scenario. Evaluation of the present-day simulation with observations from 1497 urban air quality monitoring sites shows that simulated surface maximum daily eight-hour average (MDA8) O₃ is positively biased by 15-20 ppb on average over the domain of China. By the 2050s, we find an increase of 8-12 ppb MDA8 O₃ for CLE in which emissions of nitrogen oxides (NO_x) and anthropogenic volatile organic compounds (VOCs) are projected to increase slightly (10%), and decrease of 16-20 ppb in MFR in which emissions of NO_x and anthropogenic VOCs are projected to decline by 60%, with the highest changes occurring in summer for both scenarios. Emissions reductions in the MFR scenario in China alone will have significant remote impacts, including a 4-8 ppb decrease in surface MDA8 O₃ over the western United States. The model simulates an annual mean climate penalty up to 8 ppb O₃ throughout most of China by the 2050s along with an extension of the summer ozone season by a few days, with the largest increases correlating well with the highest predicted temperatures in summer. Our results point to a range of possible futures for ozone air quality in China and demonstrate the effectiveness of emissions controls in reducing China's health burden due to air pollution.