US surface ozone trends and extremes from 1980 to 2014: quantifying the roles of rising Asian emissions, domestic controls, wildfires, and climate

Abstract. US surface O₃ responds to varying global-to-regional precursor emissions, climate, and extreme weather, with implications for designing effective air quality control policies. We examine these conjoined processes with observations and global chemistry-climate model (GFDL-AM3) hindcasts over 1980–2014. The model captures the salient features of observed trends in daily maximum 8 h average O₃: (1) increases over East Asia (up to 2 ppb yr⁻¹), (2) springtime increases at western US (WUS) rural sites (0.2–0.5 ppb yr⁻¹) with a baseline sampling approach, and (3) summertime decreases, largest at the 95th percentile, and wintertime increases in the 50th to 5th percentiles over the eastern US (EUS). Asian NO_x emissions have tripled since 1990, contributing as much as 65 % to modeled springtime background O₃ increases (0.3–0.5 ppb yr⁻¹) over the WUS, outpacing O₃ decreases attained via 50 % US NO_x emission controls. Methane increases over this period contribute only 15 % of the WUS background O₃ increase. Springtime O₃ observed in Denver has increased at a rate similar to remote rural sites. During summer, increasing Asian emissions approximately offset the benefits of US emission reductions, leading to weak or insignificant observed O₃ trends at WUS rural sites. Mean springtime WUS O₃ is projected to increase by  ∼  10 ppb from 2010 to 2030 under the RCP8.5 global change scenario. While historical wildfire emissions can enhance summertime monthly mean O₃ at individual sites by 2–8 ppb, high temperatures and the associated buildup of O₃ produced from regional anthropogenic emissions contribute most to elevating observed summertime O₃ throughout the USA. GFDL-AM3 captures the observed interannual variability of summertime EUS O₃. However, O₃ deposition sink to vegetation must be reduced by 35 % for the model to accurately simulate observed high-O₃ anomalies during the severe drought of 1988. Regional NO_x reductions alleviated the O₃ buildup during the recent heat waves of 2011 and 2012 relative to earlier heat waves (e.g., 1988, 1999). The O₃ decreases driven by NO_x controls were more pronounced in the southeastern US, where the seasonal onset of biogenic isoprene emissions and NO_x-sensitive O₃ production occurs earlier than in the northeast. Without emission controls, the 95th percentile summertime O₃ in the EUS would have increased by 0.2–0.4 ppb yr⁻¹ over 1988–2014 due to more frequent hot extremes and rising biogenic isoprene emissions.