气候变化领域集成服务门户

US surface O-3 responds to varying global-toregional 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 O-3: (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 NOx emissions have tripled since 1990, contributing as much as 65% to modeled springtime background O-3 increases (0.3-0.5 ppb yr(-1)) over the WUS, outpacing O-3 decreases attained via 50% US NOx emission controls. Methane increases over this period contribute only 15% of the WUS background O-3 increase. Springtime O-3 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 O-3 trends at WUS rural sites. Mean springtime WUS O-3 is projected to increase by similar to 10 ppb from 2010 to 2030 under the RCP8.5 global change scenario. While historical wildfire emissions can enhance summertime monthly mean O-3 at individual sites by 2-8 ppb, high temperatures and the associated buildup of O-3 produced from regional anthropogenic emissions contribute most to elevating observed summertime O-3 throughout the USA. GFDL-AM3 captures the observed interannual variability of summertime EUS O-3. However, O-3 deposition sink to vegetation must be reduced by 35% for the model to accurately simulate observed high-O-3 anomalies during the severe drought of 1988. Regional NOx reductions alleviated the O-3 buildup during the recent heat waves of 2011 and 2012 relative to earlier heat waves (e.g., 1988, 1999). The O-3 decreases driven by NOx controls were more pronounced in the southeastern US, where the seasonal onset of biogenic isoprene emissions and NOx -sensitive O-3 production occurs earlier than in the northeast. Without emission controls, the 95th percentile summertime O-3 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.