March 1st, 2017
Within the United States, ground-level ozone has been recognized since the 1940s as an air pollutant that is detrimental to public health. Ground-level ozone responds to varying global-to-regional precursor emissions and climate, with implications for designing effective U.S. air quality control policies under the lowered national air quality standard (70 ppb set in 2015). This study examines these conjoined processes with observations and global chemistry-climate model hindcasts (GFDL-AM3) over the course of 35 years, from 1980 to 2014.
Springtime ozone measured at Western U.S. national parks has increased by 0.2-0.5 ppb yr-1 over the past 20-25 years, despite stringent U.S. domestic emission controls. Notably, observations in the Denver metropolitan area show an ozone increase similar to remote rural sites. Many prior studies show that global models have difficulty simulating ozone increases measured at these sites. Focusing on days when the airflow is predominantly from the Pacific, the authors find that they can reconcile the ozone increases at Western U.S. sites from observations and GFDL model simulations. This finding lends confidence in the application of the GFDL model to attribute these observed ozone trends to specific sources.
Nitrogen oxides (NOx) are an important class of air polluting chemical compounds, which react in the atmosphere to form ozone and acid rain. Rapid economic growth has led to a tripling of NOx emissions from Asia in the past 25 years. This study shows that rising Asian NOx emissions contribute as much as 65% to modeled springtime background ozone increases (0.3-0.5 ppb yr-1) over the Western U.S., outpacing ozone decreases attained via 50% U.S. domestic NOx emission controls. Methane increases over this period contribute only 15% of the Western U.S. background ozone increase. During summer, increasing Asian emissions approximately offset the benefits of U.S. emission reductions, leading to weak or insignificant observed ozone trends at Western U.S. rural sites. These new findings indicate that a global perspective is necessary when designing a strategy to meet U.S. ozone air quality objectives.
In contrast, in the Eastern U.S., surface ozone declined throughout its probability distribution following regional NOx emission controls in the last decade. The authors find that the ozone decreases were most pronounced in the Southeast, where the seasonal onset of biogenic hydrocarbon emissions from vegetation and NOx-sensitive ozone production occurs earlier than in the Northeast, implying that controlling NOx emissions will continue to provide long-term ozone air quality benefits in the Southeastern U.S. during all seasons. Without emission controls, the 95th percentile summertime ozone in the Eastern U.S. would have increased by 0.2-0.4 ppb yr-1 over 1988-2014 due to more frequent hot extremes and rising hydrocarbon emissions from vegetation. Regional NOx emission reductions also alleviated the ozone buildup during the recent heat waves of 2011 and 2012 relative to earlier heat waves (e.g., 1988; 1999). Continued implementation of U.S. NOx emission controls guards against rising pollution levels triggered either by climate change or by global emission growth.