GFDL - Geophysical Fluid Dynamics Laboratory

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

March 1st, 2017

Meiyun Lin, Larry W. Horowitz, Richard Payton, Arlene M. Fiore, Gail Tonnesen. Atmospheric Chemistry and Physics. DOI: 10.5194/acp-17-2943-2017.


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.

Figure 1: Surface ozone has decreased during pollution episodes in the Eastern U.S. but shows increases at Western US rural sites during spring, partly due to rising Asian anthropogenic NOx emissions. Shown are the 1988-2014 trends in springtime daily maximum 8-h average ozone at surface sites for the 95th, 50th, and 5th percentiles from observations (left) and GFDL-AM3 model simulations (right). Larger circles indicate sites with statistically significant trends (p<0.05).
Figure 2: Ozone air quality and climate linkages in the Eastern USA. (Top) July mean ozone anomalies at Penn State University from 1980 to 2014 as observed (black) and simulated by the GFDL model with time-varying anthropogenic emissions (purple), correlating with anomalies in daily maximum temperature (gray lines). (Bottom) The 1988-2014 trends in biogenic isoprene emissions from vegetation and in the 95th percentile summertime ozone in the model with constant anthropogenic emissions. Stippling or larger circles indicate significant trends (p<0.05).