Dr Tzung-May Fu and her colleagues at the Department of Atmospheric and Oceanic Sciences, School of Physics recently published a paper entitled “Positive but variable sensitivity of August surface ozone to large-scale warming in the southeast United States” on Nature Climate Change. This study shows that climate warming will lead to increased surface ozone levels over polluted forested areas, threatening the future health of humans and ecosystems. In addition, the sensitivity of ozone to climate warming shows great interannual and interdecadal variability with potential links to large-scale climate interdecadal variability. These findings resolve a long-standing debate among the atmospheric chemistry community regarding with the response of ozone to climate warming. These findings also have important implications for the prediction and management of future ozone air quality over regions like East Asia (including China), South Asia, Western Europe, and the southeast U.S.

Surface ozone, a major air pollutant toxic to human and ecosystems, is produced by the photochemical oxidation of volatile organic compounds (VOCs) in the presence of sunlight and nitrogen oxides (NOx). Even in the absence of changes in anthrpogenic emissions of VOCs and NOx, climate warming may affect surface ozone levels, especially over polluted forested areas (e.g., East Asia, South Asia, Western Europe, and the southeast U.S.) in summer. This is because forests emit a particular compound, isoprene, which is an important precursor to ozone. The flux of isoprene from forests is non-linearly dependent on climate conditions. However, for a long time the atmospheric chemistry community have not reached a consensus on whether climate warming will increase or decrease surface ozone levels over these polluted forested areas. This lack of consensus is mainly due to the high uncertainty related to the NOx-sequestration efficiency during the photochemical cascade of isoprene oxidation.

Fu and her colleagues used observations and simulations to diagnose the surface ozone levels over the southeast United States during 1988-2011. They showed that climate warming will lead to increases in surface ozone levels. A key to their discovery was recognizing that the interannual variation of August surface temperatures over the southeast U.S. during 1988-2011 was mainly a manifestation of a large-scale temperature oscillation in which almost the entire North American continent was in the same phase. Thus, during 1988-2011 and on the interannual timescale, August surface ozone over the southeast U.S. was perturbed by large-scale temperature variations, which offers a unique opportunity to diagnose the sensitivity of ozone to large-scale warming.

The study found that the sensitivity of ozone to large-scale warming over the southeast U.S. was consistently positive during 1988-2011. However, that sensitivity varied interannually by a factor of three (2.4 to 7.1 ppb/K). Fu and her colleagues found that such variability was mainly driven by the variability in ozone transport by regional atmospheric circulation. They further showed that, over a longer timespan (1948-2012), the sensitivity of regional ozone transport to temperature changed signs on interannual and interdecadal timescales, with potential links to the Atlantic Multidecadal Oscillation.

This study showed that prediction of future ozone changes will require climate-chemistry models with more realistic representations of climate variability and its drivers, such as models initialized with observed ocean conditions. Long-term management of ozone air quality must consider the variability of the sensitivity of ozone to climate warming to ensure consistent attainment of ozone air quality standards in the future.

This work was mostly conducted by Dr. Tzung-May Fu and her undergraduate student Ms Yiqi Zheng (now PhD student at Yale University), in collaboration with researchers from Harvard University and Princeton University. This work was funded by the Ministry of Science and Technology of China and the National Natural Science Foundation of China.

Link to the full paperhttp://dx.doi.org/10.1038/nclimate2567

Link to the Fu Group websitehttps://www.phy.pku.edu.cn/atmoschem