Estimate of OH trends over a decade in North American cities

Qindan Zhu
Postdoctoral Fellow
Department of Earth, Atmospheric and Planetary Sciences


The hydroxyl radical (OH) is the most important oxidant on global and local scales in the troposphere. Urban OH controls the removal rate of primary pollutants and triggers the production of ozone. Interannual trends of OH in urban areas are not well documented or understood due to the short lifetime and high spatial heterogeneity of OH. Zhu and her colleagues have utilized machine learning with observational inputs emphasizing satellite remote sensing observations to predict surface OH in 49 North American cities from 2005 to 2014. They observed decadal changes in the summertime OH with wide variation among different cities. In 2014, compared to the summertime OH in 2005, three cities showed a significant increase of OH whereas in 27 cities OH decreased in 2014. The year-to-year variation of OH is mapped to the decline of the NO2 column. Zhu concludes that the cities in this analysis are either in the NOx limited regime or at the transition from a NOx suppressed regime to a NOx limited regime. The results emphasize that, in the future, controlling NOx emissions will be most effective in regulating the ozone pollution in these cities.


Quindan ZhuQindan Zhu completed her undergrad at Peking University with double majors in environmental science and mathematics. After earning her bachelor's degree in 2017, she went to the University of California at Berkeley where she earned her PhD working with Prof. Ron Cohen. Her research topics include satellite observations of air pollutants and OH chemistry. Zhu is now a NOAA climate and global change postdoctoral fellow working on chemistry-climate interaction with Prof. Arlene Fiore at MIT.