Supporting data for: Estimate of OH trends over one decade in North American cities

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. We utilize
machine learning with observational inputs emphasizing satellite remote
sensing observations to predict surface OH in 49 North American cities
from 2005 to 2014. We observe decadal changes in the summertime OH with
wide variation among different cities. Compared to the summertime OH in
2005, 3 cities show an significant increase of OH whereas in 27 cities OH
decreases in 2014. The year to year variation of OH is mapped to the
decline of the NO2 column. We conclude that these 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 result
emphasizes that controlling NOx emissions will be most effective in
regulating the ozone pollution over cities.