Spatiotemporally mapping of the relationship between NO2 pollution and urbanization for a megacity in Southwest China during 2005-2016.

Nitrogen dioxide (NO2) significantly contributes to air pollution. Long-term NO2 exposure is harmful to human health. The NO2 pollution in China has surpassed developed countries and attracts international attention. To understand the spatial and temporal distributions of NO2 across Chengdu in Southwest China, a random forest (RF) model was developed based on NO2 environmental monitoring data, the Ozone Monitoring Instrument (OMI) satellite retrievals, and geographic covariates. The RF model showed good performance with a cross validation R2 of 0.77, and a root mean square error (RMSE) of 11.0 μg/m3. The ground-level NO2 concentrations of Chengdu for 2005-2016 were predicted using the developed model with the multiyear population weighted NO2 concentration being 41.7 ± 11.7 μg/m3. The predicted NO2 concentrations exhibited a clear seasonal variation trend with winter being the highest and summer being the lowest. Furthermore, higher NO2 concentrations in the downtown areas were observed than that in the rural areas indicating the former being attributed to more anthropogenic sources. The population weighted NO2 concentrations with deseasonlization were relatively high during 2011-2013. The NO2 concentration increased at a rate of 0.81 μg/m3/year before 2011 (43.4 ± 11.2 μg/m3) and decreased at a rate of -1.03 μg/m3/year after 2013 (44.8 ± 12.8 μg/m3).