Spatiotemporally mapping of the relationship between NO pollution and urbanization for a megacity in Southwest China during 2005-2016.

Nitrogen dioxide (NO) significantly contributes to air pollution. Long-term NO exposure is harmful to human health. The NO pollution in China has surpassed developed countries and attracts international attention. To understand the spatial and temporal distributions of NO across Chengdu in Southwest China, a random forest (RF) model was developed based on NO environmental monitoring data, the Ozone Monitoring Instrument (OMI) satellite retrievals, and geographic covariates. The RF model showed good performance with a cross validation R of 0.77, and a root mean square error (RMSE) of 11.0 μg/m. The ground-level NO concentrations of Chengdu for 2005-2016 were predicted using the developed model with the multiyear population weighted NO concentration being 41.7 ± 11.7 μg/m. The predicted NO concentrations exhibited a clear seasonal variation trend with winter being the highest and summer being the lowest. Furthermore, higher NO 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 NO concentrations with deseasonlization were relatively high during 2011-2013. The NO concentration increased at a rate of 0.81 μg/m/year before 2011 (43.4 ± 11.2 μg/m) and decreased at a rate of -1.03 μg/m/year after 2013 (44.8 ± 12.8 μg/m).