Recent reduction in NO x emissions over China: synthesis of satellite observations and emission inventories

Tropospheric nitrogen dioxide (NO 2 ) column densities detected from space are widely used to infer trends in terrestrial nitrogen oxide (NO x ) emissions. We study changes in NO 2 column densities using the Ozone Monitoring Instrument (OMI) over China from 2005 to 2015 and compare them with the bottom-up inventory to examine NO x emission trends and their driving forces. From OMI measurements we detect the peak of NO 2 column densities at a national level in the year 2011, with average NO 2 column densities deceasing by 32% from 2011 to 2015 and corresponding to a simultaneous decline of 21% in bottom-up emission estimates. A significant variation in the peak year of NO 2 column densities over regions is observed. Because of the reasonable agreement between the peak year of NO 2 columns and the start of deployment of denitration devices, we conclude that power plants are the primary contributor to the NO 2 decline, which is further supported by the emission reduction of 56% from the power sector in the bottom-up emission inventory associated with the penetration of selective catalytic reduction (SCR) increasing from 18% to 86% during 2011–2015. Meanwhile, regulations for vehicles also make a significant contribution to NO x emission reductions, in particular for a few urbanized regions (e.g., Beijing and Shanghai), where they implemented strict regulations for vehicle emissions years before the national schedule for SCR installations and thus reached their NO 2 peak 2–3 years ahead of the deployment of denitration devices for power plants.