Slowing-down reduction and Possible Reversal Trend of Tropospheric NO2 over China during 2016 to 2019

Atmospheric nitrogen dioxide (NO₂) over China at national level has been kept reducing since 2011 as seen from both satellite observations, ground-based measurements and bottom-up emission inventory. After 2016, however, a significant slowing-down of the reduction trend and/or even a reversal trend were found in numerous provinces, particularly in those with heavy NO₂ level, based on satellite observations. Error analysis on satellite data excluded cloud contamination, instrument anomalies from the main reasons of this change. Ground-based measurements show strong positive temporal correlations with satellite observations and similar spatial patterns of year-to-year changes of NO₂ in 2018 and 2019 winter time. The reason for the recent change from surface emission perspective cannot be traced down since the national emission inventory was not updated since 2015. We therefore call on immediate attentions from both scientist community and policy makers to this phenomenon.