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

TL;DR

This study documents a slowdown and possible reversal in the decline of tropospheric NO2 levels over China from 2016 to 2019, highlighting the need for further investigation into emission sources and policy implications.

Contribution

It reveals a recent slowdown and potential reversal in NO2 reduction trends over China, using satellite and ground-based data, and emphasizes the importance of updated emission inventories.

Findings

NO2 reduction trend slowed after 2016

Satellite and ground measurements show consistent patterns

Potential reversal trend observed in 2018-2019

Abstract

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 (Liu et al., 2016; Irie et al., 2016; Krotkov et al., 2016; Foy et al., 2016; Liu et al 2017). These studies used data before 2015-2016. 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 correlations with satellite observations and similar patterns of year-to-year changes of NO2 in 2018 and 2019 winter time. The temporal variations of Satellite NO2 over China are believed largely determined by surface emission from power plant and transportation. The reason for the recent change from 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. Further efforts should be made to understand the reasons causing this change and to make associated air pollution controlling actions.