Long-term trends in NO2 columns related to economic developments and air quality policies from 1997 to 2016 in China.

This study detected the long-term trends in NO2 concentrations in China from 1997 to 2016 based on the NO2 columns from GOME, SCIAMACHY, and GOME-2A. Both differences in the time-overlapped NO2 columns from GOME vs. SCIAMACHAY and SCIAMACHAY vs. GOME-2A showed seasonal variations, and the annual NO2 columns from GOME were 0.9% higher than those from SCIAMACHY, which exceeded that from GOME-2A by 14%. The long-term trends of the NO2 columns on a provincial scale could be simulated by cubic models (0.60 < R2 < 0.96, p < 0.05) and presented three shapes: first decreasing then increasing and decreasing again; first decreasing then increasing; and continuously decreasing. The peak years of NO2 columns in 17 provinces occurred in 2011 and 2012. These trends in NO2 columns were determined by the economic developments and enacted air quality policies in nearly all the provinces except for Xizang and Qinghai Provinces, where the trends were determined by natural NOx emission sources. In detail, the panel data analysis showed that the simulated model had fixed effects, and the thermal power generation, consumption of diesel oil in agriculture, passenger traffic by highways, and freight traffic by highways significantly increased NO2, while the air quality policies in the 12th five-year plan decreased NO2 columns from 1997 to 2016. The benefits to decreasing NO2 columns from the air quality policies issued in the 10th and 11th five-year plans were offset by the quickly increasing economies.