Spatial and temporal distribution characteristics of ground-level nitrogen dioxide and ozone across China during 2015-2020

In recent years, the emissions control in nitrogen oxides (NO x ) was conducted across China, but how the concentrations of NO x and its product ozone (O3) in the atmosphere varied in space and time remains uncertain. Here, the spatial and temporal distributions of nitrogen dioxide (NO2) and O3 in 348 cities of China based on the hourly concentrations data during 2015-2020 were investigated, and the relationships among NO2, O3 and meteorological and socioeconomic parameters were explored. It is shown that higher NO2 and O3 concentrations were mainly distributed in North, East and Central China, which are economically developed and densely populated regions. The annual mean concentrations of NO2 increased from 2015 to 2017 but decreased from 2017 to 2020. The annual variations in O3 generally exhibited an upward trend in 2015-2019 but decreased by 5% from 2019 to 2020. About 74% and 78% of cities had a decline in NO2 and O3 in 2020, respectively, compared to 2019, due to the limits of the motorized transports and industrial production activities during COVID-19 lockdown. The monthly mean concentrations of NO2 showed an unusual decrease in February in all regions due to the reduced emissions during the Chinese Spring Festival holidays. Compared to 2019, the mean concentrations of NO2 in January, February and March, 2020 during COVID-19 lockdown decreased by 16%, 28% and 20%, respectively; O3 increased by 13% and 14% in January and February, respectively, but decreased by 2% in March, 2020. NO2 and O3 concentrations are likely associated with anthropogenic and natural emissions. In addition, meteorological parameters can affect NO2 and O3 concentrations by influencing the production process, the diffusion and local accumulation, and the regional circulations.