Spatio-temporal variations in NO2 and SO2 over Shanghai and Chongming Eco-Island measured by Ozone Monitoring Instrument (OMI) during 2008–2017

Abstract

High resolution satellite maps are useful for the identification of pollution hotspots. In this work, nitrogen dioxide (NO2) and sulfur dioxide (SO2) observations from the Ozone Monitoring Instrument (OMI) have been averaged and gridded for Shanghai and surrounding areas with a spatial resolution of 0.01° × 0.01°. The pollution maps have been used to investigate the spatio-temporal variations in NO2 and SO2 from 2008 to 2017. The averaged tropospheric NO2 and planetary boundary layer (PBL) SO2 columns in Shanghai are on average 1.40 × 1016 molec·cm−2 and 1.21 × 1016 molec·cm−2, respectively. As the government has put a lot of effort to improve the air quality, both the NO2 and SO2 columns reveal a significant decline in Shanghai during the past decade, with a reduction of ∼24% and ∼56%, respectively, which is also confirmed by the good agreement between satellite observations and ground-based in-situ measurements. The NO2 and SO2 time series also show distinct seasonal characteristics with higher pollution level during winter and lower in the summer. Due to the impact of ship and industrial emissions, the hotspots of NO2 and SO2 are concentrated in the northern part of Shanghai, while the pollution level is lower in the southern and eastern regions. Decreasing trends in NO2 and SO2 are found for most areas of Shanghai during the past decade, though the strongest decrease in the northern part of the city (1.4 × 1015 molec·cm−2·year−1 for NO2 and 3.0 × 1015 molec·cm−2·year−1 for SO2). Comparisons with the China Multi-Resolution Emissions Inventory (MEIC) indicate that the driving factors for the decline of NO2 and SO2 are mainly denitration and desulfurization in the power sector, and the reduction of emissions in the industry sector. We also use aerosol optical depth (AOD) measurements from MODerate-resolution Imaging Spectroradiometer (MODIS) to investigate the relationship between gas phase pollutants and formation of secondary aerosol. Compared to SO2, the contribution of NO2 to secondary aerosol formation increased in the central part of Shanghai. Although Chongming Eco-Island has developed rapidly in recent years, both the SO2 and NO2 levels decreased significantly. This is mainly related to the strict emission control policy and improved urban planning, and indicates that ecological development is achievable under the guidance of policies.