Spatial and temporal distribution of NO2 and SO2 in Inner Mongolia urban agglomeration obtained from satellite remote sensing and ground observations

Nitrogen dioxide (NO2) and sulfur dioxide (SO2) are important pollution gases which can affect air quality, human health and even climate change. Based on the combination of tropospheric NO2 and SO2 column density products derived from OMI satellite with the ground station observations, this study analyzed the spatial-temporal distribution of NO2 and SO2 amount in Inner Mongolia urban agglomerations. It shows that NO2 increased continually from 2005 to 2011 at a rate of 14.3% per year and then decreased from 2011 to 2016 at a rate of −8.1% per year. SO2 increased from 2005 to 2007 with a rate 9.7% per year. While with a peak value in 2011, SO2 generally showed a decreasing trend of −1.6% per year from 2007 to 2016. With regard to the spatial pattern, the highest levels of NO2 occur in Hohhot and Baotou, followed by Wuhai and Ordos, the least in Bayannur. Compared with NO2, the spatial distribution of SO2 is slightly different. The pollution of SO2 is the most serious in Wuhai, followed by Hohhot and Baotou, and the lightest in Ordos and Bayannur. The diurnal variations of NO2 and SO2 are basically the same, which decrease from 0:00 to 6:00, then increase to a peak value at 8:00, and decrease from 8:00 to 15:00. The diurnal variation of NO2 and SO2 is highly related to the diurnal variation of both anthropogenic emission and boundary layer height. Differently, the long-term spatial-temporal distribution of NO2 and SO2 are more closely related to human activities.