Spatial and temporal distribution of NO_2 and SO_2 in Inner Mongolia urban agglomeration obtained from satellite remote sensing and ground observations

Nitrogen dioxide (NO_2) and sulfur dioxide (SO_2) are important pollution gases which can affect air quality, human health and even climate change. Based on the combination of tropospheric NO_2 and SO_2 column density products derived from OMI satellite with the ground station observations, this study analyzed the spatial-temporal distribution of NO_2 and SO_2 amount in Inner Mongolia urban agglomerations. It shows that NO_2 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. SO_2 increased from 2005 to 2007 with a rate 9.7% per year. While with a peak value in 2011, SO_2 generally showed a decreasing trend of −1.6% per year from 2007 to 2016. With regard to the spatial pattern, the highest levels of NO_2 occur in Hohhot and Baotou, followed by Wuhai and Ordos, the least in Bayannur. Compared with NO_2, the spatial distribution of SO_2 is slightly different. The pollution of SO_2 is the most serious in Wuhai, followed by Hohhot and Baotou, and the lightest in Ordos and Bayannur. The diurnal variations of NO_2 and SO_2 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 NO_2 and SO_2 is highly related to the diurnal variation of both anthropogenic emission and boundary layer height. Differently, the long-term spatial-temporal distribution of NO_2 and SO_2 are more closely related to human activities.