Effect assessment of NO_x and SO₂ control policies on acid species in precipitation from 2005 to 2016 in China based on satellite monitoring

Abstract. The effects of NO_x and SO₂ policies on acid species in precipitation was assessed in China from 2005 to 2016, based on the OMI measured SO₂ and NO₂ columns. The results showed that the SO₂ and NO₂ columns in the atmospheric boundary layer (ABL) could be used to indicate the variations of S / N in precipitations (R = 0.90, incept = 0.97, P < 0.05). The spatial distribution of S / N was lower in eastern China than the west, which had a negative logarithmic relationship with population densities (R = 0.78, P < 0.05). The OMI-derived S / N decreased significantly from 2005 to 2016 (17.21 and 10.70 in 2005 and 2016, respectively), mainly due to the controlling S and N policies enacted at different times. The ABL SO₂ columns showed a decreasing trend from 2005 to 2016, while NO₂ presented an increasing tendency from 2005 to 2011 then decreasing until 2016. The temporal variations of SO₂ and NO₂ were not only determined by their emissions but also affected by precipitation amount, which induced the highest SO₂ and NO₂ concentrations in 2011 during the study time. With the combined acidification effects of S and N, the acidity had increased from 2005 to 2011, then decreased until 2016. The acidity in 2016 has declined 11.0 % and 25.4 %, respectively, compared with those in 2005 and 2011, indicating the policies on joint controlling SO₂ and NO₂ had some effects.