The composition, seasonal variation, and potential sources of the atmospheric wet sulfur (S) and nitrogen (N) deposition in the southwest of China

The composition, seasonal variation, and potential sources of sulfate (S) and nitrogen (N) deposition in precipitation in the southwest of China from 2003 to 2013 were investigated. The results showed that the concentration of SO4 (2-), NO3 (-), and NH4 (+) in rainwater were 10.57-1360, 7.16-523.71, and 7.54-1020 mu eq l(-1), with an annual volume-weighted mean (VWM) concentration of 103.99, 46.73, and 97.30 mu eq l(-1), respectively. The annual wet deposition of SO4 (2-), NO3 (-), and NH4 (+) was 21.66, 8.16, and 17.49 kg S (N) ha(-1), respectively. The temporal variations of the ions showed that the abrupt decreasing breakpoints were in 2008 for SO4 (2-) and in 2009 for NO3 (-) and NH4 (+), and increasing trends were observed after 2010 for the three ions. These trends reflected the effect of economy recession and the policy of controlling SO2 and NOx emissions. The acid rain type of precipitation was shifted from sulfur to a mixed one. The ions of SO4 (2-), NO3 (-), and NH4 (+) presented high values in winter and spring and low values in autumn and summer. A highly positive linear correlation between SO4 (2-) and NO3 (-) (R (2) = 0.71), SO4 (2-) and NH4 (+) (R (2) = 0.74), and NO3 (-) and NH4 (+) (R (2) = 0.84) existed while a strong negative correlation was found between the three main ionic concentrations and precipitation. The SO4 (2-) was mainly from fossil fuel combustion (60.53 %), aged sea salt (19.03 %), agriculture (11.38 %), crust (6.66 %), and biomass burning (2.40 %); the NO3 (-) was mainly from fossil fuel combustion (75.41 %), biomass burning (9.67 %), aged sea salt (7.97 %), and agriculture (6.96 %); and the NH4 (+) was mainly from agriculture (86.38 %), fossil fuel combustion (10.52 %), and aged sea salt (3.09 %).