Long-term monitoring of precipitation chemistry provides a great opportunity to examine the evolution of air pollutant emissions and effectiveness of air pollution control measures. We evaluated the characteristics and trends of precipitation chemistry at both annual and seasonal scales based on the records of 1994–2013 at Fushan Experimental Forest (FEF) of northeastern Taiwan. The results showed that 77% of the weekly precipitation had pH < 5.0. The two-decadal average annual pH was 4.62, without a significant inter-annual trend, possibly due to the concurrent declines of both acidic pollutants and base cations. There was a significant positive relationship between [SO42− + NO3−] and [Ca2 + + NH4+] indicating that their deposition was likely dominated by NH4NO3, (NH4)2SO4, Ca(NO3)2, and CaSO4. There was a significant negative relationship between precipitation pH and the difference between [SO42− + NO3−] and [Ca2 + + NH4+], not just [SO42− + NO3−], suggesting that precipitation acidity was not solely determined by acidic pollutants but by the balance between acidic pollutants and base cations. We also found temporal decreases of Ca2 + and NH4+ concentrations in precipitation which contributed to the low acid neutralization capacity of precipitation. Annual deposition of NO3− and SO42 − was 23 and 55 kg ha− 1 yr− 1, which is much higher than most forest sites in the industrialized countries suggesting that acid deposition is still a major environmental issue in Taiwan. Annual deposition of NH4+, Ca2 + and NO3− showed significant decreasing trends during the 20-year period, which was mostly due to the decreases in the summer deposition associated with air pollution mitigation strategies. Winter deposition showed no decreasing patterns for the same period. The high contribution to annual acid deposition from autumn-winter and spring rains (50%) associated with northeast monsoon implies that long-range transport of anthropogenic emissions from East Asia played a key role on acid depositions at FEF and possibly many areas in the region. Therefore, intergovernmental cooperation is urgently needed to effectively mitigate the threat of acid deposition in East Asia.
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