Long-term trend of wintertime precipitation chemistry at a remote islet site influenced by anthropogenic emissions from continental East Asia

Abstract

In this study, the trends and factors influencing wintertime precipitation chemistry at a remote site downwind of continental East Asia were investigated. A total of 336 precipitation samples were collected from 2005 to 2019 at Pengjia Islet, a site ~56 km north of the main island of Taiwan. Overall, the samples had a volume-weighted mean pH of 4.81 (ranging from 3.03 to 7.00), with 74% of samples considered as acid rain (pH < 5). PMF analysis of the samples identified sea-salt, anthropogenic emissions, and dust as sources of major ions in precipitation. Anthropogenic emissions were mostly associated with polluted air from East Asia. Four major air mass transport pathways to Pengjia Islet were identified, in which 87% originated from northern China (i.e. polluted air masses) with a relatively low pH and high ionic concentrations compared to the remaining samples. Wintertime pH values associated with the most common trajectory from China significantly increased at a rate of +1.7% yr−1 (p < 0.01) from 2005 to 2019, similar to those observed at other sites in East Asia in the previous two decades. The increasing pH trend in this study was attributed to the decrease in nss-SO42- concentrations and an increase in Ca2+ concentrations, of which the former aligns with the decrease in nss-SO42- at sites in China caused by the decline of SO2 emissions. The increase of Ca2+ concentrations is supported by the increase of coarse mode aerosol mass concentrations transported to northern Taiwan in winter. By splitting the dataset into two periods (i.e. 2005-2011 and 2012–2019), we found a close relationship between the changes in air-quality control in China and trends in precipitation chemistry at Pengjia Islet, including a consistently decreasing nss-SO42-/NO3− ratio in rainwater. Such a finding has important implications for the effectiveness of air pollution control policies on the regional scale. Additionally, changes in regional transport patterns of air masses might also contribute to the changes in precipitation chemistry at Pengjia Islet.