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East Asia is one of the largest acid rain affected regions of the world. As the largest East Asian country, China has experienced severe acid rain problem. Early observations in Nanking in the late 1970s showed a significant occurrence of acid rain. Following this finding network observations were made and showed that acid rain had become a regional environmental problem, particularly in Southwest China.
To monitor the long-term development of regional acid rain in China, an operational acid rain network was established in the early 1990s by China Meteorological Administration. This network consisted of 88 sites by 2005 and was extended to 365 sites by 2014. The observed distributions of rainwater pH indicate that severe acid rain (pH<4.5) area shrank in Southwest China and extended in South and East China during 1992-2006.
The worst situation of acid rain was recorded in 2006, with about 40% and 13% of area in mainland China being affected by acid rain and severe acid rain, respectively. Afterwards the frequency of acid rain has been continuously decreasing and the annual pH value has been increasing. By 2015, only about one fourth of area in mainland China was affected by acid rain and the area with annual pH less than 4.5 nearly disappeared.
These dramatic changes coincide well with the estimated emissions of SO2 and NOx. Reductions of SO2 emission, driven by a series of emission control policies and measures, have played a key role in the mitigation of acid deposition in China. In recent years, reduced emission of NOx contributes also to the observed mitigation.
These views are supported by long-term measurements of precipitation chemistry from four global atmosphere watch (GAW) stations in China. The observed concentrations of sulfate and nitrate in rainwater have generally followed the trends in estimated SO2 and NOx emissions. In addition, decrease trends in the ratio S/N for the GAW stations can be derived from the measurements, indicating an ever-increasing importance of NOx emission.
Acid rain has been monitored also in other East Asian countries. As a joined effort, the Acid Deposition Monitoring Network in East Asia (EANET) has been established, with about 60 sites delivering observational data. EANET data for the 2000-2015 period show mixed trends in precipitation chemistry.
In the past decade, increase trends of pH value were observed at some sites, while no change or a decrease were observed at the other sites. The same is true for sulfate, nitrate and ammonium in rainwater. Currently most of the East Asian countries are affected more or less by acid rain. Although the acidity of rainwater in China is not higher than in other East Asia countries, the concentrations of sulfate, nitrate and ammonium in rainwater are higher than those in other East Asian countries.
Therefore, further strong reduction in S and N emissions is needed for all countries in East Asia.