Abstract:

【Objective】 Acid rain is an environmental problem of global concerns. The Yangtze River Delta region of China (YRD) has become one of the major acid rain regions in the world. Currently, SO₂ emissions in China have decreased because of the implementation of a series of SO₂ control measures. However, NO₂ emissions have not decreased significantly, and this may lead to a progressive change in the type of acid rain. Exploring the changing trends of acid rain types in the YRD region will provide a theoretical basis for the restoration and sustainable management development of forest ecosystems stressed by acid rain. 【Method】 In this study, we explored the characteristics of acid rain in the YRD region by integrating the characteristics of precipitation and atmospheric emissions in major cities. 【Result】 The annual average pH values and frequency of acid rain were 4.87±0.28 and (57.12±18.67)% in the YRD region. The SO₂ content in the atmosphere significantly decreased year by year. However, no consistent pattern of NO₂ content was found in different cities. The NO₂ content decreased slowly in Shanghai and Nanjing. In contrast, the NO₂ content in the atmosphere clearly increased in Suzhou and Ningbo. The precipitation pH values were still mainly influenced by SO₂ in the YRD region. However, the {\mathrm{SO}}_4^{2-} / {\mathrm{NO}}_3^{-} in precipitation decreased from 7.5 in the 1990s to 2.0, which suggests that the type of acid rain has changed from sulfuric acid rain (SAR) to mixed acid rain (MAR) in the YRD region. 【Conclusion】 The Yangtze River Delta region experiences stress because of acid rain, and the {\mathrm{SO}}_4^{2-} / {\mathrm{NO}}_3^{-} of acid rain has decreased year by year. In the future, the change in acid rain types from SAR to MAR, or to nitric acid rain (NAR), might complicate the ongoing challenge of ecosystem stability and increase the risks to ecosystem functioning in the YRD region.

Key words: Yangtze River Delta, acid rain, sulfur dioxide(SO₂), nitrogen oxide (NO_x), {\mathrm{SO}}_4^{2-}