Hysteresis response of wet nitrate deposition to emission reduction in Chinese terrestrial ecosystems

Abstract China is currently undergoing a massive socio-economic transformation, and has implemented a series of emission reduction measures to control air pollution. The response of atmospheric deposition to emission reduction measures provides direct evidence for the efficiency of environmental policies. Here, we consistently measured SO42−, NO3−, and NH4+ concentrations in rainfall in 43 typical Chinese ecosystems from 2013 to 2018 to reveal their spatiotemporal dynamics and the factors influencing them. The results showed that wet acid deposition (defined as 2SO42− + NO3−) in China exhibited an overall decline throughout 2013–2018, mostly due to decreased SO42− deposition. Despite evidence of a decline in NOX emissions in recent years, wet NO3− deposition in Chinese terrestrial ecosystems showed no significant trends during 2013–2018, implying a hysteresis response of wet NO3− deposition to emission reductions. The potential reasons for this response include a lack of evaluation in natural NOX emissions (e.g. NOX emissions from soil), large variations in wet scavenging, increased atmospheric oxidation capacity, which improves the conversion efficiency of NOX to HNO3, and higher NH3 emission levels to enhance the conversion of HNO3 to nitrate. Wet NH4+ deposition exhibited a relatively stable trend in recent years, which is consistent with the NH3 emissions in China. Energy consumption (especially coal combustion) and traffic can explain approximately 76% of the spatial variation in acid deposition. Our results demonstrate that controlling atmospheric SO2 emissions over recent decades has been effective in China, whereas N deposition in China requires more attention. Further, simultaneous control of NOX and NH3 emissions should be emphasized in the future to reduce acid deposition and the associated ecological risks.