Abstract
Elevated emissions of sulfur dioxide, nitrogen oxides and ammonia in China have resulted in high levels of sulfur and nitrogen deposition, being contributors to soil acidification, especially in and near large cities. However, knowledge gaps still exist in the way that large cities shape spatial patterns of acid deposition. Here, we assessed the patterns of pH, sulfate, nitrate and ammonium in bulk precipitation and throughfall in southern China’s forests by synthesizing data from published literature. Concentrations and fluxes of sulfate, nitrate and ammonium in bulk precipitation and throughfall exhibited a power-law increase with a closer distance to the nearest large cities, and accordingly pH showed a logarithmic decline. Our findings indicate the occurrence of urban ‘acid islands’ with a critical radius of approximately 70 km in southern China, receiving potential acid loads of more than 2 keq ha−1 yr−1. These urban acid islands covered an area of 0.70 million km2, accounting for nearly 30% of the land area in southern China. Despite a significant capacity to neutralize acids in precipitation, our analysis highlights a substantial contribution of ammonium to potential acid load. Our results suggest a joint control on emissions of multiple acid precursors from urban areas in southern China.
Highlights
Ammonia can neutralize sulfuric and nitric acids in cloud water and precipitation, subsequent ammonium (NH4+) deposition has a potential to generate significant acidification[11]
Concentrations and fluxes of hydrogen ion, sulfate, nitrate and ammonium in bulk precipitation and throughfall generally were high in southern China, but large spatial heterogeneity existed (Figs 2–5)
In line with the hypothesis of urban acid islands, our results demonstrate a power-law increase of concentrations and fluxes of hydrogen ion, sulfate, nitrate and ammonium in bulk precipitation and throughfall with a closer distance to urban hotspots
Summary
Ammonia can neutralize sulfuric and nitric acids in cloud water and precipitation, subsequent ammonium (NH4+) deposition has a potential to generate significant acidification[11]. This is due to nitrification, causing the production of two protons (NH4+ + 2O2 → NO3− + H2O + 2H+). We tested the hypothesis of urban acid islands by synthesizing data from published literature on pH, nitrate, sulfate and ammonium in bulk precipitation and throughfall in southern China’s forests. We estimated the critical radius of the urban acid islands, based on a maximum critical load of acidification for soils in southern China, and discussed the policy implications for a better control of acid deposition in China
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