Spatial boundary of urban ‘acid islands’ in southern China

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.

Biomass burning drives atmospheric nutrient redistribution within forested peatlands in Borneo

“…Total annual S flux to this peatland was also extraordinarily high, totaling 42 kg S ha −1 yr −1 , of which we estimated 19 kg ha −1 yr −1 was nss--SO 4 2 -S. Assuming -SO 4 2 in throughfall is a robust index of total (wet+fog+dry) atmospheric deposition in this peatland site (Weathers et al 2006, Ponette-González et al 2010a, our sample site ranks among the regions with the highest predicted total S deposition globally (i.e., East Asia 20-50 kg S ha −1 yr −1 ; Vet et al 2014aVet et al , 2014b. The S flux reported here is comparable to estimates for forests located in Southern China's 'urban acid islands', areas <67 km from large cities where acid deposition exceeds critical loads (Du et al 2015).…”

“…Emissions inventories and long-term network data show reductions in SO 2 and NO x in North America and Europe with concomitant declines in the deposition of S and oxidized N (JRC/PBL 2011, Lehmann and Gay 2011, Tørseth et al 2012, US EPA 2014. In contrast, emissions and deposition of these compounds are rising in many Asian regions due to rapid urbanization, industrialization, as well as agricultural intensification and expansion (Dentener et al 2006, Vet et al 2014a, Du et al 2015. Although empirical data on P deposition are patchy, Asian ecosystems may become the new P deposition 'hotspots' (Wang et al 2015).…”

Wet acid deposition in Chinese natural and agricultural ecosystems: Evidence from national‐scale monitoring

“…As in the concept of the “acid island,” acid deposition (mainly including pH, SO 4 2− , and NO 3 − ) decreased with distance from the center of big cities [ Beirle et al ., ; Du et al ., ]. Du et al [] reported that the critical radius of deposition in the acid island is around 70 km in southern China.…”

“…As in the concept of the “acid island,” acid deposition (mainly including pH, SO 4 2− , and NO 3 − ) decreased with distance from the center of big cities [ Beirle et al ., ; Du et al ., ]. Du et al [] reported that the critical radius of deposition in the acid island is around 70 km in southern China. As discussed above, those fields of agricultural and natural ecosystems in China, which occupy about 95% of the total land area, might lie beyond the critical radius (70 km) and have lower acid deposition than those sites in or near cities.…”

“…Acid deposition through precipitation might be highest in areas closest to city centers, especially for megalopolises [ Chan and Yao , ]. Du et al [] reported the existence of urban “acid islands” within a critical radius of approximately 70 km of urban centers in southern China. We speculated that the agricultural and natural ecosystems of China, which occupy 95% of the total land area, would have lower levels of acid deposition than areas in or near cities because of their lower level of local industrial development and greater distance from urban centers.…”