Long-term trends and response of wet ammonia deposition to changes in anthropogenic emissions in the Pearl River delta of China

The Pearl River Delta (PRD) region has been identified as a significant hotspot of wet ammonium deposition. However, the absence of long-term monitoring data in the area hinders the comprehension of the historical trends and changes in wet NH₄⁺-N deposition in response to emissions, which interferes with the ability to make effective decisions. This study has analyzed the long-term trends of wet NH₄⁺-N deposition flux and has quantified the effect of anthropogenic emissions and meteorological factors at a typical urban site and a typical forest site in the PRD region from 2009 to 2020. It revealed a significant decreasing trend in wet NH₄⁺-N flux in both the typical urban and forest areas of the PRD region, at -6.2%/year (p < 0.001) and -3.3%/year (p < 0.001), respectively. Anthropogenic emissions are thought to have contributed 47%-57% of the wet NH₄⁺-N deposition trend over the past 12 years compared to meteorological factors. Meteorological conditions dominated the inter-annual fluctuations in wet NH₄⁺-N deposition with an absolute contribution of 46%-52%, while anthropogenic emissions change alone explained 10%-31%. NH₃ emissions have the greatest impact on the urban area among anthropogenic emission factors, while SO₂ emissions have the greatest impact on the forest area. Additionally, precipitation was identified as the primary meteorological driver for both sites. Our findings also imply that the benefits of NH₃ emissions reductions might not immediately emerge due to interference from weather-related factors.