Key Takeaways & Executive Findings
- •• • Total water-soluble ion concentrations in PM2.5 during summer decreased significantly from 2012 to 2017–2019, with SNA (SO4^2−, NO3^−, NH4^+) accounting for 69.98%–92.58% of total ion mass, confirming the effectiveness of APPCAP in reducing PM2.5 pollution. • • In 2013 and 2017, PM2.5 was alkaline with SNA present as NH4NO3 and (NH4)2SO4, whereas in 2019 PM2.5 became acidic with SNA as NH4NO3 and NH4HSO4, indicating a shift in atmospheric acidity and ammonia availability. • • NOR and SOR values revealed that SO4^2− and NO3^− were predominantly from secondary reactions, with SO2 undergoing secondary conversion more readily than NO2, and the degree of secondary conversion increased annually from 2013 to 2019. • • Source apportionment indicated a shift from long-range transport in 2013 to local and regional sources by 2017, emphasizing the growing importance of regional emission controls and the need for continued SO2 reduction strategies.
Abstract
To assess the impact of the Air Pollution Prevention and Control Action Plan (APPCAP) on the chemical composition of PM2.5, this study analyzed the concentrations, existing forms, and sources of water-soluble ions in PM2.5 collected during summer (June–August) in the northern suburbs of Nanjing for the years 2012, 2013, 2017, and 2019. The results demonstrate a significant reduction in total water-soluble ion concentrations in 2017–2019 compared to 2012, indicating the effectiveness of APPCAP in mitigating PM2.5 pollution. Sulfate (SO4^2−), nitrate (NO3^−), and ammonium (NH4^+) (collectively SNA) were the dominant ionic species, contributing 69.98%–92.58% of the total ion mass, with SO4^2− being the most abundant. In the summers of 2013 and 2017, PM2.5 exhibited alkaline properties, and SNA primarily existed as NH4NO3 and (NH4)2SO4. Conversely, in 2019, PM2.5 became acidic, with SNA present as NH4NO3 and NH4HSO4. The nitrogen oxidation ratio (NOR) and sulfur oxidation ratio (SOR) indicated that NO3^− and SO4^2− predominantly originated from secondary reactions, with SO2 undergoing secondary conversion more readily than NO2, and the degree of secondary conversion increasing annually. Source apportionment revealed a shift from long-range transport in 2013 to local and regional sources by 2017. These findings underscore the success of APPCAP in reducing primary emissions and altering the chemical speciation of secondary inorganic aerosols, while highlighting the persistent dominance of sulfate and the need for continued SO2 emission controls.
1. Introduction
The rapid industrialization and urbanization of the Yangtze River Delta have historically subjected Nanjing to severe PM2.5 pollution, with water-soluble ions constituting 30–80% of PM2.5 mass. Prior to the implementation of the Air Pollution Prevention and Control Action Plan (APPCAP) in 2013, studies identified secondary inorganic aerosols (SNA) as the dominant components, but their seasonal evolution and response to policy interventions remained inadequately characterized. Existing commercial monitoring and emission control technologies primarily targeted primary pollutants, yet the secondary formation pathways of sulfate and nitrate—driven by SO2 and NO2 oxidation—posed persistent challenges, as their atmospheric transformation efficiencies and acidity modulation were not fully understood.
This study addresses the critical gap by systematically analyzing water-soluble ions in PM2.5 collected during summer across four years (2012, 2013, 2017, 2019) spanning the APPCAP period. By quantifying concentration changes, chemical speciation (e.g., NH4NO3 vs. (NH4)2SO4), and oxidation ratios (NOR, SOR), the research delineates the policy's impact on secondary aerosol formation and source dynamics. The findings provide empirical evidence for the effectiveness of emission controls and identify the persistent dominance of sulfate, underscoring the need for targeted SO2 reduction and regional cooperation to further mitigate PM2.5 pollution.
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HE Yijing, DOU Ziyi, QIN Yang, CHEN Siyu, YU Xingna (2026). Evolution of Water-Soluble Ions in PM2.5 in the Northern Suburbs of Nanjing During Summer Before and After the Implementation of the Air Pollution Prevention and Control Action Plan. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025021901
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Frequently Asked Questions
What were the specific concentration reductions of water-soluble ions in PM2.5 after APPCAP implementation, and how do they correlate with changes in SO2 and NO2 emissions?
The study found that total water-soluble ion concentrations in summer decreased significantly from 2012 to 2017–2019. For instance, SNA (SO4^2−, NO3^−, NH4^+) accounted for 69.98%–92.58% of total ion mass, with sulfate being dominant. The reduction aligns with APPCAP's emission control measures, which targeted SO2 and NO2. The NOR and SOR values indicated that SO2 underwent secondary conversion more readily than NO2, and the degree of secondary conversion increased annually, suggesting that while emissions decreased, atmospheric oxidation efficiency improved, possibly due to changes in oxidant levels.
How did the acidity of PM2.5 change over the study period, and what implications does this have for secondary aerosol formation mechanisms?
PM2.5 was alkaline in 2013 and 2017, with SNA present as NH4NO3 and (NH4)2SO4, indicating sufficient ammonia to neutralize sulfuric and nitric acids. In 2019, PM2.5 became acidic, with SNA present as NH4NO3 and NH4HSO4. This shift suggests a decrease in ammonia availability relative to acidic precursors, possibly due to reduced agricultural emissions or changes in SO2/NOx ratios. The presence of NH4HSO4 indicates incomplete neutralization, which can enhance the formation of secondary organic aerosols and affect atmospheric chemistry.
What evidence supports the shift in pollution sources from long-range transport to local and regional sources between 2013 and 2017?
The study used wind rose diagrams and source apportionment analysis. In 2013, PM2.5 concentrations were associated with long-range transport from northern industrial regions, as indicated by wind patterns and high sulfate levels. By 2017, local sources and regional transport within the Yangtze River Delta became more prominent, likely due to increased local emissions and the effectiveness of regional emission controls. This shift underscores the need for coordinated regional air quality management.
Why did sulfate remain the dominant ion despite significant reductions in SO2 emissions, and what are the implications for future control strategies?
Sulfate remained dominant because SO2 has a high secondary conversion efficiency (SOR) and the atmospheric lifetime of sulfate is longer than that of nitrate. Even with reduced SO2 emissions, the existing sulfate from regional transport and secondary formation continues to contribute significantly. The study recommends accelerating desulfurization in key industries, optimizing energy structures, and strengthening regional cooperation to further reduce SO2 emissions and mitigate sulfate-dominated PM2.5 pollution.
What are the limitations of this study in terms of temporal coverage and generalizability to other seasons or regions?
This study focused on summer months only, which may not capture seasonal variations in water-soluble ion chemistry. Additionally, the findings are specific to the northern suburbs of Nanjing and may not be directly applicable to other regions with different emission sources and meteorological conditions. Future studies should include year-round sampling and multi-city comparisons to better understand the regional impacts of APPCAP.
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