Key Takeaways & Executive Findings
- •• • Water-soluble ions averaged 26.04 μg m−3, comprising 52.1% of PM2.5 mass; SNA (NO3−, NH4+, SO42−) accounted for 87.6% of total ions, underscoring the dominance of secondary inorganic aerosols in Taiyuan's autumn-winter PM2.5. • • The NO3−/SO42− mass ratio of 1.83 confirms a regime shift from sulfate to nitrate dominance, implying that controlling NOx emissions is now more critical than SO2 for mitigating PM2.5 pollution. • • During moderate pollution, SO42− surged to 5.31 times clean-day levels, while during heavy pollution, NO3− surged to 5.37 times, indicating that nitrate formation is the key driver of severe haze episodes. • • PMF source apportionment revealed that secondary sources contributed 45.4% on clean days but escalated to 64.7% and 63.4% on lightly and moderate-to-heavy polluted days, respectively, highlighting the increasing role of secondary formation in pollution exacerbation.
Abstract
To investigate the pollution characteristics and temporal variations of water-soluble ions in atmospheric fine particulate matter (PM2.5) during autumn and winter in Taiyuan City, continuous sampling and analysis were conducted using a Swiss Metrohm ion chromatograph in autumn and winter 2023. The results show that the daily mean concentration of nine water-soluble ions was 26.04 μg m−3, accounting for 52.1% of the average PM2.5 mass concentration. Ion concentrations ranked in descending order: NO3−, NH4+, SO42−, Cl−, K+, Na+, Ca2+, F−, and Mg2+. Secondary inorganic ions (SNA) constituted 87.6% of total water-soluble ions. The mass ratio of NO3− to SO42− reached 1.83, indicating a shift from sulfate-dominated to nitrate-dominated aerosol chemistry. During pollution episodes, water-soluble ion concentrations increased exponentially, with distinct ion-specific trends: on moderately polluted days, SO42− increased to 5.31 times that on clean days, whereas on heavily polluted days, NO3− increased to 5.37 times, highlighting nitrate as a primary driver of severe pollution. Comparison with historical data reveals a recent increase in the proportion of water-soluble ions in PM2.5, with higher proportions during more polluted periods. Analysis of NH4+ forms and PM2.5 acidity suggests that acidic components contribute more under heavier pollution. Positive matrix factorization (PMF) identified four major sources: secondary sources, combustion and motor vehicles, industrial sources, and dust. The secondary source contributions were 45.4%, 64.7%, and 63.4% on clean, lightly polluted, and moderate-to-heavy polluted days, respectively, indicating a significantly higher secondary contribution on polluted days.
1. Introduction
Urban air quality management in northern China faces a persistent challenge: fine particulate matter (PM2.5) concentrations remain elevated during autumn and winter, despite stringent emission controls. Water-soluble ions constitute a major fraction of PM2.5, yet their source dynamics and chemical evolution are region-specific. In Taiyuan, a coal-dependent industrial city, historical controls focused on sulfur dioxide, but recent observations suggest a shift in aerosol chemistry. Existing studies have documented sulfate dominance, but the transition to nitrate-driven pollution has not been systematically quantified. This research addresses that gap by providing a comprehensive characterization of water-soluble ions during the critical 2023 autumn-winter period, using high-resolution ion chromatography and positive matrix factorization (PMF) to resolve source contributions.
The study's significance lies in its empirical evidence of a chemical regime shift: the NO3−/SO42− ratio of 1.83 indicates that nitrate now surpasses sulfate as the primary secondary inorganic species. This finding has direct implications for emission reduction strategies, as controlling NOx emissions becomes more urgent. Furthermore, the exponential increase in ion concentrations during pollution episodes, with nitrate surging 5.37-fold on heavily polluted days, identifies nitrate formation as a key driver of severe haze. By comparing with historical data, the research reveals a rising proportion of water-soluble ions in PM2.5, suggesting that secondary formation is becoming more efficient. These insights are essential for refining air quality models and designing targeted mitigation measures in Taiyuan and similar industrial cities.
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LAN Jie, FENG Kun, LIAO Nan, WANG Duo, ZHANG Yuqia, SONG Yaqi (2026). Characteristics and Source Apportionment of Water-Soluble Ions in PM2.5 in Taiyuan during Autumn and Winter 2023. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025062404
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Frequently Asked Questions
What are the implications of the NO3−/SO42− ratio of 1.83 for emission control policies in Taiyuan?
The ratio indicates that nitrate is now the dominant secondary inorganic species, surpassing sulfate. This suggests that reducing NOx emissions from combustion and vehicle sources should be prioritized over SO2 controls to effectively lower PM2.5 levels. The shift likely results from successful desulfurization efforts and increasing vehicle emissions.
How does the contribution of secondary sources vary across different pollution levels, and what does this imply for pollution mitigation?
PMF analysis shows secondary sources contribute 45.4% on clean days, rising to 64.7% on lightly polluted days and 63.4% on moderate-to-heavy polluted days. This indicates that secondary formation is a major driver during pollution episodes, so controlling precursor gases (NOx, SO2, NH3) is crucial, especially during stagnant meteorological conditions.
What are the limitations of using PMF for source apportionment in this study, and how were they addressed?
PMF requires sufficient sample size and assumes source profiles are constant over time. The study used 2023 autumn-winter data, which may not capture seasonal variations. To address this, the authors compared with historical studies and used a robust PMF protocol (USEPA PMF 5.0). However, uncertainties remain due to potential collinearity of sources and unmeasured species.
How do the concentrations of water-soluble ions in Taiyuan compare with other Chinese cities, and what does this indicate about regional pollution?
The average concentration of 26.04 μg m−3 is comparable to other northern cities like Xi'an and Beijing, but the NO3−/SO42− ratio of 1.83 is higher, indicating a stronger nitrate influence. This suggests that Taiyuan's pollution is more influenced by local combustion and vehicle emissions, and regional transport may also play a role.
What is the significance of the finding that SO42− increases 5.31-fold on moderately polluted days but NO3− increases 5.37-fold on heavily polluted days?
This differential increase suggests that sulfate formation is more responsive to moderate pollution conditions, possibly due to heterogeneous reactions, while nitrate formation becomes dominant during heavy pollution, likely due to increased ammonia availability and lower temperatures. This implies that controlling NOx emissions is particularly critical for preventing severe haze episodes.
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