Key Takeaways & Executive Findings
- •• • SNA averaged 33.19 ± 18.72 μg m−3, constituting 47.13% of PM2.5 mass in Taiyuan winter, underscoring its dominance and the need for targeted control of precursor gases. • • Daytime nitrate formation was favored at RH > 55%, indicating that high humidity promotes gas-to-particle conversion of HNO3, a critical factor for predicting nitrate episodes. • • Nighttime nitrate production correlated with RH and AWC, with higher AWC enhancing NO2-to-nitrate conversion, highlighting the role of aqueous-phase chemistry in nocturnal pollution. • • PM2.5 pH ranged from 4.3 to 5.2, and sulfate formation was dominated by H2O2 oxidation, with NO2 oxidation secondary, providing quantitative constraints for modeling sulfate production.
Abstract
Secondary inorganic aerosols (SNA), comprising sulfate, nitrate, and ammonium, are critical contributors to PM2.5 pollution in the Fenwei Plain, yet their formation mechanisms remain poorly characterized. Wintertime observations in Taiyuan revealed SNA as the dominant PM2.5 component, with a mean mass concentration of 33.19 ± 18.72 μg m−3, accounting for 47.13% of total PM2.5 mass. SNA concentrations increased markedly with pollution severity, but even under relatively clean conditions, SNA maintained a high mass fraction. Diurnal variation and correlation analyses indicated that nitrate formation pathways differed between day and night, largely governed by relative humidity (RH). During daytime, high RH (>55%) facilitated the partitioning of gaseous HNO3 to particulate nitrate. At night, RH positively correlated with nitrate concentration and nitrogen oxidation rate (NOR), with increased aerosol liquid water content (AWC) promoting NO2-to-nitrate conversion. The PM2.5 pH ranged from 4.3 to 5.2, and sulfate formation was primarily driven by H2O2 oxidation, with NO2 oxidation as a secondary pathway. These findings enhance understanding of SNA formation in the Fenwei Plain and provide a scientific basis for air quality policy.
1. Introduction
Secondary inorganic aerosols (SNA) are a major driver of PM2.5 pollution in the Fenwei Plain, yet the region's complex emission and meteorological conditions have hindered elucidation of their formation pathways. Previous studies in other Chinese basins have identified diverse mechanisms, including gas-phase oxidation, heterogeneous hydrolysis, and aqueous-phase reactions, but their relative importance in the Fenwei Plain remains unresolved. This knowledge gap impedes the design of effective emission control strategies, as mitigation priorities differ depending on whether sulfate or nitrate formation is governed by photochemistry, humidity, or aerosol acidity.
This study addresses this bottleneck by conducting wintertime field observations in Taiyuan, a representative city in the Fenwei Plain, to quantify SNA concentrations and analyze their formation mechanisms. By integrating diurnal variation patterns, correlation analyses with meteorological parameters, and aerosol pH measurements, the authors delineate the dominant pathways for nitrate and sulfate production. The findings provide essential empirical data to refine chemical transport models and inform policy decisions aimed at reducing PM2.5 levels in this heavily polluted region.
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WEI Ying, MU Ling, WU Zhijun, ZONG Taomou, KONG Xiangyu, LI Chenhui, LI Jiajie, LI Yongqi (2026). Formation Mechanisms of Secondary Inorganic Components in Fine Particulate Matter in a Typical City of the Fenwei Plain. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025022704
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Frequently Asked Questions
What are the dominant formation pathways for nitrate during daytime and nighttime, and how does relative humidity influence them?
During daytime, high RH (>55%) promotes the partitioning of gaseous HNO3 to particulate nitrate, likely via gas-to-particle conversion. At night, RH positively correlates with nitrate concentration and NOR, with increased aerosol liquid water content (AWC) enhancing NO2-to-nitrate conversion, possibly through heterogeneous hydrolysis of N2O5.
What is the pH range of PM2.5 in Taiyuan during winter, and how does it affect sulfate formation?
The pH ranged from 4.3 to 5.2. In this acidic range, sulfate formation is primarily driven by H2O2 oxidation, with NO2 oxidation as a secondary pathway. This indicates that transition metal ion catalysis may be less important under these conditions.
How does the SNA mass fraction change with pollution level, and what implications does this have for control strategies?
SNA mass fraction remains high even at low pollution levels, and its concentration increases significantly with pollution severity. This suggests that controlling SNA precursors (SO2, NOx, NH3) is crucial for reducing PM2.5, especially during haze episodes.
What are the key uncertainties in extrapolating these findings to other cities in the Fenwei Plain?
The study focuses on Taiyuan, which may have unique emission sources and meteorological conditions. Extrapolation requires consideration of regional variability in precursor emissions, aerosol acidity, and RH patterns. Future multi-city observations are needed to validate the generality of these mechanisms.
How do the sulfate formation pathways identified here compare to those in other polluted regions like the North China Plain?
In the North China Plain, sulfate formation is often dominated by transition metal ion catalysis or NO2 oxidation under neutral conditions. In Taiyuan, the lower pH (4.3-5.2) favors H2O2 oxidation, suggesting that regional differences in aerosol acidity significantly alter the dominant sulfate production pathway.
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