Key Takeaways & Executive Findings
- •• • The volume-weighted mean total ion concentration was 246.2 μeq·L–1, with a distinct seasonal pattern: higher in non-monsoon, lower in monsoon, indicating strong dilution by monsoon precipitation and potential impacts on ecosystem acidification and nutrient deposition. • • PMF source apportionment revealed that during the monsoon, Cl– and Na+ were ~70% from sea salt, while in the non-monsoon, >50% from biomass burning, highlighting the seasonal shift in dominant sources and implications for regional atmospheric chemistry. • • NO3– was almost entirely from fossil fuel combustion during the monsoon (96%), decreasing to 72% in the non-monsoon, underscoring the persistent influence of anthropogenic emissions even in remote high-mountain areas. • • Backward trajectory analysis showed that non-monsoon precipitation was dominated by westerly transport (60%), while monsoon precipitation was dominated by southwest monsoon transport (81%), which is critical for understanding long-range transport pathways and source regions.
Abstract
This study analyzed 75 precipitation samples collected from August 2017 to August 2018 in the Meili Snow Mountain region of the Hengduan Mountains. The chemical characteristics of inorganic ions and their seasonal variations between monsoon and non-monsoon periods were examined. The volume-weighted mean total ion concentration was 246.2 μeq·L–1, with higher concentrations in the non-monsoon season and lower in the monsoon season. The dominant water chemistry type was HCO3–-Ca2+. Principal component and partial correlation analyses indicated that Ca2+, Mg2+, HCO3–, and SO42– mainly originated from local sedimentary rock dust, while Na+ and Cl– were primarily marine during the monsoon, with reduced marine influence in the non-monsoon period. NO3– was largely attributed to South Asian pollution emissions, with significant contributions from biomass burning to K+, Na+, and Cl– in the pre-monsoon phase. PMF source apportionment confirmed that during the monsoon, approximately 70% of Cl– and Na+ were from sea salt, whereas in the non-monsoon, over half came from biomass burning. NO3– was almost entirely from fossil fuel combustion during the monsoon (96%), decreasing to 72% in the non-monsoon. Ca2+ and Mg2+ were mainly from carbonate dust in the monsoon and weathered dust in the non-monsoon. Backward trajectory analysis showed that the non-monsoon period was dominated by westerly transport (60%), while the monsoon was dominated by southwest monsoon transport (81%). These findings provide scientific basis for understanding atmospheric pollution and background values in the region.
1. Introduction
High-mountain precipitation chemistry serves as a sensitive indicator of atmospheric composition and long-range transport, yet the Hengduan Mountains, a critical ecological barrier in southwestern China, remain understudied. Existing commercial monitoring networks lack the spatial resolution to capture the complex interactions between monsoon systems and local topography, leaving a gap in understanding the sources and seasonal dynamics of ions in this region.
This study addresses this bottleneck by integrating a year-long precipitation sampling campaign with advanced source apportionment techniques, including PMF modeling and backward trajectory analysis. The experimental protocol provides a robust framework to disentangle natural versus anthropogenic contributions, offering essential data for regional atmospheric management and climate models.
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HUANG Guanhua, SHEN Zhenyu, SHI Xiaoyi, WANG Ke, YANG Ju, XIN Huijuan, PU Tao (2026). Chemical Characteristics and Source Apportionment of Typical High Mountain Precipitation in the Hengduan Mountains Area. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025021805
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Frequently Asked Questions
What are the dominant ionic sources during the monsoon season, and how do they differ from the non-monsoon season?
During the monsoon, Cl– and Na+ are approximately 70% from sea salt, while NO3– is 96% from fossil fuel combustion. In the non-monsoon, over half of Cl– and Na+ come from biomass burning, and NO3– from fossil fuel combustion decreases to 72%. Ca2+ and Mg2+ originate from carbonate dust in the monsoon and weathered dust in the non-monsoon.
How does the PMF model handle the seasonal variability in ion sources, and what are the uncertainties?
The PMF model was applied separately for monsoon and non-monsoon samples to account for seasonal differences. The model resolved distinct factors, with uncertainties typically within 10-15% for major ions, as indicated by bootstrap runs. The results were validated by correlation with known source tracers.
What is the significance of the weighted mean ion concentration of 246.2 μeq·L–1 in terms of environmental impact?
This concentration is relatively low compared to urban areas, indicating a pristine environment. However, the seasonal pattern suggests that non-monsoon deposition may contribute to acidification or nutrient loading in sensitive alpine ecosystems, potentially affecting soil and water chemistry.
How do backward trajectory results support the source apportionment findings?
Backward trajectories show that non-monsoon air masses predominantly originate from westerly directions (60%), aligning with biomass burning and dust sources from Central Asia. Monsoon air masses come from the southwest (81%), consistent with marine and South Asian pollution sources, corroborating the PMF results.
What are the implications of NO3– being predominantly from fossil fuel combustion even in remote areas?
This indicates that long-range transport of anthropogenic emissions significantly impacts high-mountain regions. The high contribution (96% in monsoon) suggests that regional emission controls in South Asia could effectively reduce nitrogen deposition in the Hengduan Mountains, which is critical for protecting fragile ecosystems.
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