The characterization of water-soluble inorganic ions in PM_2.5 during a winter period in Xi'an, China

ABSTRACT

The characteristics of water-soluble inorganic ions (WSIIs) during a winter period in a suburb of Xi'an, China, were investigated. Our results show that the total mass concentration of the dominant WSIIs (8) was 91.27 µg m^–3, accounting for 50.1% of the total mass concentration of PM_2.5 (particulates with a size of 2.5 µm or less). Secondary inorganic aerosols (SO₄²⁻, NO₃⁻ and NH₄⁺) were the most abundant ions, accounting for up to 95.12% of the total ions. By using the anion and cation equivalence ratio method, PM_2.5 was shown to have weak alkalinity, and the chemical forms of WSIIs were mainly (NH₄)₂SO₄ and NH₄NO₃. The sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) suggested that larger proportions of SO₄²⁻ and NO₃⁻ were formed by gas-phase SO₂ and NO₂ in the sampling site. Ratio analysis also indicated that anthropogenic sources significantly contributed to WSII pollution. Among the anthropogenic sources, fixed pollution sources were found to be dominant over mobile sources.

KEYWORDS:

• Acidic balance
• PM_2.5
• ratio analysis
• water-soluble inorganic ions
• Xi'an

Acknowledgments

This research was supported by the Key Consulting Project of Chinese Academy of Engineering (No. 2015–06-XZ-01). The authors thank Rui Feng for his participating in the sampling campaign and the anonymous reviewers for the valuable comments.

Additional information

Funding

the Key Consulting Project of Chinese Academy of Engineering 2015-06-XZ-01.