# Seasonal Characteristics and Source Apportionment of Water-Soluble Inorganic Ions of PM2.5 in a County-Level City of Jing–Jin–Ji Region

## TL;DR

This study examines the seasonal patterns and sources of water-soluble inorganic ions in PM2.5 in a county-level city in China's Jing–Jin–Ji region.

## Contribution

The study provides new insights into PM2.5 composition and sources in a county-level city, which is under-researched compared to urban areas.

## Key findings

- Nitrate was the most abundant ion in PM2.5, followed by ammonium and sulfate.
- Secondary sources and biomass burning were the main contributors to water-soluble inorganic ions in PM2.5.
- Ammonium salts were present as different compounds depending on the season, with ammonium chloride appearing in winter.

## Abstract

Water-soluble inorganic ions (WSIIs) are major components of PM2.5 and play a prominent role in atmospheric acidification. Previous studies have mainly focused on urban areas, whereas research pertaining to county-level cities remains comparatively limited. To fill this gap, PM2.5 samples were collected from March 2018 to February 2019 in Botou, a county-level city in the Jing–Jin–Ji region. Seasonal variation of WSII were studied, and their sources was apportioned by Positive Matrix Factorization (PMF) model. Annual PM2.5 concentrations were 79.15 ± 48.44 mg/m3, which is 2.26 times of the Level II standard limit specified the National Ambient Air Quality Standard. Nitrate (NO3−) was the most abundant ion, followed by ammonium (NH4+) and sulfate (SO42−). The secondary inorganic aerosols (SIA, i.e., SO42−, NO3−, and NH4+) constituted 35.1± 4.7% of PM2.5 mass. PM2.5 mass, SO42−, NO3−, NH4+, K+, and Cl− showed highest concentrations in winter. Ammonium salts were existed as ammonium sulfate ((NH4)2SO4) and ammonium nitrate (NH4NO3) in spring, summer, and autumn, while it also can be existed as ammonium chloride (NH4Cl) in winter. PMF analysis shows that the sources of WSIIs dominated by secondary source and followed by biomass burning. These results highlight the need for improved controls on gaseous precursors (NH3, NO2 and SO2) and biomass burning to effectively reduce PM2.5.