Chemical and Optical Characteristics and Sources of PM_2.5 Humic-Like Substances at Industrial and Suburban Sites in Changzhou, China

• Main Authors: Ye Tao, Ning Sun, Xudong Li, Zhuzi Zhao, Shuaishuai Ma, Hongying Huang, Zhaolian Ye, Xinlei Ge
• Format: Article
• Language: English
• Published: MDPI AG 2021-02-01
• Series: Atmosphere
• Subjects: fine particle; HULIS; industrial region; light-absorbing characteristics; positive matrix factorization (PMF)
• Online Access: https://www.mdpi.com/2073-4433/12/2/276

The chemical and optical properties and sources of atmospheric PM_2.5 humic-like substances (HULIS) were investigated from October to December 2016 in both industrial and suburban areas in Changzhou, China, during polluted and fair days. The average PM_2.5 concentration in the industrial region was 113.06 (±64.3) μg m⁻³, higher than 85.27 (±41.56) μg m⁻³ at the suburban site. The frequency of polluted days was significantly higher in the industrial region. In contrast, the chemical compositions of PM_2.5 at the two sampling sites exhibited no statistically significant differences. Rapidly increased secondary inorganic ions (SNA = NH₄⁺ + SO₄²⁻ + NO₃⁻) concentrations suggested secondary formation played an important role in haze formation. The daily mean concentration of humic-like substance (HULIS) was 1.8–1.9 times that of HULIS-C (the carbon content of HULIS). Our results showed that HULIS accounted for a considerable fraction of PM_2.5 (industrial region: 6.3% vs. suburban region: 9.4%). There were no large differences in the mass ratios of HULIS-C/WSOC at the two sites (46% in the industrial region and 52% in the suburban region). On average, suburban HULIS-C constituted 35.1% of organic carbon (OC), higher than that (21.1%) in the industrial region. Based on different MAE (mass absorption efficiency) values under different pollution levels, we can infer that the optical properties of HULIS varied with PM levels. Moreover, our results showed no distinct difference in E₂/E₃ (the ratio of light absorbance at 250 nm to that at 365 nm) and AAE_300–400 (Absorption Angstrom Exponent at 300–400 nm) for HULIS and WSOC. the MAE₃₆₅ (MAE at 365 nm) value of HULIS-C was different under three PM_2.5 levels (low: PM_2.5 < 75 μg m⁻³, moderate: PM_2.5 = 75–150 μg m⁻³, high: PM_2.5 > 150 μg m⁻³), with the highest MAE₃₆₅ value on polluted days in the industrial region. Strong correlations between HULIS-C and SNA revealed that HULIS might be contributed from secondary formation at both sites. In addition, good correlations between HULIS-C with K⁺ in the industrial region implied the importance of biomass burning to PM_2.5-bound HULIS. Three common sources of HULIS-C (i.e., vehicle emissions, biomass burning, and secondary aerosols) were identified by positive matrix factorization (PMF) for both sites, but the contributions were different, with the largest contribution from biomass burning in the industrial region and secondary sources in the suburban region, respectively. The findings presented here are important in understanding PM_2.5 HULIS chemistry and are valuable for future air pollution control measures.