Abstract
The chemical and optical properties and sources of atmospheric PM2.5 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 PM2.5 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 PM2.5 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 PM2.5 (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 AAE300–400 (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 PM2.5 levels (low: PM2.5 < 75 μg m−3, moderate: PM2.5 = 75–150 μg m−3, high: PM2.5 > 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 PM2.5-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 PM2.5 HULIS chemistry and are valuable for future air pollution control measures.
Highlights
Brown carbon (BrC), a class of organic matter in aerosols, has strong light absorption in the UV–vis region (300–700 nm)
PM2.5 and its chemical constituents were slightly higher in the industrial area than those in the suburban area, the relative mass fractions of Humic-like substances (HULIS)-C in WSOC were very similar at the two sites throughout the entire sampling period, similar to results for Hong Kong (53.4%) [7] and urban Shanghai (50%) [35]
Water-soluble HULIS-C accounted for 21.1% of organic carbon (OC) in the industrial region and 35.1% of OC in the suburban region, illustrating the important role of HULIS in atmospheric aerosols
Summary
Brown carbon (BrC), a class of organic matter in aerosols, has strong light absorption in the UV–vis region (300–700 nm). HULIS has not been studied thoroughly, and knowledge of HULIS optical properties, chemical structural characteristics, and sources is limited but important in assessing its effects on air quality, climate, and human health [8]. The individual as well as combined influences of various factors on the formation of HULIS in different regions needs further investigation since assessment of health effects associated with HULIS should address their chemical composition and optical properties instead of only focusing on the mass concentration. PM2.5 samples were collected at two sites (located in industrial and suburban regions) in Changzhou, a neighborhood city of Shanghai in the Yangtze River Delta (YRD) region, China. For HULIS quantification, an aliquot of 20 μL of the aqueous solution was injected into an HPLC system coupled to an evaporative light-scattering (ELSD3000) detector
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