Chemical and Optical Characteristics and Sources of PM2.5 Humic-Like Substances at Industrial and Suburban Sites in Changzhou, China

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.

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“…This result is in agreement with other studies worldwide showing that HULIS-C accounted for about half of WSOC in urban areas [24,29,31,35,37,123]. The HULIS-C/PM 2.5 (2.9%) and HULIS-C/OC ratio (22.2%) values were slightly lower than those obtained in some previous studies in urban sites [24,27]. It is interesting to note that the HULIS-C/OC polar plot (Figure 4) showed two spots under northeasterly and southeasterly sectors (such as the HULIS-C polar plot), corresponding to a ratio value of 0.6 approximately.…”

“…The fraction of HULIS-C in WSOC was 48.2%. This result is in agreement with other studies worldwide showing that HULIS-C accounted for about half of WSOC in urban areas [24,29,31,35,37,123]. The HULIS-C/PM 2.5 (2.9%) and HULIS-C/OC ratio (22.2%) values were slightly lower than those obtained in some previous studies in urban sites [24,27].…”

“…HULIS represent a significant fraction of organic aerosol. Indeed, the carbon content of HULIS (HULIS-C) accounts for 25 to 75% of WSOC [24,25]. Previous studies have identified that WSOC and HULIS-C result both from primary emissions such as biomass burning and atmospheric secondary reactions [22][23][24][26][27][28].…”

“…Indeed, the carbon content of HULIS (HULIS-C) accounts for 25 to 75% of WSOC [24,25]. Previous studies have identified that WSOC and HULIS-C result both from primary emissions such as biomass burning and atmospheric secondary reactions [22][23][24][26][27][28]. Kuang et al [29] have also shown that secondary formation processes contribute to 69% of HULIS at an urban site in China.…”