Secondary Organic Aerosols in PM2.5 in Bengbu, a Typical City in Central China: Concentration, Seasonal Variation and Sources

Shiwei Zhang,Li Li,Jialiang Feng,Chaojun Ge,Hao Tang,Liang Li,Qing Li

doi:10.3390/atmos12070854

Abstract

To investigate the concentration, seasonal variation, and sources of secondary organic aerosols (SOAs) in the inland areas of central China, 244 seasonal PM2.5 samples were collected from January to October 2019 at one urban site and one suburban site simultaneously in Bengbu of Anhui Province. Concentrations of organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and typical organic tracers, including saccharides, tracers of SOAs from isoprene, terpenes, and toluene, were measured. Results showed that Bengbu has high pollution levels of organic aerosols, with annual average OC concentrations of about 9.5 μg m−3. About 60% of the OC in PM2.5 in Bengbu was water soluble. Different seasonal trends were found for the SOA tracers of isoprene, monoterpene, seisquiterpene, and toluene. The highest seasonal average concentration of the isoprene SOA tracers was observed in summer and of the monoterpene and seisquiterpene SOA tracers in autumn. A stronger correlation was found between the 2-methylglyceric acid-to-2-methyltetrol ratio (MGA/MTL) and ambient temperature than that between MGA/MTLs and NOX concentration, suggesting that temperature has an important impact on the MGA/MTL ratio besides NOX concentration. The OC/EC-based method, WSOC-based method, tracer yield method, and positive matrix factorization (PMF)-based method were used to estimate the concentration and sources of secondary organic carbon (SOC), and the PMF-based method was believed to be able to give reasonable results. SOC was the main contributor of WSOC in PM2.5 in Bengbu, while biomass burning made an important contribution to WSOC in autumn and winter (~40%). SOC was mainly associated with SOA tracers in summer and mainly associated with secondary ions in spring and winter, suggesting different formation mechanisms in different seasons.

Highlights

Introduction iationsSecondary organic aerosols (SOAs) in atmospheric fine particles (PM2.5 ) are mainly formed by the homogeneous and heterogeneous reactions of volatile organic compounds (VOCs) emitted from biogenic [1] and anthropogenic [2] sources
The results showed that the linear fitting equations were SOCWSOC-based = 0.41 × SOCEC-based + 2.57 (R2 = 0.31, p < 0.01), SOCPMF-based = 0.40 × SOCEC-based + 1.85 (R2 = 0.38, p < 0.01), and SOCPMF-based = 0.57 × SOCWSOC-based + 1.05 (R2 = 0.36, p < 0.01)
The significant correlations between these results indicated the rationality of the estimation methods, but the large positive intercepts in the equations related to SOCEC-based suggested that the organic carbon (OC)/elemental carbon (EC)-based method may underestimate the secondary organic carbon (SOC) concentration when the concentration is low, because part of the primary organic carbon (POC) from the OC - EC × (OC/EC) method may still be secondary

Summary

Introduction

Secondary organic aerosols (SOAs) in atmospheric fine particles (PM2.5 ) are mainly formed by the homogeneous and heterogeneous reactions of volatile organic compounds (VOCs) emitted from biogenic [1] and anthropogenic [2] sources. Monoterpenes, and sesquiterpenes are major biological VOCs, and benzene, toluene, ethylbenzene, and xylene (BTEX) may be the most important anthropogenic VOCs for SOA formation [4,5,6,7,8,9,10]. Many studies on SOA formation from typical VOC precursors have been conducted, and a variety of SOA tracers have been found. 2-methyltetrols (2-methyl-threitol and 2-methyl-erythritol) are proved to be tracers of isoprene SOAs [7,8]; 3-hydroxyglutaric acid (HGA), 3-hydroxy-4,4-dimethylglutaric acid.

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