Abstract
Abstract. Water-soluble organic carbon (WSOC) accounts for a large proportion of aerosols and plays a critical role in various atmospheric chemical processes. In order to investigate the primary sources and secondary production of WSOC in downtown Beijing, day and night fine particulate matter (PM2.5) samples in January (winter), April (spring), July (summer) and October (autumn) 2017 were collected and analyzed for WSOC and organic tracers in this study. WSOC was dominated by its moderately hydrophilic fraction and showed the highest concentration in January and comparable levels in April, July and October 2017. Some typical organic tracers were chosen to evaluate the emission strength and secondary formation of WSOC. Seasonal variation of the organic tracers suggested significantly enhanced formation of anthropogenic secondary organic aerosols (SOAs) during the sampling period in winter and obviously elevated biogenic SOA formation during the sampling period in summer. These organic tracers were applied into a positive matrix factorization (PMF) model to calculate the source contributions of WSOC as well as its moderately and strongly hydrophilic portions. The secondary sources contributed more than 50 % to WSOC, with higher contributions during the sampling periods in summer (75.1 %) and winter (67.4 %), and the largest contributor was aromatic SOC. In addition, source apportionment results under different pollution levels suggested that controlling biomass burning and aromatic precursors would be effective to reduce WSOC during the haze episodes in cold seasons. The impact factors for the formation of different SOA tracers and total secondary organic carbon (SOC) as well as moderately and strongly hydrophilic SOC were also investigated. The acid-catalyzed heterogeneous or aqueous-phase oxidation appeared to dominate in the SOC formation during the sampling period in winter, while the photochemical oxidation played a more critical role during the sampling period in summer. Moreover, photooxidation played a more critical role in the formation of moderately hydrophilic SOC, while the heterogeneous or aqueous-phase reactions had more vital effects on the formation of strongly hydrophilic SOC.
Highlights
Organic compounds account for a considerable fraction (20 %–60 %) of atmospheric aerosols (Huang et al, 2014; Zhang et al, 2020), and water-soluble organic carbon (WSOC) generally composes 30 %–70 % of organic carbon (OC) (Zhang et al, 2018; Yang et al, 2019; Chen et al, 2020)
The fact that Water-soluble organic carbon (WSOC) exhibited mild temporal variation with no sudden increase in October implied that short-term outdoor biomass burning after the harvest season in the surrounding areas of Beijing was well controlled over the sampling period in autumn
Based on the WSOC and related secondary organic aerosols (SOAs) tracer analysis for the PM2.5 samples collected in downtown Beijing in four seasons of 2017, the moderately hydrophilic fraction of WSOC dominated in WSOC throughout the sampling period, and it showed the highest proportion to WSOC during the sampling period in summer
Summary
Organic compounds account for a considerable fraction (20 %–60 %) of atmospheric aerosols (Huang et al, 2014; Zhang et al, 2020), and water-soluble organic carbon (WSOC) generally composes 30 %–70 % of organic carbon (OC) (Zhang et al, 2018; Yang et al, 2019; Chen et al, 2020). To raise effective control measures targeting the specific SOA precursors, some recent studies introduced the SOA tracers into the PMF model to investigate the secondary sources of organic aerosols (Kang et al, 2018a, b; Geng et al, 2020). The formation mechanisms of secondary organic carbon (SOC) in the moderately and strongly hydrophilic fractions of WSOC may differ from each other due to their different water solubility in the cloud droplets or the aqueous phase of aerosols. The characteristics of WSOC and the selected organic tracers were investigated, and the contributions of primary and secondary sources to WSOC and its moderately and strongly hydrophilic fractions were quantified. The key influencing factors for the formation of different SOA tracers, the moderately hydrophilic SOC and strongly hydrophilic SOC were explored and compared, so as to gain insights into the possible formation mechanism of different types of SOA
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