Abstract
Despite the significant role water-soluble organic carbon (WSOC) plays in climate and human health, sources and formation mechanisms of atmospheric WSOC are still unclear; especially in some heavily polluted areas. In this study, near real-time WSOC measurement was conducted in Beijing for the first time with a particle-into-liquid-sampler coupled to a total organic carbon analyzer during the springtime, together with collocated online measurements of other chemical components in fine particulate matter with a 1 h time resolution, including elemental carbon (EC), organic carbon (OC), multiple metals, and water-soluble ions. Good correlations of WSOC with primary OC, as well as carbon monoxide, indicated that major sources of WSOC were primary instead of secondary during the study period. The positive matrix factorization model-based source apportionment results quantified that 68 ± 19% of WSOC could be attributed to primary sources, with predominant contributions by biomass burning during the study period. This finding was further confirmed by the estimate with the modified EC-tracer method, suggesting significant contribution of primary sources to WSOC. However, the relative contribution of secondary source to WSOC increased during haze episodes. The WSOC/OC ratio exhibited similar diurnal distributions with O3 and correlated well with secondary WSOC, suggesting that the WSOC/OC ratio might act as an indicator of secondary formation when WSOC was dominated by primary sources. This study provided evidence that primary sources could be major sources of WSOC in some polluted megacities, such as Beijing. From this study, it can be seen that WSOC cannot be simply used as a surrogate of secondary organic aerosol, and its major sources could vary by season and location.
Highlights
Organic carbon (OC) is one of the major constituents of atmospheric fine particulate matter (PM2.5 ), including water-soluble organic carbon (WSOC) and water-insoluble organic carbon (WIOC) [1,2,3].WSOC, which accounts for around 20–70% of total OC mass, has attracted growing attention due to its significant roles in aerosol hygroscopic properties, direct and indirect aerosol radiative forcing, as well as adverse effects on human health [4,5,6]
This study suggested that the WSOC/OC ratio might act as a better indicator of secondary formation in cases when primary sources are the dominant source of WSOC
The contributions by primary and secondary WSOC were estimated for the first time based on high time-resolved monitoring data in Beijing using the modified elemental carbon (EC)-tracer method and the Positive Matrix Factorization (PMF) model
Summary
WSOC, which accounts for around 20–70% of total OC mass, has attracted growing attention due to its significant roles in aerosol hygroscopic properties, direct and indirect aerosol radiative forcing, as well as adverse effects on human health [4,5,6]. Atmosphere 2020, 11, 395 properties of ambient particles and the formation of cloud condensation nuclei (CCN), which contributes to the indirect radiative effects of aerosols [7,8,9,10]. In view of its light absorbing properties, WSOC has been used as a proxy for water-soluble brown carbon in many studies [11,12], which could substantially influence the radiation balance of the atmosphere [13]. It is essential to study sources and formation mechanisms of WSOC in order to better understand the roles played by WSOC in the abovementioned areas
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