Source apportionment of carbonaceous aerosols in Beijing with radiocarbon and organic tracers: insight into the differences between urban and rural sites

Carbonaceous aerosol is a dominant component of fine particles in Beijing. However, it is challenging to apportion its sources. Here, we applied a newly developed method which combined radiocarbon (C-14) with organic tracers to apportion the sources of fine carbonaceous particles at an urban (IAP) and a rural (PG) site of Beijing. PM2.5 filter samples (24 h) were collected at both sites from 10 November to 11 December 2016 and from 22 May to 24 June 2017. C-14 was determined in 25 aerosol samples (13 at IAP and 12 at PG) representing low pollution to haze conditions. Biomass burning tracers (levoglucosan, mannosan, and galactosan) in the samples were also determined using gas chromatography-mass spectrometry (GC-MS). Higher contributions of fossil-derived OC (OCf) were found at the urban site. The OCf / OC ratio decreased in the summer samples (IAP: 67.8 +/- 4.0% in winter and 54.2 +/- 11.7% in summer; PG: 59.3 +/- 5.7% in winter and 50.0 +/- 9.0% in summer) due to less consumption of coal in the warm season. A novel extended Gelencser (EG) method incorporating the C-14 and organic tracer data was developed to estimate the fossil and non-fossil sources of primary and secondary OC (POC and SOC). It showed that fossil-derived POC was the largest contributor to OC (35.8 +/- 10.5% and 34.1 +/- 8.7% in wintertime for IAP and PG, 28.9 +/- 7.4% and 29.1 +/- 9.4% in summer), regardless of season. SOC contributed 50.0 +/- 12.3% and 47.2 +/- 15.5% at IAP and 42.0 +/- 11.7% and 43.0 +/- 13.4% at PG in the winter and summer sampling periods, respectively, within which the fossil-derived SOC was predominant and contributed more in winter. The non-fossil fractions of SOC increased in summer due to a larger biogenic component. Concentrations of biomass burning OC (OCbb) are resolved by the extended Gelencser method, with average contributions (to total OC) of 10.6 +/- 1.7% and 10.4 +/- 1.5% in winter at IAP and PG and 6.5 +/- 5.2% and 17.9 +/- 3.5% in summer, respectively. Correlations of water-insoluble OC (WINSOC) and water-soluble OC (WSOC) with POC and SOC showed that although WINSOC was the major contributor to POC, a non-negligible fraction of WINSOC was found in SOC for both fossil and non-fossil sources, especially during winter. In summer, a greater proportion of WSOC from non-fossil sources was found in SOC. Comparisons of the source apportionment results with those obtained from a chemical mass balance model were generally good, except for the cooking aerosol.

Automated summary

Carbonaceous aerosol is a dominant component of fine particles in Beijing. A newly developed method combining C-14 and organic tracers was applied to trace its sources, with findings showing higher fossil-derived OC in urban areas and a decrease in fossil carbon ratio in summer due to less coal consumption. Non-fossil fractions of secondary organic carbon increased in summer, with a larger biogenic component contributing. Comparisons with a chemical mass balance model yielded generally good results, except for cooking aerosol.