Sources and formation of carbonaceous aerosols in Xi'an, China: primary emissions and secondary formation constrained by radiocarbon

Abstract

Abstract. To investigate the sources and formation mechanisms of carbonaceous aerosols, a major contributor to severe particulate air pollution, radiocarbon (14C) measurements were conducted on aerosols sampled from November 2015 to November 2016 in Xi'an, China. Based on the 14C content in elemental carbon (EC), organic carbon (OC) and water-insoluble OC (WIOC), contributions of major sources to carbonaceous aerosols are estimated over a whole seasonal cycle: primary and secondary fossil sources, primary biomass burning, and other non-fossil carbon formed mainly from secondary processes. Primary fossil sources of EC were further sub-divided into coal and liquid fossil fuel combustion by complementing 14C data with stable carbon isotopic signatures. The dominant EC source was liquid fossil fuel combustion (i.e., vehicle emissions), accounting for 64 % (median; 45 %–74 %, interquartile range) of EC in autumn, 60 % (41 %–72 %) in summer, 53 % (33 %–69 %) in spring and 46 % (29 %–59 %) in winter. An increased contribution from biomass burning to EC was observed in winter (∼28 %) compared to other seasons (warm period; ∼15 %). In winter, coal combustion (∼25 %) and biomass burning equally contributed to EC, whereas in the warm period, coal combustion accounted for a larger fraction of EC than biomass burning. The relative contribution of fossil sources to OC was consistently lower than that to EC, with an annual average of 47±4 %. Non-fossil OC of secondary origin was an important contributor to total OC (35±4 %) and accounted for more than half of non-fossil OC (67±6 %) throughout the year. Secondary fossil OC (SOCfossil) concentrations were higher than primary fossil OC (POCfossil) concentrations in winter but lower than POCfossil in the warm period. Fossil WIOC and water-soluble OC (WSOC) have been widely used as proxies for POCfossil and SOCfossil, respectively. This assumption was evaluated by (1) comparing their mass concentrations with POCfossil and SOCfossil and (2) comparing ratios of fossil WIOC to fossil EC to typical primary OC-to-EC ratios from fossil sources including both coal combustion and vehicle emissions. The results suggest that fossil WIOC and fossil WSOC are probably a better approximation for primary and secondary fossil OC, respectively, than POCfossil and SOCfossil estimated using the EC tracer method.

Measurement report: Dual-carbon isotopic characterization of carbonaceous aerosol in Beijing and Xi'an: distinctions in primary versus secondary sources

“…In this study, the ffossil(EC) values in Xi'an during winter 2016/2017 are comparable with those previously measured during winter 2015/2016 and winter 2008/2009 (Ni et al, 2018(Ni et al, , 2019b, as illustrated in Fig. 3b, pointing to relative constant contribution of fossil fuel combustion vs. biomass burning to EC in Xi'an over the past decade.…”

“…Compared with earlier observations in Xi'an ( Fig. 3b), we found that the δ 13 CEC values in January 2017 from this study are comparable with wintertime δ 13 CEC in 2015/2016 (Ni et al, 2019b), but much more depleted than wintertime δ 13 CEC in 2008(Ni et al, 2018 and January 2003 (Cao et al, 2011). This suggests that fossil sources of EC in Xi'an have changed in the past decade, with decreasing relative contribution from coal combustion.…”

“…To eliminate this effect, F 14 C is divided by F 14 C of non-fossil sources (F 14 Cnf). F 14 Cnf is estimated as 1.09 ± 0.05 for OC and 1.10 ± 0.05 for EC (see details in Ni et al, 2019b), using a tree growth https://doi.org/10.5194/acp-2020-455 Preprint. Discussion started: 22 June 2020 c Author(s) 2020.…”

“…Duplicate analysis of δ 13 C of EC showed an analytical precision better than ± 0.3‰. This method was detailed in Ni et al, (2019b), where impacts of potential charred OC on the isolated EC were evaluated using an isotope-mass-balance based sensitivity analysis. We concluded that the expected differences in δ 13 CEC is smaller than 1‰ under the assumption that the fraction of charred OC in the isolated EC is at most 20%.…”

Significant Contribution of Primary Sources to Water-Soluble Organic Carbon During Spring in Beijing, China

“…In some previous field studies, WSOC was found to be primarily from secondary sources, and it has been widely used as a proxy of SOA, especially in regions in absence of biomass burning influences [16,20]. However, some recent studies have highlighted the non-negligible influence of fossil sources on atmospheric WSOC, especially in East Asia regions based on dual-carbon isotope analysis [21][22][23][24]. Source testing studies also provide supporting evidence that, in addition to biomass burning [25][26][27], other primary non-biomass burning emission sources such as vehicles [28,29], coal combustion [22], and industry (e.g., steel and smelting industry) [30] could also directly emit some amount of WSOC (e.g., formic and acetic acids).…”