Sources and formation of carbonaceous aerosols in Xi'an, China: primary emissions and secondary formation constrained by radiocarbon
Authors: Ni Haiyan , Huang Ru-Jin , Dusek Ulrike 
Published: March 2020

&lt;p&gt;To investigate the sources and formation mechanisms of carbonaceous aerosols, a major contributor to severe particulate air pollution, radiocarbon&amp;#160;(&lt;span&gt;&lt;sup&gt;14&lt;/sup&gt;C&lt;/span&gt;) measurements were conducted on aerosols sampled from November&amp;#160;2015 to November&amp;#160;2016 in Xi'an, China. Based on the&amp;#160;&lt;span&gt;&lt;sup&gt;14&lt;/sup&gt;C&lt;/span&gt;&amp;#160;content in elemental carbon&amp;#160;(EC), organic carbon&amp;#160;(OC) and water-insoluble OC&amp;#160;(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&amp;#160;EC were further sub-divided into coal and liquid fossil fuel combustion by complementing&amp;#160;&lt;span&gt;&lt;sup&gt;14&lt;/sup&gt;C&lt;/span&gt;&amp;#160;data with stable carbon isotopic signatures.&lt;/p&gt;&lt;p&gt;The dominant EC&amp;#160;source was liquid fossil fuel combustion (i.e., vehicle emissions), accounting for 64&amp;#8201;% (median; 45&amp;#8201;%&amp;#8211;74&amp;#8201;%, interquartile range) of&amp;#160;EC in autumn, 60&amp;#8201;% (41&amp;#8201;%&amp;#8211;72&amp;#8201;%) in summer, 53&amp;#8201;% (33&amp;#8201;%&amp;#8211;69&amp;#8201;%) in spring and 46&amp;#8201;% (29&amp;#8201;%&amp;#8211;59&amp;#8201;%) in winter. An increased contribution from biomass burning to&amp;#160;EC was observed in winter (&lt;span&gt;&amp;#8764;28&lt;/span&gt;&amp;#8201;%) compared to other seasons (warm period;&amp;#160;&lt;span&gt;&amp;#8764;15&lt;/span&gt;&amp;#8201;%). In winter, coal combustion (&lt;span&gt;&amp;#8764;25&lt;/span&gt;&amp;#8201;%) and biomass burning equally contributed to&amp;#160;EC, whereas in the warm period, coal combustion accounted for a larger fraction of&amp;#160;EC than biomass burning. The relative contribution of fossil sources to&amp;#160;OC was consistently lower than that to&amp;#160;EC, with an annual average of&amp;#160;&lt;span&gt;47&amp;#177;4&lt;/span&gt;&amp;#8201;%. Non-fossil OC&amp;#160;of secondary origin was an important contributor to total&amp;#160;OC (&lt;span&gt;35&amp;#177;4&lt;/span&gt;&amp;#8201;%) and accounted for more than half of non-fossil&amp;#160;OC (&lt;span&gt;67&amp;#177;6&lt;/span&gt;&amp;#8201;%) throughout the year. Secondary fossil&amp;#160;OC&amp;#160;(SOC&lt;span&gt;&lt;sub&gt;fossil&lt;/sub&gt;&lt;/span&gt;) concentrations were higher than primary fossil&amp;#160;OC&amp;#160;(POC&lt;span&gt;&lt;sub&gt;fossil&lt;/sub&gt;&lt;/span&gt;) concentrations in winter but lower than POC&lt;span&gt;&lt;sub&gt;fossil&lt;/sub&gt;&lt;/span&gt;&amp;#160;in the warm period.&lt;/p&gt;&lt;p&gt;Fossil WIOC and water-soluble&amp;#160;OC&amp;#160;(WSOC) have been widely used as proxies for POC&lt;span&gt;&lt;sub&gt;fossil&lt;/sub&gt;&lt;/span&gt;&amp;#160;and SOC&lt;span&gt;&lt;sub&gt;fossil&lt;/sub&gt;&lt;/span&gt;, respectively. This assumption was evaluated by (1)&amp;#160;comparing their mass concentrations with POC&lt;span&gt;&lt;sub&gt;fossil&lt;/sub&gt;&lt;/span&gt;&amp;#160;and SOC&lt;span&gt;&lt;sub&gt;fossil&lt;/sub&gt;&lt;/span&gt;&amp;#160;and (2)&amp;#160;comparing ratios of fossil WIOC to fossil&amp;#160;EC to typical primary&amp;#160;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&amp;#160;OC, respectively, than POC&lt;span&gt;&lt;sub&gt;fossil&lt;/sub&gt;&lt;/span&gt;&amp;#160;and SOC&lt;span&gt;&lt;sub&gt;fossil&lt;/sub&gt;&lt;/span&gt;&amp;#160;estimated using the EC&amp;#160;tracer method.&lt;/p&gt;