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. Sources of organic carbon (OC) and elemental carbon (EC) in Xi'an, China, areinvestigated based on 1-year radiocarbon and stable carbon isotopemeasurements. The radiocarbon results demonstrate that EC is dominated byfossil sources throughout the year, with a mean contribution of 83±5  %( 7±2 µg m−3 ). The remaining 17±5  % ( 1.5±1 µg m−3 )is attributed to biomass burning, with a highercontribution in the winter ( ∼24  %) compared to the summer( ∼14  %). Stable carbon isotopes of EC ( δ13CEC ) are enriched in winter ( - 23.2 ± 0.4  ‰) anddepleted in summer ( - 25.9 ± 0.5  ‰), indicating the influenceof coal combustion in winter and liquid fossil fuel combustion in summer. Bycombining radiocarbon and stable carbon signatures, relative contributionsfrom coal combustion and liquid fossil fuel combustion are estimated to be 45 %(median; 29 %–58 %, interquartile range) and 31 % (18 %–46 %) inwinter, respectively, whereas in other seasons more than one half of EC isfrom liquid fossil combustion. In contrast with EC, the contribution ofnon-fossil sources to OC is much larger, with an annual average of 54±8  % ( 12±10 µg m−3 ). Clear seasonal variations are seenin OC concentrations both from fossil and non-fossil sources, with maxima inwinter and minima in summer because of unfavorable meteorologicalconditions coupled with enhanced fossil and non-fossil activities in winter,mainly biomass burning and domestic coal burning. δ13COC exhibited similar values to δ13CEC , and showed strongcorrelations ( r2=0.90 ) in summer and autumn, indicating similarsource mixtures with EC. In spring, δ13COC is depleted(1.1 ‰–2.4 ‰) compared to δ13CEC , indicating theimportance of secondary formation of OC (e.g., from volatile organic compoundprecursors) in addition to primary sources. Modeled mass concentrations andsource contributions of primary OC are compared to the measured mass andsource contributions. There is strong evidence that both secondary formationand photochemical loss processes influence the final OC concentrations.

atmospheric chemistry and physics

Source apportionment of carbonaceous aerosols in Xi'an, China: insights from a full year of measurements of radiocarbon and the stable isotope &amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C