Accuracy and precision of 14C-based source apportionment of organic and elemental carbon in aerosols using the Swiss_4S protocol

摘要

Aerosol source apportionment remains a critical challenge for understandingthe transport and aging of aerosols, as well as for developing successfulair pollution mitigation strategies. The contributions of fossil andnon-fossil sources to organic carbon (OC) and elemental carbon (EC) incarbonaceous aerosols can be quantified by measuring the radiocarbon(C) content of each carbon fraction. However, the use of C instudying OC and EC has been limited by technical challenges related to thephysical separation of the two fractions and small sample sizes. There is nocommon procedure for OC/EC C analysis, and uncertainty studies havelargely focused on the precision of yields. Here, we quantified theuncertainty in C measurement of aerosols associated with theisolation and analysis of each carbon fraction with the Swiss_4S thermal–optical analysis (TOA) protocol. We used an OC/EC analyzer(Sunset Laboratory Inc., OR, USA) coupled to a vacuum line to separate thetwo components. Each fraction was thermally desorbed and converted to carbondioxide (CO) in pure oxygen (O). On average, 91 % of theevolving CO was then cryogenically trapped on the vacuum line, reducedto filamentous graphite, and measured for its C content viaaccelerator mass spectrometry (AMS). To test the accuracy of our setup, wequantified the total amount of extraneous carbon introduced during the TOAsample processing and graphitization as the sum of modern and fossil(C-depleted) carbon introduced during the analysis of fossilreference materials (adipic acid for OC and coal for EC) and contemporarystandards (oxalic acid for OC and rice char for EC) as a function of samplesize. We further tested our methodology by analyzing five ambient airborneparticulate matter (PM) samples with a range of OC and ECconcentrations and C contents in an interlaboratory comparison. Thetotal modern and fossil carbon blanks of our setup were 0.8 ± 0.4 and 0.67 ± 0.34 μg C, respectively, based onmultiple measurements of ultra-small samples. The extraction procedure(Swiss_4S protocol and cryo-trapping only) contributed 0.37 ± 0.18 μg of modern carbon and 0.13 ± 0.07 μg offossil carbon to the total blank of our system, with consistent estimatesobtained for the two laboratories. There was no difference in the backgroundcorrection between the OC and EC fractions. Our setup allowed us toefficiently isolate and trap each carbon fraction with theSwiss_4S protocol and to perform C analysis ofultra-small OC and EC samples with high accuracy and low C blanks.