ALBERT

Description: Accuracy and precision of 14 C-based source apportionment of organic and elemental carbon in aerosols using the Swiss_4S protocol Atmospheric Measurement Techniques Discussions, 8, 3933-3965, 2015 Author(s): G. O. Mouteva, S. M. Fahrni, G. M. Santos, J. T. Randerson, Y. L. Zhang, S. Szidat, and C. I. Czimczik Aerosol source apportionment remains a critical challenge for understanding the transport and aging of aerosols, as well as for developing successful air pollution mitigation strategies. The contributions of fossil and non-fossil sources to organic carbon (OC) and elemental carbon (EC) in carbonaceous aerosols can be quantified by measuring the radiocarbon ( 14 C) content of each carbon fraction. However, the use of 14 C in studying OC and EC has been limited by technical challenges related to the physical separation of the two fractions and small sample sizes. There is no common procedure for OC/EC 14 C analysis, and uncertainty studies have largely focused on the precision of yields. Here, we quantified the uncertainty in 14 C measurement of aerosols associated with the isolation 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 vacuum line to separate the two components. Each fraction was thermally desorbed and converted to carbon dioxide (CO 2 ) in pure oxygen (O 2 ). On average 91% of the evolving CO 2 was then cryogenically trapped on the vacuum line, reduced to filamentous graphite, and measured for its 14 C content via accelerator mass spectrometry (AMS). To test the accuracy of our set-up, we quantified the total amount of extraneous carbon introduced during the TOA sample processing and graphitization as the sum of modern and fossil ( 14 C-depleted) carbon introduced during the analysis of fossil reference materials (adipic acid for OC and coal for EC) and contemporary standards (oxalic acid for OC and rice char for EC) as a function of sample size. We further tested our methodology by analyzing five ambient airborne particulate matter (PM 2.5 ) samples with a range of OC and EC concentrations and 14 C contents in an interlaboratory comparison. The total modern and fossil carbon blanks of our set-up were 0.8 ± 0.4 and 0.67 ± 0.34 μg C, respectively, based on multiple measurements of ultra-small samples. The Swiss_4S protocol and the cryo-trapping contributed 0.37 ± 0.18 μg of modern carbon and 0.13 ± 0.07 μg of fossil carbon to the estimated blanks, with consistent estimates obtained for the two laboratories. There was no difference in the background correction between the OC and EC fractions. Our set-up allowed us to efficiently isolate and trap each carbon fraction with the Swiss_4S protocol and to perform 14 C analysis of ultra-small OC and EC samples with high accuracy and low 14 C blanks.