On the quantification of atmospheric carbonate carbon by thermal/optical analysis protocols

Abstract. The objective of this work was to study the possibility of identifying and quantifying atmospheric carbonate carbon (CC) by thermal-optical analysis. Three different temperature protocols, two modified NIOSH-like protocols (RT-QUARTZ-840 and RT-QUARTZ-700), and the EUSAAR_2 protocol were tested on filter samples containing known amounts of CC with the semi-continuous Sunset analyzer. Carbonate was quantified by the manual integration of the sharp peak appeared at the maximum temperature step of the inert mode. High recoveries of CC were achieved by all the thermal protocols. Using the EUSAAR_2 thermal protocol, more than 95% of CC evolved as OC during the maximum temperature step in inert atmosphere for CC amounts up to 56 μg. Using the RT-QUARTZ-840 protocol specifically developed for on-line analyses, CC completely evolves as OC during the maximum temperature step in the inert node, regardless of the CC concentration. However, the quantification of CC by the RT-QUARTZ-840 protocol suitable for the semi-continuous analyzer implies a high level of uncertainty (manual integration, residual contribution of organic carbon). Therefore, it is advisable to determine CC with an independent method (e.g. by acidic decomposition of CO₃^2- and subsequent detection of CO₂) when other sample aliquots are available. The comparison of the peak integration method with the direct determination of the CC sample content by acidic CO₂ release showed that the peak integration method provides always higher CC concentrations of about 33%. Nevertheless, the determination of CC with the RT-QUARTZ-840 protocol may be considered in cases where on line monitoring instruments are used and for areas where CC concentrations are expected to be significant e.g. Southern European countries. This case study suggests that users of the semi continuous Sunset analyzer can manually integrate the sharp peak (if present) at the maximum temperature step of the He mode (between 128–130 and 160–165 s when using the RT-QUARTZ-840 protocol) and calculate the CC concentration though with a rather high error.