Quantifying Residual Elemental Carbon by Thermal-Optical Analysis Using an Extended IMPROVE_A Protocol with Higher Maximum Temperature

Thermal-optical analysis (TOA) has long been used to quantify organic carbon (OC) and elemental carbon (EC) on quartz-fiber filter samples collected in national ambient air monitoring networks. In the routine analysis of samples from the Chemical Speciation Network (CSN), we observed a considerable fraction of filter punches that remain gray or black in color after TOA was completed, suggesting the presence of EC that was not fully evolved at the highest temperature specified by the IMPROVE_A protocol (840 °C). In this work, we explored the operational conditions necessary to evolve and quantify such residual EC. First, four heavily loaded CSN samples were analyzed to evaluate modifications to the IMPROVE_A protocol. We found that adding a higher temperature step at 930 °C more effectively evolved the residual EC than did lengthening the duration of the 840 °C step. Compared with the standard IMPROVE_A results, the modified protocol evolved additional EC of 1.08 to 4.45 µg cm-2 in mass, or 0.12 to 0.50 µg m-3 in concentration. This excess EC accounts for 27.1% to 45.3% of the total EC and 7.6% to 25.1% of the total carbon by the standard IMPROVE_A protocol. We then analyzed over 2600 samples from CSN using the extended IMPROVE_A protocol with higher maximum temperature (930 °C). A total of 168 samples (6.4% of the total samples analyzed) contained measurable EC at the added higher temperature step. Among these samples, the average fraction of the evolvable residual EC mass in total EC is 5.7%, and up to 28% for samples with high total EC mass loading (i.e., 95th percentile and above).

Our results suggest that EC measured from urban CSN sites using the standard IMPROVE_A protocol should be considered a lower limit, and that a modified maximum heating temperature of 930 °C or above can be used to better quantify EC from CSN sites impacted by fresh urban emissions.