Issues with the OC and EC Fraction Data in Recent CSN Data

PHILIP K. HOPKE, Yunle Chen, David Q. Rich, Judith Chow, University of Rochester

     Abstract Number: 11
     Working Group: Source Apportionment

Abstract
Since the first work in 2004, there has been use of the carbon fractions (OC1, OC2, OC3, OC4, OP, EC1, EC2, and EC3 obtained in thermal optical analysis of quartz filters using the IMPROVE protocol) to help distinguish spark-ignition vehicular emissions from those of heavy-duty diesel vehicles. It also resulting in the identification of an OP factor that includes significant contributions to sulfate and has been attributed to aged air containing SOA and secondary inorganic species. In part, EPA changed its sampling and analytical protocols in the 2007-2009 period to provide well separated individual peaks in the thermogram for such source apportionment work. However, after October 1, 2018, there was a change in the laboratory and instruments used to implement the IMPROVE_A protocol. Although this switch resulted in similar total OC and EC values, the change to an instrument that did not wait until the carbon signal returns to the baseline before starting the next temperature increment resulted in drastic shifts in the OC4 and EC2 results and caused a swap in resolved concentrations of gasoline and diesel vehicles in the resulting source apportioned PM2.5 mass values. It also affected the OP factor contributions. The only current recourse is to reanalyze those samples with an instrument designed to fully implement the IMPROVE_A protocol.