Issues with the Organic and Elemental Carbon Fractions in Recent U.S. Chemical Speciation Network Data

Abstract

The carbon fractions of organic (OC1, OC2, OC3, OC4, OP) and elemental (EC1, EC2, and EC3) carbon obtained from IMPROVE (Interagency Monitoring of PROtected Visual Environments) thermal/optical carbon analysis of quartz-fiber filters have been implemented to help distinguish spark-ignition engine exhaust particulate matter (PM) contributions from those of heavy-duty diesel exhaust. The Positive Matrix Factorization (PMF) source factor containing the optical pyrolysis correction (OP) typically includes significant contributions to sulfate and/or nitrate. This factor is associated with aged secondary organic and inorganic species. The U.S. EPA changed the sampling and analytical protocols for its Chemical Speciation Network (CSN) in the 2007–2009 period to provide these well separated individual peaks in the thermogram for aerosol characterization, source apportionment, visibility, and health studies. However, after October 1, 2018, there was a change in the laboratory and instruments used to implement the IMPROVE_A protocol for CSN samples. Although this switch resulted in similar total organic and elemental carbon (OC and EC) values, the protocol changes resulted in large shifts in the OC4, OP, and EC2 fractions, resulting in a swap in the resolved contributions from gasoline and diesel engine exhaust apportionments of PM2.5 mass concentration. It also affected the OP factor contributions. As a result, using the most recent CSN carbon fractions for source apportionments to separate gasoline from diesel vehicles will not result in correct results. The only current recourse is to reanalyze remnants of those samples using the original and temporally consistent IMPROVE_A protocol.