The IMPROVE_A Temperature Protocol for Thermal/Optical Carbon Analysis: Maintaining Consistency with a Long-Term Database

TLDR

Since 1987, the IMPROVE network has reported thermally derived organic and elemental carbon fractions, which aid source apportionment and assess quartz‑fiber filter adsorption of organic vapors. The IMPROVE_A protocol establishes specific temperature plateaus for OC1–OC4 and EC1–EC3 using helium and helium/oxygen carrier gases, and the updated hardware more accurately reflects sample temperatures than previous equipment. Calibration confirms the new temperatures better represent sample temperatures, leaving only thermal fractions affected while preserving long-term consistency of OC and EC measurements, and shows that O₂ levels above 100 ppmv alter thermal fractions but minimally impact the TOR OC/EC split.

Abstract

Thermally derived carbon fractions including organic carbon (OC) and elemental carbon (EC) have been reported for the U.S. Interagency Monitoring of PROtected Visual Environments (IMPROVE) network since 1987 and have been found useful in source apportionment studies and to evaluate quartz-fiber filter adsorption of organic vapors. The IMPROVE_A temperature protocol defines temperature plateaus for thermally derived carbon fractions of 140 degrees C for OC1, 280 degrees C for OC2, 480 degrees C for OC3, and 580 degrees C for OC4 in a helium (He) carrier gas and 580 degrees C for EC1, 740 degrees C for EC2, and 840 degrees C for EC3 in a 98% He/2% oxygen (O2) carrier gas. These temperatures differ from those used previously because new hardware used for the IMPROVE thermal/optical reflectance (IMPROVE_TOR) protocol better represents the sample temperature than did the old hardware. A newly developed temperature calibration method demonstrates that these temperatures better represent sample temperatures in the older units used to quantify IMPROVE carbon fractions from 1987 through 2004. Only the thermal fractions are affected by changes in temperature. The OC and EC by TOR are insensitive to the change in temperature protocol, and therefore the long-term consistency of the IMPROVE database is conserved. A method to detect small quantities of O2 in the pure He carrier gas shows that O2 levels above 100 ppmv also affect the comparability of thermal carbon fractions but have little effect on the IMPROVE_TOR split between OC and EC.