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

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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 °C for OC1, 280 °C for OC2, 480 °C for OC3, and 580 °C for OC4 in a helium (He) carrier gas and 580 °C for EC1, 740 °C for EC2, and 840 °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.

INTRODUCTION
The Interagency Monitoring of Protected Visual Environments (IMPROVE) network1 has acquired inhalable particulate matter (PM10) and fine particulate matter (PM2.5) mass,2 PM2.5 elemental,3,4 ionic,5 and carbon,6–9 measurements at U.S. National Parks and Wilderness Areas since 1987.10 The sampling locations have been expanded to approximately 170 sites since 2000. IMPROVE measurements have been used to determine single source impacts on visibility in mandatory Class I areas,11–13 understand atmospheric processes,14–17 verify long-range transport and chemical conversion models,18–20 identify and quantify multiple source contributions using receptor models, 21–25 and track long-term trends in aerosol concentrations and regional haze.26–32 Chemical extinction budgets,33,34 constructed from measured PM2.5 sulfate, nitrate, crustal, organic carbon (OC), and elemental carbon (EC) from 2000 through 2004 establish the baseline for tracking improvements toward natural conditions in 2065 required by the U.S. Regional Haze Rule.35

As a long-term trends network, IMPROVE has kept abreast of new findings in aerosol measurement technology and evaluated the consequences of new knowledge on its sampling, laboratory analysis, and data reporting methods. As hardware wears out and becomes obsolete, the effects of changes on the long-term database are evaluated, typically by redundant sampling and/or analysis with the old and new hardware.

Carbonaceous material that consists of OC, EC, and carbonate accounts for a substantial fraction of PM2.5 mass in most atmospheric environments. EC is more thermally resistant and light absorbing than OC and they are often separated by thermal and optical methods. However, this separation is operationally, rather than fundamentally, defined. Watson et al.36 showed that different carbon methods report different EC abundances for the same samples by up to an order of magnitude. The IMPROVE network has adopted the thermal/optical reflectance (IMPROVE_TOR) method6 since its inception. The IMPROVE carbon analysis method has been adopted for aerosol studies in other countries (e.g., refs 37–39) and will be applied to samples from the U.S. Chemical Speciation Network (CSN, including the Speciation Trends Network [STN]) using modified IMPROVE carbon samplers (i.e., URG 3000N carbon sampler; URG Corp.) after 2007.40

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