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