Citation:

CONNY, J. M. OPTIMIZATION OF THERMAL OPTICAL ANALYSIS FOR THE MEASUREMENT OF BLACK CARBON IN REGIONAL PM_2.5: A CHEMOMETRIC APPROACH. National Institute of Standards and Technology (NIST), Gaithersburg, MD, 2006.

Impact/Purpose:

The goal of this task is to develop methods and models to reduce the uncertainty in quantifying local and regional air pollutant source impacts on ambient samples collected in speciated PM, air toxic, and semi-continuous measurement networks. A combination of high resolution sampling, organic and inorganic analytical methods, and models will be developed and evaluated to reduce the uncertainty in source apportionment:

(1) semi-continuous inorganic species sampling

(2) inorganic analysis

(3) organic analysis for medium flow samples

(4) multivariate receptor models for ambient samples

(5) regional and local models

In addition, this task contributes to two additional tasks that have research focused on reducing the uncertainty in source apportionment: Identify Sources of Human Exposure (21176), and NAAQS implementation (21179).

Description:

Thermal-optical analysis (TOA) is the principal method of the U.S. EPA's National Air Monitoring System for determining refractory carbon from combustion, or elemental carbon (EC), in particulate matter <2.5 µm (PM_2.5). To isolate and quantify EC from organic carbon (OC) in PM, TOA combines a thermal protocol for removing carbonaceous material on a particle-laden quartz fiber filter with a system to optically monitor the filter during heating. The optical nature of TOA, in particular the thermal-optical transmission method (TOT), necessarily implies that what is measured is the mass of the light-absorbing component of EC, i.e., black carbon (BC). In 2003 the National Institute of Standards and Technology and the U.S. Environmental Protection Agency entered an agreement to optimize the TOA thermal protocol for PM_2.5 based on an understanding of TOA's optical behavior, and thus, for measuring BC in PM_2.5. While historically TOA has been used in studies concerned with the effect of PM on health and visibility, TOT is potentially a powerful method for measuring light-absorbing aerosol mass from combustion that is responsible for positive aerosol radiative forcing in global climate change [1]. Unfortunately, TOA is problematic in that different thermal protocols produce different BC results on the same sample material, and no protocol has been shown to produce BC measurements with proven accuracy and comparability with other methods. The principal focus of this research is the TOT method as implemented on Sunset Laboratory's Dual-Optics Carbon Analyzer. Since this instrument measures BC by the thermal-optical reflection method (TOR) along with TOT, an investigation of the reflection method was possible. A summary of the research on the TOA optimization is presented here.

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