The chemical, physical, and morphological complexity of atmospheric aerosol black carbon (BC) presents major problems in measurement accuracy. Methods based on thermal-optical analysis (TOA) are widely used for ambient air samples because no prior knowledge of the aerosol's absorption coefficient is required. Nevertheless, different TOA thermal desorption protocols result in wide BC to total carbon (BC/TC) variation. We created three surface models of the following response variables: BC/TC, maximum laser attenuation in the He phase (Lmax), and maximum laser attenuation at the end of the He phase (LHe4). A two-level central-composite factorial design comprised of four factors considered the temperatures and duration of all desorption steps in TOA's inert (He) phase and the initial step in TOA's oxidizing (O2-He) phase. The Lmax surface was used to optimize the production of pyrolized organic carbon (OC char) in the He phase. Unpyrolized OC is measured as native BC, causing a positive bias. The LHe4 surface was used to minimize the loss of char in the He phase, which served as a surrogate indicator for the loss of native BC in the He phase, a source of negative bias. The intersection between the Lmax and LHe4 surfaces revealed conditions that minimize potential biases, leading to an optimized thermal desorption protocol. Our data suggest the following operating conditions when TOA is operated in the fixed-step-durations, laser-transmittance mode (i.e., TOT): step 1 in He, 190 C for 60 s; step 2 in He, 365 C for 60 s; step 3 in He, 610 C for 60 s; step 4 in He, 835 C for 72 s. For steps 1-4 in He-O2, our data suggest using 550 C for 180 s, 700 C for 60 s, 850 C for 60 s, and 900 C for 90 s.
Citation: Aerosol Science and Technology
Issue: No. 9
Pub Type: Journals
atmospheric aerosol, black carbon, central-composite factorial design, elemental carbon, response surface methods, thermal-optical analysis