HR: 1340h
AN: A33B-1199    [Abstracts]
TI: Evaluating and Improving Measurements of Black Carbon Aerosol
AU: * Kirchstetter, T W
EM: twkirchstetter@lbl.gov
AF: Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, One Cyclotron Rd, MS70-108B, Berkeley, CA 94720, United States
AU: Novakov, T
EM: tnovakov@lbl.gov
AF: Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, One Cyclotron Rd, MS70-108B, Berkeley, CA 94720, United States
AB: Evaluation and optimization of black carbon (BC) measurement methods are difficult because neither a BC standard or a BC reference measurement method exist. In this study, we generate BC particles with an inverted diffusion flame [Kirchstetter and Novakov, Atmos. Environ., 2007]. This remarkably stable flame generates nearly constant concentrations of BC particles that contain essentially no organic carbon, so the quantification of BC mass using the thermal-optical analysis (TOA) methods is straightforward. In this case, the TOA measurement of BC can be used to evaluate the accuracy of other BC measurement methods. We applied this diffusion flame to evaluate BC measurements made with the widely-used aethalometer. Laboratory measurements of the flame-generated BC, as well as field measurements of diesel vehicle soot, illustrate that the aethalometer's response diminishes as its sampling filter becomes darkened with soot. When sampling BC at constant concentration, the aethalometer reports decreasing BC concentrations because it treats the BC attenuation coefficient as a constant whereas it actually decreases with increasing BC mass on the filter. Experiments with mixed-composition aerosol evaluated the aethalometer's response to aerosol mixtures characterized by increasing light-scattering material relative to light-absorbing BC. The aethalometer exhibits the greatest particle-loading effect when sampling aerosols with the lowest single scattering albedo (SSA between 0.15 and 0.62), and exhibits a much smaller particle-loading effect when sampling aerosols with SSA as high as 0.86. This indicate that the aethalometer responds to light-scattering aerosol in addition to light-absorbing BC aerosol, and that the response to light-scattering increases the apparent absorption and thus minimizes (i.e., offsets) the particle-loading effect. Moreover, it suggests that in many ambient environments, the particle loading effect may not be an issue. A modified aethalometer calibration is presented for situations where the aerosol SSA is known to be below about 0.6, such as near BC emission sources. When it can be verified that the aerosol SSA is greater than about 0.85, time-resolved BC data need no correction for the particle loading effect provided 5%-10% uncertainty is acceptable. These conclusions pertain only to temporally resolved BC measurements; time-averaged measurements of BC are not influenced by the particle-loading sampling artifact.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0345 Pollution: urban and regional (0305, 0478, 4251)
DE: 0365 Troposphere: composition and chemistry
DE: 0394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting