AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Measurement Uncertainties of Black Carbon Concentration Using Photoacoustic Method
MAJI NAJI, Kevin Thomson, National Research Council Canada
Abstract Number: 417 Working Group: Instrumentation and Methods
Abstract Photoacoustic spectroscopy (PAS) has been successfully developed as a commercially available method for the measurement of atmospheric black carbon concentration. With any aerosol measurement instrument, it is desirable to have a method of calibration that can be deployed by the user rather than requiring return to the instrument manufacture.
The Droplet Measurement Technology offers a field-calibration option with their PAX and PASS-3 instruments; however, the accuracy of the PAS measurement is dependent on accurate PAS signal calibration so the robustness and uncertainty of the field calibration method should be considered prior to report BC concentrations.
As an example, the PAX instrument, calibrates the scattering and absorption coefficients reported by the instrument using highly scattering and high absorbing aerosols respectively. In the first step, the PAX scattering measurement is calibrated using highly scattering materials, with a particle size around 200-260 nm. The uncalibrated scattering signal is compared to extinction measurement made through the measurement cell. For performing absorption measurement PAX uses PAS resonance method to measure laser light absorption of a highly absorbing aerosol. The absorption coefficient is determined through the difference of the extinction and calibrated scattering measurement.
There are several possible uncertainties introduced through the calibration process. For example truncation error in the scattering measurements which may vary with particle size and type. The calibration is performed at very high concentration levels relative to those that would likely be measured during operation and so putting high demands on the linearity of the instrument. In the interest of understanding the robustness of the instrument’s calibration, we investigate the calibration method.
By measuring the scattering calibration, using Ammonium sulfate aerosol particles at different concentration and particle sizes, using a TSI 3936 Scanning mobility particle sizer (SMPS) spectrometer along with PAX, the uncertainty of PAX scattering calibration to the truncation effect and particle size is determined.
A mini-CAST (RSG mini-CAST Model 5201) soot generator is used to generate highly absorptive black carbon (BC) particles. By selecting different BC particle size using SMPS the effect of the BC particles size and concentration on the absorption calibration variation of the PAX is studied.