Photo Acoustic Soot Spectrometer
Introduction
The 3 wavelength Photo Acoustic Soot Spectrometer (PASS) manufactured by Droplet Measurement Technologies (Boulder, Colorado) measures the optical absorption coefficient of suspended aerosol particles using a variation of the photo acoustic technique which is also used to measure optical absorption by gases. This can then be converted to black carbon mass loading based on an assumed specific absorption of black carbon (typically 9m2/g @514nm) This instrument has a significant advantage over other instruments such as the PSAP and MAAP which also measure aerosol optical absorption in that the measurement is made on aerosol in suspension rather than collected on a filter, so no corrections for filter effects are needed. The 3 wavelength measurement also allows the absorbing angstrom exponent to be determined, giving an indication of the type and quantity of brown carbon present in addition to the soot (see the PSAP and MAAP page for a discussion of angstrom exponent and brown carbon). The PASS also measures aerosol scattering coefficient and for high particle loadings total extinction, at the same 3 wavelengths as the absorption measurement.
Principle of Operation
The operation of the PASS is based on the fact that when particles absorb laser radiation they heat up and that heat is rapidly passed to the surrounding air which causes expansion or an increase in pressure. If the aerosol sample is illuminated with a modulated laser beam the periodic heating will produce a periodic variation in pressure (i.e. a sound wave) with the same frequency as the laser modulation. In the PASS the aerosol are illuminated in a resonant chamber with a laser beam which is modulated at the resonant frequency of the chamber, thus forming a standing wave which amplifies the pressure variations. The intensity of sound in the chamber is measured using a sensitive calibrated microphone. ¼ wavelength acoustic filters are used to isolate the laser windows and inlet and exhaust lines from the chamber to reduce noise input. A piezo electric oscillator is incorporated at the opposite end of the chamber from the microphone to calibrate the resonant frequency of the chamber (which is dependant on temperature, pressure and relative humidity). The absorption coefficient (Babs) is calculated from the sound intensity (Pm) using the equation below:
Where PL is the laser intensity, Ares is the cross sectional area of the resonator, γ is the ratio of isobaric and isochoric specific heats for air (1.4), f0 is the resonant frequency and Q the quality factor of the chamber (typically about 80).
As many gas phase species also absorb light at specific wavelengths, and could cause an interference with the aerosol measurement, the wavelengths of operation have been carefully selected to fall in regions of the spectrum where there is no significant gas phase absorption by species which are present in the atmosphere with sufficient concentration. A further interference could occur if particles were partially vapourised by the laser beam, while beam intensities are not sufficient to vapourise dry aerosol, they are sufficient to at least partially remove water vapour, therefore the sample flow is dried to <40% humidity before entering the instrument.
The scattering coefficient is measured using a photomultiplier tube attached to the chamber, and extinction is measured by measuring the reduction in laser intensity transmitted through the chamber when aerosol is present.
Calibration
Routine calibration is based on the fact that the instrument independently measures extinction, scattering and absorption, which are related as follows:
When calibrating the instrument, first a zero is obtained with no aerosol present, and then measured scattering coefficient is related to extinction using a high concentration of non absorbing aerosol. Finally absorption is related to extinction using a high concentration of strongly absorbing aerosol, the scattering component of extinction already having been calibrated.
Laboratory calibration of light absorption may also be performed using a known concentration of a gas such as O3 or NO2 which absorbs at the wavelength of operation, and which have well known absorption cross sections. We also perform calibrations with absorbing PSL calibration particles which have traceable absorption cross sections (see Lack et al, 2009). In addition to the manual calibration the instrument performs automatic acoustic calibrations and filter checks to account for any gas-phase contributions to absorption at the measurement wavelengths.
Specifications
Laser wavelength (power): 781nm (2W), 532nm (0.5W) and 405nm (0.2W)
Modulation frequency: 1500Hz
Range: 0-100,000Mm-1
Sensitivity: 1Mm-1
Flow Rate: 1lpm
Further Reading
W. P. Arnott, H. Moosmüller, C. F. Rogers, T. Jin, R. Bruch, Photoacoustic spectrometer for measuring light absorption by aerosol: instrument description, Atmospheric Environment 33, pp2845-2852, 1999.
W. P. Arnott, J. W. Walker, H. Moosmüller, R. A. Elleman, H. H. Jonsson, G. Buzorius, W. C. Conant, R. C. Flagan, and J. H. Seinfeld, Photoacoustic insight for aerosol light absorption aloft from meteorological aircraft and comparison with particle soot absorption photometer measurements: DOE Southern Great Plains climate research facility and the coastal stratocumulus imposed perturbation experiments, Journal of Geophysical Research, Vol. 111, D05S02, doi:10.1029/2005JD005964, 2006.
Lack, D. A., Cappa, C. D., Cross, E. S., Massoli, P., Ahern, A. T., Davidovits, P. and Onasch, T. B., "Absorption enhancement of coated absorbing aerosols: Validation of the photoacoustic technique for measuring the enhancement", AS&T, 43, 1006-1012, 2009.