HR: 1340h
AN: A33B-1179    [Abstracts]
TI: Direct Measurement of Aerosol Absorption Using Photothermal Interferometry
AU: * Sedlacek, A J
EM: sedlacek@bnl.gov
AF: Brookhaven National Laboratory, Atmospheric Sciences Division Bldg. 815E, Upton, NY 11973-5000, United States
AU: Lee, J A
EM: leej@bnl.gov
AF: Brookhaven National Laboratory, Atmospheric Sciences Division Bldg. 815E, Upton, NY 11973-5000, United States
AB: Efforts to bound the contribution of light absorption in aerosol radiative forcing is still very much an active area of research in large part because aerosol extinction is dominated by light scattering. In response to this and other technical issues, the aerosol community has actively pursued the development of new instruments to measure aerosol absorption (e.g., photoacoustic spectroscopy (PAS) and multi-angle absorption photometer (MAAP)). In this poster, we introduce the technique of photothermal interferometry (PTI), which combines the direct measurement capabilities of photothermal spectroscopy (PTS) with high-sensitivity detection of the localized heating brought about by the PT process through interferometry. At its most fundamental level, the PTI technique measures the optical pathlength change that one arm of an interferometer (referred to as the ‘probe' arm) experiences relative to the other arm of the interferometer (called the ‘reference' arm). When the two arms are recombined at a beamsplitter, an interference pattern is created. If the optical pathlength in one arm of the interferometer changes, a commensurate shift in the interference pattern will take place. For the specific application of measuring light absorption, the heating of air surrounding the light- absorbing aerosol following laser illumination induces the optical pathlength change. This localized heating creates a refractive index gradient causing the probe arm of the interferometer to take a slightly different optical pathlength relative to the unperturbed reference arm. This effect is analogous to solar heating of a road causing mirages. As discussed above, this altered optical pathlength results in a shift in the interference pattern that is then detected as a change in the signal intensity by a single element detector. The current optical arrangement utilizes a folded Jamin interferometer design (Sedlacek, 2006) that provides a platform that is robust with respect to sensitivity-reducing vibrations enabling this technique to be used in field campaigns. A series of calibration and intercomparison experiments have recently been carried out in our laboratory to evaluate the performance of the PTI technique towards aerosol absorption measurement and monitoring. Since PTI is a calorimetric technique, calibration can be performed using an absorbing gas of known concentration and known absorption cross-section. Following this calibration, a series of intercomparison experiments using laboratory-generated nigrosin aerosols and a 3-? Particle Soot Absorption Photometer (PSAP) were carried out where correlation between the PTI and PSAP was measured to be 0.96±0.02. (Sedlacek and Lee, 2007) Extension of this intercomparison to the measurement of ambient aerosols reveals continued agreement between the two instruments except for periods of high relative humidity whereupon the PSAP reported a larger absorption coefficient. (Sedlacek and Lee, 2007) A discussion of the PTI technique, along with the results of this intercomparison and some preliminary results examining absorption enhancement brought about by coating black-dyed PSL particles with dibutyl phthalate will be presented. References: Sedlacek, A. J., and Lee, J., (2007) Photothermal interferometric aerosol absorption spectroscopy, Aerosol Sci. Tech. (in press). Sedlacek, A. J. (2006). Real-time detection of ambient aerosols using photothermal interferometry: Folded Jamin interferometer, Rev. Sci. Instrum. 77:064903.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0394 Instruments and techniques
DE: 4801 Aerosols (0305, 4906)
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting