Keith A. Gillis

Photoacoustic Measurements of Amplification of the Absorption Cross Section for Coated Soot Aerosols

A quantitative understanding of the absorption and scattering properties of mixed soot and aerosol particles is necessary for evaluating the Earths energy balance. Uncertainty in the net radiative forcing of atmospheric aerosols is relatively large and may be limited by oversimplified models that fail to predict these properties for bare and externally mixed soot particles. In this laboratory study of flame-generated soot, we combine photoacoustic spectroscopy, particle counting techniques, and differential mobility analysis to obtain high-precision measurements of the size-dependent absorption cross section of uncoated and coated soot particles. We investigate how the coating of soot by nonabsorbing films of dibutyl phthalate (chosen as a surrogate for sulfuric acid) affects the particles’ morphology and optical properties. Absorption measurements were made with photoacoustic spectroscopy using a laser at λ = 405 nm. We report measurements and model calculations of the absolute cross section, mass absorption coefficient, and amplification of the absorption cross section. The results are interpreted and modeled in terms of a core–shell geometry and Lorenz–Mie theory of scattering and absorption. We discuss evidence of soot particle and collapse as a result of the coating process and we demonstrate the ability to resolve changes in the coating thickness as small as 2 nm.

Thermodynamic properties of seven gaseous halogenated hydrocarbons from acoustic measurements: CHClFCF3, CHF2 CF3, CF3 CH3, CHF2CH3, CF3CHFCHF2, CF3CH2CF3, and CHF2CF2CH2F

Measurements of the speed of sound in seven halogenated hydrocarbons are presented. The compounds in this study are 1-chloro-1,2,2,2-tetrafluoroethane (CHClFCF3 or HCFC-124), pentafluoroethane (CHF2 CF3 or HFC-125), 1,1,1-trifluoroethane (CF3CH3 or HFC-143a), 1,1-difluoroethane (CHF2CH3 or HFC-152a), 1,1,1,2,3,3-hexafluoropropane (CF3CHFCHF2 or HFC-236ea), 1,1,1,3,3,3-hexafluoropropane (CF3CH2CF3 or HFC-236fa), and 1,1,2,2,3-pentafluoropropane (CHF2CF2CH2F or HFC-245ca). The measurements were performed with a cylindrical resonator at temperatures between 240 and 400 K and at pressures up to 1.0 MPa. Ideal-gas heat capacities and acoustic virial coefficients were directly deduced from the data. The ideal-gas heat capacity of HFC-125 from this work differs from spectroscopic calculations by less than 0.2% over the measurement range. The coefficients for virial equations of state were obtained from the acoustic data and hard-core square-well intermolecular potentials. Gas densities that were calculated from the virial equations of state for HCFC-124 and HFC-125 differ from independent density measurements by at most 0.15%, for the ranges of temperature and pressure over which both acoustic and Burnett data exist. The uncertainties in the derived properties for the other five compounds are comparable to those for HCFC-124 and HFC-125.Measurements of the speed of sound in seven halogenated hydrocarbons are presented. The compounds in this study are 1-chloro-,2,2,2-tetrafluoroethane (CHCIFCF{sub 3} or HCFC-124), pentafluoroethane (CHF{sub 2}CF{sub 3} or HFC-125), 1,1,1-trifluoroethane (CF{sub 3}CH{sub 3} or HFC-143a), 1,1-difluoroethane (CHF{sub 2}CH{sub 3} or HFC-152a), 1,1,2,3,3-hexafluoropropane (CF{sub 3}CHFCHF{sub 2} or HFC-236ea), 1,1,1,3,3,3-hexafluoropropane (CF{sub 3}CH{sub 2}CF{sub 3} or HFC-236fa), and 1,1,2,2,3-pentafluoropropane (CHF{sub 2}CF{sub 2}CH{sub 2}F or HFC-245ca). The measurements were performed with a cylindrical resonator at temperatures between 240 and 400 K and at pressures up to 1.0 MPa. Ideal-gas heat capacities and acoustic virial coefficients were directly deduced from the data. The ideal-gas heat capacity of HFC-125 from this work differs from spectroscopic calculations by less than 0.2% over the measurement range. The coefficients for virial equations of state were obtained from the acoustic data and hard-core square-well intermolecular potentials. Gas densities that were calculated from the virial equations of state for HCFC-124 and HFC-125 differ from independent density measurements by at most 0.15%, for the ranges of temperature and pressure over which both acoustic and Burnett data exist. The uncertainties in the derived properties of the other five compounds are comparable to those for HCFC-124 and HFC-125.

Improved determination of the Boltzmann constant using a single, fixed-length cylindrical cavity

We report improvements to our previous (Zhang et al 2011 Int. J. Thermophys. 32 1297) determination of the Boltzmann constant kB using a single 80 mm long cylindrical cavity. In this work, the shape of the gas-filled resonant cavity is closer to that of a perfect cylinder and the thermometry has been improved. We used two different grades of argon, each with measured relative isotopic abundances, and we used two different methods of supporting the resonator. The measurements with each gas and with each configuration were repeated several times for a total of 14 runs. We improved the analysis of the acoustic data by accounting for certain second-order perturbations to the frequencies from the thermo-viscous boundary layer. The weighted average of the data yielded kB = 1.380 6476 × 10−23 J K−1 with a relative standard uncertainty ur(kB) = 3.7 × 10−6. This result differs, fractionally, by (−0.9 ± 3.7) × 10−6 from the value recommended by CODATA in 2010. In this work, the largest component of the relative uncertainty resulted from inconsistent values of kB determined with the various acoustic modes; it is 2.9 × 10−6. In our previous work, this component was 7.6 × 10−6.

Practical determination of gas densities from the speed of sound using square-well potentials

The relationships between the first three density virial coefficients (B, C, and D) and the first four acoustic virial coefficients (βa,γa,δa andεa are rederived and a published error relatingD toδa is corrected. We observe that even it thenth and higher-density virial coefficients of a hypothetical gas are identifically zero, thenth and higher acoustic virial coefficients are not zero; they depend on the temperature derivatives of the 1st through (n-1)th density virial coefficients. Thus, two density virial coefficients may suffice for a fit to acoustic data with a cubic pressure dependence. These results are exploited by extending the pressure range of fits to preciously published speed-of-sound data without either introducing additional parameters or degrading the fits. We deduce gas densities from fits to speed-of-sound data with acoustic virial coefficients having the temperature dependencies calculated from square-well potentials. The estimated densities differ from independent measurements be a few tenths of a percent in an important range of conditions, These estimates require nop p T data whatsoever.

Thermodynamic properties of CF3CHFCHF2, 1,1,1,2,3,3-hexafluoropropane

We report the thermodynamic properties of 1,1,1,2,3,3-hexafluoropropane (known in the refrigeration industry as R236ea or HFC-236ea) in the temperature and pressure region commonly encountered in thermal machinery. The properties are based on measurements of the vapor pressure, the density of the compressed liquid (PVT), the refractive index of the saturated liquid and vapor, the critical temperature, and the speed of sound in the vapor phase. The surface tension was determined from the capillary rise. From these data we deduce the ideal-gas heat-capacity, the saturated liquid and vapor densities, the equation of state of the vapor phase, the surface tension, and estimates of the critical pressure and density. The data determine the coefficients for a Carnahan-Starlings-DeSantis (CSD) equation of state. The CSD coefficients found in REFPROP 4.0 database are based on these present measurements.

Thermodynamic properties of two gaseous halogenated ethers from speed-of-sound measurements: Difluoromethoxy-difluoromethane and 2-difluoromethoxy-1,1,1-trifluoroethane

We present measurements of the speed of sound in gaseous difluoromethoxy-difluoromethane (CHF2-O-CHF2) and 2-difluoromethoxy-1,1,1-trifluoroethane (CF3-CH2-O-CHF2). These measurements were performed in an all-metal apparatus between 255 and 384 K. We have obtained ideal-gas heat capacities and second acoustic virial coefficients from analysis of these measurements. Two methods of correlating the second acoustic virial coefficients, a square well model of the intermolecular interaction and a function due to Pitzer and Curl, are presented.

Thermodynamic properties of CHF2-O-CHF2, bis(difluoromethyl) ether

Abstract Defibaugh, D.R., Gillis, K.Aa Moldover M.R., Morrison, G. and Schmidt, J.W., 1992. Thermodynamic properties of CHF2-O-CHF2, bis(difluoromethyl) ether. Fluid Phase Equilibria, 81: 285–305. We have measured the thermodynamic properties of bis(difluoromethyl) ether, CHF2-O- CHF2, a candidate alternative refrigerant that is also known as E134. From the data we obtained the coefficients of a Carnahan-Starling-DeSantis equation of state and a polyno- mial representation of the ideal-gas heat capacity. This representation of the thermodynamic properties of E134 is consistent with the computer package REFPROP distributed by the National Institute of Standards and Technology to represent the properties of many candidate refrigerants. The representation is based on measurements of the refractive index of the saturated liquid and vapor, and the speed of sound of the dilute vapor. These measurements provide the boiling point, critical parameters, and the ideal-gas heat capacity of E134. Measurements on less pure samples were used to estimate the density of saturated liquid E134 and compressed liquid E134, and the interfacial tension. The pure samples appeared to be stable during the measurements; under similar conditions impure samples were not. Azeotropy in mixtures of E134 with CHF2CH2F (also known as R143) was discovered.

Accurate acoustic measurements in gases under difficult conditions

Accurate measurements of the speed of sound in gases are often made using metal resonators with small transducers that perturb the resonance frequencies in minor and predictable ways. We extend this method to gases that may be corrosive and to high temperatures by using remote transducers coupled to a resonator by acoustic waveguides. Thin metal diaphragms separate the waveguides from the resonator. Thus, only metal parts come into contact with the test gas. In the present apparatus, any gas compatible with gold and stainless steel can be studied.

Photoacoustic Spectrometer with a Calculable Cell Constant for Measurements of Gases and Aerosols

We benchmark the performance of a photoacoustic spectrometer with a calculable cell constant in applications related to climate change measurements. As presently implemented, this spectrometer has a detection limit of 3.1 × 10(-9) W cm(-1) Hz(-1/2) for absorption by a gas and 1.5 × 10(-8) W cm(-1) Hz(-1/2) for soot particles. Nonstatistical uncertainty limited the accuracy of the instrument to ∼1%, and measurements of the concentration of CO(2) in laboratory air agreed with measurements made using a cavity ring-down spectrometer, to within 1%. Measurements of the enhanced absorption resulting from ultrathin (<5 nm), nonabsorbing coatings on nanoscale soot particles demonstrate the sensitivity of this instrument. Together, these measurements show the instruments ability to quantitatively measure the absorption coefficient for species of interest to the climate and atmospheric science communities. Because the system constant is known, in most applications the acoustic response of this instrument need not be calibrated against a sample of known optical density, a decided advantage in field applications. Routine enhancements, such as improved processing of the photoacoustic signal and higher laser beam power, should further increase the instruments precision and sensitivity.

Standard photoacoustic spectrometer: Model and validation using O2 A-band spectra

We model and measure the absolute response of an intensity-modulated photoacoustic spectrometer comprising a 10 cm long resonator and having a Q-factor of approximately 30. We present a detailed theoretical analysis of the system and predict its response as a function of gas properties, resonance frequency, and sample energy transfer relaxation rates. We use a low-power continuous wave laser to probe O(2) A-band absorption transitions using atmospheric, humidified air as the sample gas to calibrate the system. This approach provides a convenient and well-characterized method for calibrating the absolute response of the system provided that water-vapor-mediated relaxation effects are properly taken into account. We show that for photoacoustic spectroscopy (PAS) of the O(2) A-band, the maximum conversion efficiency of absorbed photon energy to acoustic energy is approximately 40% and is limited by finite collision-induced relaxation rates between the two lowest-lying excited electronic states of O(2). PAS also shows great potential for high-resolution line shape measurements: calculated and experimental values for the PAS system response differ by about 1%.