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

Abstract

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 instrument's 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 instrument's precision and sensitivity.