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Calibration and Application of Photoacoustic Spectroscopy for Aerosol and Gases
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Novel, practical methods to calibrate photoacoustic (PA) instruments and quantify the photolysis effect in operation of multispectral PA instruments are presented.The calibration of photoacoustic aerosol optical absorption measurements through simultaneous photoacoustic spectroscopy (PAS) of the oxygen A-band absorption is demonstrated. Aerosol absorption shows no sharp spectral structures for size-distributed aerosols. However the molecular oxygen A-band has sharp absorption lines in the near-infrared (i.e., 760-770 nm) spectral region and line-strength, shape, and broadening of these lines are well known. Since molecular oxygen is ubiquitous with a constant concentration in the troposphere, simultaneous PAS of aerosol and molecular oxygen A-band absorption yields a convenient calibration for the photoacoustic measurement of aerosol absorption coefficients without the need for pressurized and potentially toxic calibration gases.Three energy levels are involved in the electron transitions of O2 A-band. The relaxation time for each transition was taken into account during calibration. The relaxation time, which is represented by the phase shift, can be measured after calibration with carbonaceous aerosols (kerosene soot). A low-power continuous wave laser was used to probe O2 A-band absorption transitions, and atmospheric, humidified air was used as the sample gas to calibrate the system. The water-vapor-mediated relaxation effects have to be taken into account since the absorption to PA signal conversion efficiency is dependent on the collision frequency between exited oxygen molecules and water vapor molecules. For O2 A-band, measurements made using a PA system with 1500 Hz showed that the maximum conversion efficiency of absorbed photon energy to acoustic energy is approximately 40%±2%.Currently, NO2 light absorption measurements at various wavelengths in the solar spectrum are needed to determine the radiation balance of the atmosphere. Multispectral photoacoustic instruments are now available and are used to measure light absorption by aerosol and gases including NO2 without considering the photolysis effect. A 405 nm laser is commonly included in the multispectral photoacoustic instruments. The NO2 photolysis effect at 405 nm is strong. A modified version of the photoacoustic instrument was developed to quantify the photolysis effect in the NO2 light absorption measurement using multispectral photoacoustic instruments. A 405 nm laser and a 532 nm laser were used to quantify photolysis measurement in terms of the light absorption coefficient of NO2. The 532 nm laser was modulated at the resonant frequency to monitor the absorption coefficient of NO2, while the 405 nm laser, which was modulated at low frequency (<0.5Hz), was used as the photolysis light source. These two laser beams overlapped very well. The change of NO2 absorption coefficient in the PA was obtained from the 532 nm PA signal, so the photolysis caused by the 405 nm laser can be determined when cycling the 405 nm laser power. Continuous NO2 flow measurements showed an 11% absorption reduction. The 405 nm laser was also modulated at high frequency (1 kHz) to simulate the operation of multispectral instruments. The measurements showed a 6% light absorption reduction at 532 nm which was caused by photolysis effect.