Absorption coefficient calibrationCalibration: of a multi-wavelength Pphotoacoustic aerosol absorption spectrometer. calibration using ozone combined with cavity ring down spectroscopy.

Bernard J. Mason, Nicolas Wagner, Mathews Richardson, Charles Brock, Daniel Murphy

Abstract
Atmospheric aerosol both absorb solar radiation and scatter it back into space directly affecting the radiative forcing. The amount light absorbed and scattered depends on the density of the aerosol, single scatter albedo of the aerosol, its distribution through the vertical column, and the aerosol size distribution. Both the single scattering albedo and the size distribution cannot be measured remotely and are measured in situ at the surface and aboard aircraft. There are several mature methods for in situ measurement of the aerosol size distributions; however, measurements of single scattering albedo are less developed. Here we describe methods for calibrating a multi wavelength photoacoustic aerosol absorption spectrometer which can be use along with a cavity ringdown aerosol extinction spectrometer for in situ measurement of single scattering albedo at three wavelength in the red, blue and green. The photoacoustic spectrometer’s sensitivity depends on laser intensity, optical alignment, pressure and temperature in the sample cell and hence it must be frequently calibrated. The calibration is performed by an automated procedure using a gasphase absorber, O3. This gas phase calibration is then validated using synthetic aerosol and absorption calculated by the difference between extinction (measured by the cavity ringdown instrument) and scattering (measured by a nephelometer). In this analysis we quantify the accuracy and stability of both calibration techniques and the dependence of the sensitivity on sample cell pressure. The attenuation, or extinction, of light by suspended airborne particles (aerosols) is a combination of light that is scattered by the particle and light that is absorbed. Photoacoustic spectroscopy is a powerful technique for measuring the absorption of atmospheric aerosol particles in-situ. Cavity ring down spectroscopy (CRDS) is a well-established technique that can be used to determine the total extinction of a gas or aerosol sample. The two techniques combined can be used to determine the amount of light that is scattered by a particle by subtracting the absorption from the total extinction. The single scattering albedo (SSA) of atmospheric aerosol, the ratio of scattering to extinction, can be determined using this instrument. The calibration of a PAS must be performed with care as it is subject to a variety of instrument dependent influences. The method of calibration discussed here is using the CRDS instrument and a purely absorbing gas. A gas which absorbs significantly at the operational wavelength(s) of the PAS is desirable for such a calibration so as to decrease the signal to noise. Ozone is a suitable calibration gas as it relatively easily generated and absorbs well at the green and red wavelengths used. A linear relationship between the PAS microphone signal and the measured extinction can be determined by varying the ozone concentration. An important part of the calibration procedure is to check the calibration coefficients against absorbing and scattering aerosol standards to ensure its applicability to aerosol measurements. By incorporating an integrating nephlometer into the measurement, to measure the amount of light scattered by the aerosol particles, it is possible to check the calibration coefficient obtained from the ozone calibration. Finally, given that the instrument is deployed on flight missions, it is important to establish how a reduced pressure will influence the obtained calibration coefficients.