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. A new technique for calibrating photoacoustic aerosolabsorption spectrometers with multiple laser passes in the acoustic cavity(multi-pass PAS) has been developed utilizing polydisperse andhighly absorbing aerosol. This is the first calibration technique formulti-pass PAS instruments that utilizes particles instead of reactive gasesand does not require knowledge of the exact size or refractive index of theabsorbing aerosol. In this new method, highly absorbing materials areaerosolized into a polydisperse distribution and measured simultaneouslywith a multi-pass PAS and a cavity-attenuated phase shift particulate mattersingle-scattering albedo (CAPS PM SSA , Aerodyne Inc.) instrument. TheCAPS PM SSA measures the bulk absorption coefficient through thesubtraction of the scattering coefficient from the extinction coefficient.While this approach can have significant errors in ambient aerosol, theaccuracy and precision of the CAPS PM SSA are high when the measuredaerosol has a low single-scattering albedo (SSA) and particles are less than 300 nm in size, in whichcase truncation errors are small. To confirm the precision and accuracy ofthe new calibration approach, a range of aerosol concentrations were sent tothe multi-pass PAS and CAPS PM SSA instruments using three differentabsorbing substances: Aquadag, Regal Black, and Nigrosin. Six repetitionswith each of the three substances produced stable calibrations, with thestandard deviation of the calibration slopes being less than 2 % at 660 nm and less than 5 % at 405 nm for a given calibration substance.Calibrations were also consistent across the different calibrationsubstances (standard deviation of 2 % at 660 nm and 10 % at 405 nm)except for Nigrosin at 405 nm. The accuracy of the calibration approach isdependent on the SSA of the calibrationsubstance but is roughly 6 % for the calibration substances used here,which all have an SSA near 0.4 at 405 nm. This calibration technique iseasily deployed in the field as it involves no toxic or reactive gases andit does not require generation of a monodisperse aerosol. Advantages to thisparticle-based calibration technique versus techniques based on ozone ornitrogen dioxide absorption include no reactive losses or impact fromcarrier gases and the broad absorption characteristics of the particles,which eliminate potentially significant errors in calibration that come withsmall errors in the peak wavelength of the laser light when utilizinggas-phase standards.

A novel approach to calibrating a photoacoustic absorption spectrometer using polydisperse absorbing aerosol