A novel multi−wavelength photoacoustic spectrometer for the measurement of the UV–vis-NIR spectral absorption coefficient of atmospheric aerosols

Abstract

A multi-wavelength photoacoustic instrument is described, which measures the wavelength dependent optical absorption coefficient (OAC) of soot or soot-containing aerosols in-situ in a range from the ultra-violet to the near-infrared region. The instrument combines a Nd:YAG disc laser (fundamental wavelength 1064 nm, harmonics at 532, 355 and 266 nm) and four photoacoustic detection cells, each purged with the same aerosol sample flow, while being irradiated with one of the four light beams. With the help of a supplementary optical arrangement to illuminate each detection cell with 532 nm light, the system is calibrated against OAC by purging the cells with known concentrations (and hereby known OAC values) of NO 2. This calibration eliminates differences in sensitivity of the detection PA cells and makes the measurement of OAC absolute. The minimum detectable OAC was determined to be 0.2 Mm −1 at 1064 nm and 35.5 Mm −1 at 266 nm, corresponding to a minimum detectable black carbon mass concentration of about 0.1–1 μg/m 3, depending on the wavelength. Comparison measurements with artificially generated soot aerosols showed good agreement of the device with a reference instrument, based on a long path extinction cell (LOPES).