A novel single-cavity multi-wavelength photoacoustic spectrometer for atmospheric research

The knowledge of the visible spectral absorption coefficient of atmospheric aerosol is essential for a realistic assessment of its regional and global impact on climate. While in recent climate models, the aerosol absorption is solely attributed to black carbon (BC) there is increasing evidence that also other constituents of the atmospheric aerosol can contribute to the absorption of visible solar radiation. These non-BC particles include brown carbon, volcanic ash, and mineral dust.

A widely used method for the measurement of the aerosol absorption coefficient is to sample on filters and measure the light attenuation across the particle-laden filter. This method is prone to systematic errors that are induced by multiple light scattering processes in the filter matrix. In particular, significant and uncorrectable errors are present for weakly absorbing but strongly scattering aerosol constituents, like brown carbon and mineral dust. Therefore, methods that use a more specific access to the aerosol absorption are required. One occasionally used method is the photoacoustic aerosol absorption measurement that was originally developed for trace gas detection. While the first instruments were suffering from a low sensitivity and a bulky setup, which made them unsuitable for field applications, the progress in laser technology and signal detection helped to develop next generation photoacoustic aerosol instruments that are more sensitive and mobile.

In this contribution we present a novel single-cavity multi-wavelength photoacoustic aerosol spectrometer that was specifically developed to clarify the open questions related to the contribution of BC and non-BC particulate matter to solar light absorption. The instrument concept in terms of optical setup, signal detection and processing, as well as the calibration strategy is discussed together with results from comprehensive laboratory and aerosol chamber tests. First results from field deployments show that this instrument is well-suited for ambient aerosol studies as well as studies related to the absorption of atmospheric particles deposited in snow and ice packs.