Validation of Integrating Cavity Absorption Spectroscopy for Cloud and Aerosol Mass Concentration Measurement

Instruments expanding the capabilities to characterize cloud properties, such as cloud mass fractions of liquid water droplets (liquid water content, LWC) and ice crystals (ice water content, IWC), are of continued interest to the research of cloud radiative properties, the prediction of cloud and precipitation formation, as well as the research of aerosol-cloud interactions. Further applications of such instruments include certification aspects in aviation, where the representative experimental simulation of clouds for the certification of aircraft for flight in icing conditions is still a work in progress.

As total condensed cloud water content measurement is routinely performed using evaporative probes, a tempting approach to solve the challenge of separating liquid and ice fractions of water is to directly determine the relative fractions of both condensed states via the different absorption spectra.
By using a flow-through integrating cavity with nearly Lambertian (diffuse) and highly reflective inner walls, as commonly used in integrating nephelometers for aerosol light scattering measurement, a homogeneous, isotropic light field is generated inside the cavity, ideally eliminating the influence of scattering effects on the absorption measurement of optically thin clouds and aerosols.
To ensure absorption proportional to the mass fractions for large water droplets and ice crystals with diameters above 1 mm, near-infrared radiation is preferable for such measurements.

We present a broadband near-infrared integrating cavity absorption spectroscopy setup for mass concentration measurement of ice crystals and liquid water droplets. The working principle is demonstrated by the analysis of the absorption spectrum measured for a stream of water droplets with defined sizes in the 100 μm range and constant mass fractions, generated by a customized, piezoelectric actuated droplet generator.