A new instrument to study fog and clouds: Insights from laboratory characterization and field deployment of the Ground-Based Fog and Aerosol Spectrometer (GFAS

Clouds and their interaction with aerosol particles remain one of the largest sources of uncertainty for quantitatively describing the climate system, primarily due to the challenges in accurately representing their microphysical properties. These properties determine, for example, how a cloud interacts with short and longwave radiation or determine its lifetime, and thus, influence the local energy budget. However, microphysical properties of clouds are highly variable in space and time and are difficult to measure with high precision.

The Ground-Based Fog and Aerosol Spectrometer (GFAS, Droplet Measurement Technologies, USA) is a newly developed instrument, designed to characterize microphysical properties of clouds and aerosol particles via forward and backward light scattering. Operating in the diameter size range of 0.4 – 40 µm, the GFAS builds on the capabilities of the Fog Monitor (Droplet Measurement Technologies, USA) but is extended by  backscattered light intensity and changes in the polarization of the backscattered light as measured variables. These new parameters provide information on the particle’s shape and refractive index, enabling differentiation between solid and liquid particles, such as snow crystals, dust, and droplets, while minimizing biases in particle sizing caused by a change in refractive index. Furthermore, the GFAS automatically aligns with the main wind direction to minimize sampling losses.

Laboratory tests have validated the GFAS’ ability to characterize various particle types. Nebulized droplets and solid materials, like dust and ash, were analysed, revealing distinct polarization signatures between solids and liquids at an optical diameter > 10 µm. The backscattering ratio was used to further refine size distributions by investigating particle phase functions.

Field deployment of the GFAS during the ARTofMELT 2023 expedition in the Arctic under extreme environmental conditions provided first valuable insights into the role of low-level clouds and fog during the onset of sea ice melt. Over six weeks, the GFAS captured 46 hours of in-cloud data, showing strong agreement with the Fog Monitor data. These results confirm the GFAS as a new and powerful tool for advancing cloud microphysical measurements, reducing uncertainties in particle characterization, and improving our understanding of cloud-climate interactions.