Fluorescence spectra of atmospheric aerosols

Abstract. This study summarizes the results of 14 years of fluorescence measurements with the spectrometric fluorescence and Raman lidar RAMSES at the Lindenberg Meteorological Observatory, Germany. The focus is on findings that can only be obtained by spectrometric measurements and not with a single discrete fluorescence receiver channel. The measurement parameters that are useful in the spectral analysis are introduced. In particular, this includes the spectrum of the fluorescence capacity, which has proven to be instructive. A new method is described that has been employed to determine the aerosol source regions. It is based on the combination of ensemble back-trajectory calculations with satellite measurements of fires and aerosol plumes. A total of 14 RAMSES measurements are discussed, which represent a selection of the most interesting measurement nights in the years 2020 to 2023. The emphasis is on biomass burning aerosol (BBA) in the free troposphere, but measurements of Saharan dust or boundary-layer aerosol are also provided. Excited at a wavelength of 355 nm, the BBA fluorescence spectrum has a rounded shape with a maximum between about 500 and 550 nm. With height, it becomes increasingly Gaussian, and a shift towards longer wavelengths is generally observed (red shift). However, BBA layers which exhibit an opposite dependence (blue shift) can be found in specific cases as well. Overall, the spectral fluorescence capacity is high with values up to over 9 × 10⁻⁶ nm⁻¹. Fluorescence spectra of Saharan dust, on the other hand, are skewed to short wavelengths. The fluorescence maxima are below 500 nm, and a linear decrease in the spectral backscattering coefficient can be seen at longer wavelengths; the spectral fluorescence capacity is low (< 1 × 10⁻⁶ nm⁻¹). Our statistical analyses show that the correlations between the characteristics of the fluorescence spectra and other parameters are relatively weak. Of the atmospheric state variables, the ambient temperature correlates best, and of the elastic-optical particle properties it is the particle depolarization ratio. In addition, indications are found for both BBA and Saharan dust that the spectral shape is influenced not only by the type but also by the source region of the aerosols, which would allow a more accurate aerosol typing if further substantiated. The importance of spectral fluorescence measurements for investigations of aerosol-cloud interaction is also highlighted. Measurement examples reveal cirrus nucleation at low supersaturation on contact with an optically extremely thin BBA filament at the tropopause, and provide the clearest indication to date of interaction, rather than coexistence, between clouds with supercooled water droplets and BBA using lidar. This groundbreaking study opens up a new field in atmospheric and aerosol research with exciting prospects for future studies, some of which will be presented.