Comment on acp-2021-21

Abstract

Aerosol particles contribute to the climate forcing through their optical properties. Measuring these aerosol optical properties is still challenging, especially considering the hygroscopic growth of aerosol particles, which alters their optical properties. Lidar and in-situ techniques can derive a variety of aerosol optical properties, like aerosol particle light extinction, 15 backscattering, and absorption. But these techniques are subject to some limitations and uncertainties. Within this study, we compared with Mie-theory modeled aerosol optical properties with direct measurements. At dried state they were with airborne and ground-based in-situ measurements; at ambient state with lidar estimates. Also, we examined the dependence of the aerosol particle light extinction-to-backscatter ratio, also lidar ratio (LR), to relative humidity. The used model was fed with measured physicochemical aerosol properties and ambient atmospheric conditions. The model considered aerosol particles in an internal 20 core-shell mixing state with constant volume fractions of the aerosol components over the entire observed aerosol particle sizerange. The underlying set of measurements was conducted near the measurement site Melpitz, Germany, during two campaigns in summer, 2015, and winter, 2017, and represent Central European background aerosol conditions. Two airborne payloads deployed on a helicopter and a balloon provided measurements of microphysical and aerosol optical properties and were complemented by the polarization Raman lidar system Polly XT as well as by a holistic set of microphysical, chemical and 25 optical aerosol measurements derived at ground level. The calculated aerosol optical aerosol properties agreed within 13% (3%) with the ground-based in-situ measured aerosol optical properties at a dried state (relative humidity below 40%) in terms of scattering at 450 nm wavelength during the winter (summer) campaign. The model also represented the aerosol particle light absorption at 637 nm within 8% (18%) during the winter (summer) campaign and agreed within 13% with the airborne in-situ aerosol particle light extinction measurements during summer. During winter, in a comparatively clean case with 30 equivalent black carbon mass-concentrations of around 0.2 µg m -3 the modeled airborne measurement-based aerosol particle light absorption, was up to 32-37% larger than the measured values during a relatively clean period. However, during a high polluted case, with an equivalent black carbon mass concentration of around 4 µg m -3 , the modeled aerosol particle light absorption coefficient was, depending on the wavelength, 13-32% lower than the measured values. Spread and magnitude of the disagreement highlighted the importance of the aerosol mixing state used within the model, the requirement of the inclusion 35 of brown carbon, and a wavelength-dependent complex refractive index of black and brown carbon when such kind of model is used to validate aerosol particle light absorption coefficient estimates of, e.g., lidar ...