High homogeneous freezing onsets of sulfuric acid aerosol at cirrus temperatures

Abstract. Homogeneous freezing of aqueous solution aerosol particles is an important process for cloud ice formation in the upper troposphere. There air temperature low, supersaturation can be high and concentration ice-nucleating too low to initiate dominate cirrus by heterogeneous nucleation processes. The most common description quantify homogeneous processes based on water activity criterion (WAC) as proposed Koop et al. (2000). WAC describes rate coefficients only a function activity, which makes this approach well applicable numerical models. In study, we investigate behavior sulfuric acid means comprehensive collection laboratory-based experiments conducted at AIDA (Aerosol Interaction Dynamics Atmosphere) simulation chamber, were part 17 measurement campaigns since 2007. recent during October 2020 with special emphasis temperatures below 200 K. Aqueous purity generated particle gas flow composed clean synthetic vapor, was added chamber. resulting chamber had number concentrations from 30 cm?3 up several thousand per cubic centimeter diameters ranging about nm 1.1 µm. measured simulated conditions wide range between 185 230 K steady increase relative humidity each experiment. At 205 K, results agree WAC-based predictions onsets. lower temperatures, however, show increasing deviation towards higher For saturation ratios onsets 1.6 1.7, whereas measurements 1.7 2.0 same range. Based experimental our direct measurements, suggest new fit line formulate onset isoline 5×108 1013 s?1. potential significant impacts thresholds, directly observed under conditions, model prediction occurrence related radiative effects are discussed.