High homogeneous freezing onsets of sulfuric acid aerosol at cirrus temperatures

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

High Homogeneous Freezing Onsets of Sulfuric Acid Aerosol at Cirrus Temperatures

Homogeneous freezing of aqueous solution aerosol particles is an important process for cloud ice formation in theupper troposphere. There the air temperature is low, the ice supersaturation can be high, and the concentration of ice-nucleating particles is too low to initiate and dominate cirrus cloud formation by heterogeneous ice nucleation processes. The most common description to quantify homogeneous freezing processes is based on the water activity criterion (WAC) as proposed by Koop et al. (2000). The WAC describes the homogeneous nucleation rate coefficients only as a function of the water activity, temperature and size of the aqueous aerosol particles, which makes this approach well applicable in numerical models. In this study, we investigate the homogeneous freezing behaviour of aqueous sulfuric acid aerosol particles by means of a comprehensive collection of laboratory homogeneous freezing experiments conducted at the AIDA (Aerosol Interaction and Dynamicsin the Atmosphere) cloud simulation chamber, which were conducted as part of 17 measurement campaigns since 2007. The most recent experiments were conducted during October 2020 with special emphasis on temperatures below 200 K. Aqueous sulfuric acid aerosol particles were generated at high purity by particle nucleation in a gas flow composed of clean synthetic air and sulfuric acid vapor, which was added to the AIDA chamber. The resulting chamber aerosol had number concentrations from 30 cm−3 up to several 1000 cm−3 with particle diameters ranging from about 30 nm to 1.1 μm. Homogeneous freezing of the aerosol particles was measured at simulated cirrus formation conditions in a wide range of temperatures between 185 K and 230 K with a steady increase of relative humidity during each experiment. At temperatures between about 205 K and about 230 K, the AIDA results agree well with the WAC-based predictions of homogeneous freezing onsets. At lower temperatures, however, the AIDA results show an increasing deviation from the WAC-based predictions towards higher freezing onsets. For temperatures between 185 K and 205 K, the WAC-based ice saturation ratios for homogeneous freezing onsets increase from about 1.6 to 1.7, whereas the AIDA measurements show an increase from about 1.7 to 2.0 in the same temperature range. Based on our experimental results, we suggest a new fit line as a parameterization for the onset conditions of homogeneous freezing of sulfuric acid aerosol particles. As a next step, we propose the new parameterization to be implemented in atmospheric models as an improved version of the WAC-based parameterization from Koop et al. (2000). The new homogeneous freezing thresholds may have significant impacts on the prediction of cirrus cloud occurrence and related cloud radiative effects.

In situ gelation of aqueous sulfuric acid solution for fuel cells

In situ polymerization with acrylamide (AM) as the monomer and divinylbenzene (DVB) as a crosslinker in aqueous sulfuric acid solution resulted in gel membranes applicable in fuel cells.