High homogeneous freezing onsets of sulfuric acid aerosol at cirrus temperatures

Abstract

Abstract. 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.

Homogeneous Freezing Events Sampled in the Tropical Tropopause Layer

“…(2000) formulation and approach the threshold indicated by laboratory experiments reported recently by Schneider et al. (2021). In other words, the measured supersaturations in the TTL are consistent with the recent laboratory experiments indicating a homogeneous freezing supersaturation threshold of about 1.75–1.95 at TTL temperatures.…”

“…Numerous laboratory experiments have shown that large supersaturations with respect to ice are required for aqueous aerosols to dilute sufficiently such that homogeneous freezing will occur (see Schneider et al. (2021) and references therein). Over the past two decades, homogeneous freezing of aqueous aerosols has been parameterized in models using the formulation presented by Koop et al.…”

Studies on the Competition Between Homogeneous and Heterogeneous Ice Nucleation in Cirrus Formation

“…Recent measurements showed that onset values for homogeneous freezing may be higher than predicted by Koop et al. (2000) at TTL temperatures (185–205 K; Schneider et al., 2021), but the effect of associated changes in s hom on nucleated ICNCs is small. Figure 2 shows s hom as a function of updraft speed for selected temperatures and monodisperse solution droplets.…”

Experimental Determination of the Relationship Between Organic Aerosol Viscosity and Ice Nucleation at Upper Free Tropospheric Conditions

“…However, the IN onsets reported in these studies still cluster close to the homogeneous freezing nucleation threshold. As shown by the superimposed parameterization of Schneider et al (2021) the homogeneous onset condition of Koop may be more uncertain than previously thought. Whether the ice formation in the studies of Ignatius et al (2016) and Wang et al (2012) resulted from homogeneous or heterogeneous ice formation,it cannot compete the much higher IN activity that is usually observed in mineral dust particles (Ansmann et al, 2019;Archuleta et al, 2005;Hoose & Möhler, 2012;Welti et al, 2009).…”

“…As shown by the superimposed parameterization of Schneider et al. (2021) the homogeneous onset condition of Koop may be more uncertain than previously thought. Whether the ice formation in the studies of Ignatius et al.…”