ALBERT

Description: Homogeneous nucleation of ice in supercooled water droplets is a stochastic process. In its classical description, the growth of the ice phase requires the emergence of a critical embryo from random fluctuations of water molecules between the water bulk and ice-like clusters, which is associated with overcoming an energy barrier. For heterogeneous ice nucleation on ice-nucleating surfaces both, stochastic and deterministic descriptions are in use. Deterministic (singular) descriptions are often favored because the temperature dependence of ice nucleation on a substrate usually dominates the stochastic time dependence, and the ease of representation facilitates the incorporation in climate models. Conversely, classical nucleation theory (CNT) describes heterogeneous ice nucleation as a stochastic process with a reduced energy barrier for the formation of a critical embryo in the presence of an ice-nucleating surface. This reduction is conveniently parameterized in terms of a contact angle α between the ice phase immersed in liquid water and the heterogeneous surface area. This study investigates various ice-nucleating agents in immersion mode by subjecting them to repeated freezing cycles to elucidate and discriminate the time and temperature dependences of heterogeneous ice nucleation. Freezing rates determined from such refreeze experiments are presented for Hoggar Mountain dust, birch pollen washing water and Arizona Test Dust (ATD) and nonadecanol coatings. For the analysis of the experimental data with CNT we assumed the same active site to be always responsible for freezing. Three different CNT-based parame-terizations were used to describe rate coefficients for heterogeneous ice nucleation as a function of temperature, all leading to very similar results: for Hoggar Mountain dust, ATD and larger nonadecanol coated water droplets, the experimentally determined increase of freezing rate with decreasing temperature is too shallow to be described properly by CNT using the contact angle as the only fit parameter. Birch pollen washing water and small nonadecanol coated water droplets show the reverse behavior with temperature dependencies of freezing rates steeper than predicted by CNT formulations. Good agreement of observations and calculations can be obtained when a prefactor β is introduced to the rate coefficient as second fit parameter. Thus, the following microphysical picture emerges: Heterogeneous freezing occurs on ice-nucleating sites that need a minimum (critical) surface area to host embryos of critical size to grow into a crystal. Fits based on CNT suggest that the critical active site area is in the range of 10–50 nm2 depending on sample, temperature, and CNT-based parameterization. Two fitting parameters are needed to characterize individual active sites. The contact angle lowers the energy barrier that has to be overcome to form the critical embryo on the site compared to the homogeneous case where the critical embryo develops in the volume of water. The prefactor β is needed to adjust the calculated slope of freezing rate increase with decreasing temperature to the measured one. When it is large, there are many nucleation attempts and nucleation occurs immediately when the temperature is low enough so that the active site can accommodate a critical embryo. This is the case for active sites of birch pollen washing water and the small droplets coated with nonadecanol. If the prefactor is low, the number of nucleation attempts is low and the increase of freezing rate with decreasing temperature is shallow. This is the case for Hoggar Mountain dust, the large droplets coated with nonadecanol, and ATD. Different hypotheses why the value of the prefactor depends on the nature of the active sites are discussed.

Description: To simulate the impacts of volcanic eruptions on the stratosphere, chemistry-climate models that do not include an online aerosol module require temporally and spatially resolved aerosol size parameters for heterogeneous chemistry and aerosol radiative properties as a function of wavelength. For phase 1 of the Chemistry-Climate Model Initiative (CCMI-1) and, later, for phase 6 of the Coupled Model Intercomparison Project (CMIP6) two such stratospheric aerosol data sets were compiled, whose functional capability and representativeness are compared here. For CCMI-1, the SAGE-4λ data set was compiled, which hinges on the measurements at four wavelengths of the SAGE (Stratospheric Aerosol and Gas Experiment) II satellite instrument and uses ground-based Lidar measurements for gap-filling immediately after the Mt. Pinatubo eruption, when the stratosphere was optically opaque for SAGE II. For CMIP6, the new SAGE-3λ data set was compiled, which excludes the least reliable SAGE II wavelength and uses CLAES (Cryogenic Limb Array Etalon Spectrometer) measurements on UARS, the Upper Atmosphere Research Satellite, for gap-filling following the Mt. Pinatubo eruption instead of ground-based Lidars. Here, we performed SOCOLv3 (Solar Climate Ozone Links version 3) chemistry-climate model simulations of the recent past (1986–2005) to investigate the impact of the Mt. Pinatubo eruption in 1991 on stratospheric temperature and ozone and how this response differs depending on which aerosol data set is applied. The use of SAGE-4λ results in heating and ozone loss being overestimated in the lower stratosphere compared to observations in the post-eruption period by approximately 3 K and 0.2 ppmv, respectively. However, less heating occurs in the model simulations based on SAGE-3λ, because the improved gap-filling procedures after the eruption lead to less aerosol loading in the tropical lower stratosphere. As a result, simulated temperature anomalies in the model simulations based on SAGE-3λ for CMIP6 are in excellent agreement with MERRA and ERA-Interim reanalyses in the post-eruption period. Less heating in the simulations with SAGE-3λ means that the rate of tropical upwelling does not strengthen as much as it does in the simulations with SAGE-4λ, which limits dynamical uplift of ozone and therefore provides more time for ozone to accumulate in tropical mid-stratospheric air. Ozone loss following the Mt. Pinatubo eruption is overestimated by 0.1 ppmv in the model simulations based on SAGE-3λ, which is a better agreement with observations than in the simulations based on SAGE-4λ. Overall, the CMIP6 stratospheric aerosol data set, SAGE-3λ, allows SOCOLv3 to more accurately simulate the post-Pinatubo eruption period.

Description: Field measurements indicating that atmospheric secondary aerosol (SOA) particles can be present in a highly viscous, glassy state have spurred numerous studies addressing low diffusivities of water in glassy aerosols. The focus of these studies is on kinetic limitations of hygroscopic growth and the plasticizing effect of water. In contrast, much less is known about diffusion limitations of organic molecules and oxidants in viscous matrices. These may affect atmospheric chemistry and gas-particle partitioning of complex mixtures with constituents of different volatility. In this study, we quantify the diffusivity of a volatile organic in a viscous matrix. Evaporation of single particles generated from an aqueous solution of sucrose and small amounts of volatile tetraethylene glycol (PEG-4) is investigated in an electrodynamic balance at controlled humidity (RH) and temperature. The evaporative loss of PEG-4 as determined by Mie resonance spectroscopy is used in conjunction with a radially resolved diffusion model to retrieve translational diffusion coefficients of PEG-4. Comparison of the experimentally derived diffusivities with viscosity estimates for the ternary system reveals a breakdown of the Stokes-Einstein relationship, which has often been invoked to infer diffusivity from viscosity. The evaporation of PEG-4 shows pronounced RH and temperature dependencies and is severely depressed for RH ≲ 30 %, corresponding to diffusivities

Description: Potassium containing feldspars (K-feldspars) have been considered key mineral dusts for ice nucleation (IN) in mixed-phase clouds. To investigate the effect of solutes on their IN efficiency, we performed immersion freezing experiments with the K-feldspar microcline, which is highly IN active. Freezing of emulsified droplets with microcline suspended in aqueous solutions of NH3, (NH4)2SO4, NH4HSO4, NH4NO3, NH4Cl, Na2SO4, H2SO4, K2SO4 and KCl, with solute concentrations corresponding to water activities aw = 0.9–1.0, were investigated by means of a differential scanning calorimeter (DSC). The measured heterogeneous ice nucleation onset temperatures, Thet (aw) deviate strongly from Thet∆awhet (aw), the values calculated from the water-activity-based approach (where Thet∆awhet (aw) = Tmelt (aw + ∆awhet) with a constant offset ∆awhet with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH3 and NH4+-salts (molalities  ~ 0.96), we find IN temperatures raised by up to 4.5 K above the onset freezing temperature of microcline in pure water (Thet (aw = 1)) and 5.5 K above Thet∆awhet (aw), revealing NH3 and NH4+ to significantly enhance the IN of the microcline surface. Conversely, more concentrated NH3 and NH4+ solutions show a depression of the onset temperature below Thet∆awhet (aw) by as much as 13.5 K caused by a decline in IN ability accompanied with a reduction in the volume fraction of water frozen heterogeneously. All salt solutions not containing NH4+ as cation exhibit nucleation temperatures Thet (aw)  ~ 0.96) at warm (252–257 K) and NH3/NH4+-rich conditions.

Description: In viscous, organic-rich aerosol particles containing iron, sunlight may induce anoxic conditions that stabilize reactive oxygen species (ROS) and carbon-centered radicals (CCRs). In laboratory experiments, we show mass loss, iron oxidation and radical formation and release from photoactive organic particles containing iron. Our results reveal a range of temperature and relative humidity, including ambient conditions, that control ROS build up and CCR persistence in photochemically active, viscous organic particles. We find that radicals can attain high concentrations, altering aerosol chemistry and exacerbating health hazards of aerosol exposure. Our physicochemical kinetic model confirmed these results, implying that oxygen does not penetrate such particles due to the combined effects of fast reaction and slow diffusion near the particle surface, allowing photochemically-produced radicals to be effectively trapped in an anoxic organic matrix.