Shortwave radiative impact of liquid-liquid phase separation in brown carbon aerosols

Atmospheric aerosol particles may undergo liquid–liquid phase separation (LLPS) when exposed to varying relative humidity. In this study, we model how the change in morphology affects the shortwave radiative forcing, in particular for particles containing organic carbon as a molecular absorber, often termed "brown carbon" (BrC). Preferentially, such an absorber will redistribute to the organic phase after LLPS. We limited our investigation to particle diameters between 0.04 and 0.5µm, atmospherically relevant organic-to-inorganic mass ratios typical for aged aerosol (1:4<OIR<4:1) and absorptivities ranging from zero (purely scattering) to highly absorbing brown carbon. For atmospherically relevant O:C ratios, core-shell morphology is expected for phase-separated particles. We compute the scattering and absorption cross sections for realistic eccentric core-shell morphologies. For the size range of interest here, we show that assuming the core-shell morphology to be concentric is sufficiently accurate and numerically much more efficient than averaging over eccentric morphologies. In the UV region, where BrC absorbs strongest, phase-separated particles may exhibit a scattering cross section up to 50% larger than those of homogenously mixed particles, while their absorption cross section is up to 20% smaller. Integrating over the full solar spectrum, due to the strong wavelength dependence of BrC absorptivity, limits the shortwave radiative impact of LLPS in the thin aerosol layer approximation. For particles with very substantial BrC absorption there will be a radiative forcing enhancement of 4%–11.8% depending on the Ångström exponent (AAE) of BrC absorptivity for the case of small surface albedos and a decrease of up to 18% for surfaces with high reflectivity. However, for those of moderate absorptivity, LLPS will have no significant shortwave radiative impact.ISSN:1680-7375ISSN:1680-736

Kinetic Limitation to Inorganic Ion Diffusivity and to Coalescence of Inorganic Inclusions in Viscous Liquid–Liquid Phase-Separated Particles

Mixed organic/inorganic aerosols may undergo liquid–liquid phase separation (LLPS) when the relative humidity drops in the atmosphere. Phase-separated particles adopt different morphologies, which will have different consequences for atmospheric chemistry and climate. Recent laboratory studies on submicron particles led to speculation whether LLPS observed for larger drops might actually be suppressed in smaller droplets. Here, we report on micron-sized droplets of a ternary mixture of ammonium sulfate (AS), carminic acid, and water at different temperatures, which were exposed to typical atmospheric drying rates ranging from 0.34 to 5.0% RH min^–1. Our results reveal that increasing the drying rate and lowering the temperature results in different morphologies after LLPS and may suppress the growth and coalescence of the inorganic-rich phase inclusions due to kinetic limitations in a viscous matrix. The coalescence time was used to estimate the viscosity of the organic-rich phase within a factor of 20, and based on the Stokes–Einstein relationship, we estimated AS diffusivity. Furthermore, we evaluated the initial growth of inclusions to quantitatively determine the AS diffusivity in the organic-rich phase, which is about 10^–8 cm² s^–1 at room temperature. Extrapolation of diffusivity to lower temperatures using estimations for the diffusion activation energy leads us to conclude that the growth of the inorganic phase is not kinetically impeded for tropospheric submicron particles larger than 100 nm

Kinetic Limitation to Inorganic Ion Diffusivity and to Coalescence of Inorganic Inclusions in Viscous Liquid–Liquid Phase-Separated Particles

Mixed organic/inorganic aerosols may undergo liquid–liquid phase separation (LLPS) when the relative humidity drops in the atmosphere. Phase-separated particles adopt different morphologies, which will have different consequences for atmospheric chemistry and climate. Recent laboratory studies on submicron particles led to speculation whether LLPS observed for larger drops might actually be suppressed in smaller droplets. Here, we report on micron-sized droplets of a ternary mixture of ammonium sulfate (AS), carminic acid, and water at different temperatures, which were exposed to typical atmospheric drying rates ranging from 0.34 to 5.0% RH min^–1. Our results reveal that increasing the drying rate and lowering the temperature results in different morphologies after LLPS and may suppress the growth and coalescence of the inorganic-rich phase inclusions due to kinetic limitations in a viscous matrix. The coalescence time was used to estimate the viscosity of the organic-rich phase within a factor of 20, and based on the Stokes–Einstein relationship, we estimated AS diffusivity. Furthermore, we evaluated the initial growth of inclusions to quantitatively determine the AS diffusivity in the organic-rich phase, which is about 10^–8 cm² s^–1 at room temperature. Extrapolation of diffusivity to lower temperatures using estimations for the diffusion activation energy leads us to conclude that the growth of the inorganic phase is not kinetically impeded for tropospheric submicron particles larger than 100 nm

Kinetic Limitation to Inorganic Ion Diffusivity and to Coalescence of Inorganic Inclusions in Viscous Liquid–Liquid Phase-Separated Particles

Mixed organic/inorganic aerosols may undergo liquid–liquid phase separation (LLPS) when the relative humidity drops in the atmosphere. Phase-separated particles adopt different morphologies, which will have different consequences for atmospheric chemistry and climate. Recent laboratory studies on submicron particles led to speculation whether LLPS observed for larger drops might actually be suppressed in smaller droplets. Here, we report on micron-sized droplets of a ternary mixture of ammonium sulfate (AS), carminic acid, and water at different temperatures, which were exposed to typical atmospheric drying rates ranging from 0.34 to 5.0% RH min^–1. Our results reveal that increasing the drying rate and lowering the temperature results in different morphologies after LLPS and may suppress the growth and coalescence of the inorganic-rich phase inclusions due to kinetic limitations in a viscous matrix. The coalescence time was used to estimate the viscosity of the organic-rich phase within a factor of 20, and based on the Stokes–Einstein relationship, we estimated AS diffusivity. Furthermore, we evaluated the initial growth of inclusions to quantitatively determine the AS diffusivity in the organic-rich phase, which is about 10^–8 cm² s^–1 at room temperature. Extrapolation of diffusivity to lower temperatures using estimations for the diffusion activation energy leads us to conclude that the growth of the inorganic phase is not kinetically impeded for tropospheric submicron particles larger than 100 nm

Interaction of Glycine with Common Atmospheric Nucleation Precursors

The interaction between the simplest amino acid glycine in three different protonation states and common atmospheric nucleation precursors (H₂O, NH₃, and H₂SO₄) has been investigated using computational methods. Each nucleation step has been thoroughly sampled, and statistical Gibbs free energies of formation have been calculated using M06-2X/6-311++G­(3df,3pd). From the stepwise Δ<i>G</i> values, the stabilities of the molecular clusters have been evaluated. Glycine in all three protonation states is found to have a favorable interaction with sulfuric acid with a higher cluster stabilizing effect than ammonia. The deprotonated glycine molecule is found to yield the highest stabilizing effect on the sulfuric acid clusters through the interaction of both the amino and carboxylic moieties, while the protonated glycine molecule is found to have a high stabilizing effect on the addition of water and ammonia. Furthermore, we find that a single sulfuric acid molecule is capable of stabilizing the glycine zwitterion. Sulfuric acid is found to be able to catalyze the spontaneous formation of the zwitterion and subsequently stabilize the formed ion. The formation of the glycine zwitterion occurs with a low Gibbs free energy barrier of 2.10 kcal/mol, indicating that this formation could occur rapidly in the atmosphere

Revising the hygroscopicity of inorganic sea salt particles

Sea spray is one of the largest natural aerosol sources and plays an important role in the Earth’s radiative budget. These particles are inherently hygroscopic, that is, they take-up moisture from the air, which affects the extent to which they interact with solar radiation. We demonstrate that the hygroscopic growth of inorganic sea salt is 8–15% lower than pure sodium chloride, most likely due to the presence of hydrates. We observe an increase in hygroscopic growth with decreasing particle size (for particle diameters <150 nm) that is independent of the particle generation method. We vary the hygroscopic growth of the inorganic sea salt within a general circulation model and show that a reduced hygroscopicity leads to a reduction in aerosol-radiation interactions, manifested by a latitudinal-dependent reduction of the aerosol optical depth by up to 15%, while cloud-related parameters are unaffected. We propose that a value of κs=1.1 (at RH=90%) is used to represent the hygroscopicity of inorganic sea salt particles in numerical models.ISSN:2041-172