Thermodynamic barrier to nucleation for manganese oxide nanoparticles synthesized by high-temperature gas-to-particle conversion

A complementary experimental and modeling study is reported here for nucleation of manganese oxide nanoparticles in premixed stagnation flames. The current synthesis occurs at relatively high flame temperature and low precursor loading. Thermodynamic analysis based on the postulated nucleation process, Mn(g) + O2(g) MnO(s), is carried out to quantify precursor supersaturation and potential impacts of the Kelvin effect on particle formation. Nucleation and growth are analyzed based on the computed temperature-time-oxygen history in the post-flame region. Agreement between measured and computed flame position for the base flame and precursor doped flames indicates that the manganese methylcyclopentadienyl tricarbonyl precursor does not inhibit flame chemistry for the conditions currently studied. Particle size distributions measured by mobility particle sizing and TEM images show reasonable agreement. Moreover, the measured particle size is predicted much more closely by a nucleation-limited mechanism rather than the size predicted by coagulation-limited growth.

The Role of Contact Angle and Pore Width on Pore Condensation and Freezing

It has recently been shown that pore condensation and freezing (PCF) is a mechanism responsible for ice formation under cirrus cloud conditions. PCF is defined as the condensation of liquid water in narrow capillaries below water saturation due to the Kelvin effect, followed by either heterogeneous or homogeneous nucleation depending on the temperature regime and presence of an ice nucleating active site. By using sol-gel synthesized silica with well-defined pore diameters, morphology and distinct chemical surface-functionalization, the role of the water-silica contact angle and pore width on PCF is investigated. We find that contact angle and pore width play an important role in determining the relative humidity required for capillary condensation as predicted by the Kelvin effect and subsequent ice nucleation at cirrus temperatures. For the pore diameters and contact angles covered in this study, 2.2–9.2 nm and 15–78°, respectively, our results reveal that the contact angle plays an important role in predicting the humidity required for pore filling while the pore diameter determines the ability of pore water to freeze. For T > 235 K and below water saturation, pore diameters and contact angles were not able to predict the freezing ability of the particles suggesting an absence of active sites, thus ice nucleation did not proceed via a PCF mechanism. Rather, the ice nucleating ability of the particles depended solely on chemical functionalization. Therefore, parameterizations for the ice nucleating abilities of particles at cirrus conditions should differ from parameterizations at mixed-phase clouds conditions. Our results support PCF as the atmospherically relevant ice nucleation mechanism below water saturation when porous surfaces are encountered in the troposphere.

Contact-line singularities resolved exclusively by the Kelvin effect: volatile liquids in air

The contact line of a volatile liquid on a flat substrate is studied theoretically. We show that a remarkable result obtained for a pure-vapour atmosphere (Phys. Rev. E, vol. 87, 2013, 010401) also holds for an isothermal diffusion-limited vapour exchange with air. Namely, for both zero and finite Young’s angles, the motion- and phase-change-related contact-line singularities can in principle be regularised solely by the Kelvin effect (curvature dependence of saturation conditions). The latter prevents the curvature from diverging and rather leads to its versatile self-adjustment. To illustrate the point, the problem is resolved for a distinguished vicinity of the contact line (‘microregion’) in a ‘minimalist’ way, i.e. without any disjoining pressure, precursor film, Navier slip or any other microphysics. This also leads to the determination of the ‘Kelvin-only’ evaporation- and motion-induced apparent contact angles. With the Kelvin-only microscales actually turning out to be quite nanoscopic, other microphysics effects may nonetheless interfere too in reality. The Kelvin-only results will then yield a limiting case within such a more general formulation.

A thermodynamic description for the hygroscopic growth of atmospheric aerosol particles

The phase state of atmospheric particulate is important to atmospheric processes, and aerosol radiative forcing remains a large uncertainty in climate predictions. That said, precise atmospheric phase behavior is difficult to quantify and observations have shown that “precondensation” of water below predicted saturation values can occur. We propose a revised approach to understanding the transition from solid soluble particles to liquid droplets, typically described as cloud condensation nucleation – a process that is traditionally captured by Köhler theory, which describes a modified equilibrium saturation vapor pressure due to (i) mixing entropy (Raoult's law) and (ii) droplet geometry (Kelvin effect). Given that observations of precondensation are not predicted by Köhler theory, we devise a more complete model that includes interfacial forces giving rise to predeliquescence, i.e., the formation of a brine layer wetting a salt particle at relative humidities well below the deliquescence point.

A thermodynamic description for the hygroscopic growth of atmospheric aerosol particles

The phase state of atmospheric particulate is important to atmospheric processes and aerosol radiative forcing remains a large uncertainty in climate predictions. That said, precise atmospheric phase behavior is difficult to quantify and observations have shown that precondensation of water below predicted saturation values can occur. We propose a revised approach to understanding the transition from solid soluble particles to liquid droplets, typically described as cloud condensation nucleation – a process that is traditionally captured by Köhler theory, which describes a modified equilibrium saturation vapor pressure due to I. mixing entropy (Raoult's law) and II. droplet geometry (Kelvin effect). Given that observations of precondensation are not predicted by Köhler theory, we devise a more complete model which includes interfacial forces giving rise to predeliquescence, i.e., the formation of a brine layer wetting a salt particle at relative humidities well below the deliquescence point.

Sensitivity of particle loss to the Kelvin effect in LES of young contrails

Different treatments of the Kelvin effect in LES modeling of early contrails are shown to cause variations in the survival rate of ice particles by up to a factor of 4 and in optical depth and mean particle size by up to 50 %. The Kelvin effect which varies exponentially with particle size, can reduce or even suppress the impact of other important ambient parameters, such as ice supersaturation, on particle survival rate. Lowering or neglecting the Kelvin effect is shown to substantially alter the evolution of the ice particle size distribution and delay the onset of particle loss. A strongly Kelvin effect dependent exponential relation between particle survival rate and particle size is shown for high EIsoot (O(1015)).