An accurate description of the evolution of organic aerosol in the Earth's atmosphere is essential for climate models. However, the complexity of multiphase chemical and physical transformations has been challenging to describe at the level required to predict aerosol lifetimes and changes in chemical composition. In this work a model is presented that reproduces experimental data for the early stages of oxidative aging of squalane aerosol by hydroxyl radical (OH), a process governed by reactive uptake of gas phase species onto the particle surface. Simulations coupling free radical reactions and Fickian diffusion are used to elucidate how the measured uptake coefficient reflects the elementary steps of sticking of OH to the aerosol as a result of a gas-surface collision, followed by very rapid abstraction of hydrogen and subsequent free radical reactions. It is found that the uptake coefficient is not equivalent to a sticking coefficient or an accommodation coefficient: it is an intrinsically emergent process that depends upon particle size, viscosity, and OH concentration. An expression is derived to examine how these factors control reactive uptake over a broad range of atmospheric and laboratory conditions, and is shown to be consistent with simulation results. Well-mixed, liquid behavior is found to depend on the reaction conditions in addition to the nature of the organic species in the aerosol particle.

Electron transport is of fundamental importance and has application in a variety of fields. Different scattering mechanisms affect electron transport in the condensed phase; hence, it is important to comprehensively understand these mechanisms and their scattering cross sections to predict electron transport properties. Whereas electron transport is well understood for high kinetic energy (KE) electrons, there is a discrepancy in the experimental and theoretical values for the Inelastic Mean Free Path (IMFP) in the low KE regime. In this work, velocity map imaging soft X-ray photoelectron spectroscopy is applied to unsupported organic nanoparticles (squalene) to extract experimental values of inelastic and elastic mean free paths (EMFPs). The obtained data are used to calculate corresponding scattering cross sections. The data demonstrate a decrease in the IMFP and increase in the EMFP with increasing electron KE between 10 and 50 eV.

The heterogeneous oxidation of organic aerosol by hydroxyl radicals (OH) can proceed through two general pathways: functionalization, in which oxygen functional groups are added to the carbon skeleton, and fragmentation, in which carbon-carbon bonds are broken, producing higher volatility, lower molecular weight products. An ongoing challenge is to develop a quantitative molecular description of these pathways that connects the oxidative evolution of the average aerosol properties (e.g. size and hygroscopicity) to the transformation of free radical intermediates. In order to investigate the underlying molecular mechanism of aerosol oxidation, a relatively compact kinetics model is developed for the heterogeneous oxidation of squalane particles by OH using free radical intermediates that convert reactive hydrogen sites into oxygen functional groups. Stochastic simulation techniques are used to compare calculated system properties over ten oxidation lifetimes with the same properties measured in experiment. The time-dependent average squalane aerosol mass, volume, density, carbon number distribution of scission products, and the average elemental composition are predicted using known rate coefficients. For functionalization, the calculations reveal that the distribution of alcohol and carbonyl groups is controlled primarily by the initial OH abstraction rate and to lesser extent by the branching ratio between secondary peroxy radical product channels. For fragmentation, the calculations reveal that the formation of activated alkoxy radicals with neighboring functional groups controls the molecular decomposition, particularly at high O/C ratios. This kinetic scheme provides a framework for understanding the oxidation chemistry of a model organic aerosol and informs parameterizations of more complex systems.

Aerosol-cloud interactions constitute the largest source of uncertainty in global radiative forcing estimates, hampering our understanding of climate evolution. Recent empirical evidence suggests surface tension depression by organic aerosol to significantly influence the formation of cloud droplets, and hence cloud optical properties. In climate models, however, surface tension of water is generally assumed when predicting cloud droplet concentrations. Here we show that the sensitivity of cloud microphysics, optical properties and shortwave radiative effects to the surface phase are dictated by an interplay between the aerosol particle size distribution, composition, water availability and atmospheric dynamics. We demonstrate that accounting for the surface phase becomes essential in clean environments in which ultrafine particle sources are present. Through detailed sensitivity analysis, quantitative constraints on the key drivers - aerosol particle number concentrations, organic fraction and fixed updraft velocity - are derived for instances of significant cloud microphysical susceptibilities to the surface phase.

Plasma-liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas.