The morphology of bi-phase aerosol particles containing phase separated hydrophobic and hydrophilic components is considered, comparing simulations based on surface and interfacial tensions with measurements made by aerosol optical tweezers. The competition between the liquid phases adopting core-shell and partially engulfed configurations is considered for a range of organic compounds including saturated and unsaturated hydrocarbons, aromatics, alcohols, ketones, carboxylic acids, esters and amines. When the solubility of the organic component and the salting-out of the organic component to the surface by the presence of concentrated inorganic solutes in the aqueous phase are considered, it is concluded that the adoption of a partially engulfed structure predominates, with the organic component forming a surface lens. The aqueous surface can be assumed to be stabilised by a surface enriched in the organic component. The existence of acid-base equilibria can lead to the dissociation of organic surfactants and to significant lowering of the surface tension of the aqueous phase, further supporting the predominance of partially engulfed structures. Trends in morphology from experimental measurements and simulations are compared for mixed phased droplets in which the organic component is decane, 1-octanol or oleic acid with varying relative humidity. The consequences of partially engulfed structures for aerosol properties are considered.

An aerosol flow tube apparatus was developed to perform the first kinetic study of the oxidation of particulatebound BaP on solid organic and salt aerosols by gas-phase ozone. The studies on azelaic acid aerosols were performed with submonolayer coatings of BaP under both dry (RH < 1%) and high relative humidity (RH ∼ 72%) conditions. The reaction exhibited pseudo-first-order kinetics for BaP loss and the pseudo-first-order rate coefficients displayed a Langmuir-Hinshelwood dependence on gas-phase ozone concentration. Under high relative humidity conditions the kinetics were faster but also displayed a similar functional dependence on the gas-phase ozone concentration. By assuming Langmuir-Hinshelwood behavior, the following parameters were obtained: ozone-surface equilibrium constant K O3 (< 1% RH) ) (1.2 ( 0.4) × 10 -15 cm -3 , K O3 (72% RH) ) (2.8 ( 1.4) × 10 -15 cm -3 , the maximum pseudo-first-order rate coefficient k max I (< 1% RH) ) (0.048 ( 0.008) s -1 , k max I (72% RH) ) (0.060 ( 0.018) s -1 . Uptake coefficients were extracted from the pseudofirst-order rate coefficients and a slight trend of decreasing uptake coefficients with increasing ozone concentration was observed. In contrast to the behavior on azelaic acid aerosols, no reaction was observed between ozone and BaP adsorbed to solid NaCl particles. These results are compared to previous studies, which have been performed on different substrates, and their atmospheric implications are discussed. We conclude that a strong substrate effect prevails in this reaction with the kinetics proceeding faster on surfaces best able to adsorb ozone.

Little quantitative information exists regarding the products of the heterogeneous reaction of polycyclic aromatic hydrocarbons (PAHs) and ozone. We have, therefore, performed the first quantitative study investigating the kinetics and products of the heterogeneous gas-surface reaction of anthracene and ozone as a function of ozone concentration and relative humidity (RH). The reaction exhibited pseudo-first-order kinetics for anthracene loss under dry conditions (RH < 1%) and the pseudo-first-order rate coefficients displayed a Langmuir-Hinshelwood dependence on the gas-phase ozone concentration, which yielded the following fitting parameters: the equilibrium constant for ozone adsorption, K(O3) = (2.8 +/- 0.9) x 10(-15) cm3 and the maximum pseudo-first-order rate coefficient, k(I)max = (6.4 +/- 1.8) x 10(-3) s(-1). The kinetics were unchanged when experiments were performed at approximately 50% and 60% RH. In the product study, a nonlinear dependence, similar to a Langmuir adsorption plot, of the anthraquinone product yield as a function of ozone concentration was observed and resulted in the following fitting parameters: K(O3) = (3.4 +/- 1.5) x 10(-15) cm3 and the maximum anthraquinone yield, ANQmax % = 30 +/- 18%. Experiments performed under higher relative humidity conditions ( approximately 50% and 60% RH) revealed that the anthraquinone yield was unaffected by the presence of gas-phase water. It is noteworthy that both the anthracene loss kinetics and the anthraquinone yields have a similar dependence on the degree of ozone partitioning to the surface. This can be understood in terms of a mechanism whereby the rate-determining steps for anthracene loss and anthraquinone formation are both driven by the amounts of ozone adsorbed on the surface. Our results suggest that at atmospherically relevant ozone concentrations (100 ppb) the anthraquinone yield from the ozonolysis of anthracene under dry and high relative humidity conditions would be less than 1%.

Aerosol optical tweezers are used to probe the phase, morphology, and hygroscopicity of single aerosol particles consisting of an inorganic component, sodium chloride, and a water insoluble organic component, oleic acid. Coagulation of oleic acid aerosol with an optically trapped aqueous sodium chloride droplet leads to formation of a phase-separated particle with two partially engulfed liquid phases. The dependence of the phase and morphology of the trapped particle with variation in relative humidity (RH) is investigated by cavity enhanced Raman spectroscopy over the RH range <5% to >95%. The efflorescence and deliquescence behavior of the inorganic component is shown to be unaffected by the presence of the organic phase. Whereas efflorescence occurs promptly (<1 s), the deliquescence process requires both dissolution of the inorganic component and the adoption of an equilibrium morphology for the resulting two phase particle, occurring on a time-scale of <20 s. Comparative measurements of the hygroscopicity of mixed aqueous sodium chloride/oleic acid droplets with undoped aqueous sodium chloride droplets show that the oleic acid does not impact on the equilibration partitioning of water between the inorganic component and the gas phase or the time response of evaporation/condensation. The oxidative aging of the particles through reaction with ozone is shown to increase the hygroscopicity of the organic component.

The influence of stearic acid, octanol, and octanoic acid monolayer coatings on the release of NO 2 into the gas phase following aqueous NO 3 − photolysis was studied using incoherent broadband cavity-enhanced absorption spectroscopy (IBBC-EAS). The different organic compounds, when present at the aqueous surface, had varying effects on the gas-phase NO 2 evolved. Stearic acid monolayers lowered the initial rate of appearance of NO 2(g) , and its steady-state concentration was the same as for uncoated solutions after ∼50 min. In the presence of octanol monolayers, both the steady-state [NO 2(g) ] and its rate of appearance decreased. A simple kinetic phase partitioning model suggests that the rate of NO 2(g) evaporation from the aqueous surface is physically inhibited by the long uncompressed stearic acid chains, whereas both NO 2 evaporation and steady-state NO 2(g) concentration decrease when octanol is present at the aqueous surface, due to the enhanced solubility of NO 2 in the less polar octanol environment. Despite its structural similarity to octanol, monolayers of octanoic acid showed a different effect and slightly increased the steady-state [NO 2(g) ]. We propose that octanoic acid enhances NO 2(g) production because of an increase in solution acidity, which increases the quantum yield of NO 2 production from nitrate photolysis.