Effect of Heterogeneous Chemical Reactions on the Köhler Activation of Aqueous Organic Aerosols

Effect of chemical aging of aqueous organic aerosols on the rate of their steady-state nucleation

We present the steady-state solution of the kinetic equation for the size and composition distribution of an ensemble of aqueous organic droplets, evolving via nucleation and concomitant chemical aging. The partial differential equation of second order for the temporal evolution of this distribution can be reduced to the canonical form of the multidimensional Fokker-Planck equation, which can be solved analytically by using the method of complete separation of variables. Its solution for the steady-state process provides the stationary distribution of droplets in the vicinity of the saddle point of the free-energy surface as well as the stationary nucleation rate in the form of the product "kinetic (Zeldovich) factor × normalization factor × exp(-free energy of nucleus formation)". Our numerical evaluations for the formation of aqueous organic aerosols in the air containing the vapors of water, 2-methylglyceric acid, and 3-methyl-4 -hydroxy-benzoic acid, as well as typical atmospheric gaseous species, indicate that the steady-state nucleation rate of such aerosols can be significantly enhanced by their concomitant chemical aging. Thus, one can expect that the application of our approach to the formation and evolution of atmospheric aqueous organic aerosols (via concurrent nucleation and chemical aging) will make aerosol models more adequate and may, once implemented in climate models, improve their forecasting accuracy.

Kinetic equation of concurrent nucleation and chemical aging of an ensemble of aqueous organic aerosols

Using the formalism of classical nucleation theory, we derive a kinetic equation for the size and composition distribution of an ensemble of aqueous organic droplets evolving via nucleation and concomitant chemical aging. This distribution can be drastically affected by the enthalpy of heterogeneous chemical reactions and the depletion of organic trace gases absorbed by aerosols. A partial differential equation of second order for the temporal evolution of this distribution is obtained from the discrete equation of balance via Taylor series expansions. Once reduced to the canonical form of the multidimensional Fokker-Planck equation, this kinetic equation can be solved via the method of complete separation of variables. This kinetic equation opens a new direction in the development of the kinetic theory of first-order phase transitions, while its applications to the formation and evolution atmospheric organic aerosols via concurrent nucleation and chemical aging may drastically improve the accuracy of global climate models.

Computational Study of the Effect of Mineral Dust on Secondary Organic Aerosol Formation by Accretion Reactions of Closed-Shell Organic Compounds

The effect of dust aerosols on accretion reactions of water, formaldehyde, and formic acid was studied in the conditions of earth's troposphere at the DLPNO-CCSD(T)/aug-cc-pVTZ//ωB97X-D/6-31++G** level of theory. A detailed analysis of the reaction mechanisms in the gas phase and on the surface of mineral dust, represented by mono- and trisilicic acid, revealed that mineral dust has the potential of decreasing reaction barrier heights. Specifically, at 0 K, mineral dust can lower the apparent energy barrier of the reaction of formaldehyde with formic acid to zero. However, when the entropic contributions to the reaction free energies were accounted for, mineral dust was found to selectively enhance the reaction of water with formaldehyde, while inhibiting the reaction of formaldehyde and formic acid, in the lower parts of the troposphere (with temperatures around 298 K). In the upper troposphere (with temperatures closer to 198 K), mineral dust catalyzes both reactions and also the reaction of methanol with formic acid. Despite the intrinsic potential of mineral dust, calculation of the catalytic enhancement parameter for a likely range of dust aerosol concentrations suggested that dust aerosols will not contribute to secondary organic aerosol formation via dimerization of closed-shell organic compounds. The main reason for this is the relatively low absolute concentration of tropospheric dust aerosol and its inefficiency in increasing the effective reaction rate coefficients.

Depletion of atmospheric organic trace gases due to their uptake by an ensemble of aqueous aerosols evolving via concurrent condensation and chemical aging

In the framework of classical nucleation theory (CNT), we demonstrate that an ensemble of aqueous hydrophilic-hydrophobic organic droplets, containing both soluble and insoluble surfactants and evolving via concurrent condensation and chemical aging, may deplete the surrounding air of low-volatility organic trace gases and thus noticeably decrease their saturation ratios. At a given liquid water content in the air, this depletion becomes stronger with increasing dispersion of the liquid phase, i.e., with increasing collective surface-to-volume ratio of droplets; this dependence becomes particularly sharp for droplets of submicron to micron radii R⪅ 1 μm. One can thus suggest that the adsorption of organic molecules on the surface of droplets may play an important (if not crucial) role in this phenomenon. As a result of such depletion, the height of the nucleation barrier and the size of a critical droplet (nucleus) will sharply increase; with a decrease of ∼0.01% of a saturation ratio, the barrier height and number of molecules in the nucleus both increase by a factor of ∼10². This may trigger the redistribution of condensable matter in the system. Some smaller supercritical, previously growing droplets will become subcritical, evaporating ones, with the newly available condensable matter enhancing the growth of larger droplets. Thus, the uptake of organic trace gases by an ensemble of aqueous organic aerosols may drastically affect their distribution with respect to size and chemical composition. Therefore, a CNT-based theoretical model, taking this effect into account and implemented in atmospheric aerosol models, would allow one to improve the forecasting accuracy of climate models.

Formation and evolution of aqueous organic aerosols via concurrent condensation and chemical aging

We review recent results on the formation and evolution of aqueous organic aerosols via concurrent nucleation/condensation and chemical aging processes obtained mostly using the formalism of classical nucleation theory In this framework, an aqueous organic aerosol was modeled as a spherical particle of liquid solution of water and hydrophilic and hydrophobic condensable organic compounds; besides these compounds, the surrounding air contained some chemically reactive, non-condensable species. Hydrophobic organic molecules on the aerosol surface can be processed by chemical reactions with some atmospheric species; this affects the hygroscopicity of the aerosol and hence its ability to become a cloud droplet. Such processing is most probably triggered by atmospheric hydroxyl radicals that abstract hydrogen atoms from surfactant molecules located on the aerosol surface (first step), resulting radicals being quickly oxidized by ubiquitous atmospheric oxygen molecules to produce surface-bound peroxyl radicals (second step). These two reactions play a crucial role in the enhancement of the Köhler activation of the aerosol. Taking them and a third reaction (next in the multistep chain of relevant heterogeneous reactions) into account, one can derive an explicit expression for the free energy of formation of a four-component aqueous droplet on a ternary aqueous organic aerosol as a function of four independent variables of state of a droplet. This approach was also applied to study a large subset of primary marine aerosols which can be initially treated using an "inverted micelle" model whereof the core consists of aqueous "salt" solution. Numerical evaluations suggest that the formation of cloud droplets on such (both aqueous hydrophilic/hydrophobic organic and marine) aerosols is most likely to occur via Köhler activation rather than via nucleation. The models allow one to determine the threshold parameters necessary for the Köhler activation of such aerosols. Furthermore, heterogeneous chemical reactions involved in the chemical aging of aerosols are most likely exothermic. Due to the release of the enthalpy of these reactions, the temperature of an aerosol particle during its chemical aging may become greater than the ambient (air) temperature. The analysis of the characteristic timescales of four most important processes involved suggests that this effect may play a significant impeding role in the formation of an ensemble of aqueous secondary organic aerosols via nucleation and, hence, must be taken into account in atmospheric aerosol and global climate models.

Does the Enthalpy of Heterogeneous Chemical Reactions Affect the Formation of Aqueous Secondary Organic Aerosols?

The chemical aging of liquid organic aerosols most likely occurs via exothermic heterogeneous chemical reactions on the aerosol's surface. Because of the enthalpy of reactions, the temperature of an aerosol particle during its chemical aging may become greater than the ambient (air) temperature. We attempt to shed light on this aspect of the formation of secondary organic aerosols considering their nucleation and chemical aging as concomitant processes. Using the model of aqueous hydrophilic-hydrophobic organic aerosols in the framework of classical nucleation theory, we evaluate characteristic time scales of the four most important processes involved in this complex phenomenon. Their analysis suggests that the release of the enthalpy of heterogeneous chemical reactions during the chemical aging of organic aerosols may play a significant impeding role in the formation of an ensemble of aqueous secondary organic aerosols via nucleation and hence must be taken into account in atmospheric aerosol and global climate models.