On the interactions between atmospheric radicals and cloud droplets: A molecular picture of the interface

Abstract

How gas-phase materials become incorporated with cloud droplets has been an intriguing subject for decades, and considerable work has been done to understand the interactions between closed-shell molecules and liquid water. The interactions between open-shell radical species and liquid-phase cloud droplets, however, are not well understood. To probe these interactions we used quantum chemistry calculations to predict the energetics of the hydroperoxy radical (HO2) in the presence of an (H2O)20 spherical water cage. Our calculations show that it is energetically favorable for the radical to bind to the outside of the cage. This configuration has the hydrogen and the terminal oxygen of the radical as its primary bonding sites. Free-energy calculations suggest that, at atmospheric conditions, there will be a partitioning between HO2 radicals that are surface-bound and HO2 radicals that dissolve into the bulk. This may have important ramifications for our understanding of radical chemistry and may lend insight into the role that clouds and aerosols play in atmospheric chemical processes.R adical species play an important role in controlling the chemistry of the atmosphere. The uptake of radicals by both aqueous atmospheric aerosols and cloud droplets can impact gas-phase chemistry by removing reactant radicals from the gas phase. For example, the uptake of HO 2 radicals by cloud droplets slows down the gas-phase loss of ozone (O 3 ) because HO 2 can react with O 3 as follows (1):Lack of HO 2 concentration can also suppress the formation of O 3 by the reaction sequenceHowever, the mechanism by which HO 2 is taken up by an aqueous medium and what happens to the HO 2 after uptake both are unclear. Knowing whether a particular radical species is adsorbed (surface-bound) or absorbed (dissolved into the bulk) is critical to an understanding of its kinetics and reaction dynamics.The current model for how a gas-phase molecule is taken up by a water droplet (2) is depicted in Fig. 1. The first step involves gas-phase diffusion to the surface of the droplet. The second step involves accommodation of the gas-phase molecule at the surface. This is the crucial step, and little is known about how a radical interacts with the surface. Studies by Gertner and Hynes (3) have looked at how closed-shell molecules interact with water surfaces, but no studies to date have explained how open-shell species might be accommodated. Once a molecule is accommodated, the third step involves diffusion within the water droplet. Reaction within the bulk, diffusion of the reaction products back to the surface, and desorption from the droplet make up the remainder of the sequence. An understanding of each of these steps is essential to comprehend reactive uptake by water droplets, which is important for atmospheric models and the interpretation of heterogeneous processes occurring in the atmosphere.Field observations have offered evidence for the uptake of HO 2 by atmospheric aerosols (4, 5), which act as nucleating sites for cloud droplets. Furtherm...

Complexes and Clusters of Water Relevant to Atmospheric Chemistry: H2O Complexes with Oxidants

“…In very recent studies of the Francisco group, [94,95] an (H 2 O) 20 spherical cage was used as a model of a cloud droplet. This model was chosen because it is the perfect balance between being large enough to contain an HOOC radical and small enough for quantum chemistry optimizations.…”

“…Three stable conformations of (H 2 O) 20 ·HOOC were located. [94] The most stable one is a cyclic structure with a HOOC radical coordinated on two outermost water molecules of the (H 2 O) 20 cluster. In the most stable conformation, HOOC forms a seven-membered ring [···HÀOÀO···HÀO(H)···HÀOÀH···].…”

Protonation of Water Clusters Induced by Hydroperoxyl Radical Surface Adsorption

“…First, we have considered the interaction between the hydroperoxyl radical and the (H 2 O) 20 water cluster in a similar way to that reported recently by Shi et al [40] We have found six different ways through which the HO 2 radical interacts with the edges of a water surface with similar stabilities, the most stable being C1 with a computed binding energy of 12.1 kcal mol…”

“…The interest in these complexes lies in the high stability of some hydrogen bonds formed by HO 2 (in particular, HO 2 has been shown to be a better proton donor than water). In addition, the interaction of HO 2 with a cluster of 20 water molecules has been studied by Shi et al, [40] who considered the radical both outside and inside the cluster. Moreover, Iyengar [41] has performed a dynamic study of the hydroperoxyl radical in water clusters of various sizes.…”

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“…However, chaotropic ions with low surface charge density and/or high polarizability (such as Cl -, Br -, I -, HO 2 -and O 2 -) will favor the gas-liquid interfaces (Garrett, 2004;Jungwirth and Winter, 2008) as they only interact weakly with water but are influenced favorably by the highly polarized surface. Aqueous radicals also prefer to reside at such interfaces (Roeselová al, 2004), as do some molecular species that prefer to hydrogen bond on the outside of clathrate-like structures, like superoxide (Shi et al 2003). Small cations are found away from the interface towards the bulk where their requirement for efficient hydration may be satisfied.…”