Trapping and Structural Characterization of the XNO2·NO3- (X = Cl, Br, I) Exit Channel Complexes in the Water-Mediated X- + N2O5 Reactions with Cryogenic Vibrational Spectroscopy

Molecular Insights into Chemical Reactions at Aqueous Aerosol Interfaces

Atmospheric aerosols facilitate reactions between ambient gases and dissolved species. Here, we review our efforts to interrogate the uptake of these gases and the mechanisms of their reactions both theoretically and experimentally. We highlight the fascinating behavior of N2O5 in solutions ranging from pure water to complex mixtures, chosen because its aerosol-mediated reactions significantly impact global ozone, hydroxyl, and methane concentrations. As a hydrophobic, weakly soluble, and highly reactive species, N2O5 is a sensitive probe of the chemical and physical properties of aerosol interfaces. We employ contemporary theory to disentangle the fate of N2O5 as it approaches pure and salty water, starting with adsorption and ending with hydrolysis to HNO3, chlorination to ClNO2, or evaporation. Flow reactor and gas-liquid scattering experiments probe even greater complexity as added ions, organic molecules, and surfactants alter the interfacial composition and reaction rates. Together, we reveal a new perspective on multiphase chemistry in the atmosphere.

An Alternative Calibration Method for Measuring N2O5 with an Iodide-Chemical Ionization Mass Spectrometer and Influencing Factors

In this work, we developed an alternative calibration method for measuring N2O5 with an iodide adduct mass spectrometer (I-CIMS). In this calibration method, N2O5 is heated and then quantified based on the decrease in the amount of NO due to its reaction with the pyrolysis product (NO3). This alternative calibration method was compared with the commonly used method utilizing NOx analyzers equipped with a photolytic converter, which gauge NO2 reduction as a result of its reaction with O3 to quantify N2O5. It is notable that the two methodologies demonstrate favorable consistency in terms of calibrating N2O5, with a variance of less than 10 %. The alternative calibration method is a more reliable way to quantify N2O5 with CIMS, considering the instability of the NO2 conversion efficiency of photolytic converters in NOx analyzers and the loss of N2O5 in the sampling line. The effects of O3 and relative humidity (RH) on the sensitivity toward N2O5 were further examined. There was minimal perturbation of N2O5 quantification upon exposure to O3 even at high concentrations. The N2O5 sensitivity exhibited a nonlinear dependence on RH as it initially rose and then fell. Besides I(N2O5)-, the collisional interaction between I(H2O)- and N2O5 also forms I(HNO3)-, which may interfere with the accurate quantification of HNO3. As a consequence of the pronounced dependence on humidity, it is advisable to implement humidity correction procedures when conducting measurements of N2O5.

Formulation of the cosmic ray-driven electron-induced reaction mechanism for quantitative understanding of global ozone depletion

This paper formulates the cosmic ray-driven electron-induced reaction as a universal mechanism to provide a quantitative understanding of global ozone depletion. Based on a proposed electrostatic bonding mechanism for charge-induced adsorption of molecules on surfaces and on the measured dissociative electron transfer (DET) cross sections of ozone-depleting substances (ODSs) adsorbed on ice, an analytical equation is derived to give atmospheric chlorine atom concentration: [Formula: see text] where Φe is the prehydrated electron (epre-) flux produced by cosmic ray ionization on atmospheric particle surfaces, [Formula: see text] is the surface coverage of an ODS, and ki is the ODS's effective DET coefficient that is the product of the DET cross section, the lifetimes of surface-trapped epre- and Cl-, and the particle surface area density. With concentrations of ODSs as the sole variable, our calculated results of time-series ozone depletion rates in global regions in the 1960s, 1980s, and 2000s show generally good agreement with observations, particularly with ground-based ozonesonde data and satellite-measured data over Antarctica and with satellite data in a narrow altitude band at 13 to 20 km of the tropics. Good agreements with satellite data in the Arctic and midlatitudes are also found. A previously unreported effect of denitrification on ozone loss is found and expressed quantitatively. But this equation overestimates tropospheric ozone loss at northern midlatitudes and the Arctic, likely due to increased ozone production by the halogen chemistry in polluted regions. The results render confidence in applying the equation to achieve a quantitative understanding of global ozone depletion.

Effects of Microhydration on the Mechanisms of Hydrolysis and Cl- Substitution in Reactions of N2 O5 and Seawater

The reaction of N₂ O₅ at atmospheric interfaces has recently received considerable attention due to its importance in atmospheric chemistry. N₂ O₅ reacts preferentially with Cl^- to form ClNO₂ /NO₃ ^- (Cl^- substitution), but can also react with H₂ O to form 2HNO₃ (hydrolysis). In this paper, we explore these competing reactions in a theoretical study of the clusters N₂ O₅ /Cl^- /nH₂ O (n=2-5), resulting in the identification of three reaction motifs. First, we uncovered an S_N 2-type Cl^- substitution reaction of N₂ O₅ that occurs very quickly due to low barriers to reaction. Second, we found a low-lying pathway to hydrolysis via a ClNO₂ intermediate (two-step hydrolysis). Finally, we found a direct hydrolysis pathway where H₂ O attacks N₂ O₅ (one-step hydrolysis). We find that Cl^- substitution is the fastest reaction in every cluster. Between one-step and two-step hydrolysis, we find that one-step hydrolysis barriers are lower, making two-step hydrolysis (via ClNO₂ intermediate) likely only when concentrations of Cl^- are high.

SN2 Reactions of N2O5 with Ions in Water: Microscopic Mechanisms, Intermediates, and Products

Reactions of dinitrogen pentoxide (N₂O₅) greatly affect the concentrations of NO₃, ozone, OH radicals, methane, and more. In this work, we employ ab initio molecular dynamics and other tools of computational chemistry to explore reactions of N₂O₅ with anions hydrated by 12 water molecules to shed light on this important class of reactions. The ions investigated are Cl^-, SO₄^2-, ClO₄^-, and RCOO^- (R = H, CH₃, C₂H₅). The following main results are obtained: (i) all the reactions take place by an S_N2-type mechanism, with a transition state that involves a contact ion pair (NO₂⁺NO₃^-) that interacts strongly with water molecules. (ii) Reactions of a solvent-separated nitronium ion (NO₂⁺) are not observed in any of the cases. (iii) An explanation is provided for the suppression of ClNO₂ formation from N₂O₅ reacting with salty water when sulfate or acetate ions are present, as found in recent experiments. (iv) Formation of novel intermediate species, such as (SO₄NO₂^-) and RCOONO₂, in these reactions is predicted. The results suggest atomistic-level mechanisms for the reactions studied and may be useful for the development of improved modeling of reaction kinetics in aerosol particles.

Mechanisms and competition of halide substitution and hydrolysis in reactions of N2O5 with seawater

S_N2-type halide substitution and hydrolysis are two of the most ubiquitous reactions in chemistry. The interplay between these processes is fundamental in atmospheric chemistry through reactions of N₂O₅ and seawater. N₂O₅ plays a major role in regulating levels of O₃, OH, NO _x , and CH₄. While the reactions of N₂O₅ and seawater are of central importance, little is known about their mechanisms. Of interest is the activation of Cl in seawater by the formation of gaseous ClNO₂, which occurs despite the fact that hydrolysis (to HNO₃) is energetically more favorable. We determine key features of the reaction landscape that account for this behavior in a theoretical study of the cluster N₂O₅/Cl^-/H₂O. This was carried out using ab initio molecular dynamics to determine reaction pathways, structures, and time scales. While hydrolysis of N₂O₅ occurs in the absence of Cl^-, results here reveal that a low-lying pathway featuring halide substitution intermediates enhances hydrolysis.

Ion reactions in atmospherically-relevant clusters: mechanisms, dynamics and spectroscopic signatures

Exploring models of reactions of N₂O₄ with ions in water in order to provide molecular-level understanding of these processes.

N2O5at water surfaces: binding forces, charge separation, energy accommodation and atmospheric implications

Studying the interactions between N₂O₅and water in nano-sized clusters, in bulk and on the surface of water.