Molecular Insights into the Spontaneous Generation of Cl2O in the Reaction of ClONO2 and HOCl at the Air-Water Interface

Chemical processes involving chlorine nitrate (ClONO2) at the surface of stratospheric aerosols are crucial to ozone depletion. Herein, we show a reaction route for the formation of Cl2O, which is a source of stratospheric chlorine, in the ClONO2 + HOCl reaction at the air-water interface. Our ab initio molecular dynamics (AIMD) simulations show that the (ClONO2)Cl···O(HOCl) halogen bond plays a key role in the reaction and is the main interaction between ClONO2 and HOCl both at the air-water interface and in the bulk liquid water. Furthermore, metadynamics-based AIMD simulations reveal two pathways: (i) The OCl fragment of HOCl binds to the Cl atom in ClONO2, resulting in the formation of Cl2O and NO3-. Simultaneously, the remaining hydrogen atom is transferred to a water molecule to form H3O+. (ii) HOCl acts as a bridge for Cl atom transfer from ClONO2 to the O atom of a water molecule, and this water molecule transfers one of its H atoms to another water molecule, forming two HOCl molecules, NO3-, and H3O+. Free-energy calculations show that the former is the energetically more favorable process. More importantly, the free-energy barrier for Cl2O formation at the air-water interface is only ∼0.8 kcal/mol, and the reaction is exothermic. These findings provide insights into the importance of fundamental chlorine chemistry and the broader implications of the aerosol air-water interface for atmospheric chemistry.

Mechanistic Insights into the Reactive Uptake of Chlorine Nitrate at the Air-Water Interface

It is well-known that the aqueous-phase processing of chlorine nitrate (ClONO₂) plays a crucial role in ozone depletion. However, many of the physical and chemical properties of ClONO₂ at the air-water interface or in bulk water are unknown or not understood on a microscopic scale. Here, the solvation and hydrolysis of ClONO₂ at the air-water interface and in bulk water at 300 K were investigated by classical and ab initio molecular dynamics (AIMD) simulations combined with free energy methods. Our results revealed that ClONO₂ prefers to accumulate at the air-water interface rather than in the bulk phase. Specifically, halogen bonding interactions (ClONO₂)Cl···O(H₂O) were found to be the predominant interactions between ClONO₂ and H₂O. Moreover, metadynamics-biased AIMD simulations revealed that ClONO₂ hydrolysis is catalyzed at the air-water interface with an activation barrier of only ∼0.2 kcal/mol; additionally, the difference in free energy between the product and reactant is only ∼0.1 kcal/mol. Surprisingly, the near-barrierless reaction and the comparable free energies of the reactant and product suggested that the ClONO₂ hydrolysis at the air-water interface is reversible. When the temperature is lowered from 300 to 200 K, the activation barrier for the ClONO₂ hydrolysis at the air-water interface is increased to ∼5.4 kcal/mol. These findings have important implications for the interpretation of experiments.

Long-term measurements of NO3 radical at a semiarid urban site: 2. Seasonal trends and loss mechanisms

This study is the first to present long-term measurements of the nitrate radical in an urban location. Extensive nitrate radical measurements were conducted together with ancillary parameters during a continuous two year campaign (2005-2007) in the semiarid location of Jerusalem. The average nighttime NO3 concentration was 27.3+/-43.5 ppt, the highest ever reported, with a seasonal average peak during summer (33.3+/-55.8 pptv) with maximum levels exceeding 800 pptv. Significant diurnal changes in NO3 concentrations were observed, caused by an unusual nighttime increase in ozone concentrations. The NO3 loss processes exhibited strong seasonal variability. Homogeneous gas-phase losses were the main removal processes during summer and spring. The heterogeneous losses of N2O5, averaged over the entire campaign, contributed to less than half of the direct losses even though they dominated the winter seasons and part of the autumn months. Statistical regression analysis showed that NO3 was inversely correlated with relative humidity and positively correlated with temperature and to a lesser extent with NO2 and O3, indicating that the heterogeneous removal processes were also important. The diurnal behavior of NO3 was examined using a one-dimensional chemical transport model. The simulations showed that NO3 trends and concentrations were influenced mainly by changes in ozone and nitrogen oxide levels and that the very high levels of NO3 can be explained by the entrainment of fresh ozone from the upper atmospheric levels. After sunset and in the early morning, the homogeneous processes are the major loss pathways, while the heterogeneous N2O5 removal pathway dominates the intermediate times.

The Inhibition of N2O5 Hydrolysis in Sulfuric Acid by 1-Butanol and 1-Hexanol Surfactant Coatings

Gas-liquid scattering experiments are used to measure the fraction of N2O5 molecules that are converted to HNO3 after colliding with 72 wt % H2SO4 containing 1-hexanol or 1-butanol at 216 K. These alcohols segregate to the surface of the acid, with saturation coverages estimated to be 60% of a close-packed monolayer for 1-hexanol and 44% of a close-packed monolayer for 1-butanol. We find that the alkyl films reduce the conversion of N2O5 to HNO3 from 0.15 on bare acid to 0.06 on the hexyl-coated acid and to 0.10 on the butyl-coated acid. The entry of HCl and HBr, however, is enhanced by the hexanol and butanol films. The hydrolysis of N2O5 may be inhibited because the alkyl chains restrict the transport of this large molecule and because the alcohol OH groups dilute the surface region, suppressing reaction between N2O5 and near-interfacial H3O+ or H2O. In contrast, the interfacial alcohol OH groups provide additional binding sites for HCl and HBr and help initiate ionization. These and previous scattering experiments indicate that short-chain alcohol surfactants impede or enhance sulfuric acid-mediated reactions in ways that depend on the chain length, liquid phase acidity, and nature of the gas molecule.

Measurement of trace atmospheric species by chemical ionization mass spectrometry: speciation of reactive nitrogen and future directions

Chemical ionization mass spectrometry (CIMS) has proven to be a powerful method for sensitive, fast time response (t approximately 1 sec) measurements of various atmospheric compounds with limits of detection (LOD) of the order of tens of pptv and lower. The rapid time response of CIMS is particularly well suited for airborne measurements and its application has largely grown out of airborne measurements in the stratosphere and upper troposphere. This work reviews some of the advances in CIMS technology that have occurred in the past decade. In particular, CIMS methods for selective measurement of reactive nitrogen species (e.g., HNO3, HO2NO2, PAN, and NH3) in the lower atmosphere (altitudes approximately 0-8 km) are described. In addition, recent developments in CIMS technology for the selective measurement of gas-phase hydroperoxides and aerosol chemical composition are briefly described.

Atmospheric Pressure Coated-Wall Flow-Tube Study of Acetone Adsorption on Ice

An atmospheric pressure variant of the coated-wall flow-tube technique in combination with a Monte Carlo simulation is presented. In a performance test of simple first-order wall loss, the Monte Carlo simulation, which uses a simplified model of transport in laminar flow, reproduced results of an analytical solution of the transport equations in a flow tube. This technique was then used to investigate the reversible adsorption of acetone on ice films between 203 and 223 K and a surface coverage of below 5% of a formal monolayer. Simulation of the experimental uptake traces allowed retrieving an adsorption enthalpy of -46 +/- 3 kJ mol(-1) for acetone on ice, which is in good agreement with other static and flow-tube methods. For the experimental conditions adopted here, the transport of acetone molecules along the ice film is governed by equilibrium thermodynamics. Therefore, the surface accommodation coefficient, S(0), and the preexponential factor, tau(0), for the activated desorption cannot be independently determined. These two main microphysical parameters describing partitioning can rather be estimated through their relation to the adsorption entropy. A first estimate for S(0) of acetone on ice in the range of 0.004-0.043 is given.

MICROPHYSICS AND HETEROGENEOUS CHEMISTRY OF POLAR STRATOSPHERIC CLOUDS

Liquid and solid particles in polar stratospheric clouds are of central importance for the depletion of stratospheric ozone. Surface-catalyzed reactions on these particles, and diffusion-controlled processes in the bulk of the particles, convert halogens, which derive from compounds of mainly anthropogenic origin, from relatively inert reservoir species into forms that efficiently destroy ozone. The microphysics of these particles under cold stratospheric conditions is still uncertain in many respects, in particular concerning phase transitions such as freezing nucleation and deposition nucleation. Furthermore, there are indications that the rates of key heterogeneous reactions have not yet been established with sufficient accuracy to enable a reliable diagnosis of observed ozone losses by means of global models. The present paper reviews the current (late 1996) knowledge of the physico-chemistry of polar stratospheric clouds and evaluates the remaining uncertainties with respect to their ozone depletion potential.