Elsevier

Physics Reports

Volume 487, Issue 5, February 2010, Pages 141-167
Physics Reports

Cosmic-ray-driven electron-induced reactions of halogenated molecules adsorbed on ice surfaces: Implications for atmospheric ozone depletion and global climate change

https://doi.org/10.1016/j.physrep.2009.12.002Get rights and content

Abstract

The cosmic-ray-driven electron-induced reaction of halogenated molecules adsorbed on ice surfaces has been proposed as a new mechanism for the formation of the polar ozone hole. Here, experimental findings of dissociative electron transfer reactions of halogenated molecules on ice surfaces in electron stimulated desorption, electron trapping and femtosecond time-resolved laser spectroscopic measurements are reviewed. This is followed by a review of the evidence from recent satellite observations of this new mechanism for the Antarctic ozone hole, and all other possible physical mechanisms are discussed. Moreover, new observations of the 11-year cyclic variations of both polar ozone loss and stratospheric cooling and the seasonal variations of CFCs and CH4 in the polar stratosphere are presented, and quantitative predictions of the Antarctic ozone hole in the future are given. Finally, a new observation of the effects of CFCs and cosmic-ray-driven ozone depletion on global climate change is also presented and discussed.

Introduction

Electron transfer reactions play an important role in many processes in physical, chemical and biological systems [1], [2], [3], [4] and planetary atmospheres [5], [6], [7]. Among electron transfer reactions, dissociative electron transfer (DET) of molecules is one of the most important processes. Indeed, DET is a fundamental process involved in many areas of physics, chemistry, biology, the environment and biomedicine, e.g., in atomic collisions with molecules [8], surface photochemistry [9], [10], atmospheric ozone depletion [11], [12], [13], [14], [15], [16], [17], electronic materials [18], femtochemistry and femtobiology [19], [20], [21], [22], [23], activation of anticancer drugs [21], [22], [23], and molecular pathways leading to DNA damage and cell death in the cellular (aqueous) environment [24].

In interactions of electrons with molecules, a well-known process is dissociative electron attachment (DEA). DET is similar to DEA, but there are some important differences. DEA occurs when a low energy (0–20 eV) free, unbound electron resonantly attaches to a molecule to form a transient anion state, which then dissociates into a neutral and an anionic fragment: e+AB→AB∗−→A+B. The physical process of DEA has been comprehensively reviewed by Schultz [25], Sanche [26] and Chutjian et al. [6]. In contrast, DET occurs by rapid electron transfer of a weakly bound electron localized at an atom/molecule or in a polar medium to a foreign molecule, forming a transient anion that then dissociates. For those molecules having strong DEA resonances with free electrons at near zero eV in the gas phase, DET reactions can effectively occur when these molecules are adsorbed on metal surfaces [9], [10], polar ice surfaces [11], [12], [13], [14], [15], [16], [17] and in polar liquids [21], [22], [23], [24], [27], [28], [29]. This is because the potential energy curve of AB∗− is lowered by the polarization potential Ep of 1–2 eV to lie below that of the neutral AB in the Franck–Condon (electron-transition) region. This review will focus on experimental studies of DET reactions of halogenated molecules adsorbed on polar ice surfaces or in polar liquids. Taking CF2Cl2 adsorbed on the H2O ice surface (Ep1.3eV [30]) as an example, the DEA and DET processes are illustrated in Fig. 1. In contrast to the case for the DEA process, the lifetime of a weakly bound trapped electron in polar media is orders of magnitude longer than that of a free electron in the gas phase or a quasi-free electron in non-polar media, and the autodetachment of the AB∗− transient state once formed cannot occur in DET. These properties can greatly enhance the capture probability of the electron and the dissociation probability of the molecule in a DET reaction, as discussed recently [24], [29].

There is a long history of studying electron-induced reactions of halogenated molecules including chlorofluorocarbons (CFCs, the major ozone-depleting molecules), starting from gas-phase studies [31]. In particular, Illenberger et al. [32] first found in 1978–1979 that DEA of CFC molecules to low energy free electrons near zero eV is an extremely efficient process, e.g., e(0eV)+CF2Cl2CF2Cl2Cl+CF2Cl. The above DEA reactions of CFCs are exothermic. The measured DEA cross section of gaseous CF2Cl2 (Eq. (1)) at near 0 eV is ∼1×10−16 cm2, which is four orders of magnitude higher than the photodissociation cross section [32]. Immediately, Peyerimhoff and co-workers [33], [34] made the first theoretical studies of the DEAs of CFCs and pointed out that this process must be considered a mechanism for the destruction of CFCs and the ozone layer in the stratosphere.

In the stratosphere below 60 km, the major source producing electrons is the atmospheric ionization by cosmic rays (CRs) [35], [36], which consist mainly of protons (90%) and alpha-particles (9%) originated from deep space. The ionization of molecules by CRs entering the atmosphere generates an enormous number of low energy secondary electrons. However, the detected density of free electrons is very low, as most of the free electrons produced by CRs are rapidly captured by stratospheric molecules (mainly O2) to produce negative ions (O2). Since the electron transfer from O2 to CFCs is ineffective, the DEA/DET process was thought to be an insignificant sink for CFCs in the general atmosphere [35], [36]. Although it was generally agreed that this understanding of negative-ion chemistry in the stratosphere was rather speculative [35], [36], the DET process has been excluded in current atmospheric chemistry models [37]. As will be reviewed below, however, a neglect of electron-induced reactions of halogenated molecules as an efficient process for the destruction of the ozone layer may be premature.

Correct understanding of how ozone holes are formed and how that relates to climate change is without doubt of great significance. The data from satellite, ground-based and balloon measurements have confirmed that anthropogenic emissions of CFCs and hydrochlorofluorocarbons (HCFCs) are related to stratospheric ozone loss, and the Montreal Protocol has successfully phased out the production and consumption of these chemicals. However, it is still necessary to obtain both correct and complete ozone depletion theory and precise atmospheric measurements in order to put the Protocol on a firmer scientific ground [38], [39]. This review is organized with the following structure. The photochemical model for the ozone hole, the cosmic-ray-driven electron-induced reaction mechanism (denoted as the “CRE” model hereafter) and the more recent justification of the CRE mechanism are briefly reviewed in Section 2. This is followed by a review of laboratory findings on electron-induced reactions of CFCs, HCFCs and other halogenated molecules adsorbed on polar ice surfaces or in polar liquids in Section 3. Section 4 gives a review of the evidence from satellite data for the CRE model for Antarctic ozone depletion, and all possible physical mechanisms rather than the CRE model are discussed. New observations of the 11-year cyclic variations of both polar O3 loss and stratospheric cooling, as well as the seasonal variations of CFCs and so-called trace gases (N2O and CH4) in the polar stratosphere, are presented in Section 5. Then, quantitative analyses of the O3 data available from satellite and ground-based measurements made in the Antarctic stratosphere in 1956–2008 are presented in Section 6. This is followed by Section 7, giving quantitative predictions of the Antarctic O3 hole in 2009–2010 and its future trend towards the 21st century. Finally, a new observation of the co-effects of CFCs and CR-driven ozone loss on global climate change is presented in Section 8, ending with the conclusions in 9.

Section snippets

The photochemical model for ozone depletion

In 1974, Molina and Rowland [40] first proposed that chlorine atoms are produced by sunlight photolysis of CFCs in the tropical upper stratosphere at ∼40 km: CF2Cl2+hνCl+CF2Cl. The resultant Cl atom then destroys ozone via the (Cl, ClO) reaction chain; this is similar to the destruction of O3 via the (NO, NO2) reaction chain first proposed by Crutzen in 1971 [41]. However, the ozone hole has been observed in the lower stratosphere at ∼18 km over the poles in each spring since the first

Electron stimulated desorption (ESD) measurements

To generate low energy anions and study the physical processes in their transport through surface overlayers [46], Lu and Madey [75] first studied anion formation in electron stimulated desorption (ESD) of CF2Cl2 adsorbed on a Ru(0001) surface with an incident electron beam at hundreds of eV. They found that the anions (Cl and F) were mainly generated by DEAs to CF2Cl2 of low energy secondary free electrons emitted from the metal substrate. Unexpectedly they observed a striking effect that

Spatial correlation between cosmic rays and ozone depletion

If the CR-driven electron reaction (CRE) mechanism plays a significant role in stratospheric ozone depletion, then a correlation between ozone depletion and the CR intensity should be observed. Due to the geomagnetic effect, the intensity of CRs is well-known to be larger at higher latitudes and has a maximum over the south and north poles, and due to atmospheric ionization, the rate of electron production by CRs has a maximum at ∼18 km above the ground. Lu and Sanche [14] thus analyzed data

New observations

In spite of the laboratory measurements and observations summarized in Sections 3 Laboratory findings of dissociative electron transfer reactions of halogenated molecules on ice, 4 Satellite observations of the cosmic-ray-driven electron reaction mechanism, one might still argue that the other effects mentioned above (solar effect, direct CR effect, and PSCs) might be responsible for the observed correlations between CR intensity and polar ozone loss. Alternatively, one might argue that the

Towards quantitative understanding of ozone depletion

The CRE mechanism states that in the winter polar stratosphere, the O3-depleting reactions depend on halogen concentrations (EESC), the intensity I of CRs for producing electrons and the amount of PSCs for trapping the electrons [14], [15]. Thus, the CRE mechanism, taking the stratospheric cooling caused by the polar ozone loss in the preceding year into account, gives the steady-state total ozone ([O3]i) in the Antarctic in a particular period in spring (i) as [O3]i=[O3]0[1k×Ii×Ii1×EESCi],

The future trend of the ozone hole

The intensity of cosmic rays is still peaking in 2009, so we should expect to observe one of the deepest ozone holes over the spring Antarctica in 2009–2010. With the measured I08=10132 for 2008, Eq. (7) gives the total ozone ratio [O3]08/[O3]0=59.2% in the spring Antarctic ozone hole, which is very close to the observed value of 60.6% for three-month (October–December) average zonal mean total ozone over Antarctica (60–90 S) from NASA OMI satellite. With I09=10455 as of June 30, 2009, Eq. (7)

Effects of CFCS and CRE-driven ozone depletion on global climate change

It is also interesting to note that CFCs and CRE-driven stratospheric ozone depletion can have significant effects on not only stratospheric but also global climate [138], [139]. There are two opposing effects of stratospheric ozone loss: it causes less absorption of solar radiation there and hence a cooler stratosphere but a warmer troposphere; the resulting colder stratosphere emits less long-wavelength radiation downward, thus cooling the troposphere. It has been concluded that overall, the

Conclusions

Laboratory measurements have demonstrated well that electrons can be effectively trapped at ice surfaces and greatly enhance reactions of dissociative electron transfer of halogenated species (inorganic and organic) adsorbed on the surfaces by orders of magnitude, compared with corresponding gas-phase reactions. The experimental findings summarized in this study have demonstrated three important features associated with these DET reactions: (1) ultrafast resonant DET reactions of molecules with

Acknowledgements

NASA satellite data sets were obtained from http://toms.gsfc.nasa.gov. The O3 data obtained at Halley were obtained from the BAS (http://www.antarctica.ac.uk/met/jds/ozone/), credited to Dr. J.D. Shanklin. Cosmic ray data obtained at McMurdo were from the Bartol Research Institute, supported by NSF grant ATM-0527878, http://neutronm.bartol.udel.edu. The data on CH4 and CF2Cl2 in the stratosphere were obtained from the NASA Goddard Space Flight Center (GDFC) CLAES and HALOE data sets. The

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