Surface Of Polar Stratospheric Clouds Research Articles

Modeling studies of the effects of the heterogeneous reaction ClOOCl + HCl → Cl2 + HOOCl on stratospheric chlorine activation and ozone depletion

The heterogeneous reaction ClOOCl + HCl → Cl2 + HOOCl was introduced into a chemical trajectory model of the stratosphere. Ten‐day trajectories ending at ozonesonde stations at various northern latitudes were run to simulate the period January–March 1994. The reaction on sulfuric acid aerosol surfaces has a negligible effect on ozone chemistry if a sticking coefficient similar to that of ClONO2 + HCl is assumed. On polar stratospheric cloud (PSC) surfaces the chemical effects of the addition of this reaction depend on the fate of proposed product HOOCl: if this species photolyzes to produce either ClO + OH or Cl + HO2, then HCl is activated by the reaction with chlorine peroxide. This heterogeneous activation of chlorine by active chlorine can have a significant effect on Arctic ozone depletion rates in the days following an air parcel's encounter with PSC surfaces. The ozone depletion rate usually increased but in some cases decreased, depending on the extent of PSC processing and on the initial [HCl]/[ClONO2] ratio. Averaged over 3 months, the column ozone loss rates between 350 and 675 K were accelerated by as much as 35% for a set of 10‐day trajectories ending at an Arctic station. If, on the other hand, HOOCl decomposes at the surface into HCl and O2, the net effect of these reactions is to convert ClOOCl into Cl2. These species are functionally equivalent, and such a conversion does not perturb the model chemistry.

ChemInform Abstract: Uptake of Chlorine Dioxide by Model Polar Stratospheric Cloud Surfaces: Ultrahigh‐Vacuum Studies.

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Model study of polar stratospheric clouds and their effect on stratospheric ozone: 2. Model results

We use the detailed microphysical/chemical/dynamical two‐dimensional model described in part I [De Rudder et al., this issue] to study the effect of heterogeneous reactions occurring on the surface of polar stratospheric clouds (PSCs) on stratospheric ozone. The calculations show that the heterogeneous reactions occurring on the surface of PSCs are the likely causes of the ozone decrease observed from 1980 to 1990 in both Antarctica and the Arctic. The calculation shows that the dense sulfate aerosol cloud produced by the eruption of Mount Pinatubo (in the Philippines, 1991) has enhanced the formation rate of type I PSCs in the Arctic and the Antarctic. In the calculation, the concentration and surface area of type I PSCs are enhanced. The effect on the ozone depletion in the Antarctic is, however, limited due to the fact that the conversion from ClONO2 to ClO on PSCs is almost “saturated” under no‐volcanic conditions. For the potential ozone depletion, the enlargement in the area covered by PSCs may therefore be more important than the increase in PSC density. The calculation also shows that, in the future, the density of PSCs in the Arctic could be enhanced owing to the potential emission of water vapor and nitrogen species by high altitude aircraft. The increase in PSC density could lead to a maximum of ozone depletion of 10% at the northern high latitudes in winter.

Model study of polar stratospheric clouds and their effect on stratospheric ozone: 1. Model description

We have included detailed microphysical processes accounting for the formation of polar stratospheric clouds (PSC) into a global chemical/dynamical two‐dimensional model to study the effect of heterogeneous reactions occurring on the surface of PSCs on stratospheric ozone. The model explicitly calculates the formation of the PSC particles (50 bin sizes) in terms of heterogeneous nucleation, condensation, and sedimentation. The transport of the particles and heterogeneous reactions on the particles are also represented in the model. The calculated PSC particles show that the distributions of PSCs in the Arctic and in Antarctica are very different. Over Antarctica, nitric acid trihydrate particles (type I PSCs) are formed from early June to late September with a maximum surface area of 15 μm2/cm3, while in the Arctic, type I PSCs are formed only in January with a maximum surface area of 5 μm2/cm3. Ice crystal clouds (type II PSCs) are present over Antarctica in August, but are not seen in the Arctic.

Uptake of Chlorine Dioxide by Model Polar Stratospheric Cloud Surfaces: Ultrahigh-Vacuum Studies

An investigation of the interaction of chlorine dioxide (OClO) with the surface of ice is reported. Experiments were carried out under ultrahigh vacuum (UHV) on films between 10 and 100 water monolayers thick. The initial OClO adsorption probability on ice at 100 K is high, approaching unity on both the crystalline and amorphous surfaces. At low coverages, OClO is quantitatively incorporated into the ice bulk, where it resides until sublimation of the ice film near 185 K. An adsorbed state of OClO is formed at moderate exposures. Chlorine dioxide desorbs from ice at approximately 130 K; desorption is first order, with an activation energy equal to 23 kJ mol-1 and a prefactor of 2 × 109 s-1. At high exposures, OClO condenses to form a multilayer film, which sublimes near 135 K; the sublimation energy is 38 kJ mol-1. If adsorbed OClO is covered with an ice film 13 monolayers thick, OClO is trapped within the film, and desorption is completely suppressed until the onset of ice sublimation.

Ozone and aerosol correlated observations at Thule, Greenland, in the period 1991–1994

Ozonesonde launches and ground‐based lidar observations have been carried out at Thule, Greenland, during the winters 1991–1992, 1992–1993 and 1993–1994 following the Mt. Pinatubo eruption. The profiles thus obtained show large amounts of aerosols of volcanic origin in the lower stratosphere, and negatively correlated laminated structures in the ozone and in the aerosol content. The presence of these structures is confined to the region below approximately 18 km. The laminae are probably due to the spatial and temporal modulation induced by the dynamics at the edge of the polar vortex: large differences exist in the aerosol and ozone concentration inside and outside the vortex. Averages of the correlation coefficient between the deviations of ozone and aerosol from their seasonal averages, calculated in 4‐km height intervals for the quasi‐simultaneous observations, are consistently negative in the 13‐ to 19‐km region during the three winters. Minima of the average correlation coefficient occur at 18.5 km with −0.56 during 1991–1992, at 18 km (−0.69) during 1992–1993 and at 14.5 km (−0.38) during 1993–1994. Short‐term trajectory analysis do not provide firm evidence for chemical phenomena occurring on the surface of polar stratospheric clouds or volcanic aerosol: such possible actions appear to be masked by the dynamical phenomena.

Sulfate Aerosols and Polar Stratospheric Cloud Formation

Before the discovery of the Antarctic ozone hole, it was generally assumed that gas-phase chemical reactions controlled the abundance of stratospheric ozone. However, the massive springtime ozone losses over Antarctica first reported by Farman et al in 1985 could not be explained on the basis of gas-phase chemistry alone. In 1986, Solomon et al suggested that chemical reactions occurring on the surfaces of polar stratospheric clouds (PSCs) could be important for the observed ozone losses. Since that time, an explosion of laboratory, field, and theoretical research in heterogeneous atmospheric chemistry has occurred. Recent work has indicated that the most important heterogeneous reaction on PSCs is ClONO[sub 2] + HCl [yields] Cl[sub 2] + HNO[sub 3]. This reaction converts inert chlorine into photochemically active Cl[sub 2]. Photolysis of Cl[sub 2] then leads to chlorine radicals capable of destroying ozone through very efficient catalytic chain reactions. New observations during the second Airborne Arctic Stratospheric Expedition found stoichiometric loss of ClONO[sub 2] and HCl in air processed by PSCs in accordance with reaction 1. Attention is turning toward understanding what kinds of aerosols form in the stratospheric, their formation mechanism, surface area, and specific chemical reactivity. Some of the latest findings, which underlinemore » the importance of aerosols, were presented at a recent National Aeronautics and Space Administration workshop in Boulder, Colorado.« less

A possible role of galactic cosmic rays in chlorine activation during polar night

Observations indicate that extremely low concentrations of HCl prevail in the stratosphere just after the end of polar night in Antarctica. This is mostly explained by the fast heterogeneous reactions of HCl with ClONO2 and HOCl. Just prior to polar night normal gas phase partitioning indicates that HOCl and ClONO2 constitute only about 30% of the inorganic chlorine reservoir, the major portion being HCl. Thus about 60% of the initial HCl would remain (after titration with HOCl and ClONO2) by the end of polar night. We suggest that further processing may be effected through the reaction of N2O5 with HCl on polar stratospheric cloud surfaces, the N2O5 being produced from NO radicals formed by the action of galactic cosmic rays. The efficiency of this process is controlled by the relative rate of the competing heterogeneous reaction of N2O5 with H2O. Moreover, galactic cosmic rays form OH radicals that further augment HCl processing. Through model calculations we show that this mechanism can considerably enhance the conversion of HCl into active chlorine, leading to an almost 50% lower HCl concentration shortly after the end of polar night compared to cases where galactic cosmic rays were neglected.

Stratospheric ozone depletion by CIONO2 photolysis

SPRINGTIME ozone depletion over Antarctica is thought1,2 to be due to catalytic cycles involving chlorine monoxide, which is formed as a result of reactions on the surface of polar stratospheric clouds (PSCs). When the PSCs evaporate, CIO in the polar air can react with NO2 to form the reservoir species C1ONO2. High concentrations of C1ONO2 can also be found at lower latitudes because of direct transport of polar air or mixing of CIO and NO2 at the edges of the polar vortex. C1ONO2 can take part in an ozone-depleting catalytic cycle18, but the significance of this cycle has not been clear. Here we present model simulations of ozone concentrations from March to May both within the Arctic vortex and at a mid-latitude Northern Hemisphere site. We find increasing ozone loss from March to May. The C1ONO2 cycle seems to be responsible for a significant proportion of the simulated ozone loss. An important aspect of this cycle is that it is not as limited as the other chlorine cycles to the timing and location of PSCs; it may therefore play an important role in ozone depletion at warm middle latitudes.

Fourier transform infrared studies of the interaction of HCl with model polar stratospheric cloud films

Heterogeneous reactions involving hydrochloric acid adsorbed on the surfaces of polar stratospheric clouds (PSCs) are postulated to contribute to polar ozone loss. Using Fourier transform infrared (FTIR) spectroscopy to probe the condensed phase, we have examined the interaction of HCl with ice and nitric acid trihydrate (NAT) films representative of types II and I PSCs, respectively. For HCl pressures in the range of 10−7 to 10−5 Torr our FTIR studies show that a small amount of crystalline HCl·6H2O formed on or in ice at 155 K. However, for higher HCl pressures we observed that the entire film of ice rapidly converted into an amorphous 4:1 H2O:HCl mixture. From HCl‐uptake experiments with PHCl = 8 × 10−7 Torr, we estimate roughly that the diffusion coefficient of HCl in ice is around 2 × 10−12 cm2/s at 158 K. For higher temperatures more closely approximating those found in the stratosphere, we were unable to detect bulk HCl uptake by ice. Our experiments also detected no bulk uptake of HCl by α‐NAT or β‐NAT under various temperature and pressure conditions. Indirect evidence suggests that HCl adsorption onto the surface of model PSC films inhibited the evaporation of both ice and NAT by 3–5 K.

Ab initio study of the chlorine nitrate protonation reaction: implications for loss of ClONO2 in the stratosphere

Ab initio quantum mechanical methods, including coupled-cluster theory, are used to determine the equilibrium geometries, dipole moments, and harmonic vibrational frequencies of ClONO2, NO2(+), and four isomers of protonated ClONO2. It was found that, for the equilibrium structures and harmonic frequencies of ClONO2, HOCl, and NO2(+), the highest-level theoretical predictions are consistent with the available experimental information concerning the reactions of ClONO2 and HOCl with HCl on the surface of polar stratospheric clouds (PSCs). The study supports a recent hypothesis that the reaction of ClONO2 on the surface of PSCs is proton catalyzed, although the mechanism is different.

Increased chlorine dioxide over Antarctica caused by volcanic aerosols from Mount Pinatubo

THE annual springtime depletion of Antarctic ozone1 has been shown to be due to the action of chlorine species, activated by reactions occurring on the surfaces of polar stratospheric clouds (PSCs)1,2. Similar reactions may also take place on the surfaces of liquid sulphuric acid aerosols when the temperature is too high to permit the formation of PSCs3–4. Such processes may have been facilitated following the eruption of Mount Pinatubo in June 1991, when unprecedented amounts of sulphur compounds were injected into the stratosphere5. Here we present observations of Antarctic chlorine dioxide abundances in the austral autumn and winter of 1991 (when aerosol concentrations were at background levels) and 1992 (greatly enhanced aerosol concentrations). We find that in 1992, unlike 1991, chlorine dioxide levels increased dramatically in the autumn, when PSCs were extremely unlikely to have been present. Model results suggest that this was mainly caused by the direct activation of chlorine nitrate on the aerosol surfaces. The effect of the Pinatubo aerosols probably contributed to the unprecedented depth and areal extent of Antarctic ozone depletion in 1992.

An infrared study of the UV photolysis of chlorine nitrate trapped in various matrices at 11 K

Photolysis of matrix isolated chlorine nitrate in argon matrix assisted by reactive matrices (solid nitrogen and oxygen) at 11 K have been carried out by visible and ultraviolet light in the 800—250 nm range. The product identification and relative measurements of the species time evolution were made by Fourier transform infrared spectrometry. Below 300 nm, two main dissociation channels are evidenced leading to ClNO + O 2 and ClONO + O, respectively. Recombination cage processes occur in matrices and the obtained results partially differ from gas-phase studies but give information on possible photoreactions in such condensed phase systems as polar stratospheric cloud surfaces.

Photochemical partitioning of the reactive nitrogen and chlorine reservoirs in the high‐latitude stratosphere

Partitioning of the major components of the reactive nitrogen and inorganic chlorine reservoirs is derived from aircraft measurements in the lower stratosphere during the winter season in both hemispheres at latitudes of about 60° to 80°. The goal of this work is to exercise the power of the correlated set of measurements from polar missions of the NASA ER‐2 to extend what can be learned from looking at the measurements individually. The results provide a consistent method for comparing distributions, and hence the controlling processes, between different areas of the near‐polar regions. The analysis provides clear evidence of the effects of heterogeneous processes in the atmosphere. Values for NO2, ClONO2, N2O5, and Cl2O2 are derived in a simplified steady state model based on in situ NO, ClO, O3, temperature, and pressure measurements; laboratory‐measured reaction rates; and modeled photodissociation rates. Values for the reservoir totals are independently derived from measurements of N2O, organic chlorine, and total reactive nitrogen. The relative abundances of the measured and derived species within the reservoirs are calculated, and the longer‐lived species HCl and HNO3 are estimated as the residuals of their respective reservoirs. The resulting latitude distributions in the Arctic outside the vortex agree reasonably well with predictions of a two‐dimensional photochemical model, indicating that partitioning in this region is largely controlled by standard homogeneous gas phase chemistry. Inside the Arctic vortex a large fraction of the HCl has been converted to reactive chlorine species ClO and Cl2O2, consistent with the extensive action of known heterogeneous reactions, presumably occurring on the surfaces of polar stratospheric clouds formed in the cold temperatures of the vortex. The partitioning in the Antarctic suggests that nearly the entire range of latitudes sampled by the ER‐2 is affected by heterogeneous processes in situ, including that portion of the “collar” region equatorward of the nominal chemically perturbed region (CPR). Consideration of heterogeneous processing in the region outside the CPR is important in predicting the possible expansion of Antarctic ozone depletion and the transport of chemically perturbed air to lower latitudes.

The heterogeneous reaction of HOCl + HCl → Cl2 + H2O on ice and nitric acid trihydrate: Reaction probabilities and stratospheric implications

The heterogeneous reaction HOCl + HCl → Cl2(g) + H2O(s) has been observed to proceed readily on cold water‐ice and nitric acid trihydrate (NAT) surfaces. For reactant partial pressures in the 10−7–10−5 torr range, the reaction probability (γ) on water‐ice is found to be 0.16 ± 0.10 at 202 K, and 0.24 +0.5/−0.15 at 195 K, as measured by the decay of HOCl vapor when the HCl partial pressure is above that of HOCl. On NAT, γ is found to have a strong dependence on the H2O vapor pressure of the NAT crystals: at 202 K, reaction probabilities of 0.17 are measured for H2O vapor pressures approaching those of ice (H2O‐rich NAT), whereas γ drops to 0.002 if the H2O pressure is lower by a factor of five (HNO3‐rich NAT). The yield of Cl2(g) relative to HOCl(g) loss is measured to be 0.87 ± 0.20. These results suggest that the heterogeneous process ClONO2 + HCl → Cl2(g) + HNO3(s), which occurs on polar stratospheric cloud surfaces, may be comprised of two steps: ClONO2 + H2O(s) → HOCl + HNO3(s), followed by HOCl + HCl → Cl2(g) + H2O(s). Also, the HOCl + HCl reaction may be directly of importance in the polar stratosphere providing an additional path for chlorine activation.

Polar ozone depletion: Current status

Rapid springtime depletion of column ozone (O3) is observed over the Antarctic during the austral spring. A much weaker springtime depletion is observed in the Arctic region. This depletion results from a complex chemical mechanism that involves the catalytic destruction of stratospheric ozone by chlorine. The chemical mechanism appears to operate between ~12–25 km in the colder regions of the polar winter vortices. During the polar night heterogeneous chemical reactions occur on the surface of polar stratospheric clouds that convert relatively inert reservoir Cl species such as HCl to active Cl species. These clouds form when temperatures drop below about 197 K and are ubiquitous throughout the polar winter region. At polar sunrise the reactive Cl species are photolysed, liberating large quantities of free Cl that subsequently catalytically destroys O3 with a mechanism involving the formation of the Cl2O2 dimer. The magnitude of the spring depletion is much greater in the Antarctic relative to the Arctic owing to the greater stability and longer duration of the southern polar vortex. Breakup of the intense high-latitude vortices in late (Antarctic) or early (Arctic) spring results in infilling of the ozone holes but adversely affects midlatitude ozone levels by diluting them with O3-depleted, ClO-rich high-latitude air. The magnitude of the Antarctic ozone depletion has been increasing since 1979 and its current depletion in October 1990 amounts to 60%. The increase in the size of the depletion is anticorrelated with increasing anthropogenic chlorofluorocarbon (CFCs) release. Adherence to the revised Montréal Protocol should result in a reduction of stratospheric halogen levels with subsequent amelioration of polar ozone depletion but the time constant for the atmosphere to return to pre-CFC levels is ~60–100 years.

Fourier transform infrared studies of model polar stratospheric cloud surfaces: Growth and evaporation of ice and nitric acid/ice

Fourier Transform Infrared (FTIR) surface studies have been used to probe the microphysical properties of nitric acid trihydrate (NAT) and ice films representative of type I and II polar stratospheric clouds (PSCs), respectively. The infrared spectra of NAT and nitric acid monohydrate (NAM) are obtained by dosing calibrated 3:1 and 1:1 mixtures of water and nitric acid onto a support at −90°C. The spectra, along with previously reported spectra for NAT and NAM, are used to monitor growth, transformation, and evaporation of ice and NAT films. Under our experimental pressure and temperature conditions, NAT is observed to grow at temperatures around 5°C higher than the ice frost point. This is in agreement with the conclusions drawn by Hanson and Mauersberger (1988a,b) from vapor pressure studies over ice and NAT solids. At temperatures 2°C warmer than NAT appearance, we observe growth of a film recently assigned by Ritzhaupt and Devlin (1990) as nitric acid dihydrate (NAD). Because it is formed at the highest temperatures, it is possible that NAD plays a role in PSC formation. However, the differences between the pressures used in the present study and atmospheric pressures, as well as uncertainties in measurements of absolute temperature, could explain NAD formation in our experiment that may not occur in the atmosphere. FTIR surface studies have also been used to probe the condensed products that result from the interaction of ice with gaseous nitric acid. The experiments indicate that upon initial exposure of ice to 1.8×10−6 torr of HNO3, a layer of ice is quantitatively converted to NAT. However, conversion of ice to NAT does not proceed indefinitely, but rather the system reaches saturation. For longer exposures or higher HNO3 pressures, NAM becomes the dominant nitric acid containing species on the surface. Evaporation studies were performed to test the feasibility of a recent denitrification mechanism proposed by Wofsy et al. (1990). The results indicate that ice coated with 0.02 μm of NAT evaporates at a temperature about 4°C higher than uncoated ice. This is consistent with the mechanism for stratospheric denitrification proposed by Wofsy et al. (1990).

ChemInform Abstract: Heterogeneous Reactions on Model Polar Stratospheric Cloud Surfaces: Reaction of N2O5 on Ice and Nitric Acid Trihydrate.

AbstractThe heterogeneous reaction of N2O5 on laboratory ice surfaces and nitric acid trihydrate (NAT) surfaces are studied in a Knudsen cell flow reactor.

Heterogeneous reactions on model polar stratospheric cloud surfaces: reaction of dinitrogen pentoxide on ice and nitric acid trihydrate

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTHeterogeneous reactions on model polar stratospheric cloud surfaces: reaction of dinitrogen pentoxide on ice and nitric acid trihydrateMichael A. Quinlan, Christa M. Reihs, David M. Golden, and Margaret A. TolbertCite this: J. Phys. Chem. 1990, 94, 8, 3255–3260Publication Date (Print):April 19, 1990Publication History Published online1 May 2002Published inissue 19 April 1990https://doi.org/10.1021/j100371a007RIGHTS & PERMISSIONSArticle Views119Altmetric-Citations57LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts

Heterogeneous conversion of COF2 to HF in polar stratospheric clouds

It is argued that the reaction COF2+H2O → 2HF+CO2 should proceed on surfaces of polar stratospheric clouds (PSCs), based on laboratory evidence for this reaction in condensed phase and on analysis of column observations of HF during the Airborne Arctic Stratospheric Expedition (AASE). If the hypothesis is confirmed, observations of COF2 and HF could provide unambiguous indication of heterogeneous processing of polar air, and could help elucidate the influence of heterogeneous chemistry on concentrations of HCl, ClNO3, and chlorine oxide radicals.