Chlorine Monoxide Research Articles

Chlorine monoxide in the Antarctic spring vortex: 2. A comparison of measured and modeled diurnal cycling over McMurdo Station, 1993

We have derived chlorine monoxide (ClO) mixing ratio profiles within the Antarctic vortex on an hourly basis from ground‐based measurements of pressure‐broadened emission line spectra. This data set has provided the first opportunity for a detailed comparison between the output of a photochemical model and the measured in situ diurnal behavior of ClO in the Antarctic spring stratosphere. We stress the importance of the diurnal behavior in furnishing a short‐term, crucial test of the catalytic chlorine chemistry which determines longer‐term ozone depletion. We obtain excellent agreement between our measured and modeled diurnal change using the rate constants recommended in the 1994 Jet Propulsion Laboratory (JPL) evaluation, giving support to current understanding of perturbed chlorine chemistry in the Antarctic spring vortex. We have furthermore found that we can use our data to narrow the listed 1994 JPL uncertainty range for the ClO dimer formation rate constant and the equilibrium constant between ClO dimer formation and thermal dissociation. We show that the new limits we set on the dimer formation rate constant reduce the uncertainty in the daily rate of chlorine catalyzed ozone loss calculated from observed ClO concentrations by ∼40% at 186–196 K. We find that a modeled total ozone loss rate including both chemistry and vertical transport, based on our measurements, agrees rather well with the amount and the linear trend of ozone loss seen throughout September in coincident balloon measurements.

Arctic ozone depletion observed by UARS MLS during the 1994–95 winter

During the unusually cold 1994–95 Arctic winter, the Microwave Limb Sounder observed enhanced chlorine monoxide (ClO) in late Dec and throughout Feb and early Mar. Late Dec ClO was higher than during any of the previous 3 years, consistent with the colder early winter. Between late Dec 1994 and early Feb 1995, 465 K (∼50 hPa) vortex‐averaged ozone (O3) decreased by ∼15%, with local decreases of ∼30%; additional local decreases of ∼5% were seen between early Feb and early Mar. Transport calculations indicate that vortex‐averaged chemical loss between late Dec and early Feb was ∼20% at 465 K, with ∼1/4 of that masked by downward transport of O3. This Arctic chemical O3 loss is not readily detectable in MLS column O3 data.

Chlorine monoxide in the Antarctic spring vortex: 1. Evolution of midday vertical profiles over McMurdo Station, 1993

We have obtained a prolonged record of emission spectra from chlorine monoxide in the vicinity of McMurdo Station, Antarctica, during formation of the austral spring ozone hole of 1993. These spectra have been processed to obtain vertical mixing ratio profiles by deconvolution of pressure‐broadened line shapes. The resulting profiles give a detailed evolution for both altitude distribution and mixing ratio of ClO during development of a major ozone hole event. In early September, very strong emission was observed from pressure‐broadened low‐altitude ClO. Deconvolutions show that this came from an unusually thick layer, extending well above 20 km in altitude. This layer decreased steadily in thickness through September, accompanied by a shift of the peak mixing ratio from ∼21 km altitude in early September to ∼17–18 km by the end of the month, indicating an apparent descent rate of order 100 meters per day, although we argue that the true descent rate is probably lower than the apparent rate. A brief, significant decrease in ClO content occured in late September when the inner vortex edge (defined by the magnitude of Ertel's potential vorticity = 5.2·10−5 at ∼19–20 km) approached McMurdo, signifying that a strong gradient in ClO exists near the inner vortex edge. A rapid and apparently final deactivation of chlorine in the lower stratosphere was observed to start about October 1–2. The findings of initially large values of ClO well above 20 km are consistent with observation of polar stratospheric cloud formation in this range during the austral winter of 1993, and with observations showing increased ozone depletion above 20 km relative to previous years.

Comparison of ClO measurements by airborne and spaceborne microwave radiometers in the Arctic winter stratosphere 1993

In February 1993 measurements of chlorine monoxide ClO, one of the key substances in catalytic ozone destruction, were performed over Scandinavia by two microwave receivers, the Submillimeter Atmospheric Sounder (SUMAS) on board the German research aircraft FALCON and the Microwave Limb Sounder (MLS) on board the Upper Atmospheric Research Satellite (UARS). High ClO concentrations (>1ppb) inside the polar vortex at approximately 20km altitude were detected by both experiments. A comparison shows good agreement of both sensors in the location of enhanced ClO.

Interhemispheric Differences in Polar Stratospheric HNO 3 , H 2 O, ClO, and O 3

Simultaneous global measurements of nitric acid (HNO(3)), water (H(2)O), chlorine monoxide (CIO), and ozone (O(3)) in the stratosphere have been obtained over complete annual cycles in both hemispheres by the Microwave Limb Sounder on the Upper Atmosphere Research Satellite. A sizeable decrease in gas-phase HNO(3) was evident in the lower stratospheric vortex over Antarctica by early June 1992, followed by a significant reduction in gas-phase H(2)O after mid-July. By mid-August, near the time of peak CIO, abundances of gas-phase HNO(3) and H(2)O were extremely low. The concentrations of HNO(3) and H(2)O over Antarctica remained depressed into November, well after temperatures in the lower stratosphere had risen above the evaporation threshold for polar stratospheric clouds, implying that denitrification and dehydration had occurred. No large decreases in either gas-phase HNO(3) or H(2)O were observed in the 1992-1993 Arctic winter vortex. Although CIO was enhanced over the Arctic as it was over the Antarctic, Arctic O(3) depletion was substantially smaller than that over Antarctica. A major factor currently limiting the formation of an Arctic ozone "hole" is the lack of denitrification in the northern polar vortex, but future cooling of the lower stratosphere could lead to more intense denitrification and consequently larger losses of Arctic ozone.

Kinetics and mechanism of the reaction of CH3 and CH3O with ClO and OClO at 298 K

A discharge-flow system equipped with a laser-induced fluorescence cell to detect the methoxyl radical and a quadrupole mass spectrometer to detect both the chlorine monoxide and chlorine dioxide radicals has been used to measure the rate constants for the reactions CH3+ ClO → products (1), CH3O + ClO → products (2), CH3+ OClO → products (3), CH3+ OClO → products (4), at T= 298 K and P= 1–3 Torr. The observed products of these reactions are CH3O for reaction (1), HOCl for reaction (2), and CH3O and ClO for reaction (3). For reaction (4), CH3OCl is a possible product. The rate constants derived for reactions (1)–(4) are: k1=(1.3 ± 0.4)× 10–10 cm3 molecule–1 s–1; k2=(1.3 ± 0.3)× 10–11 cm3 molecule–1 s–1; k3=(1.6 ± 0.3)× 10–11 cm3 molecule–1 s–1; k4=(1.5 ± 0.5)× 10–12 cm3 molecule–1 s–1. The likely mechanisms for reactions (1)–(4) are briefly discussed.

Chemical depletion of ozone in the Arctic lower stratosphere during winter 1992–93

Satellite observations of ozone and chlorine monoxide concentrations during winter 1992–1993 show that in February 1993 chlorine in the lower stratosphere was mostly in chemically reactive forms. Decreases in stratospheric ozone concentration during February and early March 1993 are consistent with chemical destruction by this reactive chlorine. Comparison with changes in the distribution of long-lived chemical and dynamical tracers shows that the observed decrease cannot have been caused solely by dynamical processes.

Aircraft measurements of CLO and HCL during EASOE 1991/92

As part of the European Arctic Stratospheric Ozone Experiment (EASOE), performed in the winter period 1991/92, stratospheric chlorine monoxide (ClO) and hydrochloric acid (HCl) have been observed using the Submillimeter Atmospheric Sounder (SUMAS). The instrument measures the thermal emission of the atmosphere in the frequency range 620–650 GHz. Due to strong tropospheric water vapor absorption in this frequency range the radiometer has to be operated on‐board a high‐flying aircraft. During EASOE several flight missions were performed over northern Europe in the periods 10–13 December 1991, 5–14 February 1992 and 7–13 March 1992 using the research aircraft FALCON operated by the German Air and Space Organization (DLR). We report on two flights in February and two in March. From a first analysis it is found that the HCl column content as observed during the March flight, increased by about 20–30% compared to the results for the mid‐February flight. On the other hand, high ClO amounts, particularly at lower altitudes, were observed in February. From the observed trends for the ClO and HCl abundances we assume that some HCl had been converted to reactive chlorine indicating a chemically disturbed Arctic vortex in February.

Millimeter‐wave measurements of chlorine monoxide at the Jungfraujoch Alpine Station

During the European Arctic Stratospheric Ozone Experiment (EASOE) in winter 91/92, first measurements with a new 204GHz radiometer were performed at the Jungfraujoch Alpine Station (7°59′E 46°33′N) in the Swiss Alps. The altitude of this site (3570m) allows detection of weak spectral lines like the ones of the chlorine monoxide J 11/2→9/2 rotational transition. At the end of January 1992, a large amount of ClO in the lower stratosphere was measured, indicating that highly perturbed airmasses had been transported towards mid latitudes. The maximum value of the ClO volume mixing ratio was about 1ppbv +/−0.4 ppbv in heights of 15–25km.

An overview of millimeter‐wave spectroscopic measurements of chlorine monoxide at Thule, Greenland, February–March, 1992: Vertical profiles, diurnal variation, and longer‐term trends

Measurements of chlorine monoxide in the stratosphere over Thule, Greenland (76.3N, 68.4W) were made quasi‐continuously during the period February 8 to March 24, 1992, using a high‐sensitivity ground based mm‐wave spectrometer. These observations give diurnal, short term, and long term changes in the mixing ratio and vertical distribution of ClO. At an equivalent time after the Antarctic winter solstice, very large concentrations (up to ∼1.5 ppbv) occur in lower stratospheric ClO, resulting in massive ozone destruction. We saw no evidence for large (∼1 to 1.5 ppbv) amounts of ClO in the 16–25 km range over Thule in February or March, in agreement with UARS (satellite) observations by the MLS mm‐wave spectrometer for this period, and in marked contrast to UARS/MLS and ER‐2 aircraft measurements over northern Europe and eastern Canada, respectively, during January, 1992. We have evidence for smaller enhancements (∼0.2 to 0.5 ppbv) in the 18–30 km range during late February‐early March, which could result from transport of residual low NO2 air following earlier PSC processing (the last of which occurred at least one month earlier, however) or the result of chemical processing by Pinatubo aerosols. Direct influence of Pinatubo aerosols on Arctic ozone during the spring of 1992 has been difficult to assess, and this enhancement of low‐altitude ClO might be a significant indicator of aerosol effects.

Chemistry of the 1991–1992 stratospheric winter: Three‐dimensional model simulations

A three‐dimensional chemistry‐transport model of the stratosphere is used to simulate the evolution of trace constituents during the 1991–1992 Arctic winter. It is shown that heterogeneous reactions on polar stratospheric clouds led in early January to almost complete activation of atmospheric chlorine inside the polar vortex, in remarkable coincidence with observations by the ER‐2 aircraft (Toohey et al., 1993) and the microwave limb sounder on the Upper Atmosphere Research Satellite (Waters et al., 1993). Sulfate aerosols resulting from the eruption of Mount Pinatubo also produced a significant increase in chlorine monoxide (ClO) concentrations at middle and high latitudes. The net chemical destruction of ozone found in the vortex at the end of the simulation (25% at 50 hPa and 25 DU), although substantial, was limited by available sunlight and the short period during which stratospheric clouds occurred.

Arctic chlorine monoxide observations during spring 1993 over Thule, Greenland, and implications for ozone depletion

We have determined the vertical distribution of chlorine monoxide (ClO), from measurements of pressure-broadened molecular-emission spectra made over Thule, Greenland, during the 1993 Arctic spring. The measurements show a weak lower stratospheric layer of chlorine monoxide inside the vortex in late February, which was, however, significantly greater in mixing ratio than that seen in observations we made in the spring of 1992. ClO was also observed in much smaller quantities in early to mid-March 1993 when Thule was outside the vortex. The amount of ClO within the vortex was severely reduced by the time it returned over Thule in late March. This reduction occurred several weeks earlier relative to the winter solstice than the decline of ClO inside the Antarctic vortex in 1993. The enhanced Arctic lower stratospheric layer seen in late February 1993 at a nearly equivalent photochemical period, and beyond. We have calculated daily ozone loss rates, due primarily to the dimer chlorine catalytic cycle, from both sets of measurements. The vertical integral of the Arctic daily percentage ozone loss when the largest ClO levels were present, at the end of February, is found to be approximately one quarter of that in the Antarctic at a photochemical period only 1 week later. The relative weakness of daily ozone depletion, combined with the early disappearance of ClO in the Arctic, suggests that hemispheric dilution by ozone-poor air from within the Arctic vortex is unlikely to be sufficient to explain the historically extreme loss of midlatitude northern hemisphere ozone which began in 1992 and persisted throughout 1993.

Use of Fourier transforms for asynoptic mapping: Applications to the upper atmosphere research satellite microwave limb sounder

Fourier analysis has been applied to data obtained from limb viewing instruments on the Upper Atmosphere Research Satellite. A coordinate system rotation facilitates the efficient computation of Fourier transforms in the temporal and longitudinal domains. Fields such as ozone (O3), chlorine monoxide (ClO), temperature, and water vapor have been transformed by this process. The transforms have been inverted to provide maps of these quantities at selected times, providing a method of accurate time interpolation. Maps obtained by this process show evidence of both horizontal and vertical transport of important trace species such as O3 and ClO. An examination of the polar regions indicates that large‐scale planetary variations are likely to play a significant role in transporting midstratospheric O3 into the polar regions. There is also evidence that downward transport occurs, providing a means of moving O3 into the polar vortex at lower altitudes. The transforms themselves show the structure and propagation characteristics of wave variations.

High-Resolution Real Time Laser Absorption Spectroscopy CIO A2Π-X2Π

Chlorine monoxide has been produced by photolysis of ozone in the presence of chlorine at a total pressure of 3.5 Torr. The absorption spectrum has been determined with a spectral resolution of 0.002 nm for all 35ClO (A2Π3/2v′-X2Π3/2v″ = 0) subbands from v′ = 7 up to the convergence limit (293-262 nm). The experimental absorption spectra for each subband have been reproduced closely by spectral simulation employing approximate treatments of the spin-orbit and isotopic dependencies of the spectroscopic constants. Lorentzian linewidths reported for 35ClO (A2Π3/2-X23/2) subbands clarify several discrepancies in previous reports. The region of spectral convergence is presented and values of D0 for the A and X states of ClO are reported as 6335 ± 7 and 22,049 ± 7 cm−1 respectively, in good agreement with previous reports (J. A. Coxon and D. A. Ramsay, Can. I. Phys.54, 1034-1042, 1976).

Balloon‐borne measurements of CLO, NO, and O3 in a volcanic cloud: An analysis of heterogeneous chemistry between 20 and 30 km

Balloon profiles of chlorine monoxide (ClO), nitric oxide (NO), and ozone (O3) were measured on March 11, 1992 from 100 to 10 mb over Greenland (67.0°N, 50.6°W). Measurements from SAGE II indicate that the aerosol surface area in the region was enhanced by sulfur from the eruption of Mt. Pinatubo, reaching 50 times background near 20 km. Concentrations of ClO were enhanced and concentrations of NO were suppressed relative to low aerosol conditions consistent with the effects of hydrolysis of N2O5 on the surface of sulfuric acid aerosols. The data are consistent with a value of 2×10−4 for the reaction probability of the heterogeneous hydrolysis of ClONO2, indicating a minor role for this reaction at a temperature of 220 K. At these temperatures, we find no evidence for the catastrophic loss of ozone predicted to occur under conditions of enhanced aerosol surface area.

Temperature-dependent rate constants and product branching ratios for the gas-phase reaction between methylperoxy and chlorine monoxide radicals

Kinetic and product branching data have been measured for the reaction of CH 3 O 2 radicals with ClO in the temperature range 225-355 K using the discharge-flow /mass spectrometry technique. The pressure-independent overall reaction rate constant is described by k (12) (225-355 K)=(3.25±0.50)×10 -12 exp((-114±38)/T) cm 3 molecule -1 s -1 . Two reaction channels were identifiied, leading to CH 3 O+ClOO (12a) and CH 3 OCl+O 2 (12b), respectively

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.

The Seasonal Evolution of Reactive Chlorine in the Northern Hemisphere Stratosphere

In situ measurements of chlorine monoxide (ClO) at mid- and high northern latitudes are reported for the period October 1991 to February 1992. As early as mid-December and throughout the winter, significant enhancements of this ozone-destroying radical were observed within the polar vortex shortly after temperatures dropped below 195 k. Decreases in ClO observed in February were consistent with the rapid formation of chlorine nitrate (ClONO(2)) by recombination of ClO with nitrogen dioxide (NO(2)) released photochemically from nitric acid (HNO(3)). Outside the vortex, ClO abundances were higher than in previous years as a result of NOx suppression by heterogeneous reactions on sulfate aerosols enhanced by the eruption of Mount Pinatubo.

Chlorine Chemistry on Polar Stratospheric Cloud Particles in the Arctic Winter

Simultaneous in situ measurements of hydrochloric acid (HCl) and chlorine monoxide (ClO) in the Arctic winter vortex showed large HCl losses, of up to 1 part per billion by volume (ppbv), which were correlated with high ClO levels of up to 1.4 ppbv. Air parcel trajectory analysis identified that this conversion of inorganic chlorine occurred at air temperatures of less than 196 +/- 4 kelvin. High ClO was always accompanied by loss of HCI mixing ratios equal to (1/2)(ClO + 2Cl(2)O(2)). These data indicate that the heterogeneous reaction HCl + ClONO(2) --> Cl(2) + HNO(3) on particles of polar stratospheric clouds establishes the chlorine partitioning, which, contrary to earlier notions, begins with an excess of ClONO(2), not HCl.

In Situ Observations of Aerosol and Chlorine Monoxide After the 1991 Eruption of Mount Pinatubo: Effect of Reactions on Sulfate Aerosol

Highly resolved aerosol size distributions measured from high-altitude aircraft can be used to describe the effect of the 1991 eruption of Mount Pinatubo on the stratospheric aerosol. In some air masses, aerosol mass mixing ratios increased by factors exceeding 100 and aerosol surface area concentrations increased by factors of 30 or more. Increases in aerosol surface area concentration were accompanied by increases in chlorine monoxide at mid-latitudes when confounding factors were controlled. This observation supports the assertion that reactions occurring on the aerosol can increase the fraction of stratospheric chlorine that occurs in ozone-destroying forms.