Observations Of ClO Research Articles

Evidence for heterogeneous chlorine activation in the tropical UTLS

Abstract. Airborne in-situ observations of ClO in the tropics were made during the TROCCINOX (Aracatuba, Brazil, February 2005) and SCOUT-O3 (Darwin, Australia, November/December 2005) field campaigns. While during most flights significant amounts of ClO (≈10–20 parts per trillion, ppt) were present only in aged stratospheric air, instances of enhanced ClO mixing ratios of up to 40 ppt – significantly exceeding those expected from gas phase chemistry – were observed in air masses of a more tropospheric character. Most of these observations are associated with low temperatures or with the presence of cirrus clouds (often both), suggesting that cirrus ice particles and/or liquid aerosol at low temperatures may promote significant heterogeneous chlorine activation in the tropical upper troposphere lower stratosphere (UTLS). In two case studies, particularly high levels of ClO observed were reproduced by chemistry simulations only under the assumption that significant denoxification had occurred in the observed air. However, to reproduce the ClO observations in these simulations, O3 mixing ratios higher than observed had to be assumed, and at least for one of these flights, a significant denoxification is in contrast to the observed NO levels, suggesting that the coupling of chlorine and nitrogen compounds in the tropical UTLS may not be completely understood.

Understanding the kinetics of the ClO dimer cycle

Abstract. Among the major factors controlling ozone loss in the polar vortices in winter/spring is the kinetics of the ClO dimer catalytic cycle. Here, we propose a strategy to test and improve our understanding of these kinetics by comparing and combining information on the thermal equilibrium between ClO and Cl2O2, the rate of Cl2O2 formation, and the Cl2O2 photolysis rate from laboratory experiments, theoretical studies and field observations. Concordant with a number of earlier studies, we find considerable inconsistencies of some recent laboratory results with rate theory calculations and stratospheric observations of ClO and Cl2O2. The set of parameters for which we find the best overall consistency – namely the ClO/Cl2O2 equilibrium constant suggested by Plenge et al. (2005), the Cl2O2 recombination rate constant reported by Nickolaisen et al. (1994) and Cl2O2 photolysis rates based on absorption cross sections in the range between the JPL 2006 assessment and the laboratory study by Burkholder et al. (1990) – is not congruent with the latest recommendations given by the JPL and IUPAC panels and does not represent the laboratory studies currently regarded as the most reliable experimental values. We show that the incorporation of new Pope et al. (2007) Cl2O2 absorption cross sections into several models, combined with best estimates for other key parameters (based on either JPL and IUPAC evaluations or on our study), results in severe model underestimates of observed ClO and observed ozone loss rates. This finding suggests either the existence of an unknown process that drives the partitioning of ClO and Cl2O2, or else some unidentified problem with either the laboratory study or numerous measurements of atmospheric ClO. Our mechanistic understanding of the ClO/Cl2O2 system is grossly lacking, with severe implications for our ability to simulate both present and future polar ozone depletion.

Spaceborne ClO observations by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) before and during the Antarctic major warming in September/October 2002

The first ClO profiles retrieved from limb emission spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) Fourier transform spectrometer on board the European research satellite ENVISAT (Environmental Satellite) are presented. These measurements were taken between 8 September and 13 October 2002. They cover the Antarctic major warming around 25 September, an event which had never been observed before. Since ClO has only weak signatures in the midinfrared, more than a dozen of the strongest lines, all situated in the central P branch and Q branch region of the 1‐0 band, were used for profile retrievals, which were based on constrained nonlinear least squares fitting. During the period of investigation, MIPAS measured typical ClO volume mixing ratios (vmrs) between 1 ± 0.4 and 2 ± 0.6 ppbv from 8 September until 23 September in contiguous parts of the dayside lower Antarctic stratosphere (400–625 K levels of potential temperature, Θ), i.e., before and at the beginning of the major warming. After 23 September no significant amounts of ClO were detected anymore, apart from some remains on 26 September. Daily averages of inside vortex daytime ClO showed a slight increase until about 17 September and a subsequent rapid ClO decrease. Further, a positive response of inside vortex nighttime ClO to increasing temperatures due to enhanced Cl2O2 dissociation was detected. Comparison with measurements of the Microwave Limb Sounder (MLS) on the Upper Atmospheric Research Satellite (UARS) from the years 1992–1997 generally showed a similar temporal development. Thus the warming event obviously happened too late to lead to a premature chlorine deactivation significantly different from previous Antarctic winters.

Law of mass action in the Arctic lower stratospheric polar vortex January–March 2000: ClO scaling and the calculation of ozone loss rates in a turbulent fractal medium

We consider the effects of power law scaling in the 1999–2000 Arctic lower stratospheric vortex from the point of view of the law of mass action and its application to the chemical kinetics of ozone loss embedded in a turbulent, macroscopic, fractal medium. The ER‐2 observations of ClO obey power law scaling; the exponent varies with time in a manner shown to be consistent with the scaling of NOy and O3, via the influences of polar stratospheric clouds and actinic solar radiation. While the microscopic rate coefficient for ClO three‐body recombination to the dimer applies as measured to three‐dimensional volumes in which the sole transport mechanism is molecular diffusion, this cannot be true in the 2.56‐dimensional space in which macroscopically fluctuating ClO reacts in the lower stratosphere. We show that the rate of loss of ozone via the ClO dimer mechanism is proportional to [ClO]2.20 in late January/early February and to [ClO]2.55 in March. Chemical ozone loss had already occurred by the date of the first flight, 20000120.

On the unexplained stratospheric ozone losses during cold Arctic Januaries

Using a combination of data from Match, POAM II, POAM III and MLS we show that the chemical loss rate of Arctic O3 during January of four cold winters (1992, 1995, 1996, and 2000) is consistently faster than can be accounted for by assuming complete activation of reactive chlorine and standard reaction kinetics. However, O3 loss rates measured during late February and early March 1996 are shown to be consistent with observations of ClO. The faster than expected O3 loss rates during January are shown to occur when air parcels are illuminated at high solar zenith angles (SZAs between ∼85 and 94°), and to result in cumulative O3 loss of ∼0.5 ppmv. The cause of the rapid January O3 loss is unclear, but may be related to a photolytic process at high SZA that is poorly represented by current photochemical models.

Tests of halogen photochemistry using in situ measurements of ClO and BrO in the lower polar stratosphere

In situ observations of the halogen oxides ClO and BrO made from the NASA ER‐2 during the Airborne Arctic Stratospheric Expedition (AASE) I and II missions are used to test current understanding of photochemical parameters. Measurements of ClO obtained during AASE I in the dark perturbed polar vortex are analyzed with respect to temperature to derive the equilibrium expression for the ClO/Cl2O2 system. Assuming photochemical steady state and complete activation of chlorine (ClO + 2Cl2O2 = Cly), observations of ClO made during AASE II are used to derive the photolysis rate of Cl2O2. The photolysis rate derived from atmospheric observations is compared to J values calculated with a photochemical model and various values for the absorption cross section of Cl2O2. The photolysis rate calculated with the cross section of Huder and DeMore [1995] is shown to be systematically too small, while those of Burkholder et al. [1990] and Cox and Hayman [1988] are too large to be consistent with atmospheric observations. Observations of BrO made during AASE II indicate that our understanding of the inorganic bromine budget in the polar regions is incomplete. A possible role for the adduct BrOOCl is investigated.

Mechanisms for midlatitude ozone loss: Heterogeneous chemistry in the lowermost stratosphere?

The question of midlatitude ozone erosion by chlorine free radical catalysis is examined. We present and analyze simultaneous, high‐resolution observations of ClO, H2O, tropopause height, particle reactive surface area, and ice saturation occurrence frequency obtained from the NASA ER‐2 aircraft. The objective is to test the hypothesis that the existence of cirrus clouds or cold aerosols in the first few kilometers above the tropopause at midlatitudes is responsible for increasing the ratio of chlorine free radicals to total inorganic chlorine, thus amplifying the rate of catalytic ozone destruction. The observations reveal a sharp decrease in ice saturation frequency at the tropopause, a marked degree of under‐saturation just above the tropopause, a corresponding sharp gradient in the product of cold aerosol reactive surface area and reaction probability, γSa, and, finally, the consistent absence of enhanced concentrations of ClO immediately above the tropopause. These results suggest that midlatitude ozone loss is not controlled in situ by the mechanism of cirrus cloud and/or cold aerosol enhancement of chlorine radicals in the vicinity of the tropopause.

Model study of stratospheric chlorine activation and ozone loss during the 1996/1997 winter

Chlorine activation and ozone depletion in the Arctic winter stratosphere of 1996–1997 have been studied with a newly developed stratospheric chemistry‐transport model (CTM). The chemistry scheme, using a Euler Backward Iterative approximation method, includes a comprehensive set of reactions on ternary aerosol and ice particles, which has been tested against a numerically exact solver. Tracer transports in the CTM are calculated from European Centre for Medium‐Range Weather Forecasts (ECMWF) meteorological analyses. Comparisons have been made with O3 and ClO measurements, and with ozone loss rates derived from observations during February and March 1997. ClO production and ozone depletion are somewhat underestimated by the model. Furthermore, uncertainties regarding the aerosol phase are tested. Assuming nitric acid trihydrate (NAT) particles to form at their melting point, while liquid aerosol is present simultaneously in the model, gives rise to the largest ClO production and the strongest ozone depletion. By correcting for an ECMWF temperature warm bias we obtain a similar large effect on calculated ClO production and ozone depletion for the 1996/1997 Arctic winter, whereas uncertainties in the chlorine abundance seem less important. An average warm bias of 1.3 K at polar stratospheric cloud temperatures on the 50 hPa model level reduces the calculated ozone depletion rates over February and March by 35%. Observations of ClO are reproduced when lower temperatures and maximum Cly abundance are assumed, but ozone depletion is slightly overestimated in that case.

An Investigation of CIO Photchemistry in the Chemically Perturbed Arctic Vortex

A new lightweight in situ instrument designed to measure ClO was flown on a balloon launched into the arctic vortex at dawn on February 3, 1995 at Kiruna, Sweden during the Second European Stratospheric Arctic and Mid-latitude Experiment (SESAME), together with instruments to measure ozone and long-lived tracers. Observations on ascent and descent at different solar zenith angles are compared to results from Lagrangian and box model calculations that assume the airmasses at similar potential temperatures had comparable photochemical histories. Between 20 and 22 km, in a region where ClO was significantly enhanced, a model constrained by currently recommended rate parameters significantly underestimates the abundances of ClO that were observed on ascent at high solar zenith angles, whereas the agreement is much better if a smaller ClO-Cl2O2 equilibrium constant, one inferred from previous ER-2 aircraft observations of ClO in the Arctic during nighttime, is assumed. On ascent, ClO is additionally enhanced in a narrow region between 20 and 21 km. We believe the most plausible explanation for this feature is rapid photolysis of OClO produced by the slow bimolecular reaction ClO + ClO over the 48 hours prior to the observations when the airmass was warmed to 225 K by adiabatic compression while in polar darkness. These results suggest that under special circumstances, OClO can be produced by a reaction other than one involving BrO, and, hence, OClO is not necessarily a universal proxy for BrO abundances in the perturbed polar vortex.

Implications of upper stratospheric trace constituent changes observed by HALOE for O3 and ClO from 1992 to 1995

Measurements from the Halogen Occultation Experiment (HALOE) of the upper stratosphere show increases in HCl and H2O and decreases in CH4 and O3 during the period 1992–1995. These changes all coincide with the decline of solar cycle 22. Using a simple photochemical model, we find that 4 major components contribute to the O3 decrease: 1) an increase in total chlorine as indicated by increasing HCl 2) an additional increase in reactive chlorine due to repartitioning of Cly by the decreasing CH4 3) a decrease in odd oxygen production due to decreased solar flux and 4) an increase in odd hydrogen loss due to increasing H2O. At 2 mbar, the Cly repartitioning is the largest cause of O3 changes. Because the Cly repartitioning coincides with decreasing solar flux, some recent observational estimates of the long‐term response of upper stratospheric O3 to solar UV irradiance variability may be too large. Compared with the HALOE O3 data, the model O3 exhibits a larger negative trend. This appears to be because the model O3 is more sensitive to the increased H2O than is the observed O3. The 20–30% decline in CH4 also implies a large increase in ClO which exceeds that expected from CFCs. Observations of ClO from the Microwave Limb Sounder (MLS) support this inference.

An estimate of the effect of unresolved structure on modeled ozone loss from aircraft observations of ClO

High resolution aircraft observations of ClO are used to estimate how much the local ozone loss rate due to the ClO dimer reaction is underestimated when small scale structure in ClO is not fully resolved. A local resolution error in the ozone loss rate, for a model with grid scale L, is defined in terms of the variance of the observations within data segments of length L. Errors in the northern and southern winter polar regions are similiar in magnitude, and are largest for gridboxes near the vortex edge. At the edge of the Arctic vortex, the resolution error increases linearly from 5–35% as the grid scale increases from 100–500 km. Errors are much smaller inside the vortex where [ClO]≥0.7 ppbv, increasing from 2–10% over the same range of grid scales.

Interhemispheric asymmetry in the 1 mbar O3 trend: An analysis using an interactive zonal mean model and UARS data

Trends in O3 calculated from solar backscattered ultraviolet (SBUV) observations near 1 mbar are more negative at high latitudes in the southern hemisphere than in the northern hemisphere [Hood et al., 1993]. A mechanism is presented that produces an interhemispheric O3 trend asymmetry similar to the observed asymmetry in the Goddard Space Flight Center dynamically interactive zonal mean model. Upper Atmosphere Research Satellite (UARS) data are then examined for evidence that the atmospheric trend asymmetry is produced by a similar mechanism. The model O3 trend asymmetry is mainly due to interhemispheric differences in odd chlorine (Cly) partitioning. The asymmetry in Cly partitioning is caused primarily by lower amounts of CH4 and NO in the southern hemisphere than the northern hemisphere high latitudes, due to differences in the dynamical behavior of the model hemispheres. Symmetric increases in Cly are accompanied by a southern hemisphere increase in ClO that is larger than in the northern hemisphere. Concentrations of CH4 and N2O retrieved by the cryogenic limb array étalon spectrometer aboard UARS during 1992 are lower in the southern hemisphere fall and winter seasons at high latitudes in the upper stratosphere than in the northern hemisphere, favoring higher southern hemisphere ClO values. However, observations of ClO by the microwave limb sounder on UARS do not show consistently higher values in the southern hemisphere compared with the northern hemisphere in 1992, 1993, or 1994. The UARS data therefore do not confirm that a mechanism similar to the model mechanism occurs in the real atmosphere and is the cause of the SBUV O3 trend asymmetry.

MLS observations of ClO and HNO3 in the 1996–97 Arctic Polar Vortex

Microwave Limb Sounder (MLS) measurements of lower stratospheric ClO and gas‐phase HNO3 are presented for the 1996–97 Arctic winter. The horizontal and vertical extent of enhanced ClO were smaller in 1997 than in 1996, as was the degree of enhancement. This is consistent with differences in the evolution and vertical structure of temperatures in the two years. Gas‐phase HNO3 abundances, which increased in early winter due to diabatic descent, exhibited a decreasing trend from mid to late winter in all six Arctic winters observed by MLS, probably due to increasing HNO3 photolysis throughout this period.

Three‐dimensional studies of the 1991/1992 northern hemisphere winter using domain‐filling trajectories with chemistry

We describe a new and computationally efficient technique for global three‐dimensional modeling of stratospheric chemistry. This technique involves integrating a photochemical package along a large number of independent trajectories to produce a Lagrangian view of the atmosphere. Although Lagrangian chemical modeling with trajectories is an established procedure, this extension of integrating chemistry along a large number of domain‐filling trajectories is a novel technique. This technique is complementary to three‐dimensional Eulerian chemical transport modeling and avoids spurious mixing caused by low resolutions or diffusive transport schemes in these models. We illustrate the technique by studying the chlorine activation in the Arctic winter lower stratosphere. A photochemical model was integrated along large ensembles of calculated trajectories between 20 and 100 mbar for the 1991/1992 winter in order to produce a three‐dimensional chemical picture. Large amounts of chlorine was activated at low altitudes (80 to 100 mbar) as well as altitudes near 50 mbar. This activated air was well contained at all levels, with little indication of mixing into lower latitudes. Model results for early January 1992 were compared to daily Microwave Limb Sounder (MLS) ClO observations at 465 K. The structure and evolution of the activated chlorine was well reproduced, giving faith in the technique, although absolute modeled ClO amounts were smaller than the MLS data. A larger number of domain‐filling isentropic trajectories were also run at 475 K to produce a higher‐resolution picture of vortex evolution in late January 1992. The model successfully reproduced the wave breaking events which characterized this period causing transport of activated air to lower latitudes.

Limits on heterogeneous processing in the Antarctic spring vortex from a comparison of measured and modeled chlorine

Forty‐day photochemical model runs are compared with ground‐based stratospheric ClO observations taken during the austral spring of 1993. Our purpose is to explore the range of required heterogeneous processing within which we can reproduce the duration and degree of chlorine activation within the Antarctic spring vortex. Heterogeneous processing on nitric acid trihydrate (NAT) polar stratospheric clouds (PSCs) or supercooled ternary solution (STS) ‐type particles is shown to be necessary to maintain chlorine in active forms during ozone hole formation in September, even for small HNO3 amounts, or chlorine deactivates sooner than observed. The lower limits for the surface areas required are quite small, however. Thus the record ozone losses observed during September of 1993 may be attributed to catalytic loss due to chlorine maintained in active forms by heterogeneous processing despite the sparse particle loading of the Antarctic lower stratosphere at that time. The ozone loss rates predicted by the model during the formation of the springtime Antarctic ozone hole indeed agree quite well with observations. The one‐dimensional model is also able to reproduce both the observed timing and rate for subsequent deactivation of chlorine. Renitrification from PSC evaporation is not required for this deactivation, as HCl reformation is very rapid at low ozone values.

Analysis of UARS data in the southern polar vortex in September 1992 using a chemical transport model

We have used a new, isentropic‐coordinate three‐dimensional chemical transport model to investigate the decay of ClO and evolution of other species in the Antarctic polar vortex during September 1992. The model simulations cover the same southern hemisphere period studied in a companion data paper by Santee et al. [this issue]. The model is initialized using the available data from the Microwave Limb Sounder (MLS) and Cryogenic Limb Array Etalon Spectrometer (CLAES) on the Upper Atmosphere Research Satellite (UARS). During the model initialization chemical inconsistencies in the UARS data became evident. Fields of odd nitrogen (NOy) derived from CLAES N2O underestimated the sum of the direct observations of the major NOy species. Results from the model integrations at 465 K and 585 K are sampled in the same way as the various UARS instruments and compared to the observations both directly and by considering average quantities in the inner and edge vortex regions. Sampling the observed species in the same way as the UARS instruments is important in removing any spurious trends due, for example, to changing solar zenith angle. While the model can reproduce the magnitude of the MLS ClO observations at 585 K, this is not possible at 465 K. The model partitions too much ClO into Cl2O2 to reproduce the observed ClO which is around 2.0 parts per billion by volume (ppbv) averaged within the polar vortex. The model also underestimates CLAES ClONO2 in the inner vortex at 465 K due to heterogeneous processing. The observations require that effectively all of the inorganic chlorine is in the form of ClO and ClONO2 in the inner vortex at this altitude. In the basic model run, the decay of ClO produces ClONO2 which is not observed by CLAES. Our results indicate the potential importance of the speculative reaction between OH and ClO producing HCl for the recovery of HCl in the Antarctic spring. By including this reaction, the decay of model ClO into HCl is enhanced, yielding better agreement with HCl data from the Halogen Occultation Experiment (HALOE) data. Similar results can also be obtained by including the reaction between HO2 and ClO to produce HCl with a 3% channel. The model generally reproduces the observed O3 destruction during September. The most significant discrepancy for O3 is in the inner vortex at 465 K where the model underestimates the observed O3 loss rate, especially when the effects of vertical motion are included.

The chlorine budget of the lower polar stratosphere: Upper limits on ClO, and Implications of new Cl2O2 photolysis cross sections

Chlorine catalytic chemistry, which destroys ozone while cycling chlorine between Cl, ClO, and Cl2O2, is the primary cause of the springtime Antarctic ozone hole. We have calculated the concentrations of Cl2O2 which are in equilibrium with midday ground‐based, aircraft, and satellite observations of ClO in the Antarctic spring lower stratosphere. Two significant conclusions are presented here: (1) Using the JPL 94 recommended rates and photolysis cross sections, more than ∼2.0 ppbv ClO in the polar lower stratosphere causes inferred total active chlorine to exceed the total chlorine budget. This limit is smaller than some reported ClO measurements. (2) Using smaller cross sections recently measured by Huder and DeMore [1995], the amount of Cl2O2 in midday equilibrium with measured ClO is approximately doubled. Activated chlorine inferred from many measurements then exceeds total chlorine in the lower stratosphere, suggesting these cross sections may be too small.

In situ measurements of CLO at mid‐latitudes: Is there an effect from Mt. Pinatubo?

Observations of ClO from 20 to 60°N made before and after the eruption of Mt. Pinatubo are compared for changes which may result from increased sulfate aerosol surface area. Using ozone as a vertical coordinate and examining data at similar latitudes and seasons, elevated abundances of ClO are found at low latitudes (20 to 30°N), an effect which decreases with increasing latitude. For the flights compared, there appear to be no differences, within the uncertainty of the measurements, at latitudes poleward of 40°N. These results are consistent with the idea that the hydrolysis of N2O5 on sulfate aerosols becomes saturated at moderate aerosol loadings.

In situ observations of ClO in the Arctic stratosphere: ER‐2 aircraft results from 59°N TO 80°N latitude

Large abundances of ClO were observed inside the arctic polar vortex during 14 flights of the NASA ER‐2 aircraft from Stavanger, Norway (59°N, 6°E) to 80°N latitude. Flights were conducted at altitudes between 14 and 20 km when the solar zenith angle was between 79° and 101°. Data are reported for three flights, January 6, January 16, and Febmary 10, that represent the main features observed during the mission. ClO mixing ratios were typically less than 50 parts per trillion by volume (pptv) outside the vortex and exceeded 100 pptv inside the vortex for all flights. ClO mixing ratios were more than 500 pptv for four days in a row in early February, reaching 1130 pptv on February 10, at an altitude of 19 km (potential temperature of 460 K). Peak ClO mixing ratios in early February were ∼ 100 times larger than those observed at mid‐latitudes for all altitudes surveyed between 15 and 19 km. These data, comparable to those obtained in the antarctic ozone hole, indicate that the springtime arctic polar vortex was extensively perturbed by heterogeneous chemistry and contained enough ClO to catalytically destroy ozone rapidly.

In situ observations of ClO in the Antarctic: ER‐2 aircraft results from 54°S to 72°S latitude

Results from a series of 12 ER‐2 aircraft flights into the Antarctic polar vortex are summarized. These in situ data define the spatial and temporal distribution of ClO as the aircraft flew at an altitude of ∼18 km from Punta Arenas (54°S latitude, 7O°W longitude) to the base of the Palmer Peninsula (72°S latitude, 75°W longitude) and back. At the southern extremity of the flight trajectory the aircraft descended to ∼13 km, turned north, and ascended to 18 km. A general pattern in the ClO distribution is reported: mixing ratios of approximately 10 parts per trillion by volume (pptv) are found at cruise altitude in the vicinity of 55°S increasing to 50 pptv at 6O°S. In the vicinity of 65°S latitude a steep gradient in the ClO mixing ratio is observed. At a fixed potential temperature, the ClO mixing ratio increases by an order of magnitude within a very few degrees of latitude through this sharp transition, thus defining the edge of a “chemically perturbed region.” From the edge of that region to the southern extension of the flights, 72°S, a dome of slowly increasing ClO best describes the distribution. Peak mixing ratios at 18 km at the southern extension of the flight track increased from 800 pptv on August 23 to 1100 pptv in early September and then increased slowly to 1200 pptv by the end of the mission on September 22.