Inorganic Chlorine Research Articles

Activation of chlorine in sulfate aerosol as inferred from aircraft observations

The abundance of reactive chlorine in the lower stratosphere is observed to increase sharply with exposure to temperatures below about 195 K, a temperature which is near the nitric acid trihydrate (NAT) equilibrium condensation point. Measurements from the NASA ER‐2 aircraft and a model of chemistry along back trajectories are used to examine the mechanism for this apparent temperature threshold in chlorine activation. The flight of July 28, 1994, from the Airborne Southern Hemisphere Ozone Experiment/Measurement s for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) campaign in the southern hemisphere is studied because it provides measurements in an ongoing activation episode. Isentropic back trajectories from the aircraft sampling points indicate that the sampled air was cooling at the rate of 20 to 30 K d−1 to temperatures below the NAT condensation point and had not been below the NAT condensation point prior to that for at least 10 days. Hence the observed amount of active chlorine should be kinetically limited by the recent parcel temperatures. The measurements show enhanced ClO and decreased HCl at temperatures below 195 K even in the absence of significant polar stratospheric cloud particle surface area. The model of chemistry along back trajectories, constrained by the ER‐2 chemical and microphysical measurements, indicates that an initial inorganic chlorine (Cly) partitioning of approximately half HCl and half C1ONO2 is consistent with the observations. At this initial Cly, partitioning, the model using heterogeneous reactions on liquid sulfate and ternary solutions with the most recent sticking coefficient evaluations closely reproduces the latitude gradient and temperature threshold of chlorine activation observed in the data. The sudden increase in activation in the model is a result of the steep exponential temperature dependence of the sticking coefficient for HCl+ ClONO2 on liquid aqueous solutions.

The TDLAS instrument for the detection of total inorganic chlorine in the stratosphere

A novel method based on tunable diode laser absorption spectroscopy is proposed for the detection of the sum of the stratospheric chlorine components HCl, ClONO2, ClOOCl and ClO. All of the above species are detected as HCl following thermal decomposition (ClONO2 and ClOOCl) to ClO which is converted to Cl via reaction with NO; the Cl atom is then rapidly reacted with a hydrocarbon to form HCl. The method has been investigated in the laboratory for detection of ClONO2, for which the conversion efficiency was found to be 100% under suitable conditions.

Development of the Antarctic ozone hole

A Lagrangian chemical model is used to simulate the formation of the Antarctic “ozone hole”: the decrease in high‐latitude southern hemisphere ozone between mid‐August and mid‐September of each year. The model benchmark simulation of HNO3, ClONO2, ClO, and ozone for September 17, 1992, is in good agreement with UARS observations. Simulations of the ozone column over the years 1979–1994 show quantitative agreement with the secular decline in Antarctic ozone and change in the area of the ozone hole as observed by the total ozone mapping spectrometer (TOMS). The model calculates that the Antarctic ozone loss and ozone hole area both increased linearly with time after the early 1970s until the early 1990s. After the early 1990s the growth of the area of the ozone hole slows as a result of the slowing of the growth rate of total inorganic chlorine. A hypothetical doubling of the 1992 atmospheric chlorine amount would expand the ozone hole to the very edge of the polar vortex.

The importance of heterogeneous bromine chemistry in the lower stratosphere

Heterogeneous conversion of bromine reservoirs (BrONO2, HOBr) on the surface of aerosol particles in the lower stratosphere has limited effect on the concentrations of reactive bromine radicals (Br, BrO). However, the heterogeneous hydrolysis of BrONO2 on sulfate aerosol particles, which produces HOBr, leads to the enhancement of the abundance of hydroxyl radicals, and consequently modifies the partitioning between inorganic chlorine compounds. Further, because of the enhanced abundance of HOBr, the heterogeneous reaction between HOBr and HCl(s) converts significant amounts of inactive chlorine (HCl) into reactive chlorine (ClO), especially during post‐volcanic periods. These processes could partially explain the unexpected high ClO/HCl and ClONO2/HCl concentration ratios at 20 km reported after the eruption of Mt. Pinatubo (1991), and could have contributed to the substantial ozone depletion observed after the large volcanic eruptions of the 1980's and 1990's. Our model simulations suggest that, owing to the heterogeneous hydrolysis of BrONO2 and the reaction HOBr+HCl(s) on sulfate aerosols, the increase in total bromine loading from 1960 (12 pptv) to 1990 (21 pptv) has led to enhanced concentrations of ClO, and consequently to an ozone depletion of 2–3% in the lower stratosphere at mid and high latitudes under post‐volcanic conditions.

ATMOS/ATLAS‐3 measurements of stratospheric chlorine and reactive nitrogen partitioning inside and outside the November 1994 Antarctic Vortex

Partitioning between HCl and ClONO2 and among the main components of the reactive nitrogen family (NO, NO2, HNO3, ClONO2, N2O5, and HO2NO2) has been studied inside and outside the Antarctic stratospheric vortex based on ATMOS profiles measured at sunrise during the 3–12 November 1994 ATLAS‐3 Shuttle mission. Elevated mixing ratios of HCl in the lower stratosphere with a peak of ∼2.9 ppbv (10−9 parts per volume) were measured inside the vortex near 500 K potential temperature (∼19 km). Maximum ClONO2 mixing ratios of ∼1.2, ∼1.4, and ∼0.9 ppbv near 700 K (∼25 km) were measured inside, at the edge, and outside the vortex, respectively. Model calculations reproduce the higher levels of HCl and NOx (NO + NO2) inside the lower stratospheric vortex both driven by photochemical processes initiated by low 03. The high HCl at low O3 results from chemical production of HCl via the reaction of enhanced Cl with CH4, Limited production of ClONO2, and the descent of inorganic chlorine from higher altitudes.

A comparison of measurements from ATMOS and instruments aboard the ER‐2 aircraft: Halogenated gases

We compare volume mixing ratio profiles of N2O, CFC‐11, CFC‐12, CCl4, SF6, and HCl in the mid‐latitude lower stratosphere measured by the ATMOS Fourier transform spectrometer on the ATLAS‐3 Space Shuttle Mission with in situ measurements acquired from the NASA ER‐2 aircraft during Nov 1994. Good agreement is found between ATMOS and in situ correlations of [CFC‐11], [CFC‐12], and [SF6] with [N2O]. ATMOS measurements of [CCl4] are 15% high compared to ER‐2 data, but agree within the systematic uncertainties. ATMOS observations of [HCl] vs [N2O] are within ∼10% of ER‐2 data for [HCl] > 1 ppbv, but exceed in situ measurements by larger fractional amounts for smaller [HCl]. ATMOS measurements of [ClONO2] agree well with values inferred from in situ observations of [ClO], [NO], and [O3]. The sum of [HCl] and [ClONO2] observed by ATMOS, supplemented by a minor contribution from [ClO] estimated with a photochemical model, is consistent with the levels of inorganic chlorine inferred from in situ measurements of chlorine source gases.

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.

Balloon observations of organic and inorganic chlorine in the stratosphere: the role of HClO4 production on sulfate aerosols.

Simultaneous observations of stratospheric organic and inorganic chlorine were made in September 1993 out of Fort Sumner, New Mexico, using JPL balloon-borne MkIV interferometer. Between 15 and 20 km, a significant fraction (20-60%) of the inorganic chlorine could not be accounted for by the sum of measured HCl, ClONO2, and HOCl. Laboratory measurements of the reaction of ClO radicals on sulfuric acid solutions have indicated that, along with HCl, small amounts of perchloric acid, HClO4, were formed. Very little is known about the fate of HClO4 in the stratosphere and we use a photochemical box model to determine the impact of this new species on the partitioning of inorganic chlorine in the stratosphere. Assuming that HClO4 is photochemically stable, it is shown that in the enhanced aerosol loading conditions resulting from Mt. Pinatubo's eruption, HClO4 could represent a significant reservoir of chlorine in the lower stratosphere, sequestering up to 0.2 ppbv (or 50%) of the total inorganic chlorine at 16 km. The occurrence of this new species could bring to closure the inorganic chlorine budget deficiency made apparent by recent ER-2 aircraft in situ measurements of HCl.

On the age of stratospheric air and inorganic chlorine and bromine release

We estimate the average transport time from the tropical tropopause to various regions of the northern hemisphere lower stratosphere (stratospheric age) using simultaneous mixing ratio measurements of CFC‐115 and CO2 measured by the Whole Air Sampler (WAS) during Airborne Arctic Stratospheric Expedition II (AASE II). Our inferred ages are consistent with those presented in previous studies. We discuss sources of uncertainties that affect age estimates in general, as well as specific uncertainties arising from inferring ages using CO2 and CFC‐115 abundances. We infer inorganic chlorine (Cly) and bromine (Bry) at various lower stratospheric locations using the WAS organic chlorine and bromine measurements in combination with modeled tropospheric halocarbon trends and with our estimated ages. Inferred Cly and Bry abundances generally increase with increasing latitude and altitude. For our analyzed locations inside the polar vortex, we estimate a maximum Cly abundance of about 2.7 parts per billion by volume (ppbv) and a maximum Bry abundance of about 13.7 parts per trillion by volume (pptv). The locations of these maxima correspond to an average N2O mixing ratio of about 100 ppbv, and to a fractional dissociation of organic chlorine and bromine of 0.85 and 0.90, respectively. Finally, we discuss the expected future limitations of using CFC‐115 to estimate stratospheric age due to the production limitations prescribed by the amendments and adjustments to the Montreal Protocol.

Measurements of stratospheric chlorine monoxide (ClO) from groudbased FTIR observations

Infrared absorption features due to ClO in the lower stratosphere have been identified from groundbased solar absorption spectra taken from Aberdeen, U.K. (57° N, 2° W) on 20 January 1995. A vertical column abundance of 3.42 (±0.47)×1015 molec cm-2 has been derived from 13 independent absorption features in the P and R branches of the (0–1) vibration-rotation band of 35ClO, spanning the spectral region 817–855 cm-1. The observed absorption features are consistent with very high levels of ClO (approximately 2.6 parts per billion by volume (ppbv)) in the altitude range 16–22 km. A comparison of this profile with a 3D chemical transport model profile indicates the observation was made inside the polar vortex and shows good qualitative agreement but the model underestimates the concentrations of ClO. Simultaneous measurements of other species were made including HCl, HF and ClONO2. These columns yield a value for HCl+ClONO2+ClO of 7.02±0.65×1015 molec cm-2. This is lower than the total inorganic chlorine (ClO y ) column of 10.7±1.6×1015 molec cm-2 estimated from mean measured (HCl+ClONO2)/HF ratios together with in-vortex HF measurements. The discrepancy is probably due to significant amounts of the ClO dimer (Cl2O2) in the lower part of the stratosphere. The measurements of highly elevated levels of ClO are used to estimate O3 loss rates at the 400, 475 and 550 K levels making assumptions about the probable distribution of ClO and Cl2O2. These are compared with loss rates derived from ozone sonde data.

Space shuttle's impact on the stratosphere: An update

To assess their impact on the stratosphere, a launch scenario of nine shuttles and three Titans per year is simulated in a two‐dimensional photochemistry and transport model that includes heterogeneous reactions on a stratospheric sulfate aerosol (SSA) layer and polar stratospheric clouds (PSCs). These rocket launches are predicted to cause small constituent changes in the stratosphere. Maximum total inorganic chlorine enhancements are computed to be about 12 parts per trillion by volume (∼0.4% on a 3 parts per billion by volume background) in the middle to upper stratosphere at northern middle to high latitudes. Maximum ozone decreases associated with these chlorine increases are calculated to be about 0.14% in the middle to upper stratosphere at northern middle to high latitudes. Column ozone decreases are predicted to be a maximum of about 0.05% at northern polar latitudes in the early spring. Model results using (1) gas phase only reactions, (2) gas phase reactions and heterogeneous reactions on the SSA layer, and (3) gas phase reactions and heterogeneous reactions on the SSA layer and PSCs have also been compared with one another. The simulations from these three versions of our model gave annually averaged global total ozone decreases of (1) 0.0056%, (2) 0.010%, and (3) 0.014%. Stratospheric effects from heterogeneous reactions promoted by the alumina emitted from these rockets could be larger than those predicted from the chlorine emissions and need to be investigated further.

Influence of Level and Form of Chlorine on the Formation of Chlorinated Dioxins, Dibenzofurans, and Benzenes during Combustion of an Artificial Fuel in a Laboratory Reactor

The formation of polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and chlorobenzenes (PCBzs) in the combustion process was studied where the chlorine level and source were varied in an artificial waste. Different levels of organic chlorine (PVC) and inorganic chlorine (CaCl2·6H2O) were added to the fuel. The flue gas samples were analyzed for PCDDs/PCDFs and PCBzs. The following combustion parameters were measured during incineration: temperature in bed, in freeboard, and in cooling convector; air flows; SO2, CO2, NO, NO2, CO, O2, and particle concentrations in the flue gas. The results from this study indicate no correlation between the quantities of formed PCDDs/PCDFs and PCBzs in the combustion process and the level of chlorine in the fuel, when the chlorine level is below 1%. However, when the level of chlorine in the fuel exceeds 1%, an increased formation rate was noted. No distinction in the formation rate of the chlorinated micropollutants was noticed between the two different ch...

The behavior of heavy metal Cr,Pb and Cd during waste incineration in fluidized bed under various chlorine additives.

Previous theoretical studies have demonstrated that chlorine-containing materials significantly affected heavy metals behavior during incineration. In this study, we examine the effects of operating temperature in both primary and second combustion chambers along with those of various chlorine-containing materials (organic or inorganic) on metal partitioning in sand-bed sorbents, fly ash, waste water, and flue gas.A bubbling fluidized bed made from 310 stainless steel (100 cm bed height; 10 cm ID; and 100 cm freeboard; 25 cm. ID) was used, in addition to two cyclones and a wet scrubber. The synthetic solid wastes used were plastics, sawdust and water. These results indicated that the fluidized medium (silica sands) can absorb a high proportion of metals in the incineration process; in addition, the extent of absorption ability follows the sequence: Pb > Cr > Cd. The effect of organic and inorganic chlorine on metal partitioning were very different; organic chlorine (P.V.C.) caused a more serious metal emission than inorganic chlorine (NaCl). The operating temperature and chemical reaction influenced absorption by the fluidized media. Within the operating temperature ranges, 500–600°C for the main combustion chamber, 600–800°C for the second combustion chamber), the effect of freeboard temperature on the metal partitioning is insignificant.

On the atmospheric impact of launching the Ariane 5 rocket

Calculations have been performed in a two‐dimensional chemical‐radiative‐transport model of the atmosphere aimed at investigating the atmospheric response to launching the Ariane 5 rocket. Various pollutants emitted from the rocket were studied: inorganic chlorine, water vapour, and aluminium oxide. The calculations also considered whether the atmospheric response differs according to the form in which inorganic chlorine is released. Both long‐term global and short‐term local responses were investigated. After 20‐model years of inorganic chlorine and water vapour emissions, with 10 launches per year, only small losses of ozone, of the order of less than 0.1%, were calculated. These losses were independent of the form of inorganic chlorine. Recovery of the atmosphere after the cessation of launches was rapid, and all but complete within 5 years. Local ozone loss after a single rocket launch was also limited, but the magnitude was dependent upon the form in which chlorine was released. The effect of releasing Al2O3 from Ariane 5 was a modest increase in the sulphate aerosol mass mixing ratio throughout most of the stratosphere.

Correlated observations of HCl and ClONO2 from UARS and implications for stratospheric chlorine partitioning

We present the first near‐global set of correlated measurements of stratospheric HCl and ClONO2 concentrations. These data, obtained from the Upper Atmosphere Research Satellite (UARS) between August 1992 and March 1993, are analyzed using the Goddard trajectory mapping technique. These data indicate that, between 20 and 30 km altitude and 60°N to 60°S latitude, total inorganic chlorine (Cly) is fairly evenly distributed between ClONO2 and HCl, with HCl the slightly more dominant reservoir. The sum of UARS measurements of HCl and nighttime ClONO2, which approximates Cly at these altitudes, closely agrees with Cly derived from a tracer‐Cly relation originally derived from aircraft data, increasing confidence in the UARS data. Comparisons between these data and two‐dimensional model results suggest that models underpredict ClONO2 (overpredict HCl) near 20 km and overpredict ClONO2 (underpredict HCl) near 30 km, although the 20‐km underprediction is not as large as analyses of aircraft measurements have indicated.

Impact of averaged photolysis rates on stratospheric chemical models

An evaluation is made of the effect of two approximations to the diurnal variation of photolysis rates on the simulation of stratospheric chemistry. The daylight average approximation uses photolysis rates that are given a value representative of the daylight average over that portion of the day that the Sun is shining, and zero otherwise. Although this distorts the diurnal cycle, the zonal average concentrations using this approximation are within about 5% of those from the fully resolved diurnal cycle for most of the important species in the stratosphere. Larger discrepancies occur in the partitioning of inorganic chlorine, although even this is in error by only about 10% for the dominant species. The 24‐hour average photolysis approximation gives perpetual daylight except in the polar night. The error associated with this is quite large, especially for those species such as NO3 and N2O5 that are produced during night. Over an integration time of a month, it can lead to differences in the simulated concentrations of major species such as ozone of the order of 20% or more.

Vacuum - UV - oxidation of chloroorganic compounds in an excimer flow through photoreactor

The application of a novel vacuum-UV excimer flow through photoreactor (Λ = 172 nm) to the degradation of chloroorganic compounds in water is presented. A sample of ground water, contaminated with trichloroethene (Tri), 1,2-dichloroethene (1,2-DCE) and tetrachloroethene (Per), was purified successfully without the addition of oxidizing agents like H 2 O 2 or O 3. During the VUV-oxidation the intermediary formation of 1,1,2-trichloroethane (1,1,2-Tri) was established by GC-MS analysis. Over 93% of the organically bound chlorine atoms were transformed to inorganic chlorine ions ( Cl −). The photomineralization of 2,4-dichlorophenol (2,4-DCP) is presented on the basis of 2,4-DCP, Cl − and TOC analyses. In few of technical applications the volume-corrected energy efficiencies | −s of the degradation of 2,4-DCP were determined for different intensities of irradiation and different flow rates (e.g. | −s(142 W, 481/min) = 16200 mg/kWh).

Effect of catalysts and chlorine source on the formation of organic chlorinated compounds

Two series of catalyzed incineration tests were performed in a 32 kW laboratory pilot plant to study the effect of metal catalysts on the formation of chlorobenzenes, chlorophenols and polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). In the first series, iron(III), tin(II) and copper(II) chlorides were added to the flame as catalysts and the combustion parameters were adjusted for optimum am[ incomplete combustion. The aim was to study the distribution of organic chlorinated compounds between particles and the gas phase in the flue gases when the combustion parameters were varied. Concentrations of particle-bound PCDD/Fs were high in the incomplete combustion tests, and those in the gas phase were still high even in the optimum combustion tests. In the second test series, copper(II), iron(III) and manganese (II) nitrates were added to the flame. Sodium chloride, representing inorganic chlorine, and tetrachloroethylene representing organic chlorine, were used as chlorine sources in the basic fuel with the different catalysts. Metals and chlorine accounted for 0.5 weight-% of the total fuel flow. The basic fuel used in these investigations was an aliphatic liquid free of aromatic hydrocarbons. Organic chlorine with the catalyst promoted the formation of particle-bound PCDD/Fs, whereas inorganic chlorine was observed to promote the formation of PCDD/Fs more effectively in the gas phase than in the particle phase. Consequently, different mechanisms of formation may be involved in the gas phase and on particle surfaces.

Estimates of total organic and inorganic chlorine in the lower stratosphere from in situ and flask measurements during AASE II

Aircraft sampling has provided extensive in situ and flask measurements of organic chlorine species in the lower stratosphere. The recent Airborne Arctic Stratospheric Expedition II (AASE II) included two independent measurements of organic chlorine species using whole air sample and real‐time techniques. From the whole air sample measurements we derive directly the burden of total organic chlorine (CCly) in the lower stratosphere. From the more limited real‐time measurements we estimate the CCly burden using mixing ratios and growth rates of the principal CCly species in the troposphere in conjunction with results from a two‐dimensional photochemical model. Since stratospheric chlorine is tropospheric in origin and tropospheric mixing ratios are increasing, it is necessary to establish the average age of a stratospheric air parcel to assess its total chlorine (ClTotal) abundance. Total inorganic chlorine (Cly) in the parcel is then estimated by the simple difference, Cly = ClTotal ‐ CCly. The consistency of the results from these two quite different techniques suggests that we can determine the CCly and Cly in the lower stratosphere with confidence. Such estimates of organic and inorganic chlorine are crucial in evaluating the photochemistry controlling chlorine partitioning and hence ozone loss processes in the lower stratosphere.

Time and temperature dependences of fractional HCl abundances from airborne data in the Southern Hemisphere during 1994

Measurements of HCl and CH4 taken by the aircraft laser infrared absorption spectrometer (ALIAS) on the ER-2 high-altitude research aircraft during the Southern Hemisphere winter of 1994 have been used to examine the abundance of HCl as a fraction of total inorganic chlorine. The fractional abundance of HCl shows a threshold behaviour as a function of temperature history; on a 10 day timescale, the abundance dropped sharply in those air parcels experiencing a temperature < 195 K, but little or no change was seen in parcels which stayed warmer than this temperature. The behaviour mirrors well the temperature behaviour calculated for the transformation of HCl into reactive forms (Cl2, HOCl) from laboratory studies of sulfate aerosols and polar stratospheric clouds. During the course of the winter, the fractional abundance of HCl outside the vortex decreased from its values in late May by about a third, while inside it dropped to near zero by early August. Some recovery was evident in October. Examples of the peel-off of low-HCl air equatorward of the wind maximum were evident in early June. Meteorological trajectories are used to show, in a case study of a flight in early August, that air parcels which experienced temperatures of < 195 K, and as a result had low fractional HCl abundances, did so largely poleward of the maximum in the polar night jet stream. Encountering temperatures of < 195 K during the previous 10 days was a necessary and sufficient condition for the transformation of HCl into reactive forms by heterogeneous reactions. The trajectories further showed that air arriving from sub-tropical latitudes had higher fractional HCl abundances than the air in the middle latitudes, and much higher fractions than the air at high latitudes. The resulting picture is one in which the fractional abundance of HCl in air at mid latitudes was the result of mixing of air from sub-tropical latitudes with air mainly from polward of the jet stream core which has experienced temperatures < 195 K. The sensitivity of the fractional abundance of HCl to the assumption that no HCl enters the stratosphere via the tropical tropopause is examined in the light of an observed profile near the equator with a volume fraction of 0.4 ppb HCl, zero ClO and tropospheric mixing ratios of CFCs at the tropical tropopause.