Understanding the kinetics of the ClO dimer cycle (original) (raw)

A re-evaluation of the ClO/Cl2O2 equilibrium constant based on stratospheric in-situ observations

Atmospheric Chemistry and Physics, 2005

In-situ measurements of ClO and its dimer carried out during the SOLVE II/VINTERSOL-EUPLEX and ENVISAT Validation campaigns in the Arctic winter 2003 suggest that the thermal equilibrium between the dimer formation and dissociation is shifted significantly towards the monomer compared to the current JPL 2002 recommendation. Detailed analysis of observations made in thermal equilibrium allowed to re-evaluate the magnitude and temperature dependence of the equilibrium constant. A fit of the JPL format for equilibrium constants yields K EQ =3.61×10 −27 exp(8167/T ), but to reconcile the observations made at low temperatures with the existing laboratory studies at room temperature, a modified equation, K EQ =5.47×10 −25 (T /300) −2.29 exp(6969/T ), is required. This format can be rationalised by a strong temperature dependence of the reaction enthalpy possibly induced by Cl 2 O 2 isomerism effects. At stratospheric temperatures, both equations are practically equivalent. Using the equilibrium constant reported here rather than the JPL 2002 recommendation in atmospheric models does not have a large impact on simulated ozone loss. Solely at large zenith angles after sunrise, a small decrease of the ozone loss rate due to the ClO dimer cycle and an increase due to the ClO-BrO cycle (attributed to the enhanced equilibrium ClO concentrations) is observed, the net effect being a slightly stronger ozone loss rate.

A re-evaluation of the ClO/Cl₂O₂ equilibrium constant based on stratospheric in-situ observations

Atmospheric Chemistry and Physics, 2005

In situ observations of ClO near the winter polar tropopause

Journal of Geophysical Research, 2003

1] Significant abundances of chlorine oxide (ClO) were observed throughout the lowermost stratosphere at high latitudes during winter from the NASA DC-8 aircraft during the SAGE III-Ozone Loss and Validation Experiment and Third European Stratospheric Experiment on Ozone 2000 (SOLVE/THESEO 2000) campaign. Mixing ratios of ClO averaging 15-20 parts per trillion by volume (pptv) were observed near the tropopause, a region where ClO abundances are usually only a pptv or less at lower latitudes. The ratio of ClO to inferred inorganic chlorine ([ClO]/[Cl y ]) was found to be largest ($7%) in air characterized by low abundances of ozone ($100-250 parts per billion by volume (ppbv)). This was the region where cirrus clouds were also observed occasionally during the measurement period, although abundances of ClO directly within cirrus clouds were not significantly different than background abundances. Nonzero instrument signals during darkness are attributed to detection of $5-15 pptv of OClO. BrO mixing ratios between 2 and 4 pptv are sufficient to produce these amounts of OClO, assuming daytime mixing ratios of ClO between 15 and 20 pptv. At these levels of ClO and BrO, approximately 10% of the ozone at these altitudes is chemically destroyed per month in springtime by reactions of ClO and BrO, representing an effective loss process for ozone near the high-latitude tropopause.

Balloon-borne in situ measurements of CLO and ozone: Implications for heterogeneous chemistry and mid-lattitude ozone loss

Geophys Res Lett, 1993

In situ measurements of chlorine oxide (C10) obtained on 31 March 1991 with a new balloon-borne instrument are compared to results from a photochemical model which incorporates hydrolysis of N2Os on sulfate aerosols. With the addition of this process, there is better agreement between calculation and measurement over most of the profile, except below 20 km where observed C10 is greater by as much as a factor of four. In a model which is constrained to reproduce the observed C10 below 20 km, ozone loss by catalytic cycles involving halogen oxides becomes larger than that from NOx, which would dominate under gas-phase or standard heterogeneous conditions. reasons. The balloon-borne measurements of C10 made in the late 1970's and 1980's were all obtained over eastern Texas and display significant variability, thus providing little insight into seasonal, latitudinal or secular trends [WMO, 1985]. More recent data from ER-2 flights at mid-latitudes have exhibited considerably less variability and have been examined for seasonal and latitudinal effects [Rodriguez et al., 1991; Toohey et al., 1991; King et al., 1991 ]. These studies have shown that models which include only gas-phase chemistry cannot reproduce the observations. On the other hand, incorporation of heterogeneous processes which reduce NOx (--NO + NO2) in fa-The performance of a new instrument for in situ measurements of C10 in the lower stratosphere, Geophys. Res. Lett., this issue, 1993. Weinstock, E.M., C.M. Schiller and J.G. Anderson, In situ stratospheric ozone measurements by long path UV absorption: developments and interpretation, J. Geophys. Res., 91, 5237-5248, 1986. of transport on column abundances of nitrogen and chlorine compounds in the Arctic stratosphere, Geophys. Res. Lett., 17, 533-536, 1990.

Measurements of ClO and O 3 from 21°N to 61°N in the lower stratosphere during February 1988: Implications for heterogeneous chemistry

Geophysical Research Letters, 1991

The decline in stratospheric ozone at northern midlatitudes in wintertime may be caused by chlorine photochemistry that has been enhanced by heterogeneous reactions. lVe examine the possibility that the heterogeneous reaction of N20• on sulfate aerosols is the cause of this decadal ozone decline by comparing ClO and 0• measurements made in the lower stratosphere during February, 1988, with results from a two-dimensional model. At midlatitudes, the abundances, latitudinal, and seasonal gradients of the observed ClO are similar to the results of a model with heterogeneous chemistry, but are in strong disagreement with the results from the model with only gas-phase chemistry. At low latitudes, agreement is best with the results of the model with only gasphase chemistry. These limited observations indicate that the amount of reactive chlorine is being enhanced, and that heterogeneous chemistry is a likely cause, but the details of the heterogeneous chemistry and the possibility of other mechanisms still need to be considered.

Balloon-borne in situ measurements of CLO and ozone: Implications for heterogeneous chemistry and mid-latitude ozone loss

Geophysical Research Letters, 1993

The Potential Impact of the Reaction OH+ClO->HCl+O2 on Polar Ozone Photochemistry

We call attention to the likely importance of the potential reaction OH + C10 ~ HC1 + 02. It may only be a minor channel for the reaction of OH with CIO, which is often ignored in models, but if it occurs it considerably increases the rate of recovery of HCt after an air parcel has encountered a polar stratospheric cloud (PSC). The net effect of this reaction on the ozone concentration depends on the relative HC1 concentration and whether the air parcel is in a PSC. When an air parcel is in a PSC and the HC1 concentration is less than the sum of the HOC1 and C1ONO2 concentrations, heterogeneous C10~ production is rate limited by the production of HC1. Under these conditions the reaction allows HC1 to be reprocessed more rapidly by the heterogeneous reactions of HC1 with HOC1 and C1ONO2. This allows high C10~ concentration to be maintained for longer, and at a slightly higher level, than would otherwise be possible which in turn leads to more ozone depletion. When there are PSCs but HC1 is in excess, or outside of the PSC regions (i.e. during the recovery phase), the reaction will always reduce the C10/HC1 ratio and hence slightly reduce the ozone loss.

Inorganic chlorine partitioning in the summer lower stratosphere: Modeled and measured [ClONO 2 ]/[HCl] during POLARIS

Journal of Geophysical Research, 2001

We examine inorganic chlorine (Cly) partitioning in the summer lower strait)sphere using in situ ER-2 aircraft observations made during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign. New steady state and numerical models estimate [C1ONO2]/[HC1] using currently accepted photochemistry. These models are tightly constrained by observations with OH (parameterized as a function of solar zenith angle) substituting for modeled HO2 chemistry. We find that inorganic chlorine photochemistry alone overestimates observed [C1ONO2]/[HC1] by approximately 55-60% at mid and high latitudes. On the basis of POLARIS studies of the inorganic chlorine budget, [C10]/[C1ONO2], and an intercomparison with balloon observations, the most direct explanation for the model-measurement discrepancy in Cly partitioning is an error in the reactions, rate constants, and measured species concentrations linking HC1 and C10 (simulated [C10]/[HC1] too high) in combination with a possible systematic error in the ER-2 C1ONO2 measurement (too low). The high precision of our simulation (+15% 1(5 for [C1ONO2]/[HC1], which is compared with observations) increases confidence in the observations, photolysis calculations, and laboratory rate constants. These results, along with other findings, should lead to improvements in both the accuracy and precision of stratospheric photochemical'models.

Simulation of ozone depletion in spring 2000 with the Chemical Lagrangian Model of the Stratosphere (CLaMS)

Journal of Geophysical Research, 2002

Simulations of the development of the chemical composition of the Arctic stratosphere for spring 2000 are made with the Chemical Lagrangian Model of the Stratosphere (CLaMS). The simulations are performed for the entire Northern Hemisphere on four isentropic levels (400-475 K). The initialization in early February is based on observations made from satellite, balloon and ER-2 aircraft platforms. Tracer-tracer correlations from balloon-borne cryosampler (Triple) and ER-2 measurements, as well as tracer-PV correlations, are used to derive a comprehensive hemispherical initialization of all relevant chemical trace species. Since significant denitrification has been observed on the ER-2 flights, a parameterization of the denitrification is derived from NO y and N 2 O observations on board the ER-2 aircraft and the temperature history of the air masses under consideration. Over the simulation period from 10 February to 20 March, a chemical ozone depletion of up to 60% was derived for 425-450 K potential temperature. Maximum vortex-averaged chemical ozone loss rates of 50 ppb d À1 or 4 ppb per sunlight hour were simulated in early March 2000 at the 425 and 450 K potential temperature levels. We show comparisons between the measurements and the simulations for the location of the ER-2 flight paths in late February and March and the location of the Triple balloon flight. The simulated tracer mixing ratios are in good agreement with the measurements. It was not possible to reproduce the exact details of the inorganic chlorine compounds. The simulation agrees with ClO x observations on the Triple balloon flight but overestimates for the ER-2 flights. The simulated ozone depletion agrees with estimates from other observations in the 425 and 450 K levels, but is underestimated on the 475 K level.

Understanding the kinetics of the ClO dimer cycle (original) (raw)

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