JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D21, PAGES 26,667-26,686, NOVEMBER 20, 1999

and partitioning in the middle stratosphere' A box model investigation using HALOE data

L. K. Randeniya, I. C. Plumb, 1 and K. R. Ryan 1 Commonwealth Scientific and Industrial Research Organization, Telecommunications and Industrial Physics, Lindfield, New South Wales, Australia

Abstract. The partitioning of the active nitrogen and chlorine families (NO•, Cl•) between 24 and 32 km and 30 ø and 75øN in summer and autumn is studied using a photochemical box model. Data obtained from the Halogen Occultation Experiment (HALOE) are used to initialize the model and to obtain measured values for NO, NO2 and HC1. When the recommended kinetic and photochemical parameters are used, the model underestimates the NO•/NO• ratios by 15-35%. As in analyses of aircraft, balloon, and laboratory data, it is found that the gas-phase processes which link NO• and HNO3 are inadequately represented by the model. The inclusion of rate expressions derived recently for the reactions of OH with NO2 and OH with HNO3 in the model results in 8-20% increases in NO•, where the larger increases are obtained at lower altitudes. Despite better agreement resulting from the use of new rate expressions, differences in excess of 20% remain between NO•/NOy values obtained from measurements and calculations. The remaining differences are particularly noticeable above about 27 kin. This implies that further studies of the reactions controlling partitioning in the NO• family are necessary. The use of branching ratios determined recently for the production of HC1 from the reaction between C10 and OH leads to better agreement for the HC1/CI• ratios obtained from measurements and calculations for altitudes above 27 kin. However, the agreement below 27 km becomes poorer as a result of calculated additional production of HC1. Overall, within the recommended uncertainties for the relevant rate coefficients, the model predictions for HC1/CI• agree with the values obtained from measurements between 24 and 32 kin.

1. Introduction

The importance of catalytic processes involving ox- ides of nitrogen in modifying the photochemical balance of atmospheric ozone was first recognized by Crutzen [1970] and Johnston [1971]. The sequence of reactions,

net

NO+Oa > NO2 + O2 (1) NO2+O > NO+O2 (2)

O3+O • 02+02

has the net result of removing odd oxygen from the at- mosphere. Halogen compounds are capable of forming

•Also at Cooperative Research Centre for Southern Hemi- sphere Meteorology, Monash University, Clayton, Victoria, Australia.

Copyright 1999 by the American Geophysical Union.

Paper number1999JD900440. 0148-0227/99/1999JD900440509.00

similar catalytic cycles that deplete ozone [Molina and Rowland, 1974; Stolarski and Cicerone, 1974]

X+Oa > XO+O2 (3)

XO+O > X+O• (4) net

O3+O • 02+02

where X denotes either C1 or Br. The global change that has occurred for ozone since about 1980 is attributed

primarily to the influence of enhanced halogen concen- trations in the atmosphere of the Earth [Stolarski et al., 1991; World Meteorological Organization (WMO), 19981.

Major contributions to the understanding of strato- spheric chemistry have come from the discovery of mech- anisms which couple the photochemistry of halogen-, nitrogen-, and hydrogen-containing radicals to form in- teracting catalytic cycles which deplete ozone. The in- teraction serves to change the partitioning within the NOy (N + NO + NO2 + NOa + 2N205 + HNO2 + HNOa + HNO4 + C1ONO2 + BrON02), Cly (C1 + 2C12 + ClO + C102 + OC10 + 2C1202 + C1ONO2

26,667

26,668 RANDENIYA ET AL.: NOr AND CLy PARTITIONING IN THE MIDDLE ATMOSPHERE

+ HC1 + HOC1 + BrC1), Br u (Br + BrO + BrC1 + BrONO2 + HOBr + HBr) and HO• (OH + HO2) fam- ilies. The coupling may occur via gas-phase reactions or heterogeneous processes occurring on the surfaces of atmospheric particles [Hofmann and Solomon, 1989; McElroy et al., 1986; Solomon, 1990; Solomon et al., 1986; Wennberg et al., 1994]. Although there has been substantial progress in the understanding of these pro- cesses, many important details need better explanation before trends in ozone can be understood and predicted [Solomon et al., 1997, 1998]. This will provide greater confidence in the ability to predict the impact on ozone of a range of plausible scenarios for the future which in- clude emissions from the surface of the Earth and from

high-flying aircraft [WMO, 1998]. Analyses of in situ and remote observations of strato-

spheric constituents have contributed greatly to the understanding of the budgets of NOv, Clu, Bru, and HO• and their relative contributions to the atmospheric ozone budget [Atmospheric Effects of Stratospheric Air- craft (AESA), 1999; WMO, 1998]. Recent studies of data obtained from ground-based instruments [Koike et al., 1994], in situ aircraft and balloon measurements [Jaegle et al., 1994; Renard et al., 1997; Sen et al., 1998], and remote satellite measurements [Lary et al., 1997; Morris et al., 1997] have demonstrated that photochem- ical models using currently recommended kinetic and photochemical parameters fail to account for the ob- served partitioning of species within the NO v family. During the Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) campaign, altitudes up to 21 km were sampled by the ER-2 aircraft, and mea- surements for altitudes above this were obtained from

balloon payloads. Analysis of the data obtained during the mission also demonstrated that the use of kinetic

and photochemical data recommended by DeMote et al. [1997] in photochemical models results in an under- estimation of the ratio of NOs (=NO+NO•)/NO v [Gao et al. , 1999].

Several studies have found that above 20 km, the HC1/C1 u ratios are underestimated by photochemical models which do not include production of HC1 via the reaction of C10 and OH [Chance et al., 1996; Dessler et al., 1996; Michelsen et al., 1996; Minschwaner et al., 1993]. Using measurements obtained from the Upper Atmosphere Research Satellite (UARS), Dessler et al. [1995] found that a two-dimensional (2-D) model un- derpredicted the C1ONO2/HC1 ratio near 20 km and overpredicted it near 30 km. Webster et al. [1994] re- ported measurements of HC1/Cly ratios during the 1993 Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE) campaign which were well below the values expected from model calculations. However, Dessler et al. [1997] found no evidence from UARS data to support very low HC1/Cly ratios for 1993 reported by Webster et al. [1994]. The Dessler et al. [1997] analysis of UARS measurements yielded HC1/C1 u values at 46 hPa which agreed with a 2-D model within the limits of

uncertainties of measurements with a tendency for the model to overestimate this ratio.

Clearly, the reliability of photochemical models de- pends on how well they can explain the partitioning of active species within families such as NO• and C1 u. The POLARIS measurements inherently have limited geographic and temporal coverage. Because of this, it is of interest to know to what extent the conclusions

drawn from these measurements about the partitioning of NO u and C1 u apply more generally in the strato- sphere. Measurements made by the Halogen Occulta- tion Experiment (HALOE) instrument [Russell et al., 1993] aboard UARS provide a basis which can be used to investigate the partitioning of NO u and C1 u families above 21 km over a wide range of latitudes.

In this work the ratios NO•/NO u and HC1/C1 u de- rived using a photochemical box model are compared with values deduced from HALOE measurements. The

photochemical box model 'is initialized with HALOE data obtained between 30 o and 75øN in the months of

June, July, and September. Because HALOE does not measure NOy, Clu, and Br•, these concentrations are inferred from the HALOE CH4 measurements and cor- relations obtained between each of these families and

CH4 during measurement campaigns when the concen- trations of all of these species were measured simultane- ously (see discussion below). Together the above quan- tities provide sufficient input to allow a calculation of the partitioning of NO u and C1 u families using the pho- tochemical box model. Since the HALOE instrument

also measures NO, NO• [Gordley et al., 1996], and HC1 [Russell et al., 1996], the NO•/NO u and HC1/C1 u ra- tios calculated by the model can be compared with the values deduced from measurement.

2. Methodology

2.1. Box Model Analysis of HALOE Data

The Co-operative Research Centre (CRC) for South- ern Hemisphere Meteorology, Australia, box model is used in this study. It employs the Sparse Matrix Vec- torised Gear (SMVGEAR II) solver [Jacobson, 1995; Jacobson and Turco, 1994] to integrate the species con- centration fields forward in time. The radiation scheme

and the reaction set are identical to those used in the

Commonwealth Scientific and Industrial Reasearch Or-

ganization (CSIRO) 2-D model [Randeniya et al., 1997]. Unless stated otherwise, kinetic and photochemical data used in the model are obtained from the recommenda-

tions given by DeMore et al. [1997]. At six grid points between 24 and 32 km, corresponding to the region sam- pled by HALOE, the species concentration fields are integrated forward in time to obtain diurnal concentra- tions. The solutions are considered converged when the diurnal concentrations of all species repeat to within 1% over a 24-hour period. The fundamental assumption behind this approach is that in the sampled air masses, the species lifetimes are short enough to establish a con-

RANDENIYA ET AL.: NO•- AND CL•- PARTITIONING IN THE MIDDLE ATMOSPHERE 26,669

strained equilibrium where production and loss terms are balanced over a 24-hour period [$alawitch et al., 1994].

The input data for box model calculations are ob- tained in two ways. First, the values for CH4, H20, Oa, and sulphate aerosol area density are obtained di- rectly from Version 18 HALOE data files [Bruhl et al., 1996; Harries et al., 1996; Herrig et al., 1998; Park et al., 1996]. Temperatures are obtained from HALOE data files where below 35 km they correspond to values obtained from the analysis of National Centers for En- vironmental Prediction retrievals. The HALOE signals used to determine temperature are affected by aerosols below 35 km [Herrig et al., 1996]. Ozone and tempera- ture profiles required by the radiation scheme to deter- mine the photo flux are also obtained from the HALOE data files. Second, the concentrations of the long-lived families NOv, Clv, and Br v are determined from their correlations with measured CH4. Functional forms for these relationships are obtained by linear/polynomial fitting of data obtained from instruments flown on var- ious balloon, aircraft, and satellite campaigns. Correla- tions from the CSIRO 2-D model [Vohralik et al., 1998] are also obtained for comparison. The NOr/NO v and HC1/C1 v ratios calculated by the photochemical model are then compared with those derived from HALOE measurements of CH4, NO•, and HC1.

In general, the retrieved HALOE profiles cover al- titudes from the upper troposphere to well above the stratosphere. Fifteen independent measurements are normally made daily at evenly spaced longitude points in each of two latitude bands with widths of up to 40 . Observations are made in one latitude band at sunrise

and in the other latitude band at sunset. Sunrise and

sunset observations change slowly in latitude (at a rate of 00-40 per day) and sweep through a band of 1200- 1500 between 80øS and 80øN over a period of about 1 month [Russell et al., 1993]. The data obtained at sunset are used in this study in order to minimize the diurnal effects on the NO and NO2 retrievals IGoralley et al., 1996]. On a given day in the months of June, July, and September where sunset data are available, eight acquisitions around the longitude circle were chosen for the analysis. Note that all eight acquisitions will fall within a latitude range of 4 ø. Test calculations at se- lected latitudes showed that the results obtained using only eight occultations from a day are not significantly different from those obtained using all occultations.

In Table 1, the selections of HALOE data which were subjected to box model analyses are summarized. Anal- ysis of data from June and July focuses attention on gas-phase processes and their rate parameters. This is because the photochemical processes in June and July in the altitude range considered are controlled primar- ily by gas-phase reactions. However, in September, the chemical partitioning is influenced by heterogeneous processes. This is particularly true for September 1993 because the aerosol levels in the atmosphere were still

Table 1. The HALOE Profiles Used in the Box Mo-

del

Number of

Year Month(s) Latitude Range Samples 1993 June, July 300 N to 66 oN 64 1993 September 30øN to 73øN 120 1994 June, July 30øN to 66øN 32 1997 June 300 N to 660 N 32

enhanced to values well above the background levels as a result of the eruption of Mr. Pinatubo in 1991 [Thomason et al., 1997].

Data obtained by the HALOE instrument have been subjected to extensive validation studies [Bruhl et al., 1996; Gordley et al., 1996; Hatties et al., 1996; Herrig et al., 1996; Park et al., 1996; Russell et al., 1996]. These studies identified the expected random and sys- tematic uncertainties of Version 17 HALOE data sets

by the scrutiny of instrument performance and data re- trieval algorithms together with comparisons to model calculations and overlapping measurements made by other techniques. HALOE data have undergone fur- ther refinement, and, as indicated in the documentation provided with HALOE data Version 18, general and specific improvements have resulted in species profiles which are in better agreement with overlapping mea- surements made by other techniques. Improvements have been substantial for both NO and HC1.

As discussed in the validation articles noted above, in the absence of substantial atmospheric variability, the precision of the data is represented well by the standard deviation of the zonal mean. The HALOE instruments

typically make 15 data acquisitions per day at evenly distributed longitudes in a latitude range of up to 40 [Russell et al., 1993]. From these measurements, the zonal means and the standard deviations were deter-

mined for the range of conditions studied here. Fur- ther comparisons between the Version 18 profiles and suitable measurements taken by other instruments (dis- cussed in the validation articles) were also made. On the basis of these comparisons and the calculated standard deviations, we estimate that in the altitude range and the seasons considered in this study, the total uncer- tainties in individual measurements are given approx- imately by 4-10% for NO, NO•., CH4, and H•.O, 4-8% for HC1, and 4-5% for Oa.

The Version 18 release also includes the surface areas

determined from the HALOE measurements of aerosol

extinction in the atmosphere. The aerosol surface ar- eas deduced from the HALOE measurements agree well with other measurements, and the estimated uncer- tainty of the values is 4-30% [Herrig et al., 1998].

2.2. Correlations Between CH4 and Chemical Families

2.2.1. Correlation of NO v with CH4. In the absence of irreversible removal of NOv, a compact lin-

26,670 RANDENIYA ET AL.' NOr AND CLv PARTITIONING IN THE MIDDLE ATMOSPHERE

ear anticorrelation is observed between NOy and N20 20 in the lower stratosphere [Fahey et al., 1990; Keim et al., 1997; Nevison et al., 1997]. The relation is a re- sult of long middle to lower stratospheric lifetimes of the species and the fact that the stratospheric NOy is 15 primarily produced via reaction of N20 with O(1D). Because of the correlation between long-lived CH4 and N20 in the stratosphere [Kawa et al., 1993; Mergen- thaler et al., 1996], a similar anticorrelation also ex- • 10 ists between NOu and CH4 [A'ondo et al., 1996]. At z higher altitudes, the NOu-N20 and NOu-CH4 correla- tions become noncompact and nonlinear as a result of significant photochemical removal of NO• through the reaction N + NO [Fahey et al., 1990; Loewenstein et al., 1993; Nevison et al., 1997].

Previous studies have shown that the linear correla-

tion between N20 and NO u holds over a wide range of 0 N20 concentrations. This is because the rates of mixing 0.6 of air in the regions where NOy production is dominant are faster than the rates of descent of denitrified air 20

from higher altitudes [Fahey et al., 1990; Loewenstein et al., 1993; Nevison et al., 1997]. Similar considera- tions are expected to apply to the correlation between NO• and CH4. In Figure 1, scatter plots of NO• versus 15 CH4, obtained from a range of measurement techniques, are shown. The data sets shown include satellite mea-

surements from the atmospheric trace molecule spec- troscopy (ATMOS), HALOE and the cryogenic limb • 10 array etalon spectrometer (CLAES)instruments [Gord- ley et al., 1996; Gunson et al., 1996; Kumer et al., 1996; Mergenthaler et al., 1996], balloon measurements [Kondo et al., 1996; Sen et al., 1998] and aircraft mea- surements from the POLARIS campaign. The data la- beled UARS are obtained by combining the measure- ments of HNOa and C1ONO2 from the CLAES instru- ment and NO and NO2 from the HALOE instrument. The noninclusion of N20, should not lead to an error

greater than 10% in NO u [Nevison et al., 1997]. Also displayed in Figure 1 are linear least squares fits to ex- perimental data and linear least squares fits to CSIRO 2-D model calculations (see discussion below in this sec- tion). The 2-D model was initialized with a steady state calculation corresponding to boundary conditions for 1980 and then forward integrated using the time- dependent boundary conditions for halo-carbons, CH4, and N20 specified in WMO [1998] for subsequent years. Data used in the figure are obtained from measurements made between late September and mid-November and altitudes from approximately 12-35 km. Because of the larger scatter in measurements equatorward of 40øN, the data have been grouped into two latitude bands of 30ø-40øN (Figure la)and 40ø-70øN (Figure lb).

As can be seen from Figure l a, for the latitudes be- tween 30 o and 40øN, the linear relation between NO• and CH4 is maintained for the CH4 mixing ratios in the range of approximately 1.0-1.6 ppmv (1 ppmv - 1 ttmol mol-1). For these CH4 mixing ratios, a linear least squares fit to the data gives

' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' '

ß ß

ß ß (a) 30øN - 40øN ß ß Fits to:

ß •• ß Measurements ß - - - 2-D model calculations

ß

ß

,B ø

ß ER-2, September ß Balloon, September .ATMOS, November

, , , I , , , I

0.8 1.0 1.2 1.4 1.6 CH,, (ppmv)

' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' '

(b) 40øN - 70øN Fits to:

Measurements

- - - 2-D model calculations

..... Kondo et al. (1996) ß

ß ER-2, September .ATMOS, November ß UARS, September ß Balloon, October

0 I I I I , , I , , , I &i ! . . . • t , . i . . . . i i i

0.4 0.6 0.8 1.0 1.2 1.4 1.6 .8 CH,, (ppmv)

Figure 1. NO• as a function of CH4 (a) for latitudes between 300 and 40øN and (b) for latitudes between 400 and 70 ON. Symbols correspond to measurements ob- tained from various balloon, satellite, and aircraft plat- forms during the months of September,October, and November. The solid line corresponds to a linear least squares fit to data for the CH4 mixing ratios in the range of 1.0-1.6 ppmv (Figure la) and 0.7-1.6 ppmv (Figure lb). The dashed line is a linear least squares fit to CSIRO 2-D model calculations for the same al- titudes, latitudes, and months. The dashed dot-line in Figure lb is the fit derived by Kondo et al. [1996] from the measurements made aboard a single balloon flight in October 1994 at 44øN.

[NOv]- 3.23(4-0.11) x ]0 -8- 1.91(4-0.09)x 10-2[CH4] (S)

resulting in a 2rr uncertainty of 2.0 ppbv (1 ppbv - i nmol tool -1) for NOy. The quoted uncertainties on slope and intercept are 2 times the standard errors. The

RANDENIYA ET At.' NO• AND CL• PARTITIONING IN THE MIDDLE ATMOSPHERE 26,671

CH4 mixing ratios used in the box model calculations 20 in this latitude range and altitude range of 24-32 km are 1.0-1.2 ppmv. The CSIRO 2-D model overestimates the NO s values by about 2 ppbv in this latitude range for CH4 mixing ratios between 1.0 and 1.6 ppmv. lS

For the higher latitude range of 400 to 70øN shown in Figure lb, the correlation is linear for CH4 mixing ratios between about 0.7 and 1.6 ppmv. A linear least •- squares fit to data in this range results in • 10

[NOv]- 2.81(4-0.08) x 10 -8- 1.56(4-0.06) x 10-2[CH4] (6)

giving 2rr uncertainty of 1.6 ppbv for NO v. For these latitudes, the mixing ratios of CH4 considered in the box model ranged from 0.7 to 1.1 ppmv. As can be seen in Figure lb, the fit derived by Kondo et al. [1996] using in situ balloon measurements obtained at 44øN agrees well with the above equation. It should be further noted that the use of either of the above equations results in estimated NO v values which overlap when the 2rr uncertainties of the fits are included. For a given CH4 mixing ratio between 0.7 and 1.6 ppmv, the 2-D model overestimates NO v by about 1-1.5 ppbv in the latitude range of 400 to 70øN.

For the months of June and July, a single correlation for NO v and CH4 was used for the latitude range of 300- 66øN. The available balloon and aircraft data and 2-D model results suggest no significant latitudinal variation in this correlation for the months of May to July. A linear least squares fit for CH4 mixing ratios in the range of approximately 0.7-1.6 ppmv gives

30øN - 70ON

ß ER-2 }May July Sept ß Balloon

Fits to:

-- Measurements

- - - 2-D model calc. July ..... 2-D model calc. May

0 , , , , I , , , i I , , • , I , %ß•, , 0.0 0.5 1.0 1.5 2.0

CH 4 (ppmv)

Figure 2. C1 v as a function of CH4 for the latitudes between 30øN and 70øN. Symbols correspond to balloon and aircraft measurements obtained during the months of May-September 1996 and May-September 1997. The solid curve is a polynomial fit to data. Also shown are polynomial fits to the results of 2-D model calculations for May and July 1997.

40øN - 70øN Fits to:

-- 2-D model calculations

......... Scaled fit

co i ß ER-2, July September •,.,•, 4• 0

0.8 1.0 1.2 1.4 1.6 1.8 2.0 CH4 (ppmv)

Figure 3. Br v as a function of CH4 for the latitudes between 300 and 70 oN. Symbols corresponds to Br v val- ues obtained from aircraft measurements made during May-September 1997, and the line corresponds to a polynomial fit to 2-D model calculations for the same latitudes and time period. The dashed line corresponds to the scaled 2-D model fit used to obtain Br v values to initialize the calculations.

[NOv]- 2.93(4-0.10)x 10 -8- 1.63(4-0.08)x 10-2[CH4] (7)

with 2or uncertainty of 2.0 ppbv for NO v. Because of the nonlinearity of the relationship be-

tween NO v and CH4 for smaller values of CH4, altitudes above 32 km were not considered in the present study. Investigation of available data indicates that at higher altitudes, the NOv-CH4 scatter plots can be very non- compact, possibly due to a combination of local photo- chemical removal of NO v and descent of air from regions of greater NO v loss rates [Fahey et al., 1990; Loewen- stein et al., 1993; Nevison et al., 1997].

2.2.2. Correlation of CI v with CH4. A scatter plot of C1 v versus CH4 obtained from aircraft and bal- loon measurements is shown in Figure 2. Descriptions of measurements and the methods used to obtain these

data sets are given by Elkins et al. [1996], Woodbridge et al. [1995], and Sen et al. [1998]. The data shown in Figure 2 are obtained from POLARIS measurements made in 1997 and MkIV balloon measurements made in 1996 and 1997. The curve obtained from the fitting of these data to a cubic polynomial is shown together with similar curves obtained from fits to 2-D model calcula- tions. Here the fits to 2-D model results lie within about

4-0.2 ppbv from the fit to the measurements. Further, both the 2-D model calculations and the measurements indicate that the seasonal and latitudinal dependence

of the C1 v and CH4 correlation is small in the North- ern Hemisphere extratropical latitudes. Therefore the

26,672 RANDENIYA ET AL.: NOr AND CLy PARTITIONING IN THE MIDDLE ATMOSPHERE

fit to experimental data shown in Figure 2 was used 32

to obtain Cly for all latitudes and seasons in the years of 1996-1997. Because of the significant change in Cly with time, a fit for measurements obtained in 1993 was also derived and used in the box model calculations ini-

tialized with the HALOE data from 1993 and 1994. The

2tr uncertainties of these fits were found to be approxi- mately 0.4 ppbv for Cly.

2.2.3. Correlation of Bry with CH4. Figure 3 shows the scatter plot of Bry and CH4 obtained from measurements made during the POLARIS deploy- ments. Descriptions of the way in which the Bry concen- trations are determined from measurements are given • 28

by Wamsley et al. [1998] and Elkins et al. [1996]. A cubic polynomial fit to 2-D model calculations is also shown in Figure 3. The fit to 2-D model results under- estimates the inorganic bromine content of the atmo- sphere by about 1-2 parts per trillion by volume (pptv) (1 pptv - 1 pmol mol- •). Because measurements of Bry at altitudes above 21 km are not available, the fit to D model results was scaled to the observations in order

to obtain Bry concentrations for the altitudes of inter- est to this study. It should be noted that the bromine chemistry has only little impact on the partitioning of 24 the NOy and Cly families for conditions investigated in the present study.

3. Results and Discussion

3.1. Comparison of NO•/NOy Ratios

Figure 4a shows the NO•/NOy vertical profiles at 52.5øN obtained from this study for July 3, 1994. This is one example of the 128 summer profiles studied as part of this investigation (see Table 1). The error bars shown on the values based on measurement were ob-

tained by combining in quadrature the uncertainties in the measured NO• mixing ratios (4-10%) and the uncer- tainties in the estimated NOy mixing ratios (4-2 ppbv). Percent differences between the calculated ratios and

the values obtained from measurements are shown in

Figure 4b. In this particular case, the NO•/NOy ra- tio is underestimated by the model by about 20-30% between 24 and 32 km.

In Figure 5, the distributions of percent differences between the calculated NO•/NOy ratios and those ob- tained from measurements are shown. All samples given in Table I have been included in this analysis. The frac- tion of samples which showed percent differences within intervals of 10% are displayed in Figure 5. For example, at 26.1 km, nearly half of the samples showed percent differences (between the calculated and measurement- based NO•/NOy ratios) in the range of-25 to -35%. Figure 5 demonstrates that at this level of analysis, the calculated NO•/NOy ratios are consistently smaller than the values derived from measurements at all alti-

tudes, latitudes, and months sampled in this study. A large majority of the calculated NO•/NOy ratios are 15-35% smaller than the values based on measurement.

(a) I l (52.5øN, 171.0øE)! 3July1994 /

,,,I,,,I,,,I,,,I .... I , I , I ,

0.0 0.2 0.4 0.6 0.8 1.0 -40 -30 -20 -10 0

NO•/NOy ratio Percent Difference 10

Figure 4. (a) An illustrative example of the profiles for the NO•/NOy ratios obtained from calculations and from measurements and (b) percent differences between them.

Figure 6 shows the NO•/NOy ratios obtained from calculations and measurements as functions of latitude

and altitude. The values shown are the averages of the ratios in approximately 100 latitude bands. The error bars shown represent averages of the combined uncer- tainties in NOy and NO• (see above). In summer, the calculated NO•/NOy ratios are smaller than the values derived from measurements by 15-30%. In autumn, the calculated values are smaller by about 20-35% than the values derived from measurements. The calculated val-

ues fall within the estimated uncertainties in only two cases: near 32øN and 25 km in autumn.

The above analysis clearly shows that the partition- ing of NOy by the model differs significantly from the measurements for a great majority of conditions ex- plored in this study. If it is assumed that all the impor- tant photochemical processes controlling NO• concen- trations have been considered, the disagreement means that some of the photochemical rates of interconversion of NO• with the reservoir molecules N205, C1ONO2, HNO4, and HNOa are poorly represented in the model. Because N205 and HNO4 mixing ratios are very small at sunset, particularly in the summer months, the pho- tochemical processes involving these species are unlikely to contribute significantly to the calculated discrepan- cies. Further, Dessler et al. [1996] found that the aver-

RANDENIYA ET AL.' NOr AND CL•- PARTITIONING IN THE MIDDLE ATMOSPHERE 26,673

0.50

0.40

0.30

0.20

0.10

0.00

0.50

0.40

0.30

0.20

0.10

0.00 i

0.50

0.40

0.30 0.20

0.10

0.00 0.50

0.40

0.30

0.20

0.10

0.00

26.1 km

0.50

o.4o• / ,,• i •.•_o •r• ] o.•o• / \ i ......... 1 o.•o• j \ • I

0.50

0.40

0.30

0.20

0.10

0.00

24,5 km

-40 -20 0 20 40

Percent Difference

Figure 5. Distributions of percent differences between the NO•/NOy r•tios obtained from calculations and from measurements. Fractions of samples (shown in Table 1) which exhibited percent differences within intervals of 10% are shown.

26,674 RANDENIYA ET AL.. NOv AND CLy PARTITIONING IN THE MIDDLE ATMOSPHERE

0.90

0.80

0.70

0.60

................. i•li .... 'From Measurements ß From Model '

32.0 km '

0.70 29.9 km

0.60

0.50

0.60

0.50

0.40

0.30

27.2 km ' ß

........ i ......... i ......... i ......... ß .........

o Z 0.50

0.40 26 1 km

0.30

0.40

0.30

0.20

ß 25.1 km ß -

ß

• ......... ß ......... , ......... = ......... . .........

0.40 - t ' t t 24.5 km 0.30 - ß -.

0.20 - •' ........ i ......... . ......... i ......... i _ _ i i i i i i "'

30øN 400 50 ø 600 700 80øN

Latitude

Figure 6. NOr/NO v ratios from calculations (circles) and from measurements (triangles with error bars) as functions of altitude and latitude for (a) June and July and (b) September.

age production and loss rates of C1ONO2 derived using simultaneous UARS measurements of C10, NO2, and C1ONO2 agreed to within a few percent with the val- ues predicted by photochemical theory for altitudes be- tween 24 and 32 km. Therefore production and loss terms which link active nitrogen species to HNOa may be considered as likely causes of the disagreement. The

important known photochemical processes which make this link under these conditions are

OH + NO2 + M

N205 + H20(aerosol) BrONO2 + H20(aerosol)

---+ HNO3 + M (8) --+ 2HNOa (9) --+ HNOa + HOBr (10)

RANDENIYA ET AL.. NO¾ AND CL¾ PARTITIONING IN THE MIDDLE ATMOSPHERE 26,675

T -r 'From Model 0.80

0.70

0.60

0.80 .............................................. •

0.60

0.50 i.e. ........ ? ....... _e ........... .e. ......... e. .......

0.60 0.50 t t t t t 272km 0.40

0.30

0,50

0.40 t t t t t 261km 0.30

0.40

0.30

0.20

0.40

0.30

0.20

30øN

i t 24.5 km.. 40 o 50 o 60 o 70 o 80øN

Latitude

Figure 6. (continued)

HNO3 +

HNO3 + OH > OH+ NO2 (11) NO2 k•.H+ d• > H•O + NOs (12) HNO3 ksOH M + 2k9N•Os/NO• + k•0BrONO2/NO•

The above processes directly control the partitioning between HNOs and NO• and therefore control NOx/NOy. Assuming chemical equilibrium it can be shown that

In Figure 7, terms in (13) averaged over 24 hours are plotted for latitudes near 650 and 32øN for June 1993

26,676 RANDENIYA ET AL.: NOv AND CLv PARTITIONING IN THE MIDDLE ATMOSPHERE

Summer '' ' ' I ' ' ' I ' ' ' I '1' ß I ' ' ' 32 i I i I I I I

30 •

' I

• k•[OH] I I

28 ' I

• k•[•O,l/[•O•] ß

I . . ß I ß ß ß I ß . ß I ß . . I . ß ,

i 32 30

28

26

24 ß ß ß I ß ß ß I ß I I I I I I I ß ß I

0 2 4 6 8 10

Autumn

ß ß ß I ß . . I . ß ß I ß ß . J I I i

0 2 4 6 8 10

Effective First Order Rate Coefficient (10'6s '1)

Figure ?. Diurnally averaged terms in the denominator and in the numerator of equation (13) at 65øN (full lines) and at 32øN (dashed lines) in summer and in autumn.

(summer) and September 1993 (autumn). Depending on the relative magnitudes of the various terms, the NO2/HNO3 ratio varies considerably with season, al- titude, latitude, and surface area density of sulphate aerosols. in the altitude range of 24-32 km, first-order photolysis rate coefficient of HNO3, J•, is the dominant term in the numerator, and the effective first-order gas phase association rate coefficient, ksOH M, is the dom- inant term in the denominator. As the altitude is in-

creased, J• increases more rapidly than the denomina- tor of (13), and, as a result, the modeled values for the ratio NO2/HNOa increase with altitude. It is also clear from Figure 7 that at 65øN in summer, the production of HNO3 above 24 km is controlled almost entirely by

the gas-phase combination of OH with NO2. The reason

for this is that in June at 65øN, the rate of formation of N205 is greatly diminished due to rapid photolysis of NO3 in the presence of near continuous sunlight through the day. At 32øN, however, (9) is relatively more im- portant with contributions to the formation of HNO3 of about 20% at 24 km and less than 5% at 32 km. This

varying role of the heterogeneous process is reflected in the summer latitudinal trend of the ratio NO,/NO u at altitudes below about 27 km (see Figure 6a). As seen in Figure 7, in September the heterogeneous hydrolysis of N205 plays a more significant role in the production of HNOa below 30 km at both latitudes where the rel-

ative contributions from this process increase at lower altitudes.

It should be noted that in 1993, the year for which the

RANDENIYA ET AL.' NOv AND CLv PARTITIONING IN THE MIDDLE ATMOSPHERE 26,677

results shown in Figure 7 were obtained, the sulphate aerosol levels were enhanced to values well above back-

ground levels because of the eruption of Mr. Pinatubo in June 1991. Therefore the calculated contributions

from heterogeneous processes are greater than those ex- pected in volcanically unperturbed atmospheres. Judg- ing from the results obtained for summer, it is unlikely that the uncertainty in the reaction probabilities for the two heterogeneous reactions (9) and (10) would be a major cause of the discrepancies between NO,/NO u values obtained from measurements and from calcula-

tions. To confirm this view, box model calculations were repeated with the reaction probability for (9) re- duced by ,50%. At :27.2 km the resulting increases in the NOx/NOu ratio were less than 3% in the summer months of 1993 and less than 8% in the autumn months

of 1993. The differences between NO,/NOv values ob- tained from measurements and calculations are gener- ally larger than 30% for these conditions.

Figure 7 shows that according to the photochemical model, photolysis is the major HNOa loss process for conditions considered in this study. In June, the rate of conversion of HNOa to NO, via photolysis is about 7,5% of the total at 24 km, increasing to about 8,5% of the to- tal near 32 km. In September, photolysis still dominates comprising 70% of the total loss at 24 km and 8,5% of the total loss near 32 km. The recommended cross sections

and their temperature dependence for this process were obtained from Burkholder et el. [1993]. The relative contributions from different wavelength regions to the calculated photolysis rate of HNOa near 6,5øN on June 2,5, 1993, are shown in Figure 8. Because of the strong absorption of radiation by the ozone Hartley band, no significant contributions to photolysis rates are calcu- lated for wavelengths between 220 and 280 nm. There- fore photolysis rates of HNOa are determined by the absorption in two d'}stinct wavelength regions. As ex- pected, at higher altitudes the rate is dominated by the absorption in the 180-230 nm region. Near 2,5 km, the contributions from the 280-340 nm region are compa- rable to those from the shorter wavelength region. This means that at higher altitudes where the remaining dis- agreement for NOx/NO v ratios are largest, the shorter wavelength region is the most important. Therefore adjustments to HNOa photolysis cross sections at wave- lengths greater than 280 nm, where the laboratory mea- surements have larger uncertainties, will have only a small impact on the photolysis rates near 30 km. The cross sections are large, and measurements are expected to be reliable in the shorter wavelength region. The ob- served temperature dependence is also small, and there is agreement to within a few percent between differ- ent measurements [Burkholder et el., 1993]. Therefore it is unlikely that the uncertainties in the photolysis cross sections of HNOa can explain all of the differences between NO,/NOy ratios obtained from measurements and calculations.

1.0

0.9

0.8

..q o.5 .o

• 0.4

.>

_• 0.3

0.2

0.1

0.0

• 25 km

- - - 33 km

I

I I

I

I

i

i

i

I

I

200 250 300 350 Wavelength (nm)

Figure 8. Relative contributions to the rate of pho- tolysis of HNOa at noon as functions of wavelength at 65øN in summer at 25 km (solid line) and at 33 km (dashed line).

Equation (13) highlights the importance of the OH concentration in determining the NO,/NO v ratio. In order to investigate the sensitivity of the uncertain- ties in the HOz budget on NO v partitioning, calcula- tions were repeated with OH concentrations reduced by 20%. As expected from (13) and Figure 7, the effects of this change on NO2/HNOa and hence on NO•/NO v are most noticeable in summer at high latitudes. Un- der these conditions, the gas phase association of OH and NO2 dominates the production of HNOa, and pho- tolysis dominates the loss of HNOa. Therefore the NO2/HNOa ratio increases by nearly 20076 when OH is decreased by 20%. However, the NO,/NO v ratios change only by about 10% near 32 km and by about 7% near 25 km. At lower latitudes in summer as well

as in both lower and high latitudes in fall, the result- ing changes in the NO•/NO v ratio is smaller than 8%. Given that the modeled and measured NO,/NO v val- ues differ by 15-35ø76, it would require large changes to kinetic parameters governing HO, photochemistry to fully explain the discrepancies between model and ob- servations. Measurements obtained during the SPADE campaign suggested that both OH and HO2 are under- estimated by rp. odels [Selewitch et el., 1994]. The inclu- sion of BrONO2 hydrolysis in the calculations generally

26,678 RANDENIYA ET AL.' NOy AND CLy PARTITIONING IN THE MIDDLE ATMOSPHERE

improved the agreement between models and observa- a2 tions [Lary et al., 1996]. Recent analysis of balloon mea- surements between 40 and 50 km indicates that while

the measured HO2 concentrations are underestimated by the models, the OH concentrations are generally in good agreement with the model predictions [Jucks et al., 1998]. a0

From the foregoing discussion it is clear that bar- ring unknown processes, some of the gas-phase produc- tion and loss rate coefficients for HNOa are inadequately represented by the recommended parameters given by DeMote et al. [1997]. Recently, it was reported that • according to new measurements, the temperature de- $ 28

pendence of the rate coefficient for (8) (in N2 bath gas) • at the limits of low pressure and high pressure is best represented by [Brown et al., 1999a, 1999b]

k0(T) - 2.47 x 10-aø[ T -2.97

cm e s -• (14)

koo(T)-l.45x 10-•[ T] -2.77

cm3s -• (15)

The fall-off expression for the rate coefficient given by these authors is similar to that given by DeMote et al. [1997] except that Brown et al. [1999a] include an addi- tional factor of 0.94 for the low pressure rate coefficient to account for slightly smaller collision efficiency of air compared to N2.

The new measurements also suggest that the pres- sure and temperature dependencies of (12) are signif- icantly different from those obtained from DeMote et al. [1997]. From the fits including new measurements, Brown et al. [1999b] recommend the following expres- sions for the evaluation of rate coefficients for (12) under stratospheric conditions:

k(T, M) - ko(T) + (16) (1 +

where

k0(T) - 2.41 x 10-•4exp(460/T) cm3s -1 (17)

Ak(T) - 2.69 x 10-X7exp(2199/T) cm a s -x (18) kc(T) - 6.51 x 10-aaexp(1335/T) cm 6 s -1 (19)

Percent differences between the rate coefficients calcu-

lated using the parameters from Brown et al. [1999b] and DeMote et al. [1997] are shown in Figure 9. The results are presented for conditions in summer, but sim- ilar results are obtained for conditions in autumn as well. The new rate coefficients for the association reac-

tion (8) are smaller by 20-30% in this altitude range. This occurs primarily because the new measurements predict a temperature dependence for ko(T) which is not as steep as the (T) -4'4 recommended by DeMote et al. [1997]. The calculated rates of production of HNOa in the stratosphere by (8) are smaller, and therefore the

OH+NO•,+M

I

!

I

I

I

I

I

I

!

I

I

!

I

I

I

I

I

I

I

I

I

OH+HNO 3

• 65ON

... 32ON

24 • I • • I -40 -20 0 20

Percent Difference in Rate Coefficient

Figure 9. Percent differences between rate coefficients calculated from Brown et al. [1999a] and DeMote et al. [1997] for reactions (8) and (12). Solid and dashed lines correspond to calculations for 650 and 32øN, respec- tively.

calculated NOx concentrations are larger when the new parameters for the rate coefficient for (8) are included in the model.

The relative changes to the rate coefficients for (12) depend on pressure and latitude. When compared with the values calculated from the parameters recom- mended by DeMote et al. [1997], the modified rate coef- ficients are larger below about 32 km and smaller above about 32 km. The largest differences below 32 km are obtained for higher pressures (lower altitudes) and lower temperatures. This means that for altitudes below 32

km, the inclusion of the new parameters for (12) in the model should also lead to higher calculated concentra- tions of NOx.

When the recent expressions for both rate coefficients are included in the box model, the calculated NO• con- centrations increase by 8-20% at altitudes between 24 and 32 km. The largest percent increases, as expected from the discussion above, are obtained for the lower altitudes. Figure 10 shows the altitude and latitude de- pendence of the NO•/NO v ratios calculated using the modified rate coefficients and those obtained from mea- surements. Figures 10a and 10b are directly compara- ble to those of Figure 6. The inclusion of modified rate

RANDENIYA ET AL.' NOr AND CLv PARTITIONING IN THE MIDDLE ATMOSPHERE 26,679

0.80 32.0 km

0.70•-. ' ' "• 0.60 •, ........ ,..- ................................... _,--J

ß

0.70 29.9 km

0.60

0.50

0.60

0.50

0.40

C• 0.30 Z

zO 0.50

0.40

0.30

0.40

0.20

0.30

0.20

30øN

24.5 km

. ,

......... , ......... i ......... i ......... ß .........

40 ø 50 ø 60 ø 70 ø 80øN

Latitude

Figure 10. NO•/NOy ratios obtained from measurements and from calculations using rate expressions for reactions (8) and (12) from Brown et al. [1999a] for (a) June and July and (b) September.

expressions for (8) and (12) resulted in significant in- creases to the modeled ratios, and, consequently, better agreement between measurements and calculations are obtained at all altitudes and latitudes. The calculated

ratios have shifted closer to the measured values by 5-

6% near 32 km, 9-10% near 27 km, and 13-15% near 25 km. Overall, the discrepancies in the order of-15 to -40%, obtained when the DeMote et al. [1997] pa- rameters were used for the two reactions, are reduced to 1.5 to-25%.

26,680 RANDENIYA ET AL.' NOr AND CLy PARTITIONING IN THE MIDDLE ATMOSPHERE

0.90

0.80

0.70

0.60

....................... ibi ....... •' From Measurements ß From Model '

l=

........ ß ......... i ......... ß ......... ß ........

0.80

0.70

0.60

0.50

0.50 27.2 km

......... i ......... i ......... i ......... ! .........

0.40- •

0.30 25.1 km.

0.20

0.40

0.30

0.20

30øN 40 ø 50 ø 60 ø 70 ø 80øN

Latitude

Figure 10. (continued)

Despite the better agreement between the modeled and measured NO•/NO• ratios obtained with the new rate coefficients, particularly at the lower altitudes of this study, percent differences in excess of 20% remain in many cases. The disagreements are particularly notice- able at altitudes above 27 km indicating more knowl- edge of processes controlling NO• partitioning is still required.

3.2. Comparison of HCI/CI• Ratios

The photochemistry of HC1 in the altitude range of interest is controlled primarily by the following reac- tions

CI+H2 > HCI+H (20) C1 q- CH4 •-• HC1 + CH3 (21)

Cl+CH20 --+ HCI+HCO (22)

RANDENIYA ET AL.: NOr AND CLv PARTITIONING IN THE MIDDLE ATMOSPHERE 26,681

Table 2. Percent Contributions to the Rate of Pro- duction of HC1 (Diurnally Averaged) Reaction 25.1 km 27.2 km 29.9 km 32.0 km C1 + H2 2.3 2.2 2.0 1.8 C1 + CH4 80.7 70.3 57.8 48.0 C1 + CH20 8.8 10.3 12.4 13.5 HO2 + C1 1.2 2.5 4.0 5.3 OH + C10 6.9 14.7 23.8 31.4

HO2 q- C1

OH + C10

> HC1+O2 (23) > HClq-02 (24) • C1 n u HO2 (25) • H20 q- C1 (26) OH + HC1

Some minor channels of production and loss of HC1 have been omitted from this list. The major products of the reaction between C10 and OH are HO2 and C1 (reac- tion (25)), and the rate of production of HC1 from (24) amounts to approximately 6% of the total reaction rate at stratospheric temperatures [Lipson et al., 1997]. The only significant loss process for HC1 is (26), whereas the relative importances of the primary production re- actions depend on altitude. In Table 2, the calculated diurnally averaged rates of production of HC1 at 52.5øN and 171.0øE on July 3, 1994, are shown as functions of altitude. The most significant altitude dependencies are obtained for (21) and (24). The inclusion of (24)in the model increases the calculated production of HC1 at all altitudes considered in this study with its relative im- portance increasing with altitude.

The profiles of the HC1/Cly ratios obtained from the analysis of the HALOE measurements obtained at 52.5øN and 171.0øE on July 3, 1994, are shown in Fig- ure 11. The results are shown from calculations which

included reaction (24) and calculations which did not include reaction (24). Also shown in Figure 11 are the diurnally averaged C1ONO2/HC1 ratios from the calcu- lations which included reacti•on (24). The uncertainties shown on the HC1/Cly values obtained from measure- ments are the combined uncertainties estimated for (:t:0.4 ppbv) and HC1 (:t:8%). In these particular calcu- lations the modeled HC1/Cly ratio at 32 km agrees bet- ter with the value obtained from measurements when

(24) is included in the calculations. Below 25 kin, the calculated HC1/Cly is larger compared to the values ob- tained from measurements. The distributions of percent differences between the HC1/Cly ratios obtained from measurements and calculations are shown in Figure 12. Calculations from all HALOE profiles listed in Table i have been included in this analysis. Percent differ- ences are grouped into 10% intervals, and the fractions of samples which exhibit differences in each interval are shown. Figure 12a shows the results for calculations that included reaction (24), and Figure 12b shows the results for those that did not include (24). The calcu- lated increase in the production of HC1 resulting from

(24) brings the modeled HC1/Cly ratio closer to the values obtained from measurements at altitudes above

about 30 km. However, the calculated additional pro- duction of HC1 leads to poorer agreement at lower alti- tudes. As seen from Figure 12a, the calculated HC1/Cly ratios at or below 25 km are larger than values obtained from measurements by 15% or more in at least 40% of the cases studied. However, in more than 95% of the samples, the disagreements are less than 25%.

As discussed in section 1, the overestimation of HC1/Cly by models at altitudes near and below 21 km has been noted previously by other authors [Jaegle et al., 1996; Webster et al., 1994]. More recent measurements yield HC1/Cly ratios which are in better agreement with mod- els [Webster et al., 1998]. It appears that the mea- sured low values for HC1/Cly ratios below 21 km are correlated with higher aerosol loading in the atmo- sphere. Fast heterogeneous chemistry occurring in cold air masses [Michelsen et al., 1999; Webster et al., 1998] and heterogeneous formation of HC104 [Jaegle eta!., 1996] have been proposed to help explain these obser- vations. Some discrepancies between models and ob-

32 (a) Long 98.2øE

Lat 52.0øN

3 July 1994

•HALOE

30 -- HCl/Cly with (24)

-- HCl/Cly

without (24)

• 28

26

24

0.0 0.2

Figure 11.

0.4 0.6 0.8 1.0 -20 0 20 40 Ratio Percent Difference

Profiles for (a) HC1/Cly ratios obtained from calculations and from measurements. Results are shown from calculations which included reaction (24) and from calculations which did not include reaction

(24). Also shown is the ratio C1ONO2/HC1 from cal- culations which included reaction (24) and (b) percent differences between HC1/Cly obtained from calculations with and without reaction (24) and from measurements.

26,682 RANDENIYA ET AL.' NOr AND CLy PARTITIONING IN THE MIDDLE ATMOSPHERE

0.6

0.4

0.2

0.0

ß (a)

32.0 km

0.6

0.4

0.2

0.0

29.8 km

0.6

0.4

0.2

0.0

0.6

0.4

0.2

0.0

27.1 km

26.1 km

0.6

0.4

0.2

0.0

0.6

0.4

0.2

0.0

-40 -20 0 20 40

Percent Difference

Figure 12. Distributions of percent differences between calculated and measurement-based HC1/C1 v ratios for (a) calculations which included reaction (24) and (b) calculations which did not include reaction (24).

servations still remain unexplained for HC1/Clv and C10/C1 v.

Figure 12 shows that C1ONO2 constitutes a signifi- cant fraction of the C1 v family. Heterogeneous chemi- cal processes which are important at temperatures be- low 210 K should not have a significant impact on the

partitioning between HC1 and C1ONO2 in the calcula- tions presented here. Further, the results of the current study do not show a trend in the HC1/Clv ratio obtained from measurements between 1993 and 1997 at altitudes

between 24 and 32 km. Therefore it is unlikely that the proposed sequestering of Clu to HC104 on sulphate

RANDENIYA ET AL.' NOy AND CLy PARTITIONING IN THE MIDDLE ATMOSPHERE 26,683

0.6

0.4

0.2

0.0

(.b) ß

ß

ß

ß

ß

ß

ß

32.0 km

0.6

0.4

0.2

0.0

0.6

0.4

0.2

0.0

0.2 _ 61 k 0.0 ': 0.6

0.4

k

0.2

0.0

0,6'

0.4

0.2

0.0

-40 -20 0

Percent Difference

20

24.5 km

40

Figure 12. (continued).

aerosol [Jaegle et al., 1996] would be important at these altitudes. Dessler et al. [1995] have shown that in the altitude range of interest, the ratio C1ONO2/HC1 is ap- proximately quadratically dependent on the concentra- tion of Oa and linearly dependent on the concentrations of OH and CH4. Having noted that in the majority of the cases presented in this study, the remaining differ- ences between the calculated and measmed HC1/Cly

values at. lower altitudes are less than 25% (see Fig- ure 12a), it is likely that the combined uncertainties in the measured concentrations of O3, CH4, and H20 together with the uncertainties in the relevant rate co-

efficients [Dessler et al., 1995; Michelsen et al., 1996] may account for the disagreement. In order to illustrate this fact, the effects of the quoted uncertainties for the rate coefficient of (21) was investigated. Reaction (21)

26,684 RANDENIYA ET AL.: NOr AND CLy- PARTITIONING IN THE MIDDLE ATMOSPHERE

is the largest contributor to the production of HC1 at these altitudes, and according to the expression derived by Dessler ½t al. [1995], the ratio C1ONO2/HC1 is in- versely proportional to k2•. As seen from Table 2, the relative importance of (21) increases at lower altitudes. Calculations were performed where the activation en- ergy for this reaction was increased by 0.62 kJ tool -• (an increase of 5.4%). At stratospheric temperatures, this reduces the rate coe•cients close to the lower limit

of the range of uncertainty calculated from DeMote ½t al. [1997]. This modification to the the rate coe•cient for (21) together with the inclusion of (24) in the model brings the calculated HC1/Cly ratios between 24.5 and 32 km to within -t-10% of those obtained from measure-

ments for over 95% of the cases considered in this study. Therefore it can be concluded that within the uncertain-

ties of the rate coe•cients specified by DeMote ½t al. [1997] for (21) and (24), the HC1/Cly ratios deduced from measurements agree with values derived from the photochemical model.

4. Conclusion

The partitioning of the NOy and Cly families has been studied using HALO E measurements and a photochem- ical box model. The spatial coverage of this analysis extends from 24 to 32 km in the latitude range of 300- 75øN. This altitude range should complement the region covered in the POLARIS campaign.

The use of rate parameters obtained from DeMote ½t al. [1997] in the box model leads to an underesti- mation of the NOx/NOy ratios by 15-35% between 24 and 32 km and 30ø-75øN in the months of June, July, and September. Analysis of the results for the processes which control partitioning in the NO• family indicates that the gas-phase rate coe•cients used in the model may be responsible for the discrepancies. In fact, when the rate formulations based on recent laboratory mea- surements by Brown et al. [1999b] for OH + NO2 + M and OH + HNO3 are included in the model, sub- stantially better agreement between the modeled val- ues and the measurement-based values for NOx/NOy is obtained. Despite the better agreement obtained, par- ticularly at the lower altitudes considered in this study, differences up to 20% remain at altitudes above 27 km. Therefore further improvements to our knowledge of processes controlling the partitioning of NO• appear necessary. Although the measurement uncertainties in the photolysis cross sections for HNO3 are unlikely to explain the remaining differences, contributions from processes yet unknown cannot be discounted.

When a 6% branching ratio for HC1 from the reaction C10 + OH is included in the calculations, the modeled HC1/Cly values agree with measurement-based values above 27 kin. Moreover, the calculated additional pro- duction of HC1 from this reaction leads to poorer agree- ment between the HC1/Cly ratios obtained from model and measurements for lower altitudes. When the activa-

tion energy for the reaction of C1 with CH4 is increased close to the upper limit of the range of uncertainty given by DeMote ½t al. [1997], the model predictions agree with measurements at all altitudes between 24 and 32 kin. Therefore, within the uncertainties of the rate co- e•cients specified in DeMote ½t al. [1997], the HC1/Cly ratios derived from measurements appear to agree with photochemical model predictions for this range of alti- tudes.

Acknowledgments. This work would not have been possible without access to the results of a wide range of ex- perimental observations. In particular, on the POLARIS program, we would like to thank J.W. Elkins for CH4, Cly, and Bry data; R. S. Gao and D. W. Fahey for NO• data; G. Toon and B. Sen for MkIV balloon data; and C. Webster for CH4 data. We are grateful to A. R. Ravishankara for provid- ing laboratory rate information and to the HALOE team for help with measurement interpretation. We are also grate- ful to A. Adriaansen for his assistance in the preparation of diagrams and handling of HALO E data.

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Dessler, A. E., D. B. Considine, J. E. Rosenfield, S. R. Kawa, A. R. Douglass, and J. M. Russell III, Lower strato- spheric chlorine partitioning during the decay of the Mr. Pinatubo aerosol cloud, Geophys. Res. Lett., 24, 1623- 1626, 1997.

Elkins, J. W., et al., Airborne gas chromatograph for in situ measurements of long-lived species in the upper tropo- sphere and lower stratosphere, Geophys. Res. Lett., 23, 347-350, 1996.

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I. C. Plumb, L. K. Randeniya, and K. R. Ryan, CSIRO Telecommunications and Industrial Physics, P.O. Box 218, Bradfield Road, Lindfield, NSW 2070, Australia. (K eit h. Ryan @ tip. csiro. au)

(Received December 31, 1998; revised April 20, 1999; accepted June 18, 1999.)