Observations of stratospheric hydrogen fluoride by halogen occultation experiment (HALOE)

. The Halogen Occultation Experiment (HALOE) Hydrogen Fluoride (HF) channel on the Upper Atmospheric Research Satellite is providing the first global measurements of stratospheric HF, the dominant fluorine reservoir in the atmosphere. This paper describes the latitudinal and seasonal variations of HALOE-observed HF in terms of vertical profiles, altitude/la, titude cross sections, and column abundances. The HF global distribution shows a tracerlike structure and its column amount increases with la, titude, in agreement with previous aircraft measurements of the HF column amount. A comparison between the HALOE HF annual rate of increase of stratospheric HF. Exponential rates of 4.9-6.6% yr -(cid:127) and linear growth rates of 6-8.6% yr -(cid:127) in 1985 and 4.3-5.5% yr -(cid:127) in 1992-1993 are found. HALOE HF measurements during the 1993 Antarctic spring are briefly described. This species behaves like a conserved tracer a,nd its distribution shows an area of enhanced mixing ratios correlated with the polar vortex that has a clear latitude boundary. Finally, simulated HF distributions by the National Center for Atmospheric Research two-dimensional 1nodel are used to compare with HALOE observations of HF. Reasonable agreements in the global structure and the absolute amount of HF are found. The differences between the model and the observed results indicate the need for ilnproving treatment of atmospheric dynamics and fluorine-related by the National Center for Atmospheric Research two-dimensional model and the HALOE observed HF will also be described.


Introduction
Stratospheric hydrogen fluoride (HF) is believed to be the dominant stable reservoir of free fluorine atoms released from the photochemical breakdown of the manmade chlorofiuorocarbons (CFCs) [Stolarski and Rundel, 1975;Sze, 1978]. The measurement of HF is therefore recognized to be important in investigating the changing effects of anthropogenic products on the global ozone layer as well as on the chemical and dynamical processes in the upper atmosphere. The tropospheric source gases that contain fluorine atoms are mainly CFCs as a function of altitude and its only sink path in the stratosphere is to diffuse downward to the troposphere where it is removed by rain. HF therefore has a long lifetime in the stratosphere.
Global observations of HF in the stratosphere have been obtained by the Halogen Occultation Experiment. [Russcll el al., 1993a]. The HALOE HF data set provides us information for the first. time on the global distributions and temporal variations of this poorly known species in the stratosphere. It is the aim of this paper to report and analyze HALOE-measured HF, which no doubt will enhance our knowledge of this important fluorine-containing species.

(HALOE) on the Upper Atmosphere Research Satellite (UARS) since its launch over two and one-half years ago
Two long-term measurements of HF column abundances at ground sites were reported previously:   [1991]. They found that, the loss of CF20 on these materials was very slow or nonexistent; their upperlimit rates imply that this loss "will be insignificant on timescales of a year, such as the annual polar chlorine activation phenomenon." In this paper we describe HALOE-measured stratospheric HF, including its global distribution and seasonal variations. We will also compare the measurements of the spatial and temporal variations of HF column amount observed prior to UARS with HALOE observations. HALOE measurements of HF profiles during the 1993 Antarctic spring near the southern polar region will be discussed briefly. Finally, a comparison between the model simulations of global HF by the National Center for Atmospheric Research twodimensional model and the HALOE observed HF will also be described.

Observations of Halogen Occultation Experiment (HALOE)
HALOE is the only instrument on UARS measuring hydrogen fluoride in the stratosphere. Detailed descriptions of HALOE and its HF channel can be found in a paper by Russell e! al. [1993a]. The measurement of HF volume mixing ratio profiles is made by one of the four gas filter channels in the HALOE instrument. HALOE, which uses the solar occultation approach, observes solar radiation absorbed by atmospheric gases along the Earth limb for one sunrise and one sunset event on every satellite orbit. For the HF channel, after passing through a broadband filter centered at. 2.45 Fro, the sunlight is split into two paths, a HF gas cell path and a vacuum path. The HF gas cell functions as a narrow band filter. The ratio of the difference signal obtained from the two paths divided by the vacuum path signal along with other information (e.g., the temperature versus presstire profile determined from the COu channel) is used to retrieve HF volume mixing ratios. The HALOE instrument has made excellent measurements in all eight channels since it, started to take scientific data on October 11, 1991. There is one time period of 37 days, between June 3 and July 10, 1992, when UARS experienced a solar array anomaly which forced HALOE and the other UARS instruments to be powered off. Figure 2 also shows about 3 to 4 time periods (a few days) during which HALOE does not take data either because spacecraft sunrise or sunset, does not occur or the eclipse period is lengthened causing elevated instrument temperatures. When the temperature of the instrument exceeds a predetermined threshold defined to protect the detectors at high solar illmnination angles, HALOE is turned off by ground COlnmand. A HF validation paper is in preparation by the HALOE science team. We will discuss that among the HALOE's eight channels, sunspots have the largest effects on the retrievals of the HF channel, and efforts have been made to remove this effect. Our experiences with HALOE HF data indicate that in the current version data set, there are only relatively few profiles that have been contaminated by sunspot effects with very short duration (2 or 3 time periods in a year). Those profiles can easily be identified and eliminated froin the data set used for scientific studies.
The lowest altitudes of HALOE-retrieved vertical profiles are lower for the higher-latitude n•easurelnents than they are for the tropical regions mainly because of volcanic/sulfate aerosol effects on the pointer/tracker. The Mount Pinatubo volcanic eruption in June 1991 had a large effect on HALOE observations in its first few months in orbit [Russell el al., 1993a], but as the years went by, the volcanic aerosol effects gradually reduced. The typical lowest altitude in the 1991 and early 1992 data at midlatitudes for the HF channel is near 25 kin, while in the current data set (late 1993), the lowest altitude is about 12 kin. Although HALOE does not provide daily measurements of global HF (see Figure 2), a latitude sweep in --• 20-30 days would provide a seasonal representation of the HF morphology since the timescale of the stratospheric circulation is of the order of several months. In fact, we have checked many possible latitude sweep data sets (sunrises, sunsets only, or the combinations of sunrises and sunsets) and found that the HF pattern shown in its latitude versus pressure cross sections are quite similar among 2-4 possible latitude sweeps in a seasoil (2-3 months). This can also be demonstrated by comparing Plate lc and Plate 3 for HF zonal distributions in July and September, respectively, which will be discussed below. Plates la-ld show HALOE HF latitude versus pressure cross sections for the four time periods in 1993, which are essentially for winter, spring, summer, and fall seasons. The HF mixing ratio profiles of a selected time period are binned to latitude boxes having 4-50 width and centered at. latitude grids with 1 ø steps. A normalized Gaussian function is applied to the profiles in a latitude bin to weight the profiles differently a.ccording to their distances to the latitude grid of the bin [oeuo el, al., 1994]. The pressure levels included in the plots are between 100 and 0.32 inbar. HALOE HF retrievals above 0.32 mbar are noisy, but the mean values there, at high latitudes in particular, tend to stay constant through the lower mesosphere until HALOE loses its sensitivity in the middle to upper mesosphere. The great similarity between HALOE-observed global patterns of CH4 and HF [Russell el al., 1993a] indicates the internal consistency of the instrument.

Global Distribution of
As expected, the HALOE observations of HF latitude versus pressure cross sections (Plate 1) show an increase of HF mixing ratio with altitude. Since the source gases of stratospheric fluorine (C, FCs) enter the stratosphere mainly in the tropical region and experience photolysis in the middle to upper stratosphere, HF decreases lower down into the troposphere a.s it is washed out. The stratospheric transport circulation that consists of upwelling in the tropics and downwelling at high latitudes produces the HF minimum over the tropics and maximum over the high latitudes.
The gross pattern of HF mentioned above is generally consistent with theoretical estimates. But as seen in Plate 1, it also has some unique seasonal characteristics. In particular, the HF pattern in April to May 1993 (Plate lb) shows a "double-peak" feature of equatorial Inaximum and subtropical minimum, while during the second equinox season of October to November 1993 (Plate ld), the double-peak structure is not quite so t,o stronger wave driving) will be followed by stronger westerly a.s a consequence of enhanced wave momentum deposition during its easterly phase.
Another HF pattern that is characteristic of stratospheric tracers is seen during the solstice sea.son (Plate la and lc). Below around 10 lnbar, the region of min-iImzm HF tilts t. oward the winter hemisphere, while above 10 mbar, it tilts toward the summer hemisphere.

Column
Amount of HF HF total column abundances have been used in the past to study the seasonal and latitudinal variations of this species and to record the increasing trend of the man-lnade fluorine alnount in the atmosphere. In this section we will describe the HF column alnount above the lowest. altitude measured by the HALOE instrument. As stated before (section '2), the lowest, altitudes of HALOE HF profiles gradually decrease with the reduction of Pinatubo aerosol loading in the lower stratosphere. We will therefore lnostly use data taken in 1993, the second year of UARS in orbit. This will be compared to the HF column measurements by the ATMOS experiment in early May 198,5 t.o exa,mil•e the HF loading trend in the stratosphere.

Latitudinal Dependence of the HF Column
In section 3, four time periods in 1993 are used to describe HALOE observed HF vertical mixing ratio pro-  I I I  I I I  I I I  I I I  I I I  I I I  I I I  I I I   - In the midlat. it,udes (40 o-60 ø lat,itudes), a relat. ively "flat," area in t,he HF column versus latitude diagram is found equat,orward of t,he wint,er/spring polar region (Figures 4c and 4d, and Figure 8). This feature is not quite so obvious in t.he HF column amount. observed outside t. he nort.hern wint,er/spring polar region (Figures 4a and 4b). This feat, ure is consist. ent, wit,h t.he weak gradient. region shown in t,he HF lat.it. ude versus pres- The a +bcos(latitude) curve fitted for t, he January to February period also has a different latitudinal gradient, than those for other seasons ( Figure 5). As discussed earlier, strong mixing from waves in t, he winter midlatit, udes has the effect, of flattening the HF isopleths.

Seasonal Variation of the HF Column Amount
The HALOE tangent point passes a fixed latitude less than 20 times during a year (Figure 2). We use the limited data set, fi'om one latitude region to study the seasonal variations of the HF columl•S. The HF colunto as a functiol• of altitude is provided in t.t•e HALOE level 2 files. We interpolate t,he HF column data at a fixed altitude, such as 15 kin, and then plot. them against t. ime (Figtire 6a and 6b)    Like other tracers, the global distribution of stratospheric HF is shaped by the coupling among atlnospheric radiative, dynamical, and chemical processes. The fact, that those long-lived species have a similar pattern in their cross sect. ions of latitude versus height provides a useful tool in studying atmospheric transport processes. Analysis of the correlations between two tracer species as functions of altitude and latitude should reflect the differences of the chemical-transport. coupling effects on these species. During the Antarctic winter-spring, the chemical processes of stratospheric tracers, including HALOE-measured CH4 and HF, are believed to be slower than any other seasons. Do we expect their mixing ratios to have a similar amount of downward displacement inside the polar vortex? A preliminary analysis of HALOE C, H4 and HF version 16 data in the 1992 Antarctic spring shows that these two species appear to have significant differences in their  Figures 10a-10d show HALOE-model comparisons of the HF mixing ratio vertical profiles in the la.titude bands of 15øS -15øN and 450-550 of t. he summer hemispheres for the January t.o February and July time periods. The comparisons for the tropics in January (unit CF-_,.O photolysis quantum yield case) and July and for the summer midlatitudes in February show good agreement in both profile shapes and the absolute mixing ra.tios with a. slight bias which is probably due to the 3year difference between model simulations and HALOE observations. Both model and HALOE HF profiles show the t. endency for its mixing ratios to stay constant in the stratopause region a.t higher latitudes. and they both show that in the lower stratosphere HF is produced at higher altitudes in the tropics than in the lnidlatitudes. Figure 10d shows a different vertical gradient of the HALOE and model HF profiles below 30 km (•10 mbar) for July a.t. sum•ner midla.titudes. This difference is expected because the HALOE-observed HF distribution in July (Plate lc) shows that the minimum HF region below •10 mbar is tilted toward the winter subtropics, while the model-sinlulated HF structure does not show this feature clearly.

Comparison With the Atmospheric Trace
There We conclude from Figure 10

Conclusion
The HF channel of the HALOE instrument on-board U ARS has provided global lneasurements of stratospheric HF for the first time. This paper describes HF global distributions and its seasonal variations in vertical profiles as well as its column abundances, while another paper [Ritss½ll et al., 1994 in preparation] focuses on HF data validatiol•. The behavior of the HF mixing ratio profiles during the 1993 Ant. arctic spring is also briefly described here. The HF latitude versus pressure cross sections show a familiar general st. ructure seen in other arnospheric tracers, such as N20 and CH4, characterized by a "bell-shape" isopleth. We showed the seasonal variations of the global HF observed by HALOE and found distinctive structures in HF mixing ratios for the four seasons, such as the "double-peak" in northern spring and the HF raininn]in region tilting in the summer/winter seasons. These features are believed to be controlled by atmospheric dynamical processes. Transport effects on the tracer distributions is coupled with related chemical processes, 16,704 LUO ET AL.' OBSERVATIONS OF STRATOSPHERIC HF BY HALOE depending on altitudes and latitudes. Current models that include fluorine chemistry, represented by the NCAR two-dimensional model discussed in this paper, are able t,o reasonably simulate the HF global distribu[ion. But, more realistic dynamical processes need to be included in the model to sinrelate the more detailed structure of HF. The theoretical partitioning of stratospheric fluorine (or the absolute amount of HF) is determined by some key photochemical parameters for chemical reactions involving fluorine. As others have suggested, we feel that the quantum yield for CF.20 photodissociation needs to be reexamined. The accumulation of the stratospheric HF column amount is a result of continuing releases of man-made CFCs into the earth's atmosphere. We lna. de comparisons between HALOE 1992-1993 and ATMOS 11t85 measurements of the HF column above 20 km t,o estimate its annual growth rate. The percentage increase we obtained (exponential rate of 4.9-6.6(7( yr -I and linear rates of 6-8.6% yr -1 in 1985 and 4.3-5.5(,•, yr -1 in 1992-1993) agree reasonably with those estimated from long-term ground-based measurements of the HF column amount as well as those from the historical records of tropospheric CFC releases. We did not attempt to make comparisons of the HALOE HF columns with ground-based measurements, because the HF amounts from ground to HALOE lowest, levels are unknown and they probably contribute a large amount t,o the HF COlUlIlnS.
An analysis of the correlations between HALOEmeasured halogen species HF and HC1 and that of tracers HF and CH4 is being prepared for future st, tidies. Although prior to HALOE, stratospheric HF is a poorly known species due to lack of experimental data, it, has been emphasized that it plays an important role in revealing the effect, of anthropogenic activities on the atmospheric chemical compositions. Limited measurements of the columns of HF/HC, 1 prior to HALOE show nearly linear correlations between the two. The question is how this correlation applies to their vertical profiles and how does the relationship change with latitudes and seasons. In the spring polar region this relationship is especially important in determining the roles of dynamical and chemical processes in ozone depletion. This will be the subject of a ful, ure paper.