atmospheric ca and ca & layers: midlatitude observations ......the characteristic features of...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. A12, PAGES 27,131-27,146, DECEMBER 1, 2000

Atmospheric Ca and Ca + layers' Midlatitude observations and modeling

M. Gerding, • M. Alpers, and U. von Zahn Leibniz-Institute of Atmospheric Physics, Ktihlungsborn, Germany

R. J. Rollason and J. M. C. Plane

School of Environmental Sciences, University of East Anglia, Norwich, Great Britain

Abstract. We report on a comprehensive set of observations of the upper atmospheric Ca and Ca + layers. The observations were obtained by ground-based lidars at Ktihlungsborn, Germany (54øN, 12øE), between December 1996 and December 1998. During this period, 1 12 nights of Ca soundings and 58 nights of Ca + soundings were realized. The Ca layer has

7 2 an average column abundance of 2.1.10 cm-, centered around 90.3 km with a mean peak density of 22 cm -3 at 89.9 km altitude. The Ca + dominates the total Ca amount above 90 km and has an average column abundance of 4.9.10 ? cm -2. Because the vaporization of cosmic dust is the most probable source of atmospheric metals, the column densities of the metals within the atmosphere are often compared with the abundance in chondritic CI meteorites. We show that the atmospheric Ca is severely depleted with respect to other metals such as Na and Fe, compared with their relative abundances in CI chondrites. We present a one- dimensional steady state chemistry model of the nighttime Ca and Ca + layers, based on new laboratory studies of CaO reaction kinetics. This model is able to reproduce satisfactorily the characteristic features of the annual mean layers and to provide a possible explanation for the unusual seasonal variation of the Ca layer which exhibits a pronounced summertime enhancement around 87 km.

1. Introduction

Over many decades of observations it has become well established that a layer of free metal atoms exists in the upper atmosphere between 80 and 110 km. Slipher [1929] described for the first time the observation of Na fluorescence lines in

the twilight airglow. Ca could not be detected, however, by ground-based optical spectrometers. Gadsden [1969] gives an upper limit of the total Ca abundance that was about 20 times less than that assumed by comparison with Na abundance. This sparcity of Ca is surprising because vaporization of cosmic dust and micrometeoroids is assumed to be the major source of these atmospheric metals [Junge et al., 1962], and Ca and Na are about equally abundant in much of the infalling extraterrestrial matter.

Searches for the resonance line of Ca + with optical spectrometers by Vallance-Jones [1956] and Broadfoot [1967] produced positive results at least after meteor showers, but no permanent layer of Ca + ions could be identified by these authors. The first altitude profile of Ca + ions was obtained by Istomin [1963] by using a rocket-borne ion mass spectrometer. He observed these ions tightly layered near 105- km altitude.

1Now at Alfred Wegener Institute for Polar and Marine Research, Research Unit Potsdam, Potsdam, Germany.

Copyright 2000 by the American Geophysical Union

Paper number 2000JA900088. 0148-0227/00/2000JA900088 $09.00

First lidar observations of the Ca layer by Granier et al. [1985] at Haute-Provence (France, 44øN, 6øE) showed the existence of a permanent layer of Ca atoms around 90-95 km with a column density of about 1.5' 107 cm '2. Subsequent lidar observations at midlatitudes showed in addition a high variability of Ca profiles and the occurrence of sporadic layers [Granier et al., 1989; Qian and Gardner, 1995; Alpers et al., 1996]. All these publications reported on case studies of the Ca layer only. The number of observations was too small to derive an annual mean profile or statements on seasonal variations of the Ca density.

Observations of Ca + are of particular interest because Ca + is the only metallic ion in the upper atmosphere that can be observed by ground-based lidar. Either the wavelengths of the resonance transitions for other metal ions are in the UV

spectral range, so that the lidar radiation will be absorbed by the Earth's ozone layer, or the resonance cross sections are too low to use with the lidar technique. The first lidar soundings of Ca + ions were also performed at Haute-Provence by Granier et al. [1985]. They observed Ca + concentrated mostly in 2- to 4-km-thick highly variable layers. Up to now three groups have reported lidar soundings of Ca+: Granier et al. [1989] summarized 24 nights in summer 1983 and 1984 and October 1983. Gardner et al. [1993] obtained three single Ca + profiles in December 1991 during sporadic Na and sporadic Fe events. Alpers et al. [1996] performed simultaneous common-volume observations of neutral and

ionized Ca. All publications and the rocket-borne observations of several metallic ions (reviewed by Grebowsky et al. [1998]) show the appearance of Ca + mainly in sporadic layers.

27,131

27,132 GERDING ET AL.: OBSERVATIONS AND MODELING OF CA/CA + LAYERS

Here we report on a comprehensive set of observations of the upper atmospheric Ca and Ca + layers. The observations were obtained by ground-based lidars at Ktihlungsborn, Germany (54øN, 12øE), between December 1996 and December 1998. During this period 112, nights of Ca soundings and 58 nights of Ca + soundings were realized. This data set allows us to detail the monthly mean profiles and annual means of Ca and Ca + densities. These observational results will then be interpreted by means of a new one- dimensional steady state model, which is based on a recent laboratory investigation at the University of East Anglia into the reaction kinetics of CaO with a number of mesospheric species.

2. Instrument

For the observations reported here the Institute of Atmospheric Physics (lAP) Double Lidar was modified which was formerly used by Alpers et al. [1996] for their Ca and Ca + observations at Juliusruh (55øN, 13øE). The double transmitter of the lidar is now installed within the IAP

building at Ktihlungsborn. It consists of two dye lasers, each having an oscillator and two amplifier stages. The dye lasers are simultaneously pumped by splitting an excimer laser beam and providing 50% to each dye laser.

The backscattered photons are collected in a receiving system using five parabolic mirrors of 50-cm diameter each. In 1997, the telescope was upgraded to seven mirrors. A bundle of fiber cables lead the photons to the detecting branch, where all photons pass the same rotating chopper and are seperated by wavelength with a dichroic mirror. After noise reduction with interference filters the photons are counted with two independent photomultiplier tubes. Owing to the very low number of backscattered photons from the Ca layer, compared, for example, with the Na layer, the altitude resolution of the electronic data acquisition was set to 200 m. The low signal levels also prevented daytime measurements, because of the difficulty of discriminating against scattered solar radiation. Table 1 summarizes the technical data of the IAP Double Lidar.

3. Observations of the Ca Layer From December 1996 until December 1998 we obtained

Ca profiles during 112 nights. The observations lasted typically several hours per night. Surveying these data, one feature of the Ca layer becomes quite prominent: its high temporal variability. We therefore start by showing an example of this variability in Figure 1. Its three panels display the nightly development of the Ca layer during three out of four consecutive nights in July 1997.

The peak density increased from the night July 6/7 to July 8/9 from about 13 cm '3 to about 100 cm '3. In the next night, again a much lower density of 28 cm -3 was observed. This strong density variation is also visible in column density data which represent the total layer and not only the maximum. For this parameter we measure night mean values of 1.2.107 cm -2, 6.2.107 cm -2, and 1.7.107 cm '2 during the nights July 6/7, July 8/9, and July 9/10, respectively. Figure 1 indicates that the density variation is much stronger from one night to the next than during a single night; so the night-to-night variation cannot be interpreted as the continuation of the variation during a single night. To derive climatological means of the

parameters of the Ca layer, it is necessary to average the observations of several nights within a given period. Note that during the night July 6/7 there was sporadically an increased backscatter signal from a noctilucent cloud (NLC) below the metal layer.

From our 112 night mean profiles we derive the annual variation of the Ca layer above Ktihlungsborn as follows: The nightly mean profiles were smoothed by a Hanning filter [Press et al., 1992], which calculates a weighted mean of all observations within a 91-day-wide window. At the end of the year the window cycles round to the beginning, and vice versa. For nights without observation a profile was interpolated by the same algorithm. Figure 2 shows the result of this procedure with two sets of observations: (Figure 2a) the full set consisting of 112 nights and (Figure 2b) a reduced set consisting of all 66 nights without a dominant sporadic layer. (One of our goals is to interpret our observations by using a one-dimensional steady state chemistry model of the Ca and Ca + layers. This model is only suitable for describing the permanent metal layers. For the interpretation of the sporadic layers often visible in Ca and Ca + profiles, higher- dimensional and time-dependent models such as the model of Cox and Plane [1998] are required.) The reduced data set will be used to make meaningful comparisons between the observations and a steady state model. Owing to unfavorable weather conditions, there are in both data sets fewer observations from September to December than during the rest of the year.

Between January and March, Ca is found broadly distributed over a layer of comparatively small number density (up to about 12 cm-3). Into the summer the layer width decreases somewhat, but the maximum number density increases and reaches about 35 cm '3 at 87-km altitude, regarding the full data set. In the second half of the year the width of the Ca layer increases considerably; the highest density was found in October at 91-km altitude with about 40cm -3. From December into January, the Ca abundance decreases strongly, as is visible in both panels of Figure 2.

Table 1. Technical data of the IAP Double Lidars

Transmitter

Laser type Excimer Laser Dye Laser Wavelength, nm "' 308 Ca+: 393.3663

Ca: 423.6728

Bandwidth, nm 0.5

Pulse energy, mJ 600 Ca+: 17; Ca: 22 Pulse length, ns 10 10 Pulse rate, s -• 15 15

Receiver

Number of telescope mirrors Telescope mirrors Fiber cable

Dichroic beam splitter

Interference filter

Altitude resolution

5 or7

0 = 50 cm, f = 120 cm

0 = 0.65 mm, NA = 0.22 T > 85% at 393 nm R > 95% at 423 nm

FWHM 1 nm each, centered on

resonance wavelength of specific metal

200 m

a Wavelengths given in air.

GERDING ET AL.: OBSERVATIONS AND MODELING OF CA/CA + LAYERS 27,133

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Figure 1. Series of Ca profiles of July 6/7, 8/9, and 9/10, 1997. Each profile is the result of 2000 laser pulses (about 2 min 10 s) and is smoothed by a 1.8-km-wide Hanning filter in steps of 200 m. Additionally, the data are averaged by a running mean of three profiles.

Considering only the "regular" nights without sporadic layers, the most prominent differences are the two maxima in late August and in November/December instead of a broad autumnal maximum.

4. Observations of the Ca + Layer From our 58 night mean profiles we derive the annual

variation of the Ca + ion layer above Ktihlungsborn with the same procedure as that described for the Ca layer (Figure 3). We add that it is clearly desirable to enhance the database beyond the 58 nights that we have observations.

The seasonal variation of the Ca + ion layer is dominated by changes of smaller temporal and spatial scales than those of the Ca atom layer. Much of this variability is caused by strong sporadic Ca + layers, which have been observed during most of the nights and have not been neglected in the data set. But

even without sporadic layers being present, Ca + ions are observed between 90 and 100 km. They are detected in this altitude region throughout the year with a number density of at least 5 ions/cm 3 (Figure 3). Thus our observations provide a somewhat different picture than that given by former publications that report a total absence of a Ca + layer on many nights [Granier et al., 1989; Alpers et al., 1996]. Of course, a nondetection of Ca ions depends on the detection limit of the various lidar experiments. In addition, there exists a small, but finite, probability that lasers were not in fact tuned precisely to the resonance wavelength of Ca + (which is difficult to prove positively without detection of the ions). For our 59 nights of observations, the Ca + abundance was found below the detection limit during only 1 night. In conclusion, a permanent Ca + layer exists, though it is sometimes rather weak.


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Figure 2. Annual variation o f Ca atom density. (a) Observations of all 112 nights; (b) only 66 nights without sporadic layers. The bar code like pattern at the lower ledge of the plot indicates the nights of observations. For a description of the filter applied, see text.

The Ca + layer develops its strength only in late summer/autumn and then between 90- and 105-km altitude. It

is the same season that the Ca layer is strongest. In addition, Figure 3 shows two peculiar density maxima above 100-km altitude in May/June and July/August. The statistical significance of the August peak is modest, however (see the bar code of observing nights). The overall behavior of the Ca and the Ca + layers looks as if it is partly driven by the

occurrence of high-speed meteor showers. Meteoroids from these high-speed meteor shower• should more effectively ablate Ca than slow meteroids, because Ca is one of the most refractory metallic constituents [McNeil et al., 1998;' von Zahn et al., 1999]. Candidate showers are the Perseids in August (with 60 km/s entry velocity), the Orionids in October (with 66 km/s), and the Leonids in November (with 71 km/s). We plan to test this suggestion in a future publication.


5. Mean Variations of the Ca and Ca + Layers 5.1 Annual Variation of Layer Parameters

We now proceed to a more quantitative description of the "regular" Ca and Ca + layers. Here again only nights without sporadic layers are considered with respect to model studies. To derive the annual and semiannual components of the seasonal variation, some parameters of the Ca layer are derived first. The nightly mean values of column abundance, peak number density, ccntroid height, and full width at half maximum (FWHM) were sorted by month and day. A best fit to the harmonic function

f (x) = M + A•-cos ß d - a• + A•-cos ß d - a: 365

was then calculated (M is the mean value; A•, A• are amplitudes of annual and semiannual variation; a•, a2 are phase shifts of annual and semiannual variation; and d is day of year). The results are plotted in Figure 4, where circles indicate nightly mean values and the solid and dashed lines are the results of the harmonic fits.

Because the component due to the annual variation dominates the fit of the column density, there is also a best fit for A• = 0, although the observational data between February and April are then reproduced worse. The other layer parameters are dominated by semiannual variations. Out of all parameters, the FWHM was represented best by the harmonic fit. Here the standard deviation of the amplitudes is less than 20%. The night-to-night scatter of the layer parameters varies during the year (see Figure 4). From January to March the ccntroid height and FWHM values are very variable from night to night. From May through August, these parameters

are more constant, but here the column and peak density show strong variations. So in the first quarter the Ca layer changes in shape, while in the summer the layer changes in abundance. This will be examined in detail later. For the second half of the year the smaller number of observations prohibits a detailed analysis.

5.2 Annual Mean Profiles

Based on the nightly mean profiles, monthly mean profiles of the Ca and Ca* layers can be calculated. In the case of Ca an annual mean profile is calculated for both the reduced set of 66 nights without strong sporadic layers and the complete set of 112 night mean profiles. For the case of Ca +, again we do not distinguish between different types of profiles and show the complete data set.

In the annual mean the lower ledge of the Ca layer was found at about 82-km altitude (Figure 5, left panel). The number density reaches a value of 22 cm -3 at 89 or 90 km for the 66-night or 112-night sets, respectively. Above 105 km, Ca was only observed during some summer observations, which in the annual mean profiles produces densities of up to 2 cm '3 between 105 and 115 km. At the bottom side of the Ca layer the density increases with a gradient of up to 5 cm -3 km -• or 6 cm -3 km -• (Figure 5, right panel) for the complete and the reduced data set, respectively. The gradient at the bottom side is roughly the same for both samples. We further note that the gradient at the bottom side of the Ca layer most often was found to be much steeper (10 to 20 cm -3 km -•) over periods of several minutes. The absolute gradient in the upper pan of the layer is less (mean of-1.7 cm -3 km -1 below 100 km) and decreases with altitude. Because of the influence of sporadic layers the extremal point of the upper side gradient is found

1 2 3 4 5 6 7 8 9 10 11 12 11 ..... • .............. L l ................... • ......... • ........... • • • • • • •

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month

Figure 3. Contour plot of the annual variation of Ca + ion density, using as input the observations of all 58 nights. The bar code like pattern at the lower ledge of the plot indicates the nights of observations. For a description of the filter applied, see text.


8

(a)

month month 2 3 4 5 6 7 8 9 10 11 12 2 3 4 5 6 7

I I I I I I

Al= 0,9 cm -2 A2= 0,5 cm -2 M = 2,1 cm -2

0 60 120 180 240 300 360

day of year

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-

80 ' I ' • ' • ' • ' • ' 0 60 120 180 240 300 360

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150

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Figure 4. Annual variation of Ca layer parameters (points) with a least squares harmonic fit to the reduced data set (thick lines). The crosses and thin lines mark the monthly mean results of the chemistry model discussed later. (a) Column abundance, (b) peak number density, (c) centroid height, and (d) FWHM of the layer. The numbers for the parameters are the result of the fitting procedure. For the dashed line in Figure 4a the amplitude A2 was set to 0.

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number densi• [c• Figure 5. Annual mean profile and density gradient of the Ca layer (black line, all 112 nights' dashed line, 66 nights without sporadic layers) and the Ca + layer (gray line).


Table 2. Parameters of the Mean Ca Layer (With Highest and Lowest Observed Nightly Meaned Values)

This Work (Nights This Work (All Granier et al. Qian and Without Sporadic Nights) b [ 1989] Gardner [ 1995] Ca) •

Alpers et al. [ 1996]

Observation period Dec. 1996 - Dec. 1996 - Dec. 1998 Dec. 1998

Column density, cm '2 2.1' 107 2.4' 107 (0.6-6.2' 107) (0.6-9.2' 107)

Peak number density, cm -3 22 22 (5-135) (5-148)

Altitude of peak, km 89.1 89.9 (84.7-95.9) (84.7-114.5)

Layer width (FWHM), km 7.7 9.4 (2.4-15.0) (1.8-30.4)

April, July, Aug., Oct. 1992 - Oct., Dec. Jan. 1993

2.7.107 6.3.107

Aug. 1995

2.2.107

22 ca. 100 31

89.2 91.8 86.9

(median height) 11 3.5 4.2

(rms width)

Number of nights: 66. Number of nights: 112.

3-4 km higher for the complete data set. Above 98 km the number of sporadic layers is less; so both the gradient and the number density are similar for both samples. Within both panels the two profiles for the Ca layer differ the most above 90 km, where sporadic layers are most often observed.

We observe Ca* ions only above 85-km altitude. There were local maxima of Ca* number density at several altitudes (Figure 5, left panel), which are caused by sporadic layers that at times appear at the same altitude for several consecutive observations. Because of a typical descent in layer altitude during the night and the increasing recombination of ions with decreasing altitude, Ca* mean profiles often show a maximum of number density at about 92-km altitude. The Ca* ion density equals the Ca atom density at about 90-km altitude. Because of sporadic ion layers above 90 km the absolute density gradient of Ca* ions is higher and changes faster than the gradient of neutral Ca. In contrast, the lower ledge of Ca* ions between 85 and 90 km is a feature of the permanent Ca + layer.

5.3 Summarized Properties of Ca and Ca + Layers

Tables 2 and 3 summarize the properties of the Ca and Ca + layers as observed at 54øN, 12øE (Ktihlungsbom, Germany). The layer parameters are given for the annual mean layers. The extremal night mean values show the variability of both

layers. The earlier published results of Granier et al. [1989], Gardner et al. [1993], Qian and Gardner [1995], and Alpers et al. [ 1996] are listed for comparison.

6. Variation of Nightly Mean Profiles

It becomes evident from Figure 4 that from January to March the peak height and FWHM often change strongly from one night of observation to the next. The column abundance and peak number density show less scatter but vary strongly between May and August. These different kinds of variations are now examined more quantitatively. For this purpose the nightly mean profiles were averaged for a single month with a 1-km-altitude resolution, and the standard deviations of the nightly mean profiles were calculated for each altitude bin. In Figure 6 the deviation profiles and the monthly means are compared for January and August as representative for winter and summer. The monthly mean profiles are normalized to 1 at their maximum in order to suppress the effects caused by seasonal differences in number density. The deviation profiles are normalized with the same height-independent factor.

In January the highest standard deviation is found at the bottom side of the layer. For several bins around 83 km the deviation amounts to about 50% of the number density at the layer maximum. Although the standard deviation is also large

Table 3. Parameters of the Mean Ca + Layer (With Highest and Lowest Observed Nightly Meaned Values) This Work Granier etal. [1989] b Gardneretal. [1993] c A1persetal. [1996]

Observation period Column density, cm-2

Peak number density, cm-3

Altitude of peak, km

Layer width (FWHM), km

Jan. 1997 - Dec. 1998 June, July, Aug., Oct. Dec. 1991 July-Aug. 1995 4.9.107 2.10 7_ 6.10 TM 6.0.107; 4.4.107 14.8.107 (0.7 - 21.4'107) 29 <2400 440; 570 127

(5- 726)

91.9 90- 119 95.6; 99.2; 96.3 97.6

(90.3- 110.9)

5.4 e >1.2 2.2; 0.9 8.0 (1.6- 18.4)

Number of nights: 58. Extremal values out of 24 soundings. Three profiles (of about 20-min integration) within two nights of December. Strong variation within 6 nights 1983, less variation around mean value 6' 107 cm '2 within 18 nights 1984. In 10 nights the layer width could not be calculated because of several strong maxima.

27,138 GERDING ET AL.' OBSERVATIONS AND MODELING OF CA/CA + LAYERS

110

100

9O

80

0.0

I

0.5

atom density [a.u.]

I

I January

1.0 0.5 1.0 standard deviation [a.u.]

110

80

I

i

August

0.0 0.5 1.0 0.5 1.0

atom density [a.u.] standard deviation [a.u.]

Figure 6. Standard deviation (fight panels) of the night mean profiles from the normalized monthly mean (left panels) for January and August. For each month, both profiles are normalized by the same height-independent factor, calculated from the monthly mean peak density.

in August, the highest value of almost 60% is at the same height as the layer maximum. Again in contrast to the January situation, in August the density profile and the deviation profile are of similar shape. So in summer the density variation occurs uniformly across the whole metal layer. It seems therefore that seasonally different environmental conditions cause production or reduction of Ca atoms within the layer. In January the Ca layer seems to be biased by dynamical effects. The atoms are more or less only redistributed between 80 and 100 km. This leads to stronger changes of layer height and FWHM than of total abundance. Probably these differences are related to the seasonal variation of gravity wave activity. Wave motions within the upper mesosphere cause the vertical shift of the lower ledge of the layer [Gardner and Shelton, 1985; Eska and Hi,finer, 1998], and the metal chemistry can actually amplify the wave- induced perturbations [Plane et al., 1999b]. In general, the activity of gravity waves within the upper mesosphere is higher in winter than in summer [Garcia and Solomon, 1985]; hence this may cause the higher deviations of the bottom side of the January Ca layer. In August the low temperatures in the mesopause region [She and von Zahn, 1998] can cause the fast changes in chemical equilibrium between Ca and its chemical sinks (compare with model section).

7. Comparison With Previous Observations

Granier et al. [1989] published the results of 32 nights of lidar observations of the Ca layer at 44øN, 6øE. Because their observations concentrated strongly in the second half of the

year, no seasonal variation can be deduced from these soundings. Similar to our own data, the column abundance shows a high variability during the summer. Likewise, the density maximum was observed a few kilometers lower in altitude in summer than during other seasons. For their observation period, the total abundance and the peak density obtained by Granier et al. [1989] are comparable with the data published in our work, or at least within the typical deviation. The next Ca observations were also performed at midlatitudes (40øN, 88øW) as reported by Qian and Gardner [1995]. The abundance derived from their eight observations between October and January is only slightly higher than our results, if all nights are taken into account. Within their small data set, a strong decrease of Ca abundance between December and January is notable. More recently, Ca observations were performed at Juliusruh, Germany (55øN, 13øE), by Alpers et al. [1996]. Their published Ca layer parameters fit very well with the results obtained in the present study (see Table 1).

A complete annual cycle of the Ca layer has not been obtained so far. It is therefore of interest to compare the annual variations of Ca as reported here with those of the other meteoric metals Fe, K, or Na, as shown in Plate 1. The Fe profiles were obtained at Ktihlungsborn between March and December of 1997 and 1998. Although this data set is limited, its main attributes fit very well with the data published by Kane and Gardner [1993]. The K profiles were obtained at Ktihlungsborn between June 1996 and June 1998 [Eska et al., 1998]. The Na profiles were obtained at Fort Collins, Colorado (41øN, 105øW), by She and Lowe [1998].

As Plate 1 shows, the annual variations are different for each of the four metals, in part dramatically different. The differences between Na and K are particularly striking, since these alkali elements should have very similar chemical behavior [Eska et al., 1998]. One possible reason for these differences is that the Na data were collected from an

observatory at 41øN in the midwest of the United States, whereas the K, Fe, and Ca data are from Ktihlungsborn at 54øN. However, there does not appear to be any obvious reason for such a marked change over a relatively small difference in latitude. Indeed, the latitude-dependent examination of the K layer by Eska et al. [1998] did not exhibit any evidence for this, and there are only small differences between the Fe seasonal variation reported by Kane and Gardner [1993] and the Fe data presented here. Hence we would not expect a dramatic latitudinal difference for the Na layer.

For Ca and Fe, both show a dominantly annual variation, but with a significant phase difference. A common feature of all layers is that the layer width maximizes and minimizes in winter and summer, respectively. This annual variation in layer width occurs independently of a strong semi-annual variation of density for the K layer. All metals show strong semiannual variation of the altitude of the layer, with Na and K having the lowest altitude in winter [Kane and Gardner, 1993; Eska et al., 1998] and Fe and Ca in summer [Kane and Gardner, 1993; this work]. The summer-winter difference is strongest for Ca.

Grebowsky et al. [1998] have reviewed existing measurements of metal ions in the D and lower E region by means of rocket-borne mass spectrometers. His collection comprises 12 experiments in which Ca + profiles were

GERDING ET AL.: OBSERVATIONS AND MODELING OF CA/CA* LAYERS 27,139

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o 03 • \

Ca + e' I

!No + i o:• + ß

Ca

............... >ld

..... <ld

......... <lh

.... < 1 min

<Is

O2'M 03

• 03 , H Ill III

CaO 2" CaO n ............. CaOH O •H20,M

o .... • Ca(OH)2 CaC03 Ca03 H•O

H20

Plate 2. Set of chemical reactions used for the Ca model. The line thickness marks the velocity of the reaction calculated for 90-km altitude (red, dk/dT < 0; black, dk/dT > 0).


110

• 105 -• .... 5:- • _

• 100 E -- ....... , -- • - 'r• - = 95 -- --• --

• - • 90 -- ' --

- .( _ 85- • ß - ass-spectrometer

- lidar :

80 0 10 20 30 40 50

ion density [cm -3] Figure 7. Mean profile of Ca + obtained from lidar soundings (solid line, this work; see Figure 5) and 12 soundings by rocketborne ion mass spectrometers (dashed line, J. M. Grebowsky (private communication, 1998)).

obtained. These soundings took place over a wide range of latitudes and seasons. From these J. M. Grebowsky (private communication, 1998) obtained a mean Ca + profile which is shown in Figure 7 and compared with our annual mean profile.

The mean number densities of Ca + , obtained by the two totally different experimental methods, agree surprisingly well. One would not expect, though, the fine structure of the two profiles to match. Nevertheless, both Ca + profiles start only above 85 km. Below about 100 km the lidar measures a 50% lower number density. The most likely explanation is that all of the rocket experiments were carried out during the day, whereas the lidar measurements were made exclusively at night. The Ca+/Ca ratio should be higher in the day because of photoionization of Ca and charge transfer with NO + and 02 + ions (see below). Other factors could include the different geographic conditions of the data sets and the fact that the mass spectrometer experiments were often performed during special conditions such as the presence of noctilucent clouds [Kopp et al., 1985], sporadic E layers, or meteor showers [Kopp, 1997]. One would expect an enhanced Ca + abundance at least during some of these events. The lidar observations were performed, however, independently of any special mesospheric or ionospheric conditions.

Time-resolved lidar observations of Ca + have been made

previously at three different locations. Though the Ca + abundance published by Granier et al. [1985] is similar to the numbers presented here, the Ca/Ca + abundance ratio tums out to be larger at Observatoire de Haute Provence (OHP) than at IAP (1.5 and 0.4, respectively). Gardner et al. [1993] obtained three single Ca + profiles during two nights with neutral sporadic metal layers present. Their Ca + abundance is much higher than the annual mean presented here but within the range of our nightly means. Alpers et al. [1996] presented the results for seven nights of Ca + lidar sounding which all took place in July and August. Their observed mean density values and layer shapes fit very well with the parameters observed at Ktihlungsbom.

8. A Model of the Ca and Ca + Layers

The type of model that we will employ here has been used successfully to describe the Na layer [Plane et al., 1998, 1999a], the Fe layer [Helmer et al., 1998; Plane et al., 1999b], and most recently the K layer [Eska et al., 1999]. It is a one-dimensional model extending from 65 to 110 km in 0.5-km intervals and contains the following assumptions: meteoric ablation is the major source of a metal in the mesosphere, gas-phase chemistry essentially controls the formation of the metal atom layer, and the metal chemistry is closed. That is, all the constituent species cycle between each other. This is shown in Plate 2, which is a schematic drawing of the chemistry of all Ca species that will be discussed below.

The continuity equation for the ith Ca-containing species with concentration ni is given by

dn, + - z, +v(I,, =0 ,

where Pi and L• are the chemical production and loss terms, respectively, Ii is the injection rate from meteoric ablation, and (I)i is the vertical flux of i. Summing over all calcium- containing species i and applying the law of mass action to Pi and El, we obtain

dn(Ca) = • (I,-V(I) ) = I(Ca)- V(I)(Ca) (2) dt , ' '

where n(Ca) is the total calcium concentration, /(Ca) is the total injection rate, and (I)(Ca) is the total vertical flux of all calcium species. Under time-averaged conditions, both sides of equation (2) should be zero. A further assumption is that the rate of partitioning of a metal among its constituent species is rapid on the timescale of vertical mixing. Hence a chemical steady state operates at each altitude (this is demonstrated by Plane et al. [1998]). Since the transport of all the calcium constituents is governed by the same eddy diffusion coefficient up to the turbopause at about 105 km, the vertical flux of total calcium at height z is then given by

do(Ca) _KEIdn(Ca) I• 1 dTIl (3) = + n(Ca) ß +-- , dz T dz

where K• is the eddy diffusion coefficient, H is the atmospheric scale height, and T is the temperature. Note that cI>(Ca) is equal to the integrated ablation rate of calcium from z upward, under time-averaged conditions.

In order to solve equation (3) on a monthly basis for n(Ca) as a function of z, the following input was required. The downward flux of calcium at 65 km (i.e., assuming that all significant ablation occurs at higher altitudes) was set to 44 cm -2 s -• as the vertical integral of the Ca injection rate (Figure 9). This downward flux was multiplied by a factor representing the relative monthly variation in flux. The factor was calculated from the monthly variation of meteor showers with a geocentric velocity of 30 km s -1 and faster [Jenniskens, 1994; Rendtel et al., 1995], in order to take account of the high particle temperatures required for Ca to ablate (see, e.g., von Zahn et al. [1999]). A constant background flux from sporadic micrometeoroids was then added (Figure 8). The meteoric ablation profile was taken from McNeil et al. [1998] for a bimodal velocity distribution of incoming particles around 15 km s -• and 30 km s 'l (see Figure 9).


3.0

2.5

2.0

1.5

1.0

1 2 3 4 5 6 7 8 9 10 11 12 month

Figure 8. Seasonal variation of the monthly weighting factor for meteoroid influx.

The solution of equation (3) also requires estimates of the monthly average profiles of temperature, atmospheric density, and Ke. Furthermore, profiles of the minor species such as O, H, H20, and 03 that control the production and loss of atomic Ca are required to partition n(Ca) among its constituent species (Plate 2). In order to preserve self-consistency, all of these parameters were taken off-line from the two- dimensional model of Garcia and Solomon [1994] for nighttime conditions. Plane et al. [1999a] have shown recently that this model reproduces very satisfactorily both the midlatitude thermal structure and the H20 seasonal variation in the upper mesosphere/!ower thermosphere. Above about 105 km, molecular diffusion becomes more significant than eddy diffusion; their combined contribution to Ke is illustrated in Figure 9. In fact, the vertical eddy diffusion coefficient is a rather imprecise parameterization of vertical transport. However, the estimated Ca abundance is not very sensitive to Ke. For instance, increasing Ke from Garcia and Solomon [1994] by a factor of 2 changes the total Ca abundance only by about 10%. Finally, solution of equation (3) requires an initial estimate of the mean total calcium density at 65 km, fitted as a single adjustable parameter (= 1917 cm'3), and then scaled on a monthly basis by the relative meteoric input flux.

Solution of the continuity equation therefore yields the vertical profile of n(Ca). The concentrations of the constituent Ca species at each altitude were then computed by assuming a chemical steady state governed by the set of 23 reactions listed in Table 4. Of course, this is not likely to be a comprehensive set of all the reactions involving Ca species in the mesosphere. For example, we have not considered the heterogeneous loss of gas phase calcium species onto the nanometer-sized dust particles that most likely form from the condensation of meteoric debris [Hunten et al., 1980], since this process is only likely to be significant below 80 km. Nevertheless, 14 of these reactions have now been studied in isolation in the laboratory, mostly under conditions appropriate to the upper atmosphere, so that the chemistry of calcium is becoming much better understood. As is shown in Plate 2, above the neutral Ca layer, ion-molecule chemistry dominates, with neutral chemistry below. In both cases, the rates at which atomic Ca is ionized by charge transfer to Ca + , or oxidized via 03 or 02 to form a variety of Ca-containing reservøirs, are reasonably well known. In this context it is worth noting that recombination reactions (R5) to (R7) are so

fast that it was not possible to observe them under laboratory conditions close to their low-pressure limits, and so Rice- Ramsberger-Kassel-Marcus theory coupled with ab initio quantum calculations was required to extrapolate the experimental measurements to pressures about 3 orders of magnitude lower (J. M. C. Plane and R. J. Rollason, unpublished manuscript, 2000].

By contrast, the reactions which cycle calcium ions back to atomic Ca have not been studied, and they comprise most of the fitted parameters in Table 4. Since radiative recombination between Ca + and electrons is likely to be inefficient [Bates and Dalgarno, 1962], Ca + must form molecular ions through reactions (R17)-(R19). The rates of dissociative recombination with electrons ((R22) and (R23)) are then controlled by competition with atomic O, which reduces these molecular ions back to Ca + ((R20) and (R21)).

In the neutral cycle, Ca reacts at every collision with 03 to form CaO (reaction (R1)). Although this is most likely to react with atomic O to yield Ca again, a small .fraction will react with 03, 02, H20, or CO2 to form CaO2, CaO3, Ca(OH)2, or CaCO3, respectively. J. M. C. Plane and R. J. Rollason (unpublished manuscript, 2000) have shown using high level ab initio quantum calculations that Ca(OH)2 is easily the most stable of these species. It is therefore likely that CaO3 and CaCO3, which have large dipole moments, will react rapidly at the dipole-dipole collision frequency with H20 to form Ca(OH)2 ((R10) and (R12)). The conversion of this reservoir species back to Ca, via CaOH, will then be controlled by atomic H ((R13) and (R14)). In fact, these reactions have been studied in a high-temperature flame and are known to be fast [Jensen and Jones, 1978].

Application of the steady state to this set of reactions requires the concentration profiles of the controlling minor atmospheric species. As was stated earlier, for this study all neutral species were taken from the Garcia and Solomon [1994] model. NO + , O2 +, and e- are taken from the International Reference Ionosphere (1995) (see Bilitza et al. [1993] for reference and http://nssdc.gsfc.nasa.gov/space/for recent data) for conditions of local midnight, where the electron density between 95 and 110 km is about a factor of 10 less than that at midday.

diffusion coefficient K E [cm 2/s] 1E+5 1E+6 1E+4 1E+7

110 . , ....... • , , ,,,,,,I , , , .....

' _ 105 ' - : .'' _- - _-

•-" 95 -

m - vaporization - E 90 KE ;- k,,•;e -

- .

80 - ,,,,' j - _- -

75 - . ....... • . • ...... • ........ • ....... • ........ ' 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3

vaporization rate [ 1/(cm 3 s)]

Figure 9. Annual mean profiles of diffusion coefficient K•. and meteoric ablation flux (for references, see text).

GERDING ET AL.' OBSERVATIONS AND MODELING OF CA/CA + LAYERS 27,!43

Table 4. Neutral and Ionic Gas Phase Reactions of Calcium Species in the Upper Mesosphere Number Reaction Rate Coefficient a Source

(R1) Ca 4- 03 '--> CaO + 02 8.23 X 10 © exp(-192/T) (R2) CaO + O --> Ca + 02 3.2 X 10 © (T/200) ø'5 (R3) Ca + 02 + M --> CaO2 + M 9.7 x 10 -3ø exp(-451/T) (R4) CaO + 03 --> CaO2 + 02 8.3 x 10-1øexp(-434/T) (R5) CaO + 02 + M --> CaO3 + M 7.12 x 10-28(T/200) 'ø'ø8 (R6) CaO + H20 + M ---> Ca(OH)2 + M 3.54 x 10-25(T/200) -1'28 (R7) CaO + CO2 + M --> CaCO3 + M 3.13 x 10-27(T/200) -ø'85 (R8) CaO2 + O ---> CaO + 02 5 x 10 © exp(-500/T) (R9) CaO3 + O ---> CaO2 + 02 1 x 1042 exp(-500/T) (R10) CaO3 + H20 ---> Ca(OH)2 + 02 6 x 104ø(T/200) ø'17 (R11) CaCO3 + O --> CaO2 + CO2 1 x 10 -• exp(-400/T) (R12) CaCO3 + H20 ---> Ca(OH)2 + CO2 4 X !0-10(T/200) 0'17 (R13) Ca(OH)2 + H ---> CaOH + H20 1.2 x 10 © exp(-300/T) (R14a) CaOH + H ---> CaO + H2 4.0 x 10 -ll exp(-3680/T) (R14b) CaOH + H ---> Ca + H20 1.2 x 10 © exp(-300/T) (R15) Ca + O• ---> Ca + + 02 1.8 x 10 '9 (R16) Ca + NO + ---> Ca + + NO 4.0 x 10 -lø (R17) Ca + + 03 ---> CaO + + 02 4.9 x 10'lø(T/300) ø'5 (R18) Ca + + 02 + M ---> CaO• + M 2.6 x 10-29(T/300) -1'8 (R19) Ca + + N2 + M ---> CaN• + M 8.0 x 10 '3ø (T/300) '1'52 (R20) CaO2 +, CaN2 + + O ---> CaO + + 02, N2 1 x 1040 (R21) CaO + + O ---> Ca + + 02 3.3 x 10-11exp(94/T) (R22) CaO + + e' ---> Ca + O 7.2 x 104 (200/T) ø'5

C + (R23) aO2, CaN2 + + e' ---> Ca + 02, N2 9 x 10 -9 (200/T) ø'5

1

2

3

4

4

4

4

5

5

5

5

5

6

6

6

7

7

8

9

10

5

11

5

5

a Rate coefficient units: bimolecular, cm 3 molecule 4 s-l; termolecular, cm 6 molecule '2 s '•. b Sources: 1, Helmer et al. [1993]; 2, Plane and Nien [1990]; 3, Nien et al., [1993]; 4, J. M. C. Plane and

R. J, Rollason (unpublished manusript, 2000); 5, fitted model parameter; 6, Jensen and Jones [1978], k•3 and multiplied by 4; 7, Rutherford et al. [1972]; 8, Ferguson and Fehsenfeld [1968], increased by a correction factor of 3.0 following Rowe et al. [1981 ]; 9, Ferguson and Fehsenfeld [ 1968]; 10, set to the analogous Fe + reaction rate coefficient [Rollason and Plane, 1998]; 11, estimate from detailed balance with the reverse reaction [Armentrout et al., 1982].

The unknown rate coefficients in Table 4 and the total Ca

flux at 65 km were then optimized to produce the best overall fit to the annual mean Ca layer below 92 km and the annual mean Ca + layer below 95 km. These altitude limits were imposed to reduce the effects of sporadic layers at greater heights. The resulting fits are illustrated in Figure 10. This

110

105 x

100

.... model

95 '""• .... -- • '-'Ca +

90 '

85 Ca

80 ! 0 10 20 30 40

number density [cm -3] Figure 10. Comparison between modeled (thick line) and observed (thin line) annual mean profiles of Ca (solid lines) and Ca + (dashed lines).

shows that the model is able to reproduce very well the peak heights of both layers, their relative peak abundances, and their very small bottom scale heights. The optimized yalues for the rate coefficients describing the dissociative electron recombination reactions (R22) and (R23) are significantly smaller than those required in models for the analogous reactions of other metals [e.g., Helmer et al., 1998; Plane et al., 1999a]. However, this may simply reflect the influence of sporadic Ca + layers in increasing the Ca+/Ca ratio, even below 95 km. Figure 11 illustrates the profile of total Ca, n(Ca), as well as its partitioning into the major forms of the metal. It shows that in our model, calcium is transported downward into the middle mesosphere mostly in the form of Ca(OH)2, with smaller amounts of CaOH and CaO3.

A further constraint on the temperature dependences of the unknown rate coefficients is introduced by the observations of a summertime Ca maximum below 90 km (Figure 2). This seasonal behavior is in striking contrast to the Na and Fe layers: they both exhibit summertime minima which are explained by the formation of reservoir compounds such as NaHCO3, FeO3, and Fe(OH)2 which are converted back to their respective metal atoms by reactions with large activation energies and hence slow rate coefficients during the colder summer [Helmer et al., 1998' Plane et al., 1999a, b]. There appear to be two possible reasons for the summertime Ca maximum. The first is that differential ablation of meteoric

dust, which appears to be the most likely explanation for the depletion of atomic Ca relative to Na and Fe in the upper

27,144 GERDING ET AL.' OBSERVATIONS AND MODELING OF CA/CA + LAYERS

105 -

g - 95 ] ) • 90 ":'-"'-'-• .... ?,..:. - • 85 .....

80 ] Ca(

- CaOI'"-, X 75 ' ' ....... • ........ I • ...... 1 10 100 1000

number density [cm -3] Figure 11. Modeled annual mean profiles of the most abundant Ca species.

atmosphere [McNeil et al., 1998; von Zahn et al., 1999], should leave residual meteoroids greatly enriched in calcium. Photosputtering of Ca atoms from this material could contribute an additional source, and this would be greater during summer because of the longer days and higher solar flux. Indeed, we are able to reproduce the observed increase in Ca column density by including a seasonal photosputtering flux in the model. However, there are two significant difficulties with this explanation. First, the photosputtering flux during midsummer is required to be 9 times that during the rest of the year (if limited to 79 - 85 km) in order to match the observed column density, which is rather unlikely. Second, the Ca produced from photosputtering would be redistributed vertically by eddy diffusion, giving rise to an enhanced summertime layer up to 2 km higher than that

actually observed. Finally, there is no experimental evidence that the process would be efficient enough, particularly since high-energy photons in the VUV would presumably be required.

The more likely explanation for the summertime maximum is that it arises from the gas phase chemistry. Plate 2 illustrates the reactions that increase at lower temperatures. In particular, the recent study of J. M. C. Plane and R. J. Rollason (unpublished manuscript, 2000) shows that the recombination of CaO with H20 accelerates markedly with decreasing temperature, by comparison with the CaO + 02 recombination (cf. (R5) and (R6) in Table 4). Also, since the H20 concentration roughly doubles between 80 and 90 km in summer [Plane et al., 1999a], the overall result is that formation of Ca(OH)2 becomes much more likely in comparison with CaO3 at the summer mesopause. Ca(OH)2 then reacts rapidly with H to yield Ca via (R13) and (R14). The observed summertime Ca maximum around 87 km can

therefore be reproduced by ensuring that (R8) and (R9) have relatively small activation energies and that (R9) is slow with a small preexponential factor so that CaO3 is a significant (though not dominant) reservoir (Figure 11).

Figure 12 is a contour plot of the modeled Ca layer. Comparison with the observations in Figure 2b reveals that the summertime layer in May and July/August is well reproduced in both height and density. The early winter maximum is correctly predicted to be higher, but the density is underestimated by about 40%. Figure 4 shows that the model reproduces quite satisfactorily the monthly variation in the Ca column density, peak density, and centroid height, although it fails to replicate the seasonal variation in the layer FWHM (Figure 4d).

Finally, Figure 13 is an altitude profile of the correlation coefficient between Ca density and temperature. This plot reveals a much more complex relationship below 90 km, when it is compared with the analogous plot for Na (Figure

105

lOO-

1 2 3 4 5 6 7 8 9 10 11 12 I I ...................... 1 ! I .I I ! ...................... 1 ! 1 .....................

-50

1 2 3 4 5 6 7 8 9 10 11 12

month

Figure 12. Seasonal variation of modeled Ca density based on monthly mean profiles. For smoothing and interpolation, the same filter as in Figure 2 is applied.


110

105

100

9s 90

85- -

80 -

75

x,•...... _ observations model - -•

i

i

.0 -0.5 0.0 0.5

correlation coefficient 1.0

Figure 13. Correlation coefficient for the temperature- density-correlation (observed, dashed line; modelled, solid line). Below 80 km the Ca density is too low for correlation analysis.

10a of Plane et al. [ 1999a]), where the correlation coefficient between Na and temperature approaches 1 on the entire underside of the Na layer. In the case of Ca, the negative coefficients between 84 and 88 km reflect the summer

maximum, and above 92 km they are the result of ion- molecule chemistry leading to more rapid neutralizing of Ca + at lower temperatures. As is shown in Figure 13, the model is able to reproduce most of these features, although with some discrepancies in the heights of the turning points. The apparent increase in the observed correlation coefficient above 95 km, not predicted by the model, may be a manifestation of small sporadic layers biasing the database.

9. Conclusions and Summary

On the basis of 2 years of observations, the first complete annual cycle of Ca and Ca + profiles has been presented. The column density of Ca is influenced by both annual and semiannual variations with a maximum in summer and

autumn/winter and by additional short-time variations. The mean column density in 112 nights of observations was found to be 2.4' 107 cm'2; the layer maximum of 22 cm -3 appeared at 90-km altitude. Within Ca + data the short-time variations of

sporadic layers are even more obvious. Fifty-eight observations lead to a mean column abundance of

4.9 107cm -2 and a layer maximum in 92-km altitude (29 cm-3). The high variability of the layer profile was found as a basic feature of the Ca layer even when the sporadic layers are neglected. While in summer this variation affects the total layer, in winter more often a dynamically forced shift of the lower ledge was observed.

The model presented here is able to account for many observed features of calcium in the upper atmosphere, in particular, the annual mean nighttime Ca and Ca + layers and the summertime Ca maximum around the mesopause. Results on the meteoroid ablation process as published by McNeil et al. [1998], Gerding et al. [1999], and von Zahn et al. [1999] lead to the profile and seasonal variation used for meteoric metal input. We note, though, that in our model the absolute source strength for Ca was treated as a fitting parameter.

The variability of the Ca layer compared with the Na and Fe layers probably arises from the fact that only a small fraction (<1%) of the Ca in meteoroids appears to ablate. Hence small fluctuations in the ablation efficiency will cause large variations in the observed atomic Ca. Additionally, the Ca variability may be caused by the comparatively small part of atomic Ca compared with all Ca species and other metals. This may also be the first step in understanding the high number of sporadic Ca layers observed. Although our recent laboratory study of CaO reaction kinetics (J. M. C. Plane and R. J. Rollason, unpublished manuscript, 2000) has enabled significantly more of the calcium reaction scheme in the model to be put on a quantitative basis, the reactions which return reservoir species to Ca remain largely speculative. For this reason, a detailed sensitivity analysis of the model is inappropriate here. Also, in the absence of measured photolysis cross sections for these reservoir species, it is not worth speculating about the diurnal variations of the Ca and Ca + layers. Rather, the objective in this paper has been to construct a plausible reaction scheme that produces satisfactory agreement with the observations. Laboratory work on the reactions of the calcium reservoir species, an enlargement of the observational database, and further examination of the meteoroid ablation process as the key to the amount of total calcium will allow the model to be more

tightly constrained in the future.

Acknowledgments. M.G., M.A., and U.v.Z. acknowledge the technical assistance of J. H6ffner and T. K6pnick. M.G. appreciates the support by grant A1 458/1-2 of the Deutsche Forschungsgemeinschaft, Bonn, Germany. The model development and a research studentship for R.J.R. were supported by grant GST/02/1242 from the Natural Environment Research Council of the

United Kingdom under the Laboratory Studies in Atmospheric Chemistry initiative.

Michel Blanc thanks Ernest Kopp and another referee for their assistance in evaluating this paper.

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M. Alpers and U. von Zahn, Leibniz-Institut of Atmospheric Physics, Schloss-Strasse 6, D-18225 Ktihlungsborn, Germany. (alpers @iap-kborn.de; vonzahn @ iap-kborn.de)

M. Gerding, Alfred Wegener Institute for Polar and Marine Research, Research Unit Potsdam, Telegrafenberg A43, D-!4473 Potsdam, Germany. ([email protected])

J. M. C. Plane and R. J. Rollason, School of Environmental Sciences, University of East Anglia, Norwich, Great Britain NR4 7TJ. (J.Plane@uea. ac.uk; R.Rollason@uea. ac.uk)

(Received February 25, 2000; revised May 29, 2000; accepted June 4, 2000.)

atmospheric ca and ca & layers: midlatitude observations ......the characteristic features of...

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