Measurements of methane fluxes on the landscape scale from a wetland area in North Scotland

l352-2310(94)E0063-P

MEASUREMENTS OF METHANE FLUXES ON THE LANDSCAPE SCALE FROM A WETLAND AREA IN NORTH SCOTLAND M. W. G.II I .i(;tlkK.

T. W. CHOL

IL~KTON.

K. N. BOWFR.

I. M.

STROVHFRG

and

K. M. BESH.IC.~;

and

INTRODUCTION

Methane is an important trace gas in the atmosphere being both chemically and radiatively active. It is a source of water vapour in the stratosphere where it is oxidized by its reaction with hydroxy radicals. the main sink for methane. In addition. one of the final oxidation products is CO,. Both CO2 and HZ0 are themselves greenhouse gases. At present methane contributes approximately 15% to the total enhanced greenhouse effect and concentrations have more than doubled since pre-industrial times. Over the past 40 years measurements have shown that methane concentrations are still increasing. although the rate has decreased from about 20 ppbv ye,- I in 1970 to as low as 10 ppbv yr - ’ in 1989 (Steel et ~11.. 1997). Since methane is a much more elective radiative gas than CO,. it is important to understand its sources and sinks, and how they are linked with climatological feedback processes. At present many of these sources AE 28:15-D

are known only semi-quantitatively although the major contributions come from natural wetlands, rice paddies and ruminants. In the Northern Hemisphere. methane is generally produced biogenically from anoxic soils in natural wetland areas such as peat bogs and rice paddies--Cicerone and Oremland (1988bwhere the average methane concentration of 1760 ppbv compares with 1680 ppbv in the Southern Hemisphere (Aselman and Crutzen, 1989). Natural wetlands contribute significantly to the global methane emission with Northern Hemisphere peatlands being the most important contributor, up to 60% (Mathews and Fung, 1987). Many experiments have now been conducted over such peatland areas to quantify the magnitude of this source and they have used a variety of techniques, e.g. enclosure techniques, Grill eral. ( 1988), and eddy correlation, Verma er ul. (1992). One of the main sources of uncertainty with enclosure techniques in particular is the large heterogeneity in source

242 I

M. W. GALLAGHER

2422

strengths on the microscale which makes estimating area averaged fluxes difficult. In this paper we report on the results from a study to determine the landscape scale fluxes of methane by using an aircraft to collect air samples at different heights in the mixed boundary layer. This study formed part of the U.K. Terrestrial Initiative on Global Environment Research (TIGER) programme to measure fluxes of methane to Northern peatlands using a variety of micrometeorological techniques and to assessthe validity of scaling local fluxes up to landscapescales.Full details of the micrometeorological techniques and results will be presented in an associated paper. Here we concentrate on the results of the aircraft measurementsto obtain daytime methane effluxes. In addition nocturnal methane effluxes were estimated using the boundary layer accumulation technique or “box model” by monitoring the build up of methane under a strong, low-level nocturnal inversion whose height was inferred from doppler sodar measurements.

THESITE Measurementswere made over an area of peatland called Strathy Bog, in Sutherland, Northern Scotland, located at latitude 58.46” N and longitude 4.11” W. The site was approximately 50 km from the west coast of Scotland, 30 km from the east coast and 10 km from the north coast. The strict definition of the site is a fenland since it is fed by ground and rain water and the site would normally consist of flooded pools.

et al. METEOROLOGICAL

CONDITIONS

Measurements were made over the period 18 July-4 August 1992.During the first half of this period the site was uncharacteristically dry as this period was the end of a two-year drought with no rain recorded in the previous 2 months. Average temperatures for July were 1S”C higher than the annual norm with most of the increase being due to elevated nocturnal temperatures. Throughout the measurement period the temperature at 1.5 m above the ground did not fall below 7.O”C.Insolation levels were about 25% below average.Most of the available net radiation was in the form of sensible heat flux as shown in Fig. 1, which is the temporal variation of net, sensibleand latent heat fluxes measured by a Bowen Ratio System (Campbell Instruments) from 18 July to 4 August. The site was thus uncharacteristically dry; however, rainfall increased throughout the latter half of the period reaching a total of 170 m m restoring anoxic conditions to most of the site. Generally the water table was between 10 and 20 cm below the surface.This suggests that methane fluxes at this site were likely to be smaller than average.To the west of the site, however, the bog was visibly wetter with large areas of standing water. The soil temperature measured at a depth of 20 cm was between 14 and 16°C throughout the ex_periment. Strong westerly winds prevailed for most of the period with near neutral stability in the boundary layer by day. Air trajectories to the site throughout this period are shown in Fig. 2. Towards the end of the project convective activity increased producing showery conditions.

Strathy bog 18” Jyly - 4& August 1992

20

22

24

26

28

30

1

3

Time (GMT)

kg. 1. Measurements of net radiation,sensibleand latent heat fluxesduring the experiment from 18 July to 4 August 1992.

Methane fluxes from a wetland area

2423

Tiger methaneexperiment Loch More 1992 22-30/7/92 Back trajectory analysis 950 mb winds 3 hour displacementsfrom 12:00 GMT

-30/l/93

56

-

29n/93

-

2817/93

-21/l/93 -X---

26/7/93

-t---2517/93 52 \ 51

/

50

+ -20

-15

A”

i -10

A-r+@+-5

0

5

10

I--X-

24/7/93

-

23tll93

-

2217193

Longitude (deg) Fig. 2. 950 mb air trajectories to the site from 22 to 30 July 1992.

methane concentration was independent of height at the upstream coast as the air enters the region. The Air samples were collected using the UMIST in- aircraft measurementstended to support this assumpstrumented aircraft, a modified Cessna 182.This aircraft has full GPS and omega navigation equipment enabling position tion as even when it was not possible to make vertical and local wind speed to be determined. Its measurement profiles over the upwind coast the methane concencapabilities include a five-hole Jurbulence probe, cloud aero- trations observed at the top of the boundary layer sol fast response ozone and radiometric instrumentation. inland were very similar to those observed on the The aircraft has an air sampling system comprising an isokinetic inlet mounted on a wing tip to avoid contamina- ground at the upwind coast. As the airstream passed tion from engine exhaust. A stainless steel bellows pump is over the land, at approximately the geostrophic windused to pump air into an array of stainless steel 850 ml speed, it was assumed that there was a constant upultra-clean canisters. Each canister was evacuated several ward flux of methane from the ground. This is clearly times prior to taking a sample. Each air sample took approx- not true on the small scale. It is well known that imately 60-90 s to complete during a horizontal flight leg. Airspeeds were typically 40-50 ms-’ with each sample methane fluxes differ significantly between humcovering a horizontal distance of about 311 km. The canis- mocks, dips and lawns and will certainly be larger for ters were returned to the ground where they were analysed at some reasonably extensive,very wet areas.The objecthe experimental site using a FID technique which has a res- tive of this work, however, was to obtain a large olution of 2 ppbv. In addition to the canisters, Tedlar bags were also used to collect air samples.The concentration of spatial average of the total methane flux. As a parcel of air advects across the source region methane in the canisters and bags did not change significthe diffusion equation is solved. The general continuantly during the storage period. Samples were collected from upstream, downstream and ity equation for such a parcel is AIRCRAFT

MEASUREMENTS

above the main surface measurementsite where continuous measurementsof methane were made. In each case samples azc(z) act4 -=-K(z)aZ2 +&&=0). (1) were collected from a number of heights within the planetary at boundary layer and in the free troposphere. On some occasions additional samples were collected by ground-based In this equation C(z) is the methane concentration observers using a portable sampling system, again using at height z, K(z) is the eddy diffusivity for methane, stainless steel canisters and Tedlar bags. The air samples were used to determine the methane concentrations at and SCH.is the methane source term which was set a given height roughly along an air trajectory over the constant at the surface,z = 0. Horizontal homogeneity

measurement site and as a functionof heightupwind,above of the trace gas source and of chemical and meteoroand downwindof 1% main site. logical processesis assumed.This is obviously a simplification; however, the spatial resolution of the data presented here, and the lack of any detailed knownux ESTIMATES ledge regarding the location and strengths of sources, In order to deduce a surface methane flux from this does not justify inclusion of such detail. In the model the boundary layer is divided into data set a simple one-dimensional Lagrangian diffusion model was constructed. It was assumedthat the 3 separate layers. The lowest 100 m is the surface or

constant flux layer where fined by

the edd!

diH‘usl\,lty

15 de-

111 where k is van Karman‘s constant ( :0.-l). II+ ib the friction velocity and &,,(
for < >O (stable stratilicatlon) I= I +-1.7; = I for <= I (nrutral stratifcationl =[I - 15;1- ’ ’ for L < 0 (unstable stratilicatlon)

with <=: L where L IS the hdonin- Obukhoi sionless length scale defined b)

dlmen-

where I), is the mean temperature in the layer and H is the local sensible heat flux. L \vas measured at the experimental site by tower-based sonic anemometers mounted at 5 and 10 m above the surface. In addition, f. was measured on a continuous basis using a IO0 m path length laser scintillometer (Sclntec Model SLS20). These measurements enabled corrections for atmospheric stability to be made, but these were small in the case studies considered here. This was confirmed by analysis of the aircraft soundings which showed nearly constant potential temperature profiles within the boundary layer. In the second or well-mixed layer. the diffusivity was extrapolated from the top of the surface layer using a number of parameterizatlon schemes. e.g. Stull (1989). which included using a constant diffusivity. The cubic polynominal parameterization suggested by O’Brien (1970) was also used where the diffusivity gradually declines from a maximum within the boundary layer to zero at the top of the layer. In this case the diffusivity as a function of hetght is given by

+(z-lg).

K;+

The sensitlvlty of the model. for neutral stahlc stratifcation. to these parameterl~ati~~~i scheme4 was IIIinvestigated: ho\ve\,er. the resu1th wcrc relatl\eI\ sensitive to the scheme emplo>eJ M hen cc)mpared to the errors In the aircraft measurements. The dlfTusion equation was halved usIn@ ;I >impls tinlte dlfferrncc technique to give a methane concentration which Increased with distance tra\,elled across the hitc at an! given value of: and decay5 with altitude. The concentratmn increases are “largest” at small z (cIose to thr ground). The value of the methane flu\ from the surface was adjusted until the Increase In the methane concentration In the parcel matched the obser\,ed increase in the methane concrntratlon across the site both at the surface and at a higher altitude as measured by the aircraft. When aGlable the vertical profiles of methane predicted by the model also agreed quite well with those observed by the aircraft at different horizontal positions. In this way a mean source strength for methane at the ground was obtaIned. The main sources of error in this approach arise from the difficulty in specifying the upwind methane concentration. On many occasions this was determined from ground-based samples obtained from the west coast as our aircraft was not able to fly to this area. As stated above these values were generalI!, similar but not identical to values obtained from our aircraft at altitudes of more than 2 km above the observation site. The vertical protile of methane concentration in the vicinity of the measuring site. for a typical case study IS sholvn in Fig. 3. The error analysis in the results presented includes this unccrtainty in the upstream methane concentration. The results from the main case studies are shown in Table I. These were selected for periods during which the continuous ground-level methane concentrations at the main measuring site remained nearly constant. The periods selected had a near neutral boundary layer with a well marked boundary layer top as deduced from the aircraft temperature and dew point

-7(&c,-K,)~ A:

(4)

where zA and zB are the heights of the boundary layer and surface layer. respectively, and A:=:, -zR. the primes denote differentiation with respect to 2. The difTusivity K, was set to zero to conserve methane. It can be shown that K,,,=(4:27).(k,+:, h’;)) at Z= I/3 zA, that is the diffusivity reaches a maximum value at a height of about one third the depth of the boundary layer. In the final layer. within 100 m of the top of the boundary layer, which was determined from the aircraft temperature and dew point profiles, the diffusivity was assumed to decline linearly to zero at the boundary layer top.

g

2600 J

.M ii

800 600 400 200

2

10000 E 1980

2000

2020

2040

2060

2f 90

Methane (ppbv) Fig. 3. AIrcraft measurements of the vertical profile of methane in the vlclnlty of the cxperlment on 24 July 199’ compared wlrh model predictlons.

protiles. This ellminatrd periods during which strong coni’ectlon produced substantial mlslnp betwren the boundar> laker and the free troposphere.

THE CASE STUDIES

The six cast studies are summarized in Table I. Typical mean air trajectories at the 050 mb level for the period 22-30 July are shown in Fig. 2. We were able IO examine the changes In methane concentration as air parcels were advected over horlzontal distances of 15.-40 km. The increases in methane concentration o\er these distances uere between 50 and 100 ppbv and hence \vere easily detected by the gas chromatography technique employed at the measurement site. On two of the days it \V;LSappropriate to divide the data Into two case studies with different Huxes to the east and west of the site. It is immediately apparent that the methane fluxes to the east of the site were much IoLver (by a factor of 3). This is consistent with the generally much dryer conditions observed in the vicinity of the site and to the east during this particular campaign. Overall the modelled surface Huxes which best fitted the concentration measurements varied from 0.35 I.42~~gm~‘s~’ with a mean of 0.91 f t0 0.51 pgrn- * 5 - ’ An example of the model predictions compared to the aircraft measurements is shown In Fig. 3. In most cases there was some spatial variabilit) particularly In the upper half of the well defined boundary layer. The model was run several times with different source strengths to reproduce these variations. These are included in the uncertainties in the flux estimates listed in Table I.

NOCTURNAL

METHANE

EFFLUXES

Methane effluxes were estimated during nocturnal periods by measuring the build up of methane under the low-level nocturnal temperature inversion. This technique requires reasonably steady state. light wind conditions with relatively small changes in the inversion height to produce acceptable results. The temperature inversion acts like a lid providing a natural version of the microscale enclosure technique. The

methane efflu\ can hc cstlmated from a knowledge of the depth or height of the ho\ and the rate of increase of the methane concsntratlon ivlth time. During thl prolect ;I doppler sodar (Remtech model 40) \\;I> operated and protided continuous vertical protile of wind speed. wind direction and the 3-components of turbulence. The Instrument also pro\idcd a measure of the echo return as a function of height. elevated region5 of strong echo return generaIIy corresponding to elevated temperature inversions. Reliable informatlon was obtalned up to 4OtX500 m abo\,e the ground at night. Surface momentum and heat Huues \vere measured simultaneously bv. eddy correlation using sonic anemometers at 5 m and IO m. respectiveI!. These were supplemented with measurements made using the laser scintlllometer which was operated \vlth a path length of 100 m A comparlsnn between the eddy correlation and \clntillometcr measurements of II+ is shown in Fig. 4. Good agreement uaz obtalned between the two techniques even down to very low turbulence Intensities: however. the sclntillometer was able to provide much more detailed Information on shorter time scales. The depth of the nocturnal layer 1~;~s estimated from the sodar by considering the vertical variation In echo intensity and wind shear. The top of the nocturnal layer was generally marked by a layer of strong echo return. characteristic of an elevated inversion layer and a region of strong wind shear. Figure 5 sho\vs an example of the echo returns from the sodar during the evening of 73 July IYY?. The elevated region of high echo is interpreted as the top of the nocturnal boundary layer.

FLUX C.ALCULATIONS

We have assumed that the increase in methane concentration as a [unction of time can be described by the following simple zeroth-order relation F.Ar C(r,,l+-=C(r)

(5)

where F is a flux assumed to be a horlzontally homogeneous constant source term. C(r) is the methane concentration at some time I. C(r,) is the concentration at a specified start time. At is the time interval (r -rO), and z, is the depth of the nocturnal boundary layer (NBL). This simplistic approach assumes that the height of the NBL remains constant during the observation period r, to r and that the stable boundary layer is well mixed. If this is the case then the concentration increase with time is constant and given by K(r) c’r = E zi, where the flux F = C(r,). vd is constant with time and cd is the methane emission velocity which is also assumed to be a constant. We have used a simpler linear regression of In(C(r)iC(rO)) vs time to estimate rd. and hence the methane flux. since C(r)=C(r,).e’~‘~““‘.

M. W.

2426

GALLAGHER

CI cd

0.8

-

I 2ol7~920o:oo

Sonic

I

I 30/7/92

3 l/7/92

0o:oo

I 0O:OO

Time (GMT) Fig. 4. A comparison of measurements of I,* made by a sonic anemometer and laser scintillometer from 29 to 31 July 1992.

Echo intensity 1200 1050 I%3

900

I@#j 750 600 450 El

300

Time (GMT) Fig. 5. Echo returns from the sodar for the period 17.00 on 23 July~7.00

The height of the NBL was estimated from the doppler sodar measurements using a range of criteria. These included defining the NBL depth either at the height where the turbulent kinetic energy fell to small values, ~0.05 of that measured at the surface, or where a maximum in the wind profile or back-scattered signal was encountered or where the sodar return disappeared. The surface turbulence measurements, e.g. Fig. 4, suggest that the mixing times within the NBLs used in this study are short compared to the time scales of 2-6 h over which the methane concentration changes were seen to occur, strongly supporting the assumption that the NBL was well mixed. The simple zerothorder model described above does not, however, include entrainment at the top of the NBL. The ob-

on 24 July 1992.

served changes in temperature with time within the layer do, however, seem to be determined by the radiative loss and the mixing of heat within the depth of the nocturnal boundary layer as measured by the sodar and tower micrometeorological systems supporting these assumptions. In general, once surface cooling starts, a shallow weak stable layer forms near the surface which gradually deepens and strengthens. (The strength of the NBL is often defined as the near surface potential temperature difference, e.g. Surridge and Swanepoel, 1987.) The evolution of the NBL depends on the rate of change in the accumulated cooling which is determined by the net radiative heating which can vary significantly with time. For the case where the net heating is constant the energy budget can be solved

analytically IO predict the change in NBL depth wtth time. e.g. Brost and Wyngaard (197X). Such calculations show that the depth and strength of the NBL tncreascs with the square root of ttmr. We assume here the sample case of constant Qr and that once formed the NBL depth does not signiticantly change over the ttme pertod cnnstdered for the methane flux calculations. F-or csample during the case study I A. Table 2. the net radiation remained relatively constant. at - I5 W m ‘, and the temperature decreased by I.8 C which is the expected decrease if the NBL depth is assumed constant and well mixed such that CT ir : Qr (kj CP :,I. The tnittal growth of the NBL depth with time follows the assumed square root with time dependence quite well. A more detailed esamination of the turbulence structure and evolution of the NBL at this sate will be the subject of a future paper.

The effect of changes tn the NBL depth on the flux esttmates ustng the sample calculattons above have been included in the errors of the flux estimates. Etght case studtes were selected for etght dttTerent nights. In order to inv,estigate the effects of changes tn the depth of the nocturnal layer and any possible effects due to advectton or changes in the methane source strength these periods have been analysed by dividing them Into several sub-permds. An example of a typical case study IS shown In Fig. 6 which shows the increase in methane concentratton with time for the night of the 3 ~‘4 July. This coincides with the sodar data shown in Fig. 5 which shows the formatton of a strongly stable shallow layer near the ground during the early evening as radtattve cooling sets tn. The nocturnal inversion then deepens and intensifies during this mitial period when the methane concentration meas-

Table 2. Methane tluxes deduced from the so&r dat;l and changes rn methane conccntratron at ground level during pertods wrth LI well developed nocturnal houndarl layer End ttmr ‘0 : 40 I9.00 73:30 I Y:W 2’:OO 0:30 22:W I8:W ‘2:20 18:?0 18:OCI I Y:W lY:oo 20:40 2!:30 20:40 18:Oo IS:00 23:OO

IA 2A 28 2c 3A 38 ?C 3A 4B 4c 5A 6A 68 -IA 78 7c 7D

8A XB

Start date

3 00 2210 Ol:?tl 01:30 23:30 0:w 0:W ‘7-W --.. 05:oo 05.00 x:30 0:30 03:JO 2390

7 97 7 Y2 7 92 7292 7 92 7 92 7 92 1 92 7 92 7 92 792 8 92

11.27 0.65

I 8 92

0.31 0 79 0 04 0 12

‘3 27 ‘7 27 29 20 2Y 30 30 30 31 I 2 2 2 2 7 3

06:OO Oh:00 lY:!O “.30 --._ Oh:oo

8 8 x 8 8 8

92 92 9’ 97 92 92

0.07 0.08

I.1

0 I2

0 37 0.78 0.17 0 I7 O.61 0 2-l 0 36 0.47 0.49

0.04 0.2 0.03 0.04 0.7 0.03

0. I 0. I 0. I4 01 0 I6 0.0 I 0.03 0.12

0.12 0.16 0 17

0.22 0.04

Methane concentration 2.1.?4/71Y2 31 L = 2.4 m

‘070

2000 1 I x:00

?I:00

3.00

0:oo

fJ:oo

Trme (GMT) Fig. 6. Time history of methane concentration

for the overnight run of 23-24 July 1992

7.00

ured at the surface increases rapidly. From about midnight to about 5 a.m. the nocturnal Inversion retains very similar properties and the methane concentration increases more slowly within the deeper nocturnal boundary layer. This is the main period for calculating the methane Huues. After about 5 am. convection begins to increase as the sun rtses and the nocturnal boundary layer begins to deepen and break-up with a sharp fall in the methane concentra-

tton. t’lgure 7 shnvvs the graphs used to determine the methane efflus for SIY typical case studies. A stratght line was titted using least squares analysis and was used to determtne the methane tlux. In most cases agreement v.x v’ery good. with the correlation coethcient r2 exceeding 0.Y wth small errors In the slopes. indicating that. for the cases selected here. thts simple model is adequate and justifies the assoctated assumptions made. In most cases the dominant source of

0.0s 0.04

s 9 a' 3 u'

0.03

=

0.06

0.02

g

0.04

0.01

G z

002

z 0

-0.01 ’

IX

‘t

19

’

20

I

I

I

I

21

22

23

24

Time (GMT)

Trme (GMT)

0.0s Case 7B 0.04 z

0.03

8

0.02

u’ 2

0.01

3

o0.025 1 ’ 23

’ 24

’ I

’ 2

Time (GMT)

0.15

I

I

I

’ 3

’ 4

’ 5

’ 6

’ 7

a

Time (GMT)

I

I /: ‘,

2 g

0.02 2

0.10

u’ z

5 G ‘i

0.0, -

0.05 0-

I8

19

20

21

Time (GMT)

22

23

-o.ol L----J 22

23

24

1

2

3

4

5

6

Time (GMT)

Fig. 7. Figures showing In(C(t),‘C(r,)) against time I for several nocturnal case studies (see Table 2). C(r) is the methane concentration as a function of time and Ctr,) the methane concentratton at the start of the run. Lines are best regression and 95% confidence limits.

Methane

error was in estimatmg the depth of layer from the sodar data. Case study poorest agreement due to a changes in with advection of plumes through the

RESULTS FROM THE NOCTURNAL

tluxes from a wetland

the nocturnal 7B showed the wind direction measuring site

CASE STUDIES

The results of the box model analysis are shown in Table 2 together with estimated errors. In most case studies the initial sub-period generally produced significantly higher flux estimates than subsequent subperiods. This period was generally just after or during the formation of the strongly stable surface layer when this layer was very shallow and the uncertainty in its depth was very large. If these periods are included the mean nocturnal efflux at this site is 0.45+0.28 lgrnm2 s-i (ranging from 0.0441. I pg m - 2 s - ’ ) whereas if these periods are rejected the mean efflux is 0.37+0.27 ~grn-~s-‘. The error quoted in these figures is in part due to the day to day variation in the methane flux at the site deduced by this method. Enhancements tn methane concentration of between 50 and 100 ppbv in about 4 h were typical.

COMPARISON

OF THE FLUXES MEASURED REPORTED

WITH OTHER

VALUES

Clymo urul. (1994) summarized methane flux measurements conducted in the U.K. to date. They used enclosure techniques to measure fluxes at Ellergower Moss. situated in SW Scotland. throughout the year. The values obtained were in general agreement with the results presented here. During this project flux measurements were made using the flux gradient technique. which have been reported by Hargreaves et ul. ( 1993). This technique is described by, for example, Fowler and Duyzer (1990). The response and sensitivity of the instrumentation permitted a minimum detectable flux of 3050 ngmm2s-’ under favourable conditions. These measurements were thus limited to periods of very low wind speeds and the associated errors were relatively large. Measurements from this technique were made during some of the nocturnal periods when the boundary layer box technique was employed. The fluxes obtained varied from 0.05 to 0.24 pg m -2 s- ’ in broad agreement with the results presented in this paper.

CONCLUSIONS

Measurements have been made of methane fluxes on the landscape scale. Two very different techniques have been used. During the day an aircraft measured the changes in methane concentration in the horizontal and the vertical as air was advected across the site

area

from west to east. These measurements were compared with continuous measurements of methane concentration made at a ground based site. The data were further supplemented by spot measurements on the ground including measurements at an upwind coastal site. At night measurements of changes tn the methane concentration at the main measuring site were used, as the nocturnal boundary layer formed. to determine a methane flux. A doppler sodar was used to measure the structure of the nocturnal boundary layer. During the period of the experiment the mean flux for the region by day. estimated from 6 case studies using the aircraft was 0.91 +0.51 pgm-2s-‘. It was found. however. that the fluxes to the west of the site were typically a factor of 7-3 times larger than fluxes to the east of the site. This was probably due to the much wetter state of the bog to the west of the main measuring site. The aircraft averaged fluxes were between 2 and 4 times larger than fluxes measured at the site using cuvette techniques at the surface. This may be partly explained by the dryness of the bog in the vicinity of the site and partly to the large heterogeneity of methane fluxes within the area. The nocturnal fluxes of methane estimated from the boundary layer box technique were typically a factor of 7 lower than the daytime fluxes estimated from the aircraft at 0.45kO.28 pgme2sm’. These were very similar to values measured by the flux gradient technique at the same time. It would seem therefore that these measurements were characteristic of the nocturnal flux in the vicinity of the site. The overall conclusion. therefore, is that from this site there is very little evidence of a large diurnal variation in the methane flux. The larger fluxes measured by the aircraft during the day were largely due to larger methane fluxes from the wetter western part of the bog. The values of methane flux obtained are comparable to those reported from other measurements in similar wetland areas.

.4~Lno,~ledyemenrsThis work was by the Natural Environment Research Council TIGER initrative We would also like to thank Dr R Maryon of the U.K. Meteorological Office for providing the back-traJectory analysis.

REFERENCES Aselmann I. and Crutzen P. J. t 19R9) The global distribution of natural freshwater wetlands and rice paddies. their net primary productivity, seasonality and possible methane emission. J. arm0s C/rem. 8, 307-358. Brost R. A. and Wyngaard J. C (1978) A model study of the stably stratified planetary boundary layer. J. urms SCI. 35. 1427-1440. Crcerone R. J. and Oremland R. S. (1988) Biogeochemrcal aspects of atmospheric methane. Glohcrl hi~qwchon. Cycles 2, ‘99-327. Clymo R. S., Hargreaves K. J.. Fowler D. and Gallagher M. W. (1994) Methods for determining the etllux of meth-

Group (in press). Grill P. M.. Bartlc!t K. B.. tlarris.

R. C.. Gnrham

E.. Vcrry

E. Strrlc L. P.. Dlugokcncky F. J.. l.ang P. hl.. Trans P. P.. Martin R. C. and hl;lwrw K. A. (19,921 Slowing down oI Ihc global accumuhlrion or ;Itmosphcric mcthww during the 19XOs. ,Ycrrtrrc* 359. ISI 254. Slull R. B. I 19X91 .+I Inrroclwrilwt I<) Bowtc/crr,v /.crwr .\fc,fwro/o!q,v. Kluwr Acatcmic Publishers. DwJrcch~. Surridgc A. D. and Swncp~wl D. J. (19X71 0 rhc wcolulion orthc hcighc and rcmp~rnlur~di~~rcn~~ across the nocturnal slahlr boundury klycr. Rlllr,l,/crr~-I.lr!c,r .I/cr. 40.

X7-98. Gr&x wcr S;mJs. M:lthews E. and Fune I. 119x71 Meihanc emissions horn nalural wckmds: global distribution. arca and cnvironhio~~eoc~iwnt. mcnt of ch;lrxrcrislics of sources. Glohd c-Jdc*s I. 6146.

Vcrmcl S. B.. Llllman F. G.. Billcstwh D. C’lrnwt J. Joon K. and Verry E. S. 119921 Eddy correlation mcasurcmcnls of mclh;lnc flux in a northern pc;AnJ C~~~S~SICIII.B,wArry-

Ltrwr .\fc*r. 58. 2x9..30-l.