Carbon dioxide measurements above a wheat crop, 1. Observations of vertical gradients and concentrations

Agricultural Meteorology, 12 ( 1973) 1 3 - 25 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

CARBON

DIOXIDE

OBSERVATIONS

MEASUREMENTS

OF VERTICAL

ABOVE

GRADIENTS

A WHEAT

CROP,

1.

AND

CONCENTRATIONS G. 1. PEARMAN and J. R. GARRATT

CS.I.R.O., Division of Atmospheric Physics, Aspendale, Vic. (Australia) (Accepted for publication March 26, 1973)

ABSTRACT Pearman, G. 1. and Garratt, J. R., 1973. Carbon dioxide measurements above a wheat crop, 1. Observations of vertical gradients and concentrations. Agric. Meteorol., 12: 13-25.

During 1971 measurements were made of meteorological and biological variables above and within a wheat crop grown at Rutherglen in northeast Victoria. In the present paper a brief description is given of the apparatus used to measure carbon dioxide concentration gradients and absolute concentrations above the crop, together with a summary of the observations. The vertical CO 2 gradient was found to show little variation across the crop, confirming the assumption of horizontal homogeneity generally made in micrometeorological studies. Daytime differences in the CO 2 concentration between 2 and 1 m above the crop were generally steady at about 2 p.p.m., with nocturnal differences being more variable, of reversed sign and about 1(t times greater in magnitude. The magnitudes of both daytime and nighttime differences increased through the season until the time of maximum crop growth, after which they decreased. At night vertical CO 2 differences were observed to oscillate in magnitude when wind velocities were less than approximately 1 m/sec, becoming generally larger when Richardson number exceeded a value of between 0.1 and 1. The CO 2 concentration at 2 m above the crop was found to be fairly constant during the daylight hours on single days or from day-to-day throughout the growing season ranging from about 310 to 320 p.p.m. Nocturnal values were more variable and were between 10 and 200 p.p.m, higher than the daytime values. INTRODUCTION In early 1971 t h e C.S.I.R.O. Division o f A t m o s p h e r i c Physics c o m m e n c e d a field e x p e r i m e n t near R u t h e r g l e n in n o r t h e a s t Victoria. T h e e x p e r i m e n t was d e s i g n e d t o collect m i c r o m e t e o r o l o g i c a l a n d biological data f r o m in and above a w h e a t crop and, in part, r e p r e s e n t e d a c o n t i n u a t i o n o f studies u n d e r t a k e n b y the Division in r e c e n t years. T h e m a i n i m p e t u s for the R u t h e r g l e n e x p e r i m e n t came f r o m t h e r e q u i r e m e n t to p r o v i d e basic i n p u t data for a c r o p g r o w t h s i m u l a t i o n m o d e l (Paltridge, 1 9 7 0 ) and to test the various predict i o n s o f the m o d e l . The project, w h i c h c o n t i n u e d t h r o u g h the 1972 season, i n v o l v e d the m o n i t o r i n g o f m o r e t h a n o n e h u n d r e d p a r a m e t e r s e i t h e r c o n t i n u o u s l y or o n an h o u r l y ( i n t e g r a t e d ) basis, t h r o u g h o u t the entire growing season. Details o f t h e e x p e r i m e n t a n d t h e u n i n t e r p r e t e d d a t a are p u b l i s h e d elsewhere

14

G, I. PEARMAN AND J. R. GARRATT

(Paltridge et al., 1972). In this paper we will describe the equipment and measurement techniques used to obtain carbon dioxide concentrations and COz vertical gradients of concentration over the crop, and will summarize the large amount of CO2 data collected. Throughout the paper, concentration will be taken to mean the volumetric ratio expressed as parts of CO2 per million parts of air. A second paper will deal with the vertical CO2 fluxes and the effects of a number of environmental parameters upon the growth of the crop. MEASUREMENT TECHNIQUE The carbon dioxide vertical gradient measurement system was designed to sample air from six intakes in the field and was based on an analysis technique using an infrared gas analyser* suitable for differential CO2 measurements (see Fig. 1). For the earlier part of the season (May through August), two horizontal intakes (perforated 80 cm long brass

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Fig.1. A schematic of the infrared gas analysis system for measuring carbon dioxide profiles: I ---reference level intake on field mast; 2 ---sample level intake (note that for simplicity other sample levels are not illustrated prior to the refrigerators); 3 = dry air cylinder; 4 = exhaust pumps; 5 = mixing vessels; 6 = condensation refrigerators; 7 = carbon-dioxide cylinder; 8 = nitrogen cylinder; 9 = Wosthoff mixing pump (e.g. 3% reduction gears); 10 = Wosthoff mixing pump (e.g. 1% reduction gears); 11 = diaphragm dust filters; 12 = diaphragm pumps; 13 = reference dry air cylinder. tubes) were arranged at 1 and 2 m above the crop on each of three vertical masts spatially separated in the field by some 50 m. The masts were placed in a triangular array to give information of the downwind and crosswind variations in vertical CO2 differences * URAS I1, Hartmann and Braun, Hamburg, Germany.

CARBON DIOXIDEMEASUREMENTSABOVEA WHEATCROP

15

over a period of time. For the period September through November the intakes from one of the masts were removed and one placed at crop height on each of the other two masts. Air was drawn from these intakes through 12 mm internal diameter PVC tubing by a main pump to the instrument and data acquisition caravan some 100 m away. The flow rate in each line was about 180 1/h which was found to be sufficient to avoid measurable contamination of the air system by CO2 diffusing through the walls of the tubing. All six lines were enclosed in opaque P.V.C. irrigation pipe to avoid mechanical damage and deterioration of the tubing as a result of exposure to solar radiation. At the caravan a fraction ( ~ ¼) of this main stream of air was drawn from each line for sampling, air from one preselected intake (2 m level on the main mast) being passed continuously through the reference cell of the analyser. A fraction of this "reference" air was also passed through a separate line to a rotary gas switch which allowed air from all six intakes to pass sequentially to the sample cell of the analyser. In this way, "reference" air was first compared with itself (differential zero reading) and then with air from each of the other five intakes. Each comparison took 1 min, yielding 10 cycles of comparisons each hour. Prior to flowing through the gas switch, air in each bleed line passed through 10-1 mixing vessels (to remove short-period fluctuations in carbon dioxide concentration) and then through refrigerated condenser coils (2°C) for partial removal of water vapour in the air. Because of the sensitivity of the URAS II to water vapour the air was further dried by passing it through granulated magnesium perchlorate drying towers. Bleeding of air from the main field lines to the reference and sample lines connected to the analyser was achieved using two diaphragm pumps situated close to the analyser inlets, whilst the flow of air in the five bleed lines not selected by the switch for analysis was maintained by a common exhaust vacuum pump. Air in the reference and sample lines flowed through dust filters before entering the gas analyser at a flow rate of 45 1/h. The response time of the system as a whole to a step change in CO2 concentration at the field intakes was about 5 rain. The accuracy of gradient measurements was generally close to +-0.2 p.p.m. A "null" test was performed weekly to ensure authenticity of analyser output signal, i.e., to detect leaks or other system faults. This was done by passing dry air at atmospheric pressure (obtained from a compressed air cylinder 3 in Fig.l) down all six field lines via alternative intakes located near the masts with all pumps functioning as in normal operation, the requirement being to produce a steady "zero level" analyser output with equal response to the input change in all six lines. During normal operation, only vertical differences in the CO2 concentration were obtained. However, each week the absolute CO2 concentration of the reference intake air was measured on several occasions. Gas mixing pumps* were used to obtain CO2/N2 mixtures of prescribed CO2 concentration which could then be compared to the "reference" level air from the field. The accuracy of the absolute measurement was -+4 p.p.m. The equipment operated automatically for 5 or 6 days per week throughout the grow*Gasmischpumpe, H. Wosthoff, Bochum, Germany.

16

G. I. PEARMAN AND J. R. GARRATT

ing season, except on the occasions when performing the null tests, absolute CO2 determination, calibrations of gas anatyser and changing of chemical in the drying towers. The analyser output for the whole period was transferred from recorder chart to punch cards, and in addition hourly mean vertical gradients were stored on magnetic tape. The gradient data were analysed either as running means of the 6-rain readings or as hourly means. CARBON DIOXIDE VERTICAL GRADIENTS

Diurnal cycle o f the 1 - 2 m gradient Fig.2 illustrates the diurnal cycle of ~C2_ l (the 2 m - 1 m CO2 difference) for a typical day during the period when data were available from three masts, the vertical bars representing the 95% confidence limits on the mean value for mast 1. The diagram demonstrates

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Fig.2. The diurnal cycle of the carbon dioxide difference AC2 ,, above the Rutherglen wheat crop. Differences were measured at three separate locations in the field (ca. 50 m apart) on mast 1, mast 2, and mast 3. Each point is the mean of ten measurements made in the preceding hour. The vertical bars represent the 95% confidence limits on the mean value for mast 1.

the similarity in the hourly values obtained from the three masts, indicating the horizontal uniformity in the vertical CO2 transfer over the crop. For the daytime period mast 3 was predominantly crosswind to both masts 1 and 2, the latter being downwind on mast 1. At night winds were light and wind directions highly variable.

CARBON DIOXIDE MEASUREMENTS ABOVE A WHEATCROP

17

An extensive statistical analysis (on an hourly basis) of the gradient data obtained from three masts for a 2-month period may be summarized as follows: (1) Though the fetch over a range of wind directions (generally south through west to north) was not always sufficient to ensure complete adjustment of the vertical profde (up to 4 m) to the underlying surface (on the basis of an 100/1 ratio of fetch to height) they were evidently adequate for the purposes of this experiment. (2) For any 1-h period there was generally no significant downwind or crosswind difference between masts in the variability of the individual readings of ~C2-~ (as indicated by an " F " test of variance on the hourly means of the 10 values). (3) Hourly mean gradients on any two of the masts (whether crosswind or downwind), and often on all three, were generally within 20% of each other. Greater differences occurred for any one mast mainly due to greatly reduced response in the air line from that mast (see (5) below). (4) For a given hour of the day, there were systematic differences in the mean monthly value of ~C2-1 among the three masts, though these differences were much smaller than the actual magnitude of ,5Cz-1. The main cause arose from the difficulty of defining an equivalent zero plane for all three masts due to the unevenness of the crop surface. (5) On several days during the months of July and August, significant differences were measured between the CO2 concentration in air drawn from the 2-m level on masts 1 and 2. Differences were greatest at times of rapid change in the ambient CO2 concentration, i.e., at dawn and dusk. The effect is evident in Fig.2 as a slow response to changes in vertical gradient after sunrise at mast 2. Subsequent investigation revealed this effect was related to a considerably lower flow rate in the line from mast 2, the result of freezing of water in the condenser coils giving a high resistance to air flow. The variation of ~xC2_1 through the season is illustrated in Fig.3 where the mean monthly value of 2xC2_1 for each hour of the day is plotted as a function of time of day for the months July, September and November. Values were generally about 1 p.p.m. during the daytime (sun above the horizon) early and late in the season, increasing to 2 p.p.m, or more during the peak of growing activity (September). At night (sun below the horizon) £xCz-~ was always negative, and in magnitude some 10 times greater than daytime values. Fig.3 also illustrates the increasing length of time each day that AC2-1 was positive as the season progressed towards summer, the time between sunrise and sunset being close to 8 h in July (mid-winter) and 11 h in November (late spring). An interesting feature of the data which is barely depicted in the mean monthly hourly values shown in Fig.3 was the occurrence on several days, of a marked increase in AC2-1 prior to sunset, after which it changed sign, The reason for this lies in the onset of stable stratification in the air above the crop shortly before sunset, giving a greatly reduced eddy transfer coefficient from the earlier daytime value which accompanied unstable stratification. For a given vertical flux of CO2 into the photosynthetically-active crop, an increase in vertical gradient could be expected to accompany the reduced eddy diffusivity. One other feature of the diurnal cycle deserves mention. The day-to-day variability of

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Fig.3. Monthly mean diurnal cycle of the carbon dioxide difference ,~C=_~,above the Rutherglen wheat crop, for three selected months, Note the difference in ordinate scale for positive and negative CO~ differences. AC2_] about the mean monthly values for the daytime hours was always small; for the nighttime, the variability was much increased, with hourly mean values of IAC2_~I often exceeding 20 p.p.m. The behaviour of the nocturnal vertical CO2 gradients will be described later in greater detail. Vertical carbon dioxide profiles Qualitative information on the nature of the vertical CO~ profiles above the crop was available from September onwards, when mast 3 was abandoned and its two inputs placed at crop height level, one on each of the remaining two masts. Fig.4 shows the mean monthly prof'fles for September, for alternate hours of the day, together with the corresponding mean wind profiles up to 4 m. Allowance for the zero plane displacement, (d), has been made by computing the best value of d required to make u ~x In (z-d), on a least squares basis, from a selection o f neutral wind data (z is the height above the ground). We found d to be ~ 0.96 h (height of the crop) for most of the season, and for the month of September we have d = 62 cm and h = 65 cm. It was further assumed that d for the CO2 profile was equal to d for the wind profile. A zero-plane displacement correction to the CO2 prof'de seems reasonable, since to represent the variation of CO2 concentration with height in the fully turbulent constant flux layer we use similarity with the wind speed relation. In neutral conditions, for instance, Ou/3z ~ (z-d) -1 and in the neutral fully turbulent layer, requiring u > 0 for z > d then 3C/az cc In ( z - d ) -1 ; where C is the concentration.

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Fig.4. September mean vertical profiles of wind velocity and carbon dioxide concentration above the Rutherglen wheat crop. Both variables are shown as increasing towards the right of the figure. An assumed zero plane displacement correction has been applied to all the data. Only data for alternative hours are shown, and the figures above each profile indicate the relevant hour. Plotted on a vertical In (z-d) scale, the wind profiles show the well-known diurnal variation of shape, being convex to the In (z-d) axis during the night (stable stratification) and linear or slightly concave to the same axis during the day (neutral a slightly unstable stratification). During the daytime the vertical C02 profiles in Fig.4 are of similar shape to the wind profiles (note the different height ranges for C02 and wind) being close to linear and even slightly concave near mid-afternoon. At night they show a similar increase with height of the vertical property difference per unit logarithmic height interval (AC/A On z)) as occurs for the wind profile (Au/A (In z)) under nighttime stable conditions. On several occasions during October and November an intake was placed at 10 m height above the crop. Fig.5 shows vertical profiles over the 1 - 1 0 m height range above the crop for every alternate hour for the period 4 - 5 November. During the day, when vertical mixing was relatively strong, the profiles suggest a near constant CO2 concentration above 10 m, whilst at night mixing was weak with the CO2 concentration decreasing rapidly with height up to 10 m.

Nocturnal behaviour of vertical carbon dioxide gradients On many of the nights in winter and early spring, strong inversions developed in conditions of surface cooling and light winds, with long periods of little or no turbulence present in the first few metres above the ground. In such circumstances and, in the

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Fig.5. The diurnal cycle of the carbon dioxide concentration profile up to 10 m above the Rutherglen wheat crop. Concentration increases towards the right of the figure. Only data for alternative hours are shown, and the figures above each profile indicate the hour. presence of near-surface sources of CO2 (the respiring crop and soil organic matter) large increases in the CO2 concentration and in IAf2_tl occurred. These quiescent periods lasted for 2 h or more and on many occasions were replaced by periods of turbulent activity when CO2 gradients fell to relatively small values in the presence of strong vertical mixing. On nights when skies were cloudy, wind speeds were generally greater and CO2 differences characteristically small and steady. Fig.6 shows typical observations of nighttime gradients, illustrating the periodic nature of the flfictuations. On such nights wind speeds were small, typically < 1 m/sec with a relatively large outgoing net radiation flux to clear skies. The gradients on these nights oscillated in magnitude with two distinct periods - a major oscillation of period '~3 h, and a second oscillation with a period of ~ 1 h. The dependence of the CO2 vertical gradient at night on the stability conditions in the atmospheric surface layer is demonstrated in Fig.7 where we plot hourly means of AC2_~ against Richardson number (a stability parameter) for the 2 - 1 m layer for some 20 nights. The Richardson number for the 2 - 1 m layer, is defined here as R i = (330 x 02_ ~)/(u2 - u ~)2, 02_ ~ being the dry-bulb potential temperature difference (°C) between 2 and 1 m and u the wind speed (cm/sec) at height z. Errors in u2-ul at night could well exceed 100% on some occasions when wind speeds were close to the stalling speed of the anemometers. Large positive values of Ri, arising mainly from small values of wind shear, may therefore be uncertain by a factor of 2 or more, though such errors are unimportant in this context. Of particular interest in the diagram is the large increase of I~C2_~1 in the range 0.1 < R i < 1, the range where recent work has suggested the existence of a critical Richardson number (approximately 0.2) required for the maintenance of turbulence (Webb, 1970). Throughout any one night of strong stability, one can follow the correlation between AC2_ 1 and Ri. This is illustrated in Fig.8 where the oscillatory nature of the AC2_~ and R i variation is again demonstrated with a period close to 4 h. CARBON DIOXIDE CONCENTRATIONS Measurements of the absolute CO2 concentrations above the crop were made irregularly throughout the season and are summarized in Fig.9.

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Fig.6. Nocturnal carbon dioxide gradients between 1 and 2 m above the Rutherglen wheat crop. The curves are plotted using measurements made at 6-min intervals. The data are plotted as 3 and 7 point running means. In A, the former is drawn as a continuous line to emphasize the oscillation of period, 1 h, while the 7 point running mean is drawn as a continuous line in B in order to demonstrate the longer period oscillation.

Diurnal cycle On any one day the concentrations were found to be approximately constant from about 10h00 to 16h00, with values of about 315 p.p.m. The small daytime vertical gradients of CO2 imply that during these hours, vertical mixing is effective enough to maintain CO2 concentrations near the ground which are similar to the concentration in the bulk of the lower atmosphere. Consistent with this is the fact that relatively small increases in concentration occurred on nights when near-neutral stratification predominated. Large CO2 concentrations occurred in strong nocturnal inversions and were accompanied by the steep gradients described earlier.

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Fig.7. The relationship between the nocturnal carbon dioxide difference (AC2 ~) and the Richardson number (Ri) for the 1-2 m layer of air above the Rutherglen wheat crop. Horizontal bars indicate the range of Richardson numbers from which zXC2_~ data were used to calculate each point, while vertical bars show the 95% confidence limits on each mean value. Numbers above each mean indicate the number of hours of data used in that estimate. The diurnal cycle of CO: concentration above a variety of crops including wheat has been described elsewhere (e.g., Allen, 1971), in general, daytime concentrations just above the crop are about 2 6 0 - 3 2 0 p.p.m, with nighttime values o f 3 6 0 - 5 0 0 p.p.m. Allen (1971) showed how build up o f CO2 over corn was particularly noticeable on nights when wind velocities were less than 1 m/sec. The present data are consistent with this view except that daytime values are more nearly constant at 310--320 p.p.m. Seasonal trend The seasonal trend is best described in terms o f the CO2 concentration measured around midday, when the greatest vertical mixing in the lower atmosphere takes place. These values in fact show very little variation through the season indicating the absence of a large photosynthetic sink for COs. In contrast, Denmead (1970) found day to day variations o f 50 p.p.m., and Allen (1971) described a seasonal oscillation in concentrations with a peak to peak amplitude of ~ 50 p.p.m. Data collected by the authors over southeast Australia and Bass Strait (Pearman and

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Fig.8. The hourly mean carbon dioxide difference between 1 and 2 m above the Rutherglen wheat crop, together with the Richardson number, for the night of 22-23 July, 1971. Garratt, 1973), indicate that the CO2 content of the troposphere above about 2 km varies from day to day by less than 1 p.p.m. This implies that day-to-day variations of the order of 50 p.p.m, must be confined to the convective boundary layer. In order to estimate the influence of a realistic surface sink on the COz content in the boundary layer we make the simplifying assumptions that on the time scale of hours, the boundary layer may be treated as a well mixed layer distinct from the remainder of the troposphere. The decrease (6C) in the CO2 concentration of the layer of air, of depth 7(cm), travelling at a velocity u(cm/sec) over a distance F(cm) is given by, 6C-

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variables: F = 108 cm, u = 500 cm/sec, 3' = 2"10 s cm, and Pco~ = 2 "10-3 g/cm 3 ; then 6C = 15 p.p.m. Thus, even with such a large fetch, it is difficult to understand how differences in trajectory and thus exposure to surface sources or sinks can explain day-today variations in boundary layer CO2 content of more than 15 p.p.m, unless the layer is especially shallow or wind velocities low. CONCLUSIONS

(1) Measurements of the CO2 concentration difference (AC2_ ~) between 2 and 1 m above the crop at several locations show little horizontal variation and indicate the adequacy of one mast only for the measurement of a representative CO2 profile. The variations which are observed emphasize the problem of defining a reference zero level relative to crop height because of the variable nature of the crop itself. (2) The diurnal cycles of ~C2-1 show the expected reversal of sign at sunrise and sunset, from positive values during the day (crop a sink) to negative values at night (crop a source). The daytime CO2 differences are characteristically steady and relatively small (2 p.p.m.), with nocturnal values being more variable and on most occasions, about 10 times larger in magnitude.

CARBON DIOXIDE MEASUREMENTS ABOVE A WHEAT CROP

25

(3) The seasonal cycle of the daytime AC2-1,shows a steady increase to 2 - 3 p.p.m, at the time of maximum growth rate of the crop, indicating the effect of crop photosynthesis on the near-ground CO2 gradients at day. (4) On occasions of stable stratification (nighttime) variations in AC2-1 and Ri, the local Richardson number, are found to be closely correlated. Typically, large values of [AC2-1 t (% 20 p.p.m.) occur for Ri > 0.1 1 with low level winds < 1 m/sec, whilst small values are confined to Ri < 0.1 and wind speeds generally > 1 m/sec. The data are consistent with the idea of a critical Richardson number of value 0.1 1, above which turbulence cannot be maintained. (5) The diurnal cycle of CO2 concentration (measured at 2 m) shows near constant ( 3 1 0 - 3 2 0 p.p.m.) values at daytime and much greater, and generally more variable, values during the night (up to 500 p.p.m.). The daytime CO2 concentration shows little day-to-day or seasonal variation. In the absence of local "contamination", it is difficult to see how the daytime CO2 concentration at one location can change by more than about 10 p.p.m, in one day. REFERENCES Allen, L. H., 1971. Carbon dioxide concentration over an agricultural field. Agric. Me teorol., 8: 5-24. Denmead, O. T., 1970. Transfer processes between vegetation and air: measurement, interpretation and modelling. In: 1. Setlik (Editor), Prediction and Measurement of Photo-synthetic Productivity Proc. LB.P./P.P. Tech. Meet., Trebon, 1969. Pudoc, Wageningen, pp. 149-164. Paltridge, G. W., 1970. A model of a growing pasture. Agric. Meteorol., 7 : 93-130. Paltridge, G. W., Dilley, A. C., Garratt, J. R., Pearman, G. 1., Shepherd, W. and Connor, D., 1972. The Rutherglen experiment on Sherpa Wheat: environmental and biological data. C.S.LR.O. Div. Atmos. Phys. Tech. Pap., 22: 156pp. Pearman, G. I. and Garratt, J. R., 1973. Space and time variations of tropospheric carbon dioxide in the Southern Hemisphere. Tellus, 25(3), in press. Webb E. K., 1970. Profile relationships: the log-linear range and extension to strong stability. Q. J. R. Meteorol. Soc., 96: 67-90.