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rural transect—I. Continuous measurements at the transect ends
Atmospheric Environment Vol. 24A, No. 10, pp. 2681 2688, 1990.
0004-6981/90 $3.00+0.00 Pergamon Press pie
Printed in Great Britain.
ATMOSPHERIC CARBON DIOXIDE AND SULPHUR DIOXIDE ON AN URBAN/RURAL TRANSECT--I. CONTINUOUS MEASUREMENTS AT THE TRANSECT ENDS R. D. BERRY* a n d J. J. COLLS~" University of Nottingham, School of Agriculture, Sutton Bonington, Loughborough LE12 5RD, U.K. (First received 20 February 1989 and in final form 12 May 1990)
Abstract~Carbon dioxide and sulphur dioxide concentrations have been measured on a transect between urban Nottingham and a rural area 15 km to the south-west. In Part I, the results of fixed point measurements at the ends of the transect are described. Seasonal averages showed no significant difference for CO2 or SO2 concentrations in the summer; in winter the mean CO2 concentration in the city centre was 5/A ~- ~ larger than the rural mean. These figures conceal large differences in the diurnal variation between the two sites. Key word index: Atmospheric, carbon dioxide, monitoring measurements, sulphur dioxide, transect.
INTRODUCTION
Carbon dioxide and sulphur dioxide are both emitted into the atmosphere during the combustion of most fossil fuels. They both affect plants. There, for practical purposes, their similarities end. Carbon dioxide is a primary reaction product of the combustion, has a residence time in the atmosphere of 3-4 years (Crane, 1985), and is the main substrate for photosynthesis in plants. Sulphur dioxide is a lesser, though still significant, product of impurities in fuel. Its residence time in the atmosphere is a few days (Turco et al., 1981) and it has generally been regarded as a phytotoxin. The pattern of CO z and SO2 concentration fluctuations at any location is determined by the balance between sources and sinks. In an urban area, the anthropogenic co-production of these two gases means that close to combustion sources, in the absence of differential dry deposition or atmospheric reaction effects, concurrent increases in concentration will occur following dispersion and dilution by normal atmospheric processes. These increases will be superimposed on the prevailing background concentration, since there is little vegetation for the release or emission of CO2. Rural areas are in sharp contrast. The density of the combustion sources is reduced and their size profile changes, tending to a bimodal distribution of small, low level domestic and large high level industrial (e.g. power station) sources. The large areas of vegetation absorb and release substantial quantities of CO 2 by photosynthesis and respiration, and have greater potential for the dry deposition of SO2 than unvegetated areas. *Present address: Health & Safety Executive, 403 Edgware Road, London NW2 6LN, U.K. t To whom correspondence should be addressed.
On these already complex source-sink relationships are superimposed other influences--diurnal production cycles for heating, electricity, manufacturing output and photosynthesis; annual cycles of temperature (which affect heating requirements) and plant growth rates; meteorological factors such as wind direction, wind speed and atmospheric stability. The global background CO2 concentration is currently around 345/A E-1, and is increasing at about 1.5 #1 f - 1 y r - 1 (Crane, 1985). Although the possible impacts of this increase are causing concern, the specific effects of increased CO 2 on stands of natural or cultivated plants are far from clear. Cure and Acock (1986) reviewed the literature to evaluate the predicted effects of a doubling of CO2 concentration. They concluded that yield would be increased for the great majority of agricultural species grown in the U . K . - for example, they predicted 35%, 70%, and 51% in the case of wheat, barley and potato, respectively. However, the experiments cited had been done under a variety of regimes, in which the effects of enclosure, environment, acclimation and canopy structure could not be fully evaluated. Kimball (1983) estimated a 33% increase in yield, but again warned of the confounding effects of growth chamber environment. Such yield changes are accompanied by detailed changes in the plant's response to its environment. For example, Allen et al. (1985) and Oberbauer et al. (1985) found that stomatal conductance fell with increasing atmospheric CO 2 concentration; the lower conductance conserves water (and reduces pollutant access) but may also result in higher leaf temperatures. As with CO2, the effects of SO 2 on plants have been investigated for many years. The principal route for access is through stomata; hence the uptake is influenced by the same environmental factors that influence CO2 uptake. Concentrations of around 1 # l ~ can cause visible damage and death (Last, 1982). In the
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R.D. BERRYand J. J. COLLS
absence of other pollutants, concentrations of around 100 nld -~ have been shown to reduce dry matter production, photosynthesis and yield, without producing a visible effect on the plant, (Roberts, 1984). Plants grown in controlled environments have been found to respond to much lower concentrations, particularly in the presence of other pollutants such as NOx. Barker and Neighbour (1988) exposed downy birch (Betula pubescens) to SO2 and NO 2 at concentrations of 10, 20 or 40 nl d- t above ambient. Average ambient concentrations were 5 n l d -~ SO 2 and 10nld - t NO2. The 10nld -~ fumigation was sufficient to decrease the root: shoot ratio significantly, due to reduced root mass, and to inhibit stomatal regulation of water loss from water-stressed plants. Similar behaviour at concentrations of 30 nld - t has been found for timothy grass by Lucas (1989). However, some field fumigation experiments in rural areas have not detected any reduction in crop yield for constant SO 2 exposures above 40 nl d - ~ (e.g. Baker et al., 1986). Carlson and Bazzaz (1985) reviewed the deposition and effects of SO2 and elevated CO2 separately before describing their own experiments on simultaneous exposures. They grew six annual weed species for up to 4 weeks at 0 or 240n1# -~ SO2 and 326, 595 or l l 9 0 / z l f -~ CO2. For some species, biomass was depressed in the SO2 treatments at the lowest CO2 concentration, but recovered at the higher concentrations: for others, biomass was increased by SO2 at the lowest CO2 concentration, but fell with increased C O y Leaf area increased with elevated CO2 for all plants in clean air, but was reduced for all plants in SO2. The co-production and co-occurrence of CO2 and SO2, together with their known individual effects on plants, mean that it is important to understand the detailed variations in their concentration. The project described below was designed to measure the temporal variations in CO2 and SO2 continuously at rural and urban locations, and to investigate their variation on a transect between the two sites. The continuous measurements at fixed points are reported here, and the transect measurements in Part II.
LOCATION OF SITES
Carbon dioxide and sulphur dioxide were monitored continuously for 8 months at the two sites; the urban site was in the centre of Nottingham and the rural site was at Sutton Bonington, 15 km south-west of the city centre. The urban areas and local power stations are shown in Fig. 1. There were no large industrial sources of the measured gases within 1 km of the urban site, although there were several such sources within 4 km. The only important source within 1 km of the rural site was a boiler house, 350 m N N W of the sampling point which burnt coal at an average rate of 9 t day-1 during the winter months (October-March). The inlets to both sampling systems
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Fig. 1. Locations of continuously monitored sites (M.S.) and local power stations (P.S.). were at least 40 m from small sources such as traffic and domestic chimneys. At the rural site the air was sampled at a height of 1.5 m over short grass which never exceeded 20 cm in height. The difference in the hourly-averaged CO2 concentration between the sampling point and the top of the canopy exceeded 2/~IE-t only under very stable atmospheric conditions. At the urban site the air sample was taken 4 m above a concrete walkway. No significant difference in hourly-averaged CO2 concentration was found between this height and ground-level.
GAS ANALYSIS
Prior to analysis for CO2, the air sample was filtered to remove particulate matter and dried using magnesium perchlorate (Berry, 1987). The CO2 concentration was measured by double beam, infra-red gas analyzers (Analytical Development Company, Type 225). To assess drift in the analyzer calibration the zero and span were checked on alternate hours by passing CO2free air or a standard mixture of 500 #l # - t CO2 in nitrogen through the instrument for 5-min periods. This procedure enabled the CO2 concentration to be measured to an accuracy of ± 3 #l d- t. The SO2 concentration of the sampled air was measured by flame photometric detection sulphur analyzers (Meloy, Model SA185-2). Teflon tubing was used throughout to prevent absorption of SO2 prior to analysis. It should be noted that this type of analyzer measures the total sulphur concentration of the sample and therefore, as no filter was used to remove particulate sulphate, some of the recorded SO2 concentration will have been of this form. However, the proportion of sulphur in particulate form probably averaged less than 15% (Maul et al., 1980). The calibrations of these analyzers were also checked
Carbon dioxide and sulphur dioxide--I automatically: the zero values, at 6-h intervals with sulphur-free air; the spans, daily, with 50 nl f - 1 SO2 in dried air from a permeation calibrator, (Meloy, Model CS-10 or Monitor Labs, Model 8500 PERMACAL). This gave an estimated accuracy in the measured sulphur concentration, expressed as SO2 equivalent, of +(3 n l ~ - 1 + 8% of the measured concentration). The output of each analyzer was logged at 10-min intervals and hourly-average values were calculated from these readings.
RESULTS
The monthly-averaged concentrations of CO2 and SO 2 measured at the two sites are shown in Fig. 2. The error bars show the standard deviation of the dailyaveraged values for each month. Over the four summer months there was no systematic difference in the monthly-averaged CO2 and SO2 concentrations between the two sites. Over the winter period the CO2 concentration in the city centre was on average 5 / a l l -1 higher. The unusually high average SO 2 concentration for January at the rural site can be explained by the predominance of northerly winds during that month. Such winds bring emissions from both the local boiler house and Ratcliffe power station (a 2000 M W coal-burning station, 4 km to the north), over the site. Even if the January values are ignored,
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Fig. 2. Average monthly concentrations of(a) CO 2 and (b) SO2 for the period December 1984-July 1985. The error bars show the standard deviation of the daily-averaged values recorded at each site.
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the average urban winter 802 concentration was only about 5 nl f - 1 greater than the rural value. Thus, over the winter months, the ratio of the difference between urban and rural CO2 concentrations (5/A ~ - 1) to the difference in SO 2 concentrations ( < 5 n l f -1) was more than 1000. This implies that the local emissions that contributed most to the higher urban CO2 concentration were from sources with high CO2:SO 2 emission ratios such as gas space heating and vehicle exhausts (Weber, 1970; DOE, 1985). The mean monthly concentrations might suggest that local sources usually make only small contributions to the CO2 and SO 2 concentrations at the two sites but they mask considerable differences in the diurnal variation between the sites. The mean diurnal variations of CO 2 and SO 2 for 2-month periods at the two sites are shown in Fig. 3. The curves pass through every mean hourly value and are confined between the limits of these values--i.e. they do not extend higher or lower than these values at the turning points. Therefore, although the mean hourly values are not marked they can be read directly from the curves 'on the hour'. During the winter months the monthly-averaged CO2 concentrations at the rural site (Fig. 3a) were almost constant at night but started to decrease soon after sunrise, reaching a minimum at about midday, before increasing again to the night-time value. Two processes may have contributed to the reduced CO 2 concentration during the day; the photosynthesis of the grass beneath the sampling point, and increased atmospheric turbulence during daylight hours depleting the CO2 produced by plant and soil organism respiration. At the urban site (Fig. 3b) a very different winter CO2 cycle was observed. This cycle had two m a x i m a - - a well-defined peak at dawn and a broader peak during the evening --separated by shallow troughs. The sharp increase in concentration just before dawn was probably due to the increase in emissions from local fossil fuel combustion at that time, as domestic and commercial space heating was switched on and traffic increased as people went to work. However, soon after sunrise as atmospheric turbulence increased, the local emissions were dispersed and the local CO 2 concentration decreased. In the evening, as the level of turbulence decreased, the reduced dispersion enabled the local CO2 concentration to rise again. At night, urban emissions were much reduced and thus the CO 2 concentration fell once more. At the onset of the growing season, the patterns of both the urban and rural CO 2 cycles started to change. At the rural site (Fig. 3a), the afternoon minimum of the monthly-averaged concentration showed a steady decrease from February to July and occurred at a slightly later hour in successive months• After February, the CO2 concentration did not exhibit the night-time 'plateau' observed in December and January; instead, it increased steadily from the afternoon minimum value, throughout the night, to peak just before sunrise. The increase in the daytime deple-
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R.D. BERRYand J. J. COLLS
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tion of C O 2 c a n be explained by the increased photosynthesis of the surrounding vegetation, due to the greater quantity of vegetation present and the higher temperature and level of solar radiation. This would be accompanied by increased plant respiration, and the higher soil temperature would increase the respiration of soil organisms. At night, in the absence of photosynthesis, these processes soon increased the ground-level CO2 concentration--the extent to which they did so depended on the rate at which air close to the ground was mixed with that at higher levels and thus on the degree of atmospheric stability (Berry, 1987).
In the city centre (Fig. 3b), the winter evening peak was not observed during the summer months; instead a cycle similar to that at the rural site but of smaller amplitude was found; the afternoon minima occurred slightly later in the day than at the rural site. The night-time maxima were still observed just before dawn but were slightly lower than the winter values• In the summer months, fossil fuel emissions in urban areas, particularly those from commercial and domestic space heating, are reduced and therefore a reduction in the CO2 concentration might be expected throughout the day. The increased photosynthesis and respiration of the vegetation both within and sur-
Carbon dioxide and sulphur dioxide--I rounding the urban areas might also be expected to have some effect. The study by Clarke and Faoro (1966) suggests that under conditions of nocturnal temperature inversion, the transport into an urban area of the high ground-level COz concentrations-found in summer under such conditions in the surrounding rural a r e a s - - can make a large contribution to the night-time urban COz concentrations. The fact that the monthly-averaged night-time peak values at the urban site were only slightly lower than the winter values strengthens this hypothesis. Under turbulent daytime conditions the effect at the urban site of plant activity in the local environment might be expected to be much smaller than under the more stable conditions found at night. This is confirmed by the daytime CO2 concentrations observed at the urban site: these only very rarely fell below the tropospheric background level measured at stations remote from local sources and sinks (Bolin et al., 1979, 1986) and therefore were probably little affected by local plant photosynthesis. The monthly-averaged daytime (sunrise to sunset) and night-time (sunset to sunrise) CO 2 concentrations at the two sites are shown in Fig. 4a. When comparing this figure with Fig. 2 it must be remembered that, in England, the day length in July is twice that in December. Average daytime CO z concentrations at the urban site were approximately 8 #1~-1 greater than at the rural site throughout the year. The diurnal SO2 variations did not have such welldefined characteristics (Figs 3c and 3d). However, the (a)
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daytime concentrations were substantially higher than those observed at night at both sites (Fig. 4b). This difference has been observed before (e.g. Maul et al., 1980), and is normally attributed to increased emissions and increased atmospheric turbulence during the day. The sharp increase at dawn observed at the rural site supports this explanation. It appears that at night, under stable atmospheric conditions, SO2 is depleted at ground-level, then when the stable layer breaks up after dawn, fumigation quickly increases the concentration close to the ground.
MAN-MADEEMISSIONS
The results presented above show that local variations in the CO2 concentration at the rural site are dominated by natural sources and sinks during the summer months and that these sources probably make a substantial contribution to night-time CO 2 levels in the city centre. During the winter months man-made emissions are at their greatest, and local natural contributions to CO2 levels are much reduced. Therefore only this period (from December 1984 to March 1985) was used to investigate the dispersal of these emissions and in particular to look at the correlation between CO2 and SO2 originating from local sources. To determine this correlation a 'background' level of CO2 is needed which can be subtracted from the measured CO 2 concentration to find the 'excess' CO2 due to local sources. One approach would be to measure the CO 2 concentration of the airstream entering the local environment. However, this is impracticable, and thus the approach adopted was to take as the background concentration for each hour of the day, the minimum concentration recorded for that hour at either site in the particular month; this was then subtracted from the concentrations measured at that hour on other days in the month to give the excess CO2 concentration. These minimum values for each month almost invariably occurred at the rural site on days when atmospheric stability was neutral and the SO2 concentration at the site was almost zero. Note that this was not the case during the summer months when the lowest daytime minima occurred under unstable conditions----on days with high solar radiation and low wind speeds. For the winter months, the minimum hourly CO2 concentrations at both sites were close to the background tropospheric concentration; for example, for December they ranged from 331 to 341pig -~ at the rural site and from 337 to 343 #1 f - ~ at the urban site. There was invariably one day in the month when the SO 2 concentration was not significantly above zero for each hour and thus there was no need to calculate 'excess' SO2 concentrations. Scatter diagrams of the absolute SO2 concentration versus the excess CO2 concentration at the two sites are shown in Figs 5a and 5b. The coefficients of correlation for the rural and urban sites are 0.37 and 0.90, respectively (both significant at the 0.1% level),
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R.D. BERRYand J. J. CELLS
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i.e. for both sites there is a definite positive relationship between the excess CO2 concentration calculated by the method described above and the absolute SO2 concentration. The correlation is much stronger for the urban site because it had a much lower occurrence of periods of very low SO 2 concentration ( < 5 nl : - 1) and was more remote from local natural sources. CONCENTRATION
ROSETTES
Qualitative assessments of the dependence of CO2 and SO2 concentrations on wind direction and of the relative importance of local and distant sources were made by calculating average concentrations of the gases at each site when the wind was from within 22.5 ° sectors. The main criterion adopted was that the wind direction measured at a height of 6 m at the rural site should not vary by more than 40 ° over a period of 6 h before the measurement was taken. Furthermore, the wind speed had to be greater than 2 m s-1 over this period because of the high sticking speed of the wind-vane. These criteria allowed almost 50% of the hourly measurements to be included in the rosette calculations. If it is assumed that wind direction was constant over the local environment, the fact that hourly measurements were not included in the rosette calculations if they occurred after 6-h periods when the wind dropped below 2 m s - 1 ensures that the only contributions from sources within 4 0 k m (2 m s - ~ x 6 h) of the monitoring site were from sources within the relevant sector. No allowance was made for washout of SO2 by rain. Concentration rosettes for excess CO 2 and absolute SO2 over the winter period are shown in Fig. 6,
together with a rosette showing the number of hourly values used to calculate the average concentrations. F o r most ofthe steady 6-h periods after which concentrations were used to calculate the rosettes, the wind was from either the south-west or north-east. Average CO 2 concentrations were higher in the city centre than at the rural site when tbe wind was from almost all directions, but when the wind was from the north, SO 2 concentrations were higher at the rural site. This can be explained by the fact that the most important local sources north of the rural site, Ratcliff¢ power station and the boiler house, both have a relatively high SO2:CO2 ratio compared with average urban emissions which are dominated by gas and petrol combustion processes. A prominent feature is the large average concentration from the south-east on all four rosettes. The similarity in concentration at the two sites suggests that the source of the pollution is distant and, as there is no obvious U.K. source in that direction, possibly of continental origin. Note that the number of hourly values used to calculate the sector concentrations varies greatly between sectors and thus a particularly high average concentration for a sector, for example the ESE-SE sector, does not necessarily indicate that it had a higher maximum concentration nor indeed longer periods of high concentration, than adjacent sectors with lower average concentrations. At the rural site, CO2 concentrations were considerably greater from the north-east than from the southwest, as expected, given the location of local urban areas. When the wind was from the south-west both CO2 and SO 2 concentrations were much higher at the urban site than at the rural site. Ratcliffe power station and intervening light industrial and urban domestic
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2687
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Fig. 6. Concentration rosettes for SO2 and excess CO2 at the rural and urban sites for the period December 1984-March 1985.
sources probably all made significant contributions. Graphs of cumulative frequency of occurrence vs atmospheric SO2 concentration are presented in Fig. 7 in standard log-normal form. The greater occurrence of high SOz concentrations at the rural site than the urban site can be explained by its proximity to large sources. DISCUSSION The annual cycles of the ground-level CO z concentration at both sites showed a spring maximum and autumn minimum and were therefore approximately in phase with the background tropospheric concentration cycle. If it is assumed that both cycles had almost reached their minimum value by the end of July, then each had an amplitude of about 20/A f-1--twice the background value (Bolin et al., 1979, 1986). Variations in the monthly-averaged concentration in consecutive
months at each site were of the same magnitude as the differences found between the sites. Over the summer months (April-July) there was no systematic difference in the monthly-averaged CO2 concentrations between the sites; over the winter period (December-March) the concentrations at the city centre were, on average, 5 #1 g - 1 higher than at the rural site. However, when interpreting these values it must be remembered that there were large sources close to the rural site and that the pattern of the diurnal variation at the two sites was very different. In the winter months, differences in the night-time monthly-averaged CO2 concentrations between the two sites were small. In December and January, the night-time concentration in the city centre was, on average, 8 #1 #- t higher than at the rural site, but by the end of February it was slightly higher at the rural site. As the year progressed the urban concentrations fell steadily, whilst the rural concentrations, although
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Acknowledoements--R.D.B. was supported by a National Westminster Bank Research Grant. Dr C. R. Gentle (Trent Polytechnic) helped set up the urban monitoring site.
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Fig. 7. Hourly SO 2 concentration vs cumulative percentage frequency of occurrence for the period December
1984-July 1985. varying greatly from day to day, showed an increasing trend for the remainder of the monitored period. The daytime C O 2 concentration in the city centre was on average 8/~1 # - ~ higher than at the rural site throughout the year. The dependence of SO 2 concentrations on the proximity of local combustion sources to the point of measurement makes it difficult to compare the concentrations found in this study with those observed at other sites. As with the C O 2 concentrations, variations in the monthly-averaged SO 2 concentrations in consecutive months at each of the continuously monitored sites were of the same magnitude as differences between the sites. Concentrations in the city centre were not, on average, greater than those measured at the rural site but they were much less dependent on wind direction. The most probable explanation is that concentrations at the city centre site were dominated by the many small local sources, uniformly distributed throughout the surrounding urban area, whilst a large contribution was made to the concentrations at the rural site by a few large sources close to the site. This hypothesis is supported by the graphs of cumulative frequency of occurrence vs SO2 concentration for the two sites (Fig. 7), which show that concentrations were higher at the city centre site for most of the monitored period but that there were more high-concentration episodes at the rural site. The implications of the measured levels of CO2 and SO 2 for plant growth are discussed briefly at the end of Part II of this paper.
Allen K. L. H., Jones P. and Jones J. W. (1985) Rising atmospheric CO2 and evapotranspiration. In Advances in Evapotranspiration, pp. 13-27. Am. Soc. Ag. Engs, Mich., U.S.A. Baker C. K., Coils J. J., Fullwood A. E. and Seaton G. G. R. (1986) Depression of growth and yield in winter harley exposed to sulphur dioxide in the field. New Phytol. 104, 223-241. Barker M. G. and Neighbour E. A. (1988) The control of water loss on pollutant-damaged plants. Asp. appl. Biol. 17, 133-140. Berry R. D. (1987) Atmospheric CO 2 and SO 2 in the Nottingham area. M. Phil. Thesis, University of Nottingham. Bolin B., Degens E. T., Kempe S. and Ketner P. (1979) The global carbon cycle, SCOPE Report No. 13. Wiley, New York. Bolin B., Doos B. R., Jager J. and Warrick R. A. (1986) The greenhouse effect, climatic change, and ecosystems, SCOPE Report. No. 29. Wiley, New York. Carlson R. W. and Bazzaz F. A. (1985) Plant response to SO2 and CO 2. In Sulphur Dioxide and Veoetation (edited by Winner W. E., Mooney H. A. and Goldstein R. A.). Stanford University Press, Stanford. Clark W. C. and Faoro R. B. (1966) An evaluation of CO 2 measurements as an indicator of air pollution. J. Air Pollut. Control Ass. 16, 212-218. Crane A. J. (1985) Possible effects of rising CO2 on climate. Plant, Cell Envir. 8, 371-379. Cure J. D. and Acock B. (1986) Crop responses to carbon dioxide. Agric. Forest Met, 38, 127-145. DOE (1985) Digest of Environmental Protection and Water Statistics. Department of the Environment, H.M.S.O. Kimball B. A. (1983) Carbon dioxide and agricultural yield: an assemblage and analysis of 430 prior observations. Agronomy J. 75, 779-788. Last F. T. (1982) Effects of atmospheric sulphur compounds on natural and man-made terrestrial and aquatic ecosystems. Agric. Envir. 7, 299-387. Lucas P. W. (1990) The effects of prior exposure to sulphur dioxide and nitrogen dioxide on the water relations of Timothy grass (Phleum pratense) under drought conditions. Envir. Pollut. (submitted). Maul P. R., Barber E. R. and Martin A. (1980) Some observations of the meso-scale transport of sulphur compounds in the rural East Midlands. Atmospheric Environment 14, 339-354. Oberbauer S. F., Strain B. R. and Fletcher N. (1985) Effect of CO 2 enrichment on seedling physiology and growth of two tropical species. Physiol. Plant. 65, 352-356. Roberts T. M. (1984) Effects of air pollutants on agriculture and forestry. Atmospheric Environment 18, 629~152. Turco R. D., Whitten R. C., Toon O. B., Inn E. C. Y. and Hammill P. (1981) Stratospheric hydroxyl radical concentrations; new limitations suggested by observations of gaseous and particulate sulphur. J. geophys. Res. 86, C2, 1129-1139. Weber E. (1970) Contribution to residence time of SO 2 in a polluted atmosphere. J. oeophys. Res. 75, 2909-2914.
0004-6981/90 $3.00+0.00 Pergamon Press pie
Printed in Great Britain.
ATMOSPHERIC CARBON DIOXIDE AND SULPHUR DIOXIDE ON AN URBAN/RURAL TRANSECT--I. CONTINUOUS MEASUREMENTS AT THE TRANSECT ENDS R. D. BERRY* a n d J. J. COLLS~" University of Nottingham, School of Agriculture, Sutton Bonington, Loughborough LE12 5RD, U.K. (First received 20 February 1989 and in final form 12 May 1990)
Abstract~Carbon dioxide and sulphur dioxide concentrations have been measured on a transect between urban Nottingham and a rural area 15 km to the south-west. In Part I, the results of fixed point measurements at the ends of the transect are described. Seasonal averages showed no significant difference for CO2 or SO2 concentrations in the summer; in winter the mean CO2 concentration in the city centre was 5/A ~- ~ larger than the rural mean. These figures conceal large differences in the diurnal variation between the two sites. Key word index: Atmospheric, carbon dioxide, monitoring measurements, sulphur dioxide, transect.
INTRODUCTION
Carbon dioxide and sulphur dioxide are both emitted into the atmosphere during the combustion of most fossil fuels. They both affect plants. There, for practical purposes, their similarities end. Carbon dioxide is a primary reaction product of the combustion, has a residence time in the atmosphere of 3-4 years (Crane, 1985), and is the main substrate for photosynthesis in plants. Sulphur dioxide is a lesser, though still significant, product of impurities in fuel. Its residence time in the atmosphere is a few days (Turco et al., 1981) and it has generally been regarded as a phytotoxin. The pattern of CO z and SO2 concentration fluctuations at any location is determined by the balance between sources and sinks. In an urban area, the anthropogenic co-production of these two gases means that close to combustion sources, in the absence of differential dry deposition or atmospheric reaction effects, concurrent increases in concentration will occur following dispersion and dilution by normal atmospheric processes. These increases will be superimposed on the prevailing background concentration, since there is little vegetation for the release or emission of CO2. Rural areas are in sharp contrast. The density of the combustion sources is reduced and their size profile changes, tending to a bimodal distribution of small, low level domestic and large high level industrial (e.g. power station) sources. The large areas of vegetation absorb and release substantial quantities of CO 2 by photosynthesis and respiration, and have greater potential for the dry deposition of SO2 than unvegetated areas. *Present address: Health & Safety Executive, 403 Edgware Road, London NW2 6LN, U.K. t To whom correspondence should be addressed.
On these already complex source-sink relationships are superimposed other influences--diurnal production cycles for heating, electricity, manufacturing output and photosynthesis; annual cycles of temperature (which affect heating requirements) and plant growth rates; meteorological factors such as wind direction, wind speed and atmospheric stability. The global background CO2 concentration is currently around 345/A E-1, and is increasing at about 1.5 #1 f - 1 y r - 1 (Crane, 1985). Although the possible impacts of this increase are causing concern, the specific effects of increased CO 2 on stands of natural or cultivated plants are far from clear. Cure and Acock (1986) reviewed the literature to evaluate the predicted effects of a doubling of CO2 concentration. They concluded that yield would be increased for the great majority of agricultural species grown in the U . K . - for example, they predicted 35%, 70%, and 51% in the case of wheat, barley and potato, respectively. However, the experiments cited had been done under a variety of regimes, in which the effects of enclosure, environment, acclimation and canopy structure could not be fully evaluated. Kimball (1983) estimated a 33% increase in yield, but again warned of the confounding effects of growth chamber environment. Such yield changes are accompanied by detailed changes in the plant's response to its environment. For example, Allen et al. (1985) and Oberbauer et al. (1985) found that stomatal conductance fell with increasing atmospheric CO 2 concentration; the lower conductance conserves water (and reduces pollutant access) but may also result in higher leaf temperatures. As with CO2, the effects of SO 2 on plants have been investigated for many years. The principal route for access is through stomata; hence the uptake is influenced by the same environmental factors that influence CO2 uptake. Concentrations of around 1 # l ~ can cause visible damage and death (Last, 1982). In the
2681
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R.D. BERRYand J. J. COLLS
absence of other pollutants, concentrations of around 100 nld -~ have been shown to reduce dry matter production, photosynthesis and yield, without producing a visible effect on the plant, (Roberts, 1984). Plants grown in controlled environments have been found to respond to much lower concentrations, particularly in the presence of other pollutants such as NOx. Barker and Neighbour (1988) exposed downy birch (Betula pubescens) to SO2 and NO 2 at concentrations of 10, 20 or 40 nl d- t above ambient. Average ambient concentrations were 5 n l d -~ SO 2 and 10nld - t NO2. The 10nld -~ fumigation was sufficient to decrease the root: shoot ratio significantly, due to reduced root mass, and to inhibit stomatal regulation of water loss from water-stressed plants. Similar behaviour at concentrations of 30 nld - t has been found for timothy grass by Lucas (1989). However, some field fumigation experiments in rural areas have not detected any reduction in crop yield for constant SO 2 exposures above 40 nl d - ~ (e.g. Baker et al., 1986). Carlson and Bazzaz (1985) reviewed the deposition and effects of SO2 and elevated CO2 separately before describing their own experiments on simultaneous exposures. They grew six annual weed species for up to 4 weeks at 0 or 240n1# -~ SO2 and 326, 595 or l l 9 0 / z l f -~ CO2. For some species, biomass was depressed in the SO2 treatments at the lowest CO2 concentration, but recovered at the higher concentrations: for others, biomass was increased by SO2 at the lowest CO2 concentration, but fell with increased C O y Leaf area increased with elevated CO2 for all plants in clean air, but was reduced for all plants in SO2. The co-production and co-occurrence of CO2 and SO2, together with their known individual effects on plants, mean that it is important to understand the detailed variations in their concentration. The project described below was designed to measure the temporal variations in CO2 and SO2 continuously at rural and urban locations, and to investigate their variation on a transect between the two sites. The continuous measurements at fixed points are reported here, and the transect measurements in Part II.
LOCATION OF SITES
Carbon dioxide and sulphur dioxide were monitored continuously for 8 months at the two sites; the urban site was in the centre of Nottingham and the rural site was at Sutton Bonington, 15 km south-west of the city centre. The urban areas and local power stations are shown in Fig. 1. There were no large industrial sources of the measured gases within 1 km of the urban site, although there were several such sources within 4 km. The only important source within 1 km of the rural site was a boiler house, 350 m N N W of the sampling point which burnt coal at an average rate of 9 t day-1 during the winter months (October-March). The inlets to both sampling systems
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Fig. 1. Locations of continuously monitored sites (M.S.) and local power stations (P.S.). were at least 40 m from small sources such as traffic and domestic chimneys. At the rural site the air was sampled at a height of 1.5 m over short grass which never exceeded 20 cm in height. The difference in the hourly-averaged CO2 concentration between the sampling point and the top of the canopy exceeded 2/~IE-t only under very stable atmospheric conditions. At the urban site the air sample was taken 4 m above a concrete walkway. No significant difference in hourly-averaged CO2 concentration was found between this height and ground-level.
GAS ANALYSIS
Prior to analysis for CO2, the air sample was filtered to remove particulate matter and dried using magnesium perchlorate (Berry, 1987). The CO2 concentration was measured by double beam, infra-red gas analyzers (Analytical Development Company, Type 225). To assess drift in the analyzer calibration the zero and span were checked on alternate hours by passing CO2free air or a standard mixture of 500 #l # - t CO2 in nitrogen through the instrument for 5-min periods. This procedure enabled the CO2 concentration to be measured to an accuracy of ± 3 #l d- t. The SO2 concentration of the sampled air was measured by flame photometric detection sulphur analyzers (Meloy, Model SA185-2). Teflon tubing was used throughout to prevent absorption of SO2 prior to analysis. It should be noted that this type of analyzer measures the total sulphur concentration of the sample and therefore, as no filter was used to remove particulate sulphate, some of the recorded SO2 concentration will have been of this form. However, the proportion of sulphur in particulate form probably averaged less than 15% (Maul et al., 1980). The calibrations of these analyzers were also checked
Carbon dioxide and sulphur dioxide--I automatically: the zero values, at 6-h intervals with sulphur-free air; the spans, daily, with 50 nl f - 1 SO2 in dried air from a permeation calibrator, (Meloy, Model CS-10 or Monitor Labs, Model 8500 PERMACAL). This gave an estimated accuracy in the measured sulphur concentration, expressed as SO2 equivalent, of +(3 n l ~ - 1 + 8% of the measured concentration). The output of each analyzer was logged at 10-min intervals and hourly-average values were calculated from these readings.
RESULTS
The monthly-averaged concentrations of CO2 and SO 2 measured at the two sites are shown in Fig. 2. The error bars show the standard deviation of the dailyaveraged values for each month. Over the four summer months there was no systematic difference in the monthly-averaged CO2 and SO2 concentrations between the two sites. Over the winter period the CO2 concentration in the city centre was on average 5 / a l l -1 higher. The unusually high average SO 2 concentration for January at the rural site can be explained by the predominance of northerly winds during that month. Such winds bring emissions from both the local boiler house and Ratcliffe power station (a 2000 M W coal-burning station, 4 km to the north), over the site. Even if the January values are ignored,
(a) zxRural 38O
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Fig. 2. Average monthly concentrations of(a) CO 2 and (b) SO2 for the period December 1984-July 1985. The error bars show the standard deviation of the daily-averaged values recorded at each site.
2683
the average urban winter 802 concentration was only about 5 nl f - 1 greater than the rural value. Thus, over the winter months, the ratio of the difference between urban and rural CO2 concentrations (5/A ~ - 1) to the difference in SO 2 concentrations ( < 5 n l f -1) was more than 1000. This implies that the local emissions that contributed most to the higher urban CO2 concentration were from sources with high CO2:SO 2 emission ratios such as gas space heating and vehicle exhausts (Weber, 1970; DOE, 1985). The mean monthly concentrations might suggest that local sources usually make only small contributions to the CO2 and SO 2 concentrations at the two sites but they mask considerable differences in the diurnal variation between the sites. The mean diurnal variations of CO 2 and SO 2 for 2-month periods at the two sites are shown in Fig. 3. The curves pass through every mean hourly value and are confined between the limits of these values--i.e. they do not extend higher or lower than these values at the turning points. Therefore, although the mean hourly values are not marked they can be read directly from the curves 'on the hour'. During the winter months the monthly-averaged CO2 concentrations at the rural site (Fig. 3a) were almost constant at night but started to decrease soon after sunrise, reaching a minimum at about midday, before increasing again to the night-time value. Two processes may have contributed to the reduced CO 2 concentration during the day; the photosynthesis of the grass beneath the sampling point, and increased atmospheric turbulence during daylight hours depleting the CO2 produced by plant and soil organism respiration. At the urban site (Fig. 3b) a very different winter CO2 cycle was observed. This cycle had two m a x i m a - - a well-defined peak at dawn and a broader peak during the evening --separated by shallow troughs. The sharp increase in concentration just before dawn was probably due to the increase in emissions from local fossil fuel combustion at that time, as domestic and commercial space heating was switched on and traffic increased as people went to work. However, soon after sunrise as atmospheric turbulence increased, the local emissions were dispersed and the local CO 2 concentration decreased. In the evening, as the level of turbulence decreased, the reduced dispersion enabled the local CO2 concentration to rise again. At night, urban emissions were much reduced and thus the CO 2 concentration fell once more. At the onset of the growing season, the patterns of both the urban and rural CO 2 cycles started to change. At the rural site (Fig. 3a), the afternoon minimum of the monthly-averaged concentration showed a steady decrease from February to July and occurred at a slightly later hour in successive months• After February, the CO2 concentration did not exhibit the night-time 'plateau' observed in December and January; instead, it increased steadily from the afternoon minimum value, throughout the night, to peak just before sunrise. The increase in the daytime deple-
2684
R.D. BERRYand J. J. COLLS
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tion of C O 2 c a n be explained by the increased photosynthesis of the surrounding vegetation, due to the greater quantity of vegetation present and the higher temperature and level of solar radiation. This would be accompanied by increased plant respiration, and the higher soil temperature would increase the respiration of soil organisms. At night, in the absence of photosynthesis, these processes soon increased the ground-level CO2 concentration--the extent to which they did so depended on the rate at which air close to the ground was mixed with that at higher levels and thus on the degree of atmospheric stability (Berry, 1987).
In the city centre (Fig. 3b), the winter evening peak was not observed during the summer months; instead a cycle similar to that at the rural site but of smaller amplitude was found; the afternoon minima occurred slightly later in the day than at the rural site. The night-time maxima were still observed just before dawn but were slightly lower than the winter values• In the summer months, fossil fuel emissions in urban areas, particularly those from commercial and domestic space heating, are reduced and therefore a reduction in the CO2 concentration might be expected throughout the day. The increased photosynthesis and respiration of the vegetation both within and sur-
Carbon dioxide and sulphur dioxide--I rounding the urban areas might also be expected to have some effect. The study by Clarke and Faoro (1966) suggests that under conditions of nocturnal temperature inversion, the transport into an urban area of the high ground-level COz concentrations-found in summer under such conditions in the surrounding rural a r e a s - - can make a large contribution to the night-time urban COz concentrations. The fact that the monthly-averaged night-time peak values at the urban site were only slightly lower than the winter values strengthens this hypothesis. Under turbulent daytime conditions the effect at the urban site of plant activity in the local environment might be expected to be much smaller than under the more stable conditions found at night. This is confirmed by the daytime CO2 concentrations observed at the urban site: these only very rarely fell below the tropospheric background level measured at stations remote from local sources and sinks (Bolin et al., 1979, 1986) and therefore were probably little affected by local plant photosynthesis. The monthly-averaged daytime (sunrise to sunset) and night-time (sunset to sunrise) CO 2 concentrations at the two sites are shown in Fig. 4a. When comparing this figure with Fig. 2 it must be remembered that, in England, the day length in July is twice that in December. Average daytime CO z concentrations at the urban site were approximately 8 #1~-1 greater than at the rural site throughout the year. The diurnal SO2 variations did not have such welldefined characteristics (Figs 3c and 3d). However, the (a)
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2685
daytime concentrations were substantially higher than those observed at night at both sites (Fig. 4b). This difference has been observed before (e.g. Maul et al., 1980), and is normally attributed to increased emissions and increased atmospheric turbulence during the day. The sharp increase at dawn observed at the rural site supports this explanation. It appears that at night, under stable atmospheric conditions, SO2 is depleted at ground-level, then when the stable layer breaks up after dawn, fumigation quickly increases the concentration close to the ground.
MAN-MADEEMISSIONS
The results presented above show that local variations in the CO2 concentration at the rural site are dominated by natural sources and sinks during the summer months and that these sources probably make a substantial contribution to night-time CO 2 levels in the city centre. During the winter months man-made emissions are at their greatest, and local natural contributions to CO2 levels are much reduced. Therefore only this period (from December 1984 to March 1985) was used to investigate the dispersal of these emissions and in particular to look at the correlation between CO2 and SO2 originating from local sources. To determine this correlation a 'background' level of CO2 is needed which can be subtracted from the measured CO 2 concentration to find the 'excess' CO2 due to local sources. One approach would be to measure the CO 2 concentration of the airstream entering the local environment. However, this is impracticable, and thus the approach adopted was to take as the background concentration for each hour of the day, the minimum concentration recorded for that hour at either site in the particular month; this was then subtracted from the concentrations measured at that hour on other days in the month to give the excess CO2 concentration. These minimum values for each month almost invariably occurred at the rural site on days when atmospheric stability was neutral and the SO2 concentration at the site was almost zero. Note that this was not the case during the summer months when the lowest daytime minima occurred under unstable conditions----on days with high solar radiation and low wind speeds. For the winter months, the minimum hourly CO2 concentrations at both sites were close to the background tropospheric concentration; for example, for December they ranged from 331 to 341pig -~ at the rural site and from 337 to 343 #1 f - ~ at the urban site. There was invariably one day in the month when the SO 2 concentration was not significantly above zero for each hour and thus there was no need to calculate 'excess' SO2 concentrations. Scatter diagrams of the absolute SO2 concentration versus the excess CO2 concentration at the two sites are shown in Figs 5a and 5b. The coefficients of correlation for the rural and urban sites are 0.37 and 0.90, respectively (both significant at the 0.1% level),
2686
R.D. BERRYand J. J. CELLS
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i.e. for both sites there is a definite positive relationship between the excess CO2 concentration calculated by the method described above and the absolute SO2 concentration. The correlation is much stronger for the urban site because it had a much lower occurrence of periods of very low SO 2 concentration ( < 5 nl : - 1) and was more remote from local natural sources. CONCENTRATION
ROSETTES
Qualitative assessments of the dependence of CO2 and SO2 concentrations on wind direction and of the relative importance of local and distant sources were made by calculating average concentrations of the gases at each site when the wind was from within 22.5 ° sectors. The main criterion adopted was that the wind direction measured at a height of 6 m at the rural site should not vary by more than 40 ° over a period of 6 h before the measurement was taken. Furthermore, the wind speed had to be greater than 2 m s-1 over this period because of the high sticking speed of the wind-vane. These criteria allowed almost 50% of the hourly measurements to be included in the rosette calculations. If it is assumed that wind direction was constant over the local environment, the fact that hourly measurements were not included in the rosette calculations if they occurred after 6-h periods when the wind dropped below 2 m s - 1 ensures that the only contributions from sources within 4 0 k m (2 m s - ~ x 6 h) of the monitoring site were from sources within the relevant sector. No allowance was made for washout of SO2 by rain. Concentration rosettes for excess CO 2 and absolute SO2 over the winter period are shown in Fig. 6,
together with a rosette showing the number of hourly values used to calculate the average concentrations. F o r most ofthe steady 6-h periods after which concentrations were used to calculate the rosettes, the wind was from either the south-west or north-east. Average CO 2 concentrations were higher in the city centre than at the rural site when tbe wind was from almost all directions, but when the wind was from the north, SO 2 concentrations were higher at the rural site. This can be explained by the fact that the most important local sources north of the rural site, Ratcliff¢ power station and the boiler house, both have a relatively high SO2:CO2 ratio compared with average urban emissions which are dominated by gas and petrol combustion processes. A prominent feature is the large average concentration from the south-east on all four rosettes. The similarity in concentration at the two sites suggests that the source of the pollution is distant and, as there is no obvious U.K. source in that direction, possibly of continental origin. Note that the number of hourly values used to calculate the sector concentrations varies greatly between sectors and thus a particularly high average concentration for a sector, for example the ESE-SE sector, does not necessarily indicate that it had a higher maximum concentration nor indeed longer periods of high concentration, than adjacent sectors with lower average concentrations. At the rural site, CO2 concentrations were considerably greater from the north-east than from the southwest, as expected, given the location of local urban areas. When the wind was from the south-west both CO2 and SO 2 concentrations were much higher at the urban site than at the rural site. Ratcliffe power station and intervening light industrial and urban domestic
Carbon dioxide and sulphur dioxide--I
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2687
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Fig. 6. Concentration rosettes for SO2 and excess CO2 at the rural and urban sites for the period December 1984-March 1985.
sources probably all made significant contributions. Graphs of cumulative frequency of occurrence vs atmospheric SO2 concentration are presented in Fig. 7 in standard log-normal form. The greater occurrence of high SOz concentrations at the rural site than the urban site can be explained by its proximity to large sources. DISCUSSION The annual cycles of the ground-level CO z concentration at both sites showed a spring maximum and autumn minimum and were therefore approximately in phase with the background tropospheric concentration cycle. If it is assumed that both cycles had almost reached their minimum value by the end of July, then each had an amplitude of about 20/A f-1--twice the background value (Bolin et al., 1979, 1986). Variations in the monthly-averaged concentration in consecutive
months at each site were of the same magnitude as the differences found between the sites. Over the summer months (April-July) there was no systematic difference in the monthly-averaged CO2 concentrations between the sites; over the winter period (December-March) the concentrations at the city centre were, on average, 5 #1 g - 1 higher than at the rural site. However, when interpreting these values it must be remembered that there were large sources close to the rural site and that the pattern of the diurnal variation at the two sites was very different. In the winter months, differences in the night-time monthly-averaged CO2 concentrations between the two sites were small. In December and January, the night-time concentration in the city centre was, on average, 8 #1 #- t higher than at the rural site, but by the end of February it was slightly higher at the rural site. As the year progressed the urban concentrations fell steadily, whilst the rural concentrations, although
2688
R.D. BERRYand J. J. COLLS
L
Acknowledoements--R.D.B. was supported by a National Westminster Bank Research Grant. Dr C. R. Gentle (Trent Polytechnic) helped set up the urban monitoring site.
. = - - 0 , - - - Rural Urban ~3
102 ~-
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oY
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101
102
REFERENCES
101
2
100 I
1.0
1
10.0
I
50.0
I
90.0
I
99.0
I
99.9
Cumulative Frequency / %
Fig. 7. Hourly SO 2 concentration vs cumulative percentage frequency of occurrence for the period December
1984-July 1985. varying greatly from day to day, showed an increasing trend for the remainder of the monitored period. The daytime C O 2 concentration in the city centre was on average 8/~1 # - ~ higher than at the rural site throughout the year. The dependence of SO 2 concentrations on the proximity of local combustion sources to the point of measurement makes it difficult to compare the concentrations found in this study with those observed at other sites. As with the C O 2 concentrations, variations in the monthly-averaged SO 2 concentrations in consecutive months at each of the continuously monitored sites were of the same magnitude as differences between the sites. Concentrations in the city centre were not, on average, greater than those measured at the rural site but they were much less dependent on wind direction. The most probable explanation is that concentrations at the city centre site were dominated by the many small local sources, uniformly distributed throughout the surrounding urban area, whilst a large contribution was made to the concentrations at the rural site by a few large sources close to the site. This hypothesis is supported by the graphs of cumulative frequency of occurrence vs SO2 concentration for the two sites (Fig. 7), which show that concentrations were higher at the city centre site for most of the monitored period but that there were more high-concentration episodes at the rural site. The implications of the measured levels of CO2 and SO 2 for plant growth are discussed briefly at the end of Part II of this paper.
Allen K. L. H., Jones P. and Jones J. W. (1985) Rising atmospheric CO2 and evapotranspiration. In Advances in Evapotranspiration, pp. 13-27. Am. Soc. Ag. Engs, Mich., U.S.A. Baker C. K., Coils J. J., Fullwood A. E. and Seaton G. G. R. (1986) Depression of growth and yield in winter harley exposed to sulphur dioxide in the field. New Phytol. 104, 223-241. Barker M. G. and Neighbour E. A. (1988) The control of water loss on pollutant-damaged plants. Asp. appl. Biol. 17, 133-140. Berry R. D. (1987) Atmospheric CO 2 and SO 2 in the Nottingham area. M. Phil. Thesis, University of Nottingham. Bolin B., Degens E. T., Kempe S. and Ketner P. (1979) The global carbon cycle, SCOPE Report No. 13. Wiley, New York. Bolin B., Doos B. R., Jager J. and Warrick R. A. (1986) The greenhouse effect, climatic change, and ecosystems, SCOPE Report. No. 29. Wiley, New York. Carlson R. W. and Bazzaz F. A. (1985) Plant response to SO2 and CO 2. In Sulphur Dioxide and Veoetation (edited by Winner W. E., Mooney H. A. and Goldstein R. A.). Stanford University Press, Stanford. Clark W. C. and Faoro R. B. (1966) An evaluation of CO 2 measurements as an indicator of air pollution. J. Air Pollut. Control Ass. 16, 212-218. Crane A. J. (1985) Possible effects of rising CO2 on climate. Plant, Cell Envir. 8, 371-379. Cure J. D. and Acock B. (1986) Crop responses to carbon dioxide. Agric. Forest Met, 38, 127-145. DOE (1985) Digest of Environmental Protection and Water Statistics. Department of the Environment, H.M.S.O. Kimball B. A. (1983) Carbon dioxide and agricultural yield: an assemblage and analysis of 430 prior observations. Agronomy J. 75, 779-788. Last F. T. (1982) Effects of atmospheric sulphur compounds on natural and man-made terrestrial and aquatic ecosystems. Agric. Envir. 7, 299-387. Lucas P. W. (1990) The effects of prior exposure to sulphur dioxide and nitrogen dioxide on the water relations of Timothy grass (Phleum pratense) under drought conditions. Envir. Pollut. (submitted). Maul P. R., Barber E. R. and Martin A. (1980) Some observations of the meso-scale transport of sulphur compounds in the rural East Midlands. Atmospheric Environment 14, 339-354. Oberbauer S. F., Strain B. R. and Fletcher N. (1985) Effect of CO 2 enrichment on seedling physiology and growth of two tropical species. Physiol. Plant. 65, 352-356. Roberts T. M. (1984) Effects of air pollutants on agriculture and forestry. Atmospheric Environment 18, 629~152. Turco R. D., Whitten R. C., Toon O. B., Inn E. C. Y. and Hammill P. (1981) Stratospheric hydroxyl radical concentrations; new limitations suggested by observations of gaseous and particulate sulphur. J. geophys. Res. 86, C2, 1129-1139. Weber E. (1970) Contribution to residence time of SO 2 in a polluted atmosphere. J. oeophys. Res. 75, 2909-2914.