Q. J. R. Meteorol. Soc. (2004), 130, pp. 1957–1971
doi: 10.1256/qj.03.125
Oxidized nitrogen and ozone interaction with forests. II: Multi-layer process-oriented modelling results and a sensitivity study for Douglas fir By J. H. DUYZER1∗ , J. R. DORSEY2 , M. W. GALLAGHER2 , K. PILEGAARD3 and S. WALTON2 1 TNO-MEP, Apeldoorn, the Netherlands 2 Physics Department, UMIST, Manchester, UK 3 Risø National Laboratory, Roskilde, Denmark (Received 15 July 2003; revised 2 December 2003)
S UMMARY An existing process-oriented multi-layer canopy model is applied to data from an intensive NOx and O3 surface exchange experiment, and a sensitivity study is conducted. The canopy was a mature 22.5 m Douglas fir stand. Comparison of measured data and model results shows that the model represents the concentration and fluxes of ozone well above the canopy, with adequate accuracy for concentration and fluxes below the canopy. A similar pattern is demonstrated for NO2 concentration above and below a forest canopy, with the fluxes being calculated correctly to within at least an order of magnitude below the canopy, and more accurately above. The model can reproduce the processes leading to the observed NO2 emission from forest stands. The sensitivity study demonstrates the complex interdependence of oxidized nitrogen flux controlling variables within the canopy, with NO2 emission favoured by high NO soil emission and canopy resistance and by low global radiation, leaf area index and ambient NO2 concentrations. Realistic alterations of these variables can cause reversal of the NO2 flux, leading to an ‘ecosystem compensation point’ between 17 and 35 ppbv at night, and 5 to 10 ppbv during the day for the forest canopy investigated. This highlights our improved understanding of the controls on NO2 flux, explaining and quantifying some previously reported reversals in NO2 flux above forest canopies. The effect of soil NO emission on ozone flux is investigated. During the day, reaction with NO may account for only 10% of observed ozone deposition, however, at night, this figure can rise to around 50%. The effect of volatile organic compounds on forest ozone deposition was found not to be large. K EYWORDS: NOx emission
O3 deposition
1.
I NTRODUCTION
Previous micrometeorological measurements of nitrogen dioxide (NO2 ) and nitric oxide (NO) dry deposition to vegetated canopies have shown confusing or apparently conflicting results (Hicks et al. 1986; Hicks and Matt 1988; Wesely et al. 1989). The lack of consistency in the flux measurements of nitrogen oxides (collectively termed NOx ) partly relates to classical problems such as advection from nearby sources and a poor signal-to-noise ratio from instrumentation. These problems are compounded by the fact that NOx undergoes rapid photochemical and interconversion reactions with O3 , over a time scale which is comparable to that of turbulent mixing in the atmospheric boundary layer (van Aalst 1982; Duyzer et al. 1983; Galmarini 1997). This can lead to a perturbation in the observed flux; trace gases are created or destroyed in the layer of air between the observation point and the surface. Over forests, flux measurements are more complex and harder to interpret than for other vegetation types because of potential inaccuracies in the ‘K-theory’ approximation near to the top of rough canopies (e.g. Thom 1975; Raupach 1979; Fowler and Duyzer 1989). The above difficulties, which are evident during NO2 and O3 flux measurements, make interpretation of reactive trace gas fluxes extremely difficult, leading to substantial uncertainties when extrapolating field-scale results up to regional-scale fluxes. With this ∗ Corresponding author: TNO-MEP, PO Box 342, 7300 AH, Apeldoorn, the Netherlands. e-mail:
[email protected] c Royal Meteorological Society, 2004.
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in mind, a model has been constructed to describe the chemistry, turbulent transport, canopy exchange and surface emission of reactive oxidised nitrogen and ozone. The model is used to interpret flux data from a field campaign on a case-study basis, and a sensitivity analysis conducted. Previous modelling work to investigate the influence of chemistry on NO, NO2 and O3 flux profiles has been carried out by several other groups (e.g. Lenschow 1982; Duyzer et al. 1983; Fitzjarrald and Lenschow 1983; Kramm et al. 1991; Gao et al. 1991; Duyzer 1992). All groups reported that chemical reactions affected flux profiles near the surface, especially those of NO and NO2 . The impact of chemistry on O3 fluxes was generally found to be small compared with its total flux, although here we find otherwise under certain circumstances. Fitzjarrald and Lenschow (1983) and Kramm (1989) demonstrated that flux divergences occurred at heights of only a few metres above a surface. Kramm et al. (1991) outlined the experimental requirement for simultaneous eddy correlation trace gas flux measurements made from at least two heights. These multi-height flux measurements can then be coupled together with separate chamber measurements for a final model evaluation. Gao et al. (1991) showed the dependence of the photostationary state ratio on all the flux profiles and also indicated the importance of emissions of NO from the soil to the flux profiles of NO and NO2 . Most groups were also able to show that NO2 emission and bi-directional fluxes can occur under certain conditions. It is worth noting that the reason for the slow development of models describing NOx and O3 exchange with forests is primarily the lack of high quality experimental observations of the exchange processes. This has, in the past, precluded thorough testing of such process-oriented canopy models. The aim of this paper is to continue to address this issue following the work cited above, and other studies (e.g. Duyzer and Weststrate 1995; Duyzer et al. 1995a,b; Walton et al. 1997). Models of this type are useful both to develop a better understanding of the physics of canopy transport, and for use as ‘parents’ for simpler parametrizations in large-scale models. 2.
M ULTI - LAYER CANOPY MODEL
The original basis for the model used in this paper is described by Baldocchi (1988). It was designed to produce estimates of SO2 dry deposition to a deciduous oak forest, and was adapted for a coniferous forest and organic pollutants by Deinum et al. (1995). The extended model, the results of which are presented here, simulates NOx , O3 and hydrocarbon reactions above, within and below a coniferous forest canopy. It also provides for emission of NO from the soil at the forest floor. This version of the model is identical to that described in Walton et al. (1997). However, here we apply the model to a large, mature stand of Douglas fir (Speulderbos, the Netherlands), as opposed to the small (height, h = 2.1 m) deciduous orchard described in the earlier paper. Accordingly, the full model description is not reproduced here. However, some of the important features of the model are reviewed below. The model uses first-order local turbulence closure (K-theory) to estimate turbulent diffusivities from a parametrization of the measured wind speed profile. Eight equally spaced layers from the surface to the canopy top are used, with a further 20 logarithmically spaced layers representing the overlying boundary layer. Canopy stomatal resistance is calculated as a function of air temperature, photosynthetically active radiation (PAR) and water vapour deficit. Concentrations of NOx (NO + NO2 ) and the reactive oxygen-containing species (Ox ; NO2 + O3 ) at the top of the boundary layer were chosen so that NO, NO2 and O3 concentrations matched the concentration at
OXIDIZED NITROGEN AND OZONE—INTERACTIONS WITH FORESTS. II TABLE 1. Parameter
S PEULDERBOS MODEL PARAMETERS Units Value
No. of layers below canopy No. of layers above canopy Canopy height, h Displacement height, d Roughness length, z0 Leaf length, l Leaf area index (LAI) Cuticle resistances NO NO2 O3 Soil resistances Rb + RNO,soil Rb + RNO2 ,soil Rb + RO3 ,soil Optimal uptake temperature, T opt Minimum stomatal resistance, Rs,min Stomatal resistance curvature coefficient, brs
– – m m m m – s m−1
s m−1
◦C
s m−1 W m−2
8 20 22.5 15.0 2.2 0.02 10.4
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Reference
– – Jans et al. (1994) Duyzer and Weststrate (1995) van Ek and Draaijers (1991) van Ek and Draaijers (1991) van Ek and Draaijers (1991) Wesely (1989)
5000 2500 2500 Measured 5000 600 1000 17 210 25
Measured Steingrover and Jans (1994) Measured
the 35 m measurement level. The NO2 photolysis rate, jNO2 , was determined using the measured global radiation, G, according to Bahe et al. (1980), and the NO + O3 reaction rate constant, kNO2 , was determined as a function of the measured temperature after DeMore et al. (1983). The NO soil emission in the model was set constant at the average measured value of 23 ng N m−2 s−1 . NO2 and O3 deposition to soil were likewise taken from enclosure measurements. Constants specific to Speulderbos, which are used in the model, are presented in Table 1. In the case of the Speulderbos runs, monoterpenes react with ozone according to a separate set of rate coefficients. Data from the 1993 Speulderbos experiment (Dorsey et al. 2004) were used as direct model input to try to reproduce the observed NOx and O3 fluxes both above and below the canopy. Model results along with a sensitivity study are used here to interpret the Speulderbos observations and obtain a clearer picture of the processes occurring at the atmosphere–forest interface. The field campaign, which involved European groups including the Risø National Laboratory (Denmark), the Organization for Applied Scientific Research (TNO, the Netherlands) and the University of Manchester Institute of Science and Technology (UMIST, UK), was conducted over the Speuld forest in the central Netherlands from 20 June to 2 July 1993. The site consisted of a monoculture of Douglas fir trees, with an average height of 22.5 m. A more detailed site and experimental description is given in Dorsey et al. (2004). NO soil emission was measured using chambers located on the forest floor. Continuous flux and gradient measurements of NOx and O3 were made above and within the canopy, and inside the trunk space, in order to study the effects of chemistry on the gas flux profiles. Most measurements showed NO2 emission above the canopy. Concentrations and fluxes in this paper are presented in units of mixing ratio (i.e. in ppbv and ppbv m s−1 , respectively). These were felt to be more appropriate units than, for example, µg m−3 or µg N m−2 s−1 , because comparisons are repeatedly made between ozone and nitric or nitrous oxide fluxes with reference to the atmospheric chemistry of these species. Given the 1:1 stoichiometry of the relevant chemical reactions, this allows more straightforward reference to be made to diagrams.
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M ODEL INPUT PARAMETERS FOR 29 J UNE 1993 Units Diurnal range
O3 Concentration (ppbv)
Ox concentration NOx concentration Solar (global) radiation, G Photosynthetically active radiation (PAR) Air temperature, T Wind speed at reference height, uzref Friction velocity at reference height, u∗zref NO soil flux, FNO,soil Water vapour density, Q Obukhov length, L
80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 00:00
ppbv ppbv W m−2 W m−2 ◦C m s−1 m s−1 ng N m−2 s−1 g kg−1 m
25 1 0 0 13.4 2.0 0.25 23.0 3.0 −150
– 80 – 13 – 850 – 350 – 26.0 – 4.5 – 0.9 – 23.0 – 6.0 – 500
Measured 7 m Modelled 7 m Measured 25 m Modelled 25 m
06:00
12:00
18:00
00:00
Time Figure 1.
Measured and modelled ozone concentrations at 7 and 25 m on 29 June 1993.
3.
M ODEL RESULTS
The model was run from 00h (local time) on 29 June to 00h on 30 June. Table 2 shows the variation in the Speulderbos measurement data used as model input parameters for 29 June. The modelling concentrates on this period due to the availability of reliable, continuous measurement data. Figures 1 and 2 show the O3 concentration and flux at the 7 and 25 m levels. The O3 concentrations at 25 m are in very good agreement with the data. The model is able to reproduce the O3 flux observed at 25 m to within 5% of the observed flux for the majority of the time. While this is encouraging, it is important to recall that the measured NOx and O3 concentrations at 35 m are used as model boundary conditions. It is therefore no large surprise that the model performs well at 25 m. At 7 m, the model concentrations show more deviation from the measurements and the O3 flux below the canopy is overestimated. This is due in part to the difficulty in assessing the magnitude of the turbulent diffusion coefficient below the canopy. The below-canopy concentration gradient is larger than that above the canopy due to the reduced transport; the O3 concentrations and fluxes are therefore both difficult to predict accurately for every situation. It is important to note that only the vertical
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Figure 2. As for Fig. 1, but showing ozone fluxes.
NO2 Concentration (ppbv)
10 Measured 7 m Modelled 7 m Measured 25 m Modelled 25 m
8
6
4
2
0 00:00
06:00
12:00
18:00
00:00
Time Figure 3.
As for Fig. 1, but showing NO2 concentrations.
turbulent fluxes at the 7 m level are presented here. The advection fluxes at all levels are not included in the observations presented; we assume horizontal homogeneity of O3 concentration, as does the one-dimensional model. Figures 3 and 4 show the corresponding NO2 concentrations and fluxes for both 7 and 25 m levels. The NO2 concentrations at 25 m show a very close match with the data set. However, at 7 m the NO2 concentration is largely overestimated, showing a factor of two or three higher than the measurements. One possible reason for this overestimate of NO2 below the canopy is the constant NO soil emission which is defined in the model as being 23 ng N m−2 s−1 . The model does not simulate the large spatial and temporal variation in NO soil flux and therefore, under conditions of low NO emission,
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-1
NO2 Flux (ppbv m s )
0.06
Measured 7 m Modelled 7 m Measured 25 m Modelled 25 m
0.04
0.02
0.0
-0.02 00:00
06:00
12:00
18:00
00:00
Time Figure 4.
As for Fig. 1, but showing NO2 fluxes.
can produce a higher concentration of NO2 than would actually be observed below the canopy. Higher temporal resolution soil gas-flux measurements should therefore be a priority for future measurement campaigns. An alternative, more likely, cause for the overestimate in NO2 is that the modelled transport within the canopy is too rapid. This results in the noted simultaneous overestimation of both NO2 concentration and O3 flux, making chemical reaction of the two species within and below the canopy proceed at an unrealistic rate. This is an aspect of the model due to come under closer scrutiny when further experimental results become available. We also note here that the effects of the canopy on local stability (via radiation) are not included in the model. It is likely that the region below the canopy is more stable than the free boundary layer during the day, and hence that turbulent mixing is currently overestimated by the model. In any case, the major result of the model in its current form is its ability to correctly predict the direction of the NO2 flux. 4.
M ODEL SENSITIVITY STUDY
A sensitivity study is a useful analysis to show the response of the flux to input parameters. These parameters determine the size and direction of the NO2 flux and the degree of O3 uptake by the canopy. It also gives a first impression of the trends in results which could be expected for different ecosystems, where input parameters would change. Table 3 shows the default inputs for the sensitivity studies. (a) Soil NO emission below the forest canopy The model’s O3 and NO2 flux response to a changing NO soil emission (against z/h) is shown in Fig. 5. The soil emission is given in ng N m−2 s−1 , as this is a more familiar unit for soil fluxes. The interested reader can convert this unit to ppbv m s−1 by multiplying by 1.75. For zero NO emission, the NO2 flux above and below the canopy is entirely downward with a flux of −0.015 ppb m s−1 at z/h = 1.5, indicating a small amount of deposition. On increasing the NO soil flux to 10 ng N m−2 s−1 , the NO2 flux
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TABLE 3.
D EFAULT INPUT VALUES FOR THE SENSITIVITY STUDY
Parameter Ox NOx G PAR T uzref u∗zref NO soil flux Q L
Units
Range
ppbv ppbv W m−2 W m−2 ◦C m s−1 m s−1 ng N m−2 s−1 g kg−1 m
65 4 600 250 23.0 4.0 0.6 23.0 3.0 −100
2.5
-2 -1
NO2 (NO = 0 ng N m s ) -2 -1 O3 (NO = 0 ng N m s ) -2 -1 NO2 (NO = 10 ng N m s ) -2 -1 O3 (NO = 10 ng N m s ) -2 -1 NO2 (NO = 35 ng N m s ) -2 -1 O3 (NO = 35 ng N m s )
2.0
z/h
1.5
1.0
0.5
0.0 -0.4
-0.3
-0.2
-0.1
0.0
0.1
-1
Flux (ppbv m s ) Figure 5. Sensitivity of O3 and NO2 fluxes to NO soil flux.
undergoes a change of sign, showing an upward flux below z/h = 1.25 with a downward flux above this height of −0.006 ppb m s−1 . When the soil emission flux in the model is raised once again to 35 ng N m−2 s−1 , the NO2 flux shows only emission at all heights. The flux is at its largest at the canopy base and decreases, in this case, to an almost stable emission flux of 0.01 ppb m s−1 above a height of z/h = 1.5. The NO soil emission has a large effect on the magnitude and direction of the NO2 flux. High emissions of NO from the soil are more likely to show upward fluxes of NO2 above the canopy. In the model runs, upward fluxes of NO2 could only be simulated when an NO soil emission was assumed. With zero NO soil emission no upward flux of NO2 is observed at any height. (b) Forests as a source of NO2 The relationship between NO soil emission and NO2 emission fluxes can be understood by studying the chemistry above and below the canopy. NO, which is emitted from the soil, reacts with O3 in the trunk space and crown to form NO2 beneath the canopy. Typical global radiation levels are below 10 W m−2 in this region at Speulderbos, so the NO2 created via the reaction of O3 and NO is not readily photolysed.
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The turbulent transport coefficients of pollutants below the canopy are much lower than those above, resulting in a small build-up of NO2 ; concentrations below the canopy are larger than those observed above during NO2 emission. As NO2 passes through the canopy, the radiation levels increase and NO2 photolyses into NO. A smaller emission of NO2 is detected above the canopy, and this decreases with height. The change in NO2 flux above and below the canopy is proportional to the NO soil flux, with FNO2 (↑) being the NO-induced upward component of the total NO2 flux: FNO2 (↑) ∝ FNO,soil .
(1)
(c) A sink for O3 The O3 flux is also enhanced with an increasing NO soil emission due to the extra sink for O3 below the canopy provided by the NO + O3 reaction. A soil flux rising from zero to 23 ng N m−2 s−1 produces a 10% increase in the O3 deposition flux; see also the experimental estimates of the fate of deposited ozone in Dorsey et al. (2004). During the day, the needle stomata provide a much larger sink for O3 than the chemical sink of NO below the canopy. However, at night and when stomata are expected to be closed, the destruction of O3 by NO becomes a more important deposition pathway. Since no NO2 is photolysed into NO at night, almost all the NO emitted from the soil reacts with O3 . This leads to an approximation for total nocturnal O3 flux above the canopy (in mixing ratio units, e.g. ppbv m s−1 ): FO3 ,noct ≈ FO3 ,surf + FNO,soil .
(2)
(d) NO2 concentration above canopy The ambient NO2 concentration above the canopy has a strong influence on the NO2 flux, and is shown in Fig. 6. At high concentrations, there is an NO2 deposition flux seen above the canopy. This deposition far outweighs the effects of soil NO emission when the perturbation made to NO2 concentration via the NO + O3 reaction in the trunk space is small compared to the absolute concentration aloft. At 20 ppbv, the above-canopy NO2 flux is entirely downward. On reducing the above-canopy concentration from 20 to 10 ppbv, the above-canopy deposition flux is reduced by more than 50%, with a greater upward flux of NO2 in the trunk space. At lower concentrations, the NO2 flux is directed upwards at all heights. In this case, the NO soil emission becomes much more important because it creates a large fractional change in the above-canopy NO2 concentration, causing a reversal in the NO2 gradient. Where there is a constant NO soil flux, the deposition of NO2 to the canopy is proportional to its concentration aloft, the relationship having a non-zero offset (discussed later): FNO2 (↓) ∝ CNO2 . (3) (e) Factors controlling canopy uptake The O3 flux is enlarged by almost 200% for a threefold reduction in the minimum stomatal resistance. The above-canopy NO2 flux undergoes a sign reversal from emission to deposition when the minimum stomatal resistance is lowered from 300 to 100 s m−1 . In the trunk space, the NO2 flux remains unchanged since no needle stomata are present below z/h = 0.5. These effects are shown in Fig. 7. For cases where there is a high stomatal resistance, the canopy becomes much less able to absorb any NO2 which is produced in the trunk space, causing NO2 emission from the canopy and an upward NO2 flux to be observed at all levels (at low ambient
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2.5
2.0
z/h
1.5
1.0
0.5
0.0 -0.5
NO2 (CNO2 = 1 ppbv) O3 (CNO2 = 1 ppbv) NO2 (CNO2 = 10 ppbv) O3 (CNO2 = 10 ppbv) NO2 (CNO2 = 20 ppbv) O3 (CNO2 = 20 ppbv)
-0.4
-0.3
-0.2
-0.1
0.0
0.1
-1
Flux (ppbv m s ) Figure 6.
Sensitivity of O3 and NO2 fluxes to NO2 concentration.
2.5
2.0
z/h
1.5
1.0 -1
0.5
0.0 -0.5
NO2 (Rsmin = 100 s m ) -1 O3 (Rsmin = 100 s m ) -1 NO2 (Rsmin = 200 s m ) -1 O3 (Rsmin = 200 s m ) -1 NO2 (Rsmin = 300 s m ) -1 O3 (Rsmin = 300 s m )
-0.4
-0.3
-0.2
-0.1
0.0
0.1
-1
Flux (ppbv m s ) Figure 7. Sensitivity of O3 and NO2 fluxes to minimum stomatal resistance.
NO2 concentration). At lower resistances, NO2 is deposited on passing through the canopy and a diminished emission is observed aloft. At low canopy resistances, NO2 deposition above canopy is enhanced. Consequently, a low stomatal resistance may produce a bi-directional flux of NO2 , whereby an upward flux of NO2 is observed in the trunk space, switching to NO2 deposition above the canopy. The downward flux of NO2 above canopy is inversely proportional to the canopy resistance: FNO2 (↓) ∝
1 . Rc
(4)
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2.0
z/h
1.5
1.0 -2
0.5
0.0 -0.4
NO2 (G = 0 W m ) -2 O3 (G = 0 W m ) -2 NO2 (G = 250 W m ) -2 O3 (G = 250 W m ) -2 NO2 (G = 800 W m ) -2 O3 (G = 800 W m )
-0.3
-0.2
-0.1
0.0
0.1
-1
Flux (ppbv m s ) Figure 8. Sensitivity of O3 flux to above-canopy solar radiation.
The effect of solar radiation on the O3 and NO2 flux can be seen in Fig. 8. For a rise in radiation of 800 W m−2 , the O3 deposition flux above z/h = 1 is greatly enhanced, with a 350% increase in deposition from −0.1 to −0.35 ppb m s−1 . The NO2 emission above the canopy declines considerably with increasing radiation because of the greater uptake of NO2 by the canopy at higher PAR levels, as well as increased NO2 photolysis. At 800 W m−2 , there is a very small amount of deposition observed at twice the canopy height. Since little radiation penetrates the canopy to the forest floor, the below-canopy fluxes remain almost unchanged and only the upper half of the stand (z/h > 0.5) shows a significant response to radiation. Although the (single-sided) leaf area index (LAI) during the Speulderbos model runs remained fixed (= 10.4), the model shows a variation in flux for O3 and NO2 with a changing LAI. O3 deposition is enhanced with increasing LAI because of the greater canopy surface area available for deposition. Conversely, the NO2 flux results show that, for low LAI, more NO2 emission is observed above the canopy. NO2 produced in situ below the canopy by the NO + O3 reaction is unable to deposit completely to the available foliage, so a small emission occurs. This emission becomes less pronounced with an increased LAI. Again, the NO2 flux in the trunk space remains unaltered because of the near-zero leaf area density in this region. The wind speed at a reference height above the canopy dictates the amount of turbulent mixing present in the model, both above and below the canopy. O3 deposition is enhanced at all levels during high wind speeds due to the lower aerodynamic resistance to pollutant transfer between layers. The NO2 flux shows a slightly higher emission from the canopy with rising wind speeds, because of an increase in the NO2 mixing below the canopy. The optimal stomatal uptake temperature for Douglas fir is between 20 and 25 ◦ C (Steingrover and Jans 1994). Above and below this level there is a reduction in the canopy uptake as stomata are not fully open. A fall in temperature from 20 to 15 ◦ C produces a corresponding drop in O3 flux of around 10%. As before, in the presence
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2.5 O3 (VOC = 0 ppbv) O3 (VOC = 1 ppbv) O3 (VOC = 3 ppbv)
2.0
z/h
1.5
1.0
0.5
0.0 -0.4
-0.3
-0.2
-0.1
0.0
-1
Flux (ppbv m s ) Figure 9.
Sensitivity of O3 flux to monoterpene concentrations.
of soil NO emission, the NO2 flux tends towards emission with a decrease in canopy uptake. ( f ) Hydrocarbon reactions The concentration of four major monoterpenes was determined in the air at Speulderbos: α-pinene, β-pinene, 3-carene and limonene. Several monoterpenes undergo a temperature-dependent reaction with O3 (e.g. Atkinson et al. 1982; Atkinson and Carter 1984). Several sensitivity runs were carried out using different monoterpene concentrations, the significant results of which are included in Fig. 9. There is little or no change observed for NO2 fluxes at all levels. O3 fluxes indicate a slight divergence of less than 10% at 50 m above the canopy for a total monoterpene concentration of 3 ppbv. Below the canopy there is almost no change in the estimated flux, since the O3 reaction with NO is much faster than for monoterpenes, and therefore plays a dominant role. Total monoterpene concentrations at Speulderbos were always lower than 1 ppbv, which produced less than a 5% change in the observed flux above the canopy. 5.
S UMMARY OF SENSITIVITY STUDY
No upward flux of NO2 is observed in the model when zero NO emission is present at the forest floor. It has been shown that the concentration of NO2 aloft determines not only the size of NO2 deposition to the canopy, but also the extent to which the NO soil emission perturbs the above-canopy NO2 flux. It is also found that the canopy resistance to uptake is an important factor for both emission and deposition events. NO2 emission from the canopy is enhanced when the canopy uptake is low (i.e. when stomata are closed), but deposition is enhanced by higher canopy uptake. This leads to an equation for the estimation of the observed NO2 flux above canopy which is a function of the NO soil flux, the NO2 concentration and the canopy resistance: 1 FNO2 = f FNO,soil − CNO2 − . (5) Rc
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As suggested by the sign of each term in Eq. (5), the NO soil flux causes all observed NO2 emission. The negative terms (CNO2 and Rc ) denote a contribution towards observed NO2 deposition. Large NO2 emission (FNO2 0) is observed when the NO soil flux and the canopy resistance are high and the NO2 concentration aloft is low (10 ppbv). The most common case is when the NO2 flux is low and positive, or close to zero. This occurs when FNO,soil is marginally larger than the deposition terms CNO2 and Rc , i.e. moderate NO2 concentration and canopy resistance. A compensation point is observed when both the emission and deposition terms are equivalent. Although this would conventionally be referred to as a ‘canopy compensation point’, the lack of emission of NO2 from the canopy itself suggests the use of a different term; we use ‘ecosystem compensation point’, reflecting the fact that determinants include the canopy, soil NO emission and boundary layer NO2 concentration. This can cover a wide range of concentrations for a changing canopy resistance and NO soil emission rate. Typical nocturnal ecosystem compensation points, for a soil flux of 20 ng N m−2 s−1 and for a canopy resistance of between 500 and 1000 s m−1 , range from 17 to 35 ppbv, which is not an uncommon observation at night. For an identical soil emission rate, the daytime ecosystem compensation points range from between 5 and 10 ppbv for resistances of 150 to 300 s m−1 . Such concentrations rarely occurred during the day at Speulderbos throughout the experimental period; lower concentrations dominated, hence NO2 emission was the most common observation. At Speulderbos the maximum nocturnal concentrations of NO2 reached about 25 ppbv, hence deposition was much more likely to be seen at night than during the day. This has been confirmed since this experiment by Duyzer and Weststrate (1995). As a first approximation, the behaviour of the three major determinants of NO2 flux above the canopy, as described by Eq. (5), can be represented by linear relationships. These terms should be approximately independent of each other, and should be able to show the contribution of each variable to the observed ecosystem compensation point by solution at zero flux. All have the form FNO2 = m · V + F0 , where m is some measure of the effect of the variable on NO2 flux, V is the variable considered and F0 the extrapolated value of the NO2 flux where the variable’s value is zero. We note here that, although the relationships are undetermined at present, the gradient of the first term with respect to NO2 flux is positive, while the second two terms have negative gradients. This approach will be pursued to allow parametrization of the multi-layer model results for use in regional models. 6.
C ONCLUSIONS
The performance and limitations of a multi-layer model describing ozone and reactive nitrogen transport above and below a forest canopy have been reviewed. Factors affecting transport, emission and deposition of these gases have been investigated, and conditions favourable to NO2 emission from the canopy identified. The effect of NO emission on O3 deposition during the day is found to be marginal, with around 10% of the observed O3 flux being due to reaction with NO. At night this figure is more significant, with nitrogen chemistry sometimes accounting for 50% of O3 flux (cf. the experimentally determined figure of 41% (Dorsey et al. 2004)). The effect on O3 flux of the volatile organic compound (VOC) species included in the model was negligible within and below the canopy; VOC reactions caused less than 10% of the O3 flux at all times.
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The bi-directionality of NO2 flux above the canopy can be described by an ‘ecosystem compensation point’, which is dependent partly on canopy structure and global radiation levels. The compensation point was found to vary between 5 and 10 ppbv during the day, and between 17 and 35 ppbv at night. The variation in these figures is predominantly due to uptake of NO2 by the canopy, and variable soil NO emission. This adequately explains the previously noted reports of bi-directional NO2 fluxes above forests. 7.
F URTHER WORK
The model will be modified in the light of new experimental results currently under analysis. These data will allow a more critical analysis of the ‘K-theory’ assumption, and provide an experimental constraint on the NO flux in the trunk space, and NO concentration at all heights. Improvements to the treatment of within-canopy transport are anticipated, possibly by using measured values of friction velocity and stability. Model output will be parametrized to improve a ‘two layer’ model currently under development for inclusion in regional models. ACKNOWLEDGEMENTS
The data presented here were originally compiled by Sam Walton. The experimental study (FOREXNOX) was funded by the European Commission, DGXII contract number EV5V-CT92-0060. Sam Walton received a grant from the European Environmental Research Organisation, Wageningen, the Netherlands, for a period of the work. Further work on this paper was undertaken as part of an EU Framework V project, NOFRETETE (EVK2-2001-00033). The authors wish to express their heartfelt sympathy to the family of Sam Walton following his untimely death shortly after his PhD graduation. Much of the work presented here was the result of his boundless enthusiasm. His good-natured approach to life is sadly missed by all who worked with him. R EFERENCES Atkinson, R. and Carter, W. P. L.
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