Detlef R. Matt

A preliminary multiple resistance routine for deriving dry deposition velocities from measured quantities

Because there is no simple device capable of measuring the dry deposition rates of small particles and trace gases directly, much current activity is focused on the use of an inferential technique. In this method, measurements of atmospheric concentration (C) of selected chemical species are coupled with evaluations of appropriate deposition velocity (Vd) to yield estimates of dry deposition rate from their product. Difficulties arise concerning the ability to measure C, and especially regarding the poor knowledge of Vd for many species. A multiple resistance routine for deriving deposition velocities is presented here. Current knowledge of biological processes is incorporated into a first-generation lsbig leaf’ model; formulations of resistances appropriate for describing individual leaves are combined to simulate the canopy as a whole. The canopy resistance is combined with estimates of aerodynamic and boundary-layer resistances to approximate the total resistance to transfer, from which deposition velocity is then computed. Special emphasis is given to the influence of the diurnal cycle, to the way in which the various transfer resistances can be inferred from routine data, and to the role of canopy factors (e.g., leaf area index, wetness, temperature response, and sunshade fractions).

Eddy fluxes of CO2, water vapor, and sensible heat over a deciduous forest

Fluxes of CO2, latent heat and sensible heat were measured above a fully-leafed deciduous forest in eastern Tennessee with the eddy correlation technique. These are among the first reported observations over such a surface. The influences of solar radiation, vapor pressure deficit and the aerodynamic and canopy resistances on these mass and energy exchanges are examined. Following a concept introduced by McNaughton and Jarvis (1983), examination of our data suggest that the water vapor exchange of a deciduous forest is not as strongly coupled with net radiation as is that of agricultural crops. The degree of decoupling is smaller than in the case of a coniferous forest. This difference may be attributable in part to the greater aerodynamic resistance to water vapor transfer in a deciduous forest. It appears that the concept of decoupling may be extended to the CO2 exchange of a deciduous forest as well.

The Distribution of Solar Radiation within a Deciduous Forest

Solar radiation was measured within and above an east Tennessee deciduous forest over a 2-yr period. Diurnal patterns of within-forest radiation follow the temporal variation in incident radiation but become more irregular with depth in the forest because of the highly variable penetration of beam radiation in space and in time. Seasonally, radiation in the forest increases substantially from winter to its annual maximum in early spring as solar elevations increase. Although solar elevations continue to rise each day until the summer solstice, amounts of total radiation and its beam component drop sharply in the forest with the advent of leaf expansion. Diffuse radiation in the forest continues to increase slowly follow- ing the onset of leaf expansion but reverses as the forest approaches a fully leafed state. Fol- lowing the summer solstice, forest structure remains essentially static until abscission while solar elevations decrease. In summer and early autumn, total radiation and both its beam and diffuse components decrease slowly in the forest to their annual minimum in autumn. With leaf abscission and subsequent fall, radiation increases slightly in the forest but soon declines again as solar elevations approach their annual minimum of the winter solstice.

Solar radiation within an oak—hickory forest: an evaluation of the extinction coefficients for several radiation components during fully-leafed and leafless periods

Global shortwave, photosynthetically active, net and allwave radiation was measured above, and at several levels within an oak—hickory forest with instruments mounted on a moving tram system. Profiles of radiation flux densities were quantified using extinction coefficients based on the Beer—Bouguer law. Data are reported here from periods when the forest was both fully-leafed and leafless. In the fully-leafed forest the solar radiation components are attenuated exponentially in the following manner: PAR > Q∗ = K↓ > Q↓;, where PAR is photosynthetically active radiation, Q∗ is net radiation, K↓ is shortwave radiation and Q↓ is allwave radiation. PAR attenuation is greater than that for the other components because leaves preferentially absorb PAR. This preferential absorption causes the ratio, PAR/K↓, to decrease from 0.49 above the canopy to 0.27 at the forest floor. During the leafless phenoseason, the radiation components are attenuated exponentially as follows: Q∗ > K↓ = PAR > Q↓. K↓ and PAR are attenuated in a similar manner during this phenological phase because no leaves are present to absorb PAR preferentially. The magnitude of the attenuation coefficients for Q∗, K↓ and PAR is much greater during winter leafless period because solar elevation angles are lower and the canopy consists of dark, opaque, woody biomass. Shortwave beam radiation is not attenuated in an exponential manner. Consequently, extinction coefficients for beam radiation, γ(S), were computed separately for the upper canopy and lower canopy. A comparison between measured and modeled γ(S) show periods of reasonable agreement and disagreement. Deviations from theory are attributed to clumping and gaps in the canopy.

A micrometeorological investigation of surface exchange of O3, SO2 and NO2: A case study

Data obtained in an intensive field study of the dry deposition of sulfur dioxide, ozone, and nitrogen dioxide, conducted in 1985 in central Pennsylvania, are used to illustrate the factors that must be considered to assure that high quality results are derived. In particular, the quality of the site must be such that flux measurements made above the surface are representative of surface values. For this purpose, tests involving momentum transfer and the surface energy budget are especially useful. In addition, conditions must not be changing rapidly, and the statistical uncertainty associated with flux measurement must be low. For the set of data presented here, conservative quality-assurance guidelines are used to reject potentially erroneous flux data. For ozone, most of the measured fluxes are of use in deriving surface resistances. For SO2, far fewer data points are available. For NO2, fluxes appear to lack the order of the O3 and SO2 fluxes, and do not enable surface resistances to be computed. The highest-quality SO2 and O3 data yield surface resistances in fair average agreement with model predictions for SO2, but substantially higher than predictions for O3.

Seasonal variations in the radiation regime within an oak-hickory forest

Abstract Seasonal variations in solar elevation, canopy phenology and leaf pigmentation result in a dynamic variation in the radiation regime within a deciduous forest. Measurements of several insolation components taken within an east Tennessee oak-hickory forest during eight combinations of season and phenological conditions of the canopy are presented. Insolation measurements were made with instruments mounted on a moving tram system at seven levels within, and at one level above the canopy. The attenuation of solar radiation is least during the leafless phenoseason since only woody biomass is present to intercept and absorb it. Net radiation is attenuated to a greater extent than shortwave and photosynthetically active radiation, which are attenuated to a similar degree, since the sparse woody biomass does not effectively trap the outgoing reflected and radiated radiation. Solar elevation did not influence the attenuation of insolation within the leafless forest under the conditions studied. The attenuation of insolation within the leafing, spring canopy increases progressively with increasing leaf area. The attenuation among the components, however, varies. Shortwave attenuation progresses at a faster rate with expanding leaf area than photosynthetically active radiation since newly expanded leaves are low in chlorophyll. The penetration of light into the leafing canopy is independent of solar elevations less than 40° and increases linearly with greater solar elevations. The radiation regime within the fully-leafed canopy is rather static. For daily mean values, no effects of leaf age or seasonal variation in solar elevation are evident. Penetration of light into the canopy, however, increases when solar elevation exceeds 65°. The attenuation of insolation components diminishes during the autumnal senescence—abscission phenoseason, in response to changes in leaf pigment and loss of foliage. Attenuation of insolation is greater in the autumn than during spring periods with similar leaf area since the sun is lower in the sky in the autumn. The depletion of photosynthetically active radiation with depth is minimal during the leafless, early leafing and autumnal phenoseasons. Moderate depletion occurs during the late leafing period and maximal depletion occurs during the fully-leafed phenoseason.

A comparison of estimated and measured SO2 deposition velocities

A nested-network program for obtaining data on the dry deposition of SO2 and SO4− has been initiated at a small array of locations (6 in 1985, presently 13) across North America. The procedures involved rely on the availability of models for deriving dry deposition rates from observations of air concentrations and of meteorological and surface properties known to influence the deposition velocity. At a subset of locations (i.e., 3), the results obtained by this indirect method are tested by comparison against more direct methods. One of the first comparison experiments of this series was conducted at Oak Ridge in July 1985 when the fluxes determined by inferential methods were compared to those measured by eddy correlation. The results obtained suggest that initial computer routines, developed to estimate deposition velocity for SOz on a routine basis, overestimate the deposition velocity by about 20% to a mixed-species deciduous forest. The difference is possibly due to the omission of water stress as a contributing factor in the initial computer routines, but might also be associated with chemical processes at the substomatal level.

Turbulence spectra of CO2, water vapor, temperature and velocity over a deciduous forest

Abstract This paper presents information on turbulence spectra derived from measurements made with an instrument array including a prototype rapid-response CO 2 sensor over a deciduous forest on complex terrain near Oak Ridge, TN. When compared to measurements from previous studies over flat terrain, velocity spectra appeared to be shifted toward lower frequencies. This tendency was especially apparent in lateral and vertical velocity spectra. Peaks occurred around f = 0.035–0.07 (where f is the non-dimensional frequency) in all spectra measured under near neutral conditions, except for vertical velocity which had a peak near f ≈ 0.16. Cospectra of sensible heat, water vapor and CO 2 measured during near neutral conditions were similar in shape and had peak frequencies near f ≈ 0.10–0.15. As thermal stability changed from near neutral to unstable conditions the spectral peaks generally shifted toward lower frequencies.

Seasonal variation in the statistics of photosynthetically active radiation penetration in an oak-hickory forest

Abstract The seasonal variability in the frequency distribution (FD) and higher order moments (e.g., standard deviation, skewness and kurtosis) of photosynthetically active radiation (PAR) penetration in an oak—hickory forest were examined. In a leafless, wintertime forest the FD is unimodal and skewed. In a leafing forest, the FD is highly variable and is strongly skewed. This results from higher solar elevation angles enhancing PAR penetration and new, expanding leaves causing penumbral shade and scattering the incoming radiation. The FD of PAR in the fully-leafed canopy is unimodal in the upper canopy, bimodal below crown closure and unimodal in the subcanopy trunkspace. Low solar elevation angles and leaf senescence cause the FD of PAR in the autumnal canopy to be uniformly distributed, as is indicated by its low kurtosis. FDs of PAR also provide a means of estimating sunlit leaf area. Estimated values of sunlit leaf area below crown closure agree within 15% with theoretical estimates derived with a negative binomial distribution.

Effects of sky brightness distribution upon penetration of diffuse radiation through canopy gaps in a deciduous forest

Abstract The penetration of diffuse sky radiation via canopy opening into a fully-leafed tulip poplar forest was estimated from canopy structure data obtained from canopy photographs and from sky brightness distribution approximations, using techniques originally developed by Anderson (1964). Small differences were found among mean daily penetration fractions predicted assuming an isotropic sky (UOC), a standard overcast sky (SOC), or an approximation of the weighted daily average predicted using a clear sky brightness distribution. For shorter time periods, penetration of diffuse radiation estimated using the UOC and SOC approximations differed substantially from estimates made using the clear-sky brightness distribution. Most diffuse radiation passes through openings that are within 10 degrees of the solar disk. Hence the directional distribution of diffuse radiation in the forest is strongly controlled by solar position on clear to partly cloudy days. Amounts of diffuse radiation observed in the forest under clear skies agree fairly well with those predicted by the model developved by Reifsnyder et al. (1971). In both cases, penetration increases with increasing solar elevation on clear days. We conclude that the SOC assumption produces acceptable results for time periods of a day or longer, but for shorter times with clear skies the use of the clear sky brightness distribution is necessary for most simulation or prediction modelling.