Poul Hummelshøj

Measurements of aerosol fluxes to Speulder forest using a micrometeorological technique

Abstract It has often been stated that micrometeorological and throughfall measurements of dry deposition differ by an order of magnitude with the results being highly variable and difficult to interpret or reconcile. We present measurements by the eddy correlation method of sub-micron aerosol deposition to a forest and show that they are large, typically 1 cm s −1 or more. We compare the measurements with literature values obtained by throughfall and related techniques. The results, rather than being irreconcilable, show a clear and consistent behaviour in deposition velocity across the aerosol size spectrum, despite the very different techniques involved. There would appear to be a contradiction with previously assumed model predictions of aerosol deposition velocity to forests and rough vegetated surfaces particularly for particles in the size range 0.1–1.0 μm where collection efficiencies appear to be significantly underestimated. A simple deposition velocity parameterisation as a function of stability and size is presented.

Nitric oxide emission from a Norway spruce forest floor

Although the available data on soil NO emission are increasing, only few measurements have been made in temperate forests. Emissions from these forests are generally low, but recent studies have shown that NO emission from N-affected forests can be much higher. In this study the emission of NO from the floor of a moderately N-affected Norway spruce forest in Denmark was measured using dynamic chambers during two seasons (April 1995 and September 1995). At the same time, fluxes of CO2 and NO2, soil water content inside and outside the chambers, soil temperature at 10 cm depth, soil surface temperature and air temperature were recorded. The emission of NO in April was low (< 0.3 ng N m−2 s−1), whereas it was significantly higher in September (21 ng N m−2 s−1 with a range 0–82 ng N m−2 s−1). In April the soil surface temperature was low (< 6°C) and the soil moisture was high (26% vol/vol). In September the soil surface temperature was high (16°C) and soil moisture was significantly lower (14% vol/vol) than in April. There was a high correlation between CO2 emission and NO emission. The fluxes of NO2 were very small and not related to NO emission. Generally, the variation within a single plot was small, whereas the variation between plots was large. About half of the total variation of NO emission in September could be explained by a combination of soil moisture, distance to tree trunks, and moss cover. No significant relationship could be found between NO emission and soil surface temperature or soil temperature. Other parameters such as the C/N dynamics in the soil must be taken into account to provide a better mechanistic understanding of the NO emission. An annual emission of 3.15 kg NO-N ha−1 yr−1 was estimated by the amount of NH4+ in throughfall. Compared to most of the previous published measurements of NO emission from temperate forest soils, the values presented here are high and of the same magnitude as found in N-affected forests in Central Europe.

Transformation of size distributions of emitted particles in streets

Abstract Dispersion and transformation of particulate matter in streets are studied by using a combination of a street pollution dispersion model, the Operational Street Pollution Model (OSPM) and a particle transformation model. The particle model implements a very fast implicit method for solving the changes in particle size distribution due to coagulation, condensation and dilution processes. Using available measurements of emitted particle size distributions, impact analysis of the various transformation processes affecting the size distribution of particles was performed. For this purpose, a plume model simulating dilution of diesel exhaust has been developed and coupled with the particle coagulation model. The results show that due to the rapid dilution of the exhaust plume, the coagulation is not significant. Growth of the emitted particles due to condensation of water vapour appears also to be marginal, but this conclusion depends critically on the assumption of particle hygroscopicity. The freshly emitted diesel particles are believed to be less hygroscopic. Analysis of recent street measurements of particle size distributions in the range 0.2–20 μm reveals in general very poor correlation with the street traffic, but strong dependence on the relative humidity. This indicates that the particles measured are not freshly emitted and aged aerosols are dominant. Calculations with the street pollution dispersion model OSPM, using the diesel exhaust emissions as the only source, show that the mass concentrations of emitted particles in the street are predicted to be significantly lower than the measured concentrations.

Stomatal acclimation influences water and carbon fluxes of a beech canopy in northern Germany

Summary The response of beech leaves to leaf-to-air vapour pressure deficit (VPD) is not constant throughout the growing season. In situ measurements of leaf gas exchange showed that this intra-annual variation of stomatal sensitivity to VPD cannot be described adequately by a simple annual course of phenology alone. Reduction in stomatal aperture in dry air was negatively correlated with the sum of photosynthetic photon flux density (PPFD) of the previous three weeks, but was positively correlated with the mean VPD of the previous month. These acclimation effects contribute significantly to explaining variances in stomatal sensitivity across the growing season. The observed mid-term reaction to PPFD is in agreement with recent findings regarding the blue light perception of stomatal guard cells. A mathematical formulation representing these mid-term variations in stomatal sensitivity was incorporated into an ecosystem flux model describing the carbon and water exchange between forests and the atmosphere. Model results were in good agreement with independent eddy covariance measurements above the beech canopy. The model, therefore, is useful for investigating the importance of stomatal acclimation for ecosystem carbon and water balances. At an annual scale, acclimation appears to influence the fluxes moderately (less than 10%). The model suggests, however, that stomatal acclimation optimises the seasonal course of carbon gain and water loss of beech forests significantly with respect to the varying availabilities of water and energy. Die Reaktion von Buchenblattern auf das Wasserdampfdefizit zwischen Blatt und Luft ist nicht das ganze Jahr uber konstant. Die in-situ gemessene intraannuelle Variabilitat der stomataren Sensibilitat gegenuber der Luftfeuchte kann daher mit einer einfachen phanologischen Kurve nicht adaquat beschrieben werden. Daten, die uber drei klimatisch sehr unterschiedliche Jahre an Buchenblattern im Hauptforschungsraum des Bornhoved-Projektes aufgenommen wurden, zeigten vielmehr, das das Ausmas, in dem die stomatare Leitfahigkeit als Reaktion auf trockene Luft verringert wird, einerseits negativ korreliert ist mit der mittleren Tagessumme der photosynthetisch aktiven Quantenstromdichte der letzten drei Wochen, andererseits positiv korreliert ist mit dem mittleren Wassersattigungsdefizit der Luft des jeweils letzten Monats. Eine Berucksichtigung dieser als stomatare Akklimatisation interpretierten Befunde erklart die Varianz der stomataren Sensibilitat signifikant besser. Die beobachtete mittelfristige Reaktion der Stomata auf die Witterung stimmt mit kurzlich publizierten biochemischen Beobachtungen uberein, nach denen die Blaulichtrezeptoren in Schlieszellen von Spaltoffnungen ebenfalls auf das Lichtklima reagieren. Eine mathematische Formulierung fur die mittelfristige Anpassung der stomataren Sensibilitat wurde in ein Okosystemflus-Modellsystem, das die Wasser- und CO2-Flusse zwischen Waldern und der Atmosphare berechnet, integriert. Die Ergebnise von Simulationslaufen mit dem erweiterten Modell zeigten gute Ubereinstimmung mit Eddy Kovarianz Messungen, die unabhangig uber dem Buchenwald durchgefuhrt wurden. Weitere Simulationlaufe zeigten, das die Akklimatisation die jahrlichen Bilanzen zwar nur moderat (weniger als 10%) beeinflust, aber tagliche Abweichungen von bis zu 60% auftraten. Es konnte gezeigt werden, das die stomatare Akklimatisation zur Optimierung des Kohlenstoffgewinns der Baume bei wechselnder Verfugbarkeit von Wasser und Energie beitragt.

Measurements of ammonia flux to a spruce stand in Denmark

Abstract This work demonstrates the existence of a linear relation between the deposition velocity of ammonia and the friction velocity measured above a spruce stand in the western part of Denmark. In order to estimate the ammonia deposition velocity and flux to a Norway spruce forest, concentration gradients of ammonia and several meteorological parameters were measured in a meteorology tower during two periods, 1 week in spring and 1 week in late summer 1991. The estimated deposition velocities lie in the range −0.125 to 0.201 m s−1, with a mean of 0.026 m s−1. The deposition velocity and the flux were generally largest in the afternoon. On the basis of 24-h measurements of ammonia and routine meteorological measurements the relation between deposition velocity and friction velocity is extrapolated to an estimate of the average flux for the growing season May to September 1991. The estimate gave an average flux of 87 μg NH3N m−2 h−1 (=0.02 μg NH3N m−2 s−1). The average deposition velocity for the period was 0.045 m s−1.

ATMOSPHERIC NITROGEN INPUT TO THE KATTEGAT

Abstract An overview is given of the processes involved in the atmospheric deposition of nitrogen compounds. These processes are incorporated in an atmospheric transport model that is used to calculate the nitrogen input to the Kattegat, the sea area between Denmark and Sweden. The model results show that the total atmospheric nitrogen input to the Kattegat is approximately 960 kg N km2 yr1. The nitrogen input to the Kattegat is dominated by the wet depositions of NHx (42%) and NOy (30%). The contribution from the dry deposition of NHx is 17% and that of the dry deposition of NOy is 11 %. The contribution of the atmospheric input of nitrogen to the Kattegat is about 30% of the total input including the net transport from other sea areas, runoff etc.

Atmospheric nitrogen compounds: Occurrence, composition and deposition

Abstract Traffic in cities and on highways is an important contributor to NOy atmospheric pollution in open areas. In this situation both the concentration and composition of NOy compounds show a wide variation and are dependent on meteorological and atmospheric chemical conditions. The proportion of NOz compounds (HNO3 + nitrate + PAN + PPN + N2O5 + organic nitrates) increases while that of NOx decreases with increasing ozone concentrations. The dry deposition velocity of NO2 was determined to be 0.2 cm s−1 above vegetation. The dry deposition contribution of the different NOy compounds was evaluated. The possiblity that a significant contribution is caused by a group of unidentified NOy compounds cannot be excluded. Therefore, future investigations of atmospheric pollution of sensitive ecosystems, at conditions with a relatively high atmospheric content of NOy compared to that of NH3, ought to take into consideration the possible contribution from unidentified NOy compounds. This also implies that atmospheric research into the identity, occurrence, sources and sinks of these compounds—as well as further developments of sampling and analytical methods—should be given a high priority.

Canopy physiology: interpreting the variations in eddy fluxes of water vapour and carbon dioxide observed over a beech forest

Summary Fluxes of carbon dioxide, water vapour, heat and momentum over a northern German beech forest were measured by use of the eddy covariance technique. The canopy conductance to water vapour transfer, g c , was calculated for daylight periods with sufficient fetch, sufficient energy balance closure, and dry canopy surface. In search for functional responses of the beech canopy to the environment, we related g c linearly to net ecosystem CO 2 exchange ( F c ) scaled by relative air humidity, and F c non-linearly to photon flux density, air temperature and g c . Consequently, conductance and CO 2 exchange of the canopy were assumed to be coupled as in the widely used leaf scale approach by Farquhar & von Caemmerer (1982) and Ball et al. (1987). Applying the respective canopy scale formulations left only 30% of the variance in g c and 37% of the variance in F c unexplained. Testing the equations against eddy flux data collected in a Danish beech forest resulted in an agreement within a few percent. A perspective is given how to simplify canopy water and carbon flux models by use of the framework presented in this study. Uber einem Buchenwald in Norddeutschland wurden die Bestandesflusse von Kohlendioxid, Wasserdampf und fuhlbarer Warme sowie der Impulsfluss mit Hilfe der Eddy-Kovarianz-Technik gemessen. Die Bestandesleitfahigkeit fur Wasserdampf, g c , wurde fur Tageslichtperioden mit hinreichend geschlossener Energiebilanz, ausreichender Ausdehnung des Bestandes in Richtung des Windes und trockener Kronendachoberflache berechnet. Auf der Suche nach Reaktionen des Buchenwaldes auf die einwirkenden meteorologischen Bedingungen wurde g c linear zu dem Produkt aus Bestandes-CO 2 -Fluss ( F c ) und relativer Luftfeuchte in Beziehung gesetzt und F c nicht-linear zu einer Funktion aus Photonenflussdichte, Lufttemperatur und g c . Damit wurden Leitfahigkeit und CO 2 -Austausch des Bestandes als eng miteinander gekoppelt betrachtet, analog zu dem verbreiteten, fur die Blattebene geltenden Ansatz von Farquhar & von Caemmerer (1982) und Ball et al. (1987). Die Anwendung der entsprechenden mathematischen Formulierungen auf den erhobenen Datensatz erklart 70% der Varianz von g c und 63% der Varianz von F c . Die Gleichungen wurden ebenfalls zur Berechnung des Kohlenstoffflusses uber einem danischen Buchenwald eingesetzt und fuhrten zu einer Ubereinstimmung bis auf wenige Prozent mit dem dort gemessenen Eddy-Fluss. Es wird eine Perspektive aufgezeigt, wie Bestandesmodelle fur den Wasser- und Kohlendioxidaustausch auf der Grundlage der hier entwickelten Beschreibung kunftig vereinfacht werden konnten.

Monitoring Atmospheric Constituents

CO, CH4, and non-methane hydrocarbons (NMHCs) are important precursors of photo-oxidants. Therefore, these species have been measured at a number of TOR stations in western Europe. An interlaboratory comparison of the analytical methods employed by the various participating laboratories (PLs) and subsequent data harmonisation were performed to ensure data compatibility within TOR. The relative deviations of the PLs from the CO reference values by the Fraunhofer Institute for Atmospheric Environmental Research (IFU), acting as the central laboratory, covered the range from —2.21 to +2.38 percent, while the relative deviations of the PLs from the respective CH4 reference values were found to range from +0.66 to +2.31 percent. For most of the nineteen species considered in the intercomparison of atmospheric C2—C7 NMHC measurements, the relative deviations of IFU and the various PLs from the respective reference values U* for each round were found to typically fall into the range of -35 to +35 percent.

Surface Flux Variability in Relation to the Mesoscale

This contribution to EUROTRAC deals with the variation that takes place in the surface fluxes (for example in the dry deposition of the various air pollutants) when the boundary layer flow encounters changes in the land-use type. The project has had three components: participation in the TRACT field campaign with ground based flux measurements; development of a model for integration of the flux variation over the variety of surface types in a given land-use pattern from scales of 25 m up to 10 km or more (aggregation model) and to apply it to the TRACT area; and to develop an interpretation techniques that will allow remote sensing (satellite) products to be used for estimating the necessary boundary conditions for the aggregation model. The following gives the background for the work and an overview of the results obtained.