F.G. Römer

The impact of canopy exchange on differences observed between atmospheric deposition and throughfall fluxes

To study the impact of canopy exchange on differences observed between atmospheric deposition and throughfall fluxes, several-held experiments were performed at the Speulder forest in The Netherlands. Relevant information was obtained by (i) measuring open-field precipitation and throughfall fluxes with different time resolutions, using two canopy exchange models, (ii) by comparing results from surface wash experiments using real and artificial twigs, respectively, and (iii) by comparing throughfall flux estimates with atmospheric deposition estimates from micrometeorological measurements and inferential modelling. Canopy uptake of gases through stomata was estimated using measured air concentrations and a stomatal conductance model. Specific information on canopy leaching of soil-derived sulphate was provided by a S-35 tracer experiment. Sulphur was found to behave conservatively within the canopy, with SO2 uptake more or less balancing leaching of soil-derived SO42-. Significant stomatal uptake of NO2, HNO2 and NH3 was calculated as well as uptake of H+ and NH4+ from water layers covering the tree surface. Experiments did not indicate significant uptake of NO3- in solution. Canopy uptake of H+ and NH4+ was countered by leaching of K+, Ca2+ and Mg2+. Part of the leaching of K+, Ca2+ and Mg2+ (15%) took place along with weak organic acids. No significant canopy exchange was found for Na+ and Cl-. Differences observed between atmospheric deposition and throughfall fluxes could almost completely be explained by canopy exchange, the difference between NOy deposition and NO3- throughfall flux being the only exception. Copyright (C) 1996 Elsevier Science Ltd (Less)

Fog deposition on a coniferous forest in The Netherlands

Fog deposition in December 1992 and February 1993 (two periods of several days in which fog occurred) was monitored at the location Speulderbos in The Netherlands. Fog droplet deposition was measured with eddy correlation and samples of fog water were taken with a string collector. At the same time, throughfall deposition was measured and throughfall water was sampled. The occult (fog/cloud) deposition during these periods was 3.4 and 2.0 mg m(-2) s(-1), respectively Throughfall fluxes measured during these periods were a factor of three higher. The contribution of occult deposition to the total acid deposition to forests in The Netherlands is estimated to be about 5%. A clear relationship between the friction veracity u(*) and turbulent deposition velocity v(t) of fog droplets could be derived. The measured turbulent deposition-velocity for fog is half the deposition velocity for impulse v(m). Copyright (C) 1996 Elsevier Science Ltd (Less)

The Elspeetsche Veld experiment on surface exchange of trace gases: Summary of results

Abstract A three-year experiment for the determination of deposition fluxes on heathland was conducted at the Elspeetsche Veld involving several research groups. Micro-meteorological measurements of SO 2 , NH 3 and NO 2 were made using different techniques. Furthermore, throughfall and bulk precipitation fluxes of SO 4 2− , NH 4 + and NO 3 − were also measured. From these measurements, deposition parameters were derived to yield a generalized description of the surface exchange of these trace gases, including possible interactions. For SO 2 , generally low surface resistances were found; the annual average dry deposition velocity ranged from 0.8 to 1.0 cm s −1 . The annual flux of SO 2 was estimated as 250 mol ha −1 yr −1 . For NH 3 also, low R c values were observed, yielding annual average V d of 0.8 cm s −1 . The annual flux of NH 3 was estimated as 850 mol ha −1 yr −1 . Under the environmental conditions prevailing in the Netherlands, co-deposition between SO 2 and NH 3 was estimated to be irrelevant. Surface wetness was found to be the dominating factor influencing dry deposition of the two gases. R c values for NO 2 stomatal behaviour, V d for this gas ranged from 0.1 to 0.4 cm s −1 . Fluxes measured with the throughfall method are compared with fluxes derived using micro-meteorological methods. The yearly average net throughfall flux for SO 4 and the dry deposition flux (SO 2 and SO 4 particles) amounted 500 and 350 mol ha −1 yr −1 , respectively. For NH 4 this was 1180 and 930 mol ha −1 yr −1 , respectively. The correlation between the two on a monthly basis is significant ( r =0.50 for SO 4 and r =0.52 for NH 4 , n = 12, p

The chemical composition of cloud and rainwater. Results of preliminary measurements from an aircraft

Abstract Using an aircraft, preliminary investigations have been carried out into the chemical composition of clouds. The emphasis was on testing the sampling devices under several conditions. The results clearly show a considerable spatial distribution in pollutant concentrations that must have been caused by the contribution of many plume sources and the inhomogeneity of cloudwater density as well. Rather high concentrations of H3O+. Cl−, NO−3, SO2−4, NH+4 and H2O2 were measured. Except for H2O2, these components are scavenged very fast during travel over short distances over source regions. Moreover H3O+, NO−3 and SO2−4 are likely to be formed by chemical reactions of gaseous precursors with oxidants in the liquid phase. This may explain the marked decrease in high background H2O2 concentrations (ppm range). Relatively high (molar) NO−3 /SO2−4 ratios mainly varying from 1 to 2 were measured over The Netherlands near source areas. The lower values found during a flight over southern Scandinavia are in agreement with data from literature. Occasionally the chloride concentration was found to be much higher than could be expected from rain network data.

Evaluation of automatic ammonia monitors for application in an air quality monitoring network

Abstract Six automatic ammonia analyzers were studied both in the field and in an environmental test chamber, on their suitability for the Netherlands National Air Quality Monitoring Network (NAQMN). The instruments studied were a−1 continuous-flow denuder, a−1 WO3-coated thermodenuder, a−1 V2O5-coated thermodenuder, a commercial Differential Optical Absorption Spectroscopy (DOAS monitor, and a−1 chemiluminescence NOx monitor with NH3 converter. In the environmental test chamber functional parameters such as range, precision and detection limit, were determined, while the field study was mainly addressed to operational and technical performance of the instruments. The continuous-flow denuder met all the functional requirements and appears to be a−1 suitable ammonia analyzer for various purposes, for instance determination of deposition fluxes, but it must be technically modified (e.g. by using lower liquid flow rates) to meet the requirements for operation in a−1 network. The W03 thermodenuder might be suitable provided that some improvements, such as the use of an “autorange” NOx monitor, are made. The other instruments were rejected as network monitors since they did not meet two or more requirements and cannot be sufficiently improved. The converter method is only useful when high NH3 (and low NH a) concentrations are expected, such as in stacks or stables, because of interference by NH4+ and differences between the efficiencies of the Mo and the Nt converter after exposure to high concentrations of N-components. The V2O5 thermodenuder is too inaccurate, has a−1 limited range and does not sample continuously, which may yield non-representative values because ambient NH3 concentrations can vary rapidly, especially in areas with many sources. The photoacoustic monitor and DOAS needs too frequent attention. The noise level of the DOAS system appeared to be too high: 2 μg m−3 for hourly average concentrations. The photoacoustic monitor suffered from many problems leaving only 1% useful data. The scarce results showed large discrepancies compared with the other methods.

Analysis of large scale sulphate, nitrate, chloride and ammonium concentrations in The Netherlands using an aerosol measuring network

SO4−2, NO3, Cl− and NH4+ concentrations in aerosol together with the gaseous components SO2, NO, NO2 and O3 and meteorological parameters were measured in The Netherlands in the period 1979–1986 using a measuring network. Variations in time and space of concentrations measured were investigated. From eigenvector analysis it appeared that, contrary to their primary components SO2, NO and NH3, which are mainly determined by local conditions, concentrations of SO4−2, NO3− and NH4+of different stations are strongly correlated and thus homogeneously distributed over The Netherlands. Cl− shows a more small-scale type of behaviour. In the summer months concentrations of NO3− and NH4+ are determined by conversion processes and local emissions at ground level. The relation between interpolation error and network density is discussed. It appeared that aerosol concentrations at one typical station give a first approximation of SO4−2 concentrations over the whole area of The Netherlands. Because concentrations of Cl− and concentrations of NO3− and NH4+, are more determined by local conditions, especially in summer, a denser network is required to describe their concentration patterns accurately. Measurements and model calculations show that aerosol concentrations vary in the course of the day. This appeared to be caused mainly by daytime variations in mixing height leading to high aerosol concentrations at high levels in the atmosphere, brought down by fumigation. These high aerosol concentrations at elevated levels originate from long-range transport (LRT) in the reservoir layer from foreign source areas. The relatively important contribution of LRT to aerosol concentrations in The Netherlands corresponds with the large-scale aerosol concentration patterns indicated by eigenvector analysis.

Preliminary Measurements from an Aircraft into the Chemical Composition of Clouds

Using an aircraft preliminary investigations have been carried out into the chemical composition of clouds. Rather high concentrations of H3O+, Cl-, NO 3 - , SO 4 2- , NH 4 + and H2O2 were measured. Except H2O2 these components are scavenged very fast during transport over short distances over source regions. Moreover H3O+, NO 3 - and SO 4 2- are likely to be formed by chemical reactions of gaseous precursors with oxidants in the liquid phase. These reactions may cause the strong decrease in high background H2O2 concentrations (ppm range).

Deposition of aerosol on speulder forest: Accumulation experiments

The deposition of alkaline and acidic aerosol compounds on a Douglas fir forest was measured by washing natural and artificial branches and using a certain type of Petri-dish sampler. The results were scaled to deposition on the entire canopy, using data on the vertical distribution of the leaf area index and the wind velocity profile within the canopy. The results are, within the uncertainty range of the measurements, comparable to those of net throughfall measurements corrected for canopy exchange, except for the results concerning Na and Cl. The calculated deposition velocities are high because of the high LAI and the large fraction of alkaline elements within the coarse-aerosol fraction. Theoretical data on aerosol deposition velocity compare well with the experimentally obtained values.

Particle deposition to forests.

Abstract Particle deposition to forest was studied using experimental and modelling results. Results show that the deposition of particles to forests has been underestimated until now. Particle deposition makes out reasonable contribution to the total deposition of acidifying components and base cations to forests. It was estimated that at Speulder forest the contribution of dry particle deposition to the total deposition was 18% for SO 4 , 38% for NO 3 , 23% for NH 4 , 56% for Na, 47% for K, 69% for Ca and 65% for Mg.. Deposition of compounds via fog at the Speulder forest was estimated to be small (

Fog deposition on Douglas fir forest

In December 1992 and February 1993 two periods in which fog occurred were monitored at the location Speulderbos in The Netherlands. The occult deposition during these periods was 3.4 and 2.0 mg/(m2.s) respectively. The contribution of occult deposition to the total acid deposition to forests in The Netherlands is estimated to be about 5%. High correlation between the friction velocity u, and turbulent deposition velocity vt of fog droplets could be derived.