M. F. Hovmand

Development and testing of a new variable scale air pollution model—ACDEP

A comprehensive trajectory model, Atmospheric Chemistry and Deposition model (ACDEP), has been developed to calculate the nitrogen deposition to the Danish sea waters. The model is constructed with the ability of taking into account spatially detailed emissions and land use data for Denmark and on a more coarse grid for the rest of Europe. In the ACDEP-model a one-dimensional column is advected along 96 h back-trajectories. The chemical mechanism in the model is a slightly extended version of the Carbon-Bond Mechanism IV (CBM-IV). The model describes the dry deposition processes with special emphasis on the conditions at sea. For the wet deposition processes both in-cloud and below-cloud scavenging are taken into account. The model results are tested versus one years measurements from six Danish and one Swedish monitoring station. Additional tests are performed for six selected stations from the European Monitoring and Evaluation Programme (EMEP) networks. The model is capable of reproducing both air concentrations and wet depositions of the nitrogen compounds in land and sea areas.

Review of dry deposition measurements of ammonia and nitric acid to forest

The paper describes some general aspects of the estimation of dry deposition fluxes and reviews the measurements of dry deposition of atmospheric gaseous ammonia and nitric acid in relation to forests. The dry deposition of ammonia is shown to be a bi-directional surface flux, i.e. the forest receives a nitrogen input from ammonia in the air, though some ammonia apparently is also emitted from the forest. Still, atmospheric ammonia originating from agricultural activities contributes with large amounts of nitrogen to the ecosystems. Nitric acid seems to deposit very fast, though only very few flux measurements above forest are reported. The method for estimating the fluxes of nitric acid might be questionable for some conditions. For the investigations reported the concentration levels of nitric acid have been relatively low and this compound has contributed with moderate loads of nitrogen to the ecosystem.

Measurements of ammonia and ammonium by denuder and filter pack

Abstract Simultaneous 24-hour measurements of gaseous ammonia (NH3) and particulate ammonium (NH4+) by denuder and filter pack were carried out at five different locations in Denmark. The locations represent areas surrounded by different emission densities of NH3. In general a good agreement was found for the determination of the sum of NH3 plus NH4+ by the two methods at all locations. The NH4+ determination by the two methods was in general found to be in good agreement. Even though sampling artifacts are seen for the NH3 determination by the filter pack, the NH3 concentrations are in general too low to affect the correlation of particulate NH4+. At a location surrounded by areas with NH3 emission densities the separate determinations of NH3 and NH4+ by the filter pack were in general in good agreement with the denuder determination of, respectively, NH3 and NH4+. At the other locations, the NH3 determination by filter pack frequently showed underestimation compared to the denuder. The under-estimation was frequently observed at conditions with low NH3 concentrations, westerly winds and a low ratio of particle NH4+ relative to particulate SO42− and NO3−. It probably relates to conditions with unsaturated acidic aerosols absorbing the NH3. The observations show that the ability of the filter pack method for NH3 determination is dependent on the climate, pollution climate and surrounding NH3 emission density. The use of the filter pack method and its restrictions therefore demand an individual investigation of the sampling site characteristics.

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.

Downward trends of sulphur deposition to Danish spruce forest

Abstract Determination of sulphur deposition to Danish forest and documentation of possible trends during an eight-year period was one of the aims of this study. Measurements were carried out at two sites in Denmark with even-aged Norway Spruce plantations. A western site is situated 15 km from the North Sea in the sandy soils of West Jutland, and a site 250 km east of this is situated in the loamy soils of Sealand. Subsequent measurements of gases, aerosols, precipitation and throughfall from June 1985 to June 1993 make it possible to follow a decrease in concentrations of atmospheric sulphur dioxide and sulphate in precipitation as well as in the deposition of sulphur compounds to the forest ecosystem. Taking throughfall sulphur fluxes as an estimate for atmospheric deposition to the forest we calculated the yearly average SO2 dry deposition velocity, Vd, to be 1.1 cm s−1 at the western site and 1.0 cm s−1 at the eastern site.

Ammonia and nitric acid dry deposition and throughfall

Since 1991 measurements of fluxes of ammonia have been carried out periodically at a forest location in the western part of Denmark. The ammonia deposition velocities and fluxes are estimated from gradient measurements done by denuders and micrometeorology. The deposition velocities showed a large variation, ranging from deposition mainly governed by the atmospheric transport with fast adsorption at the surface to emission. Nitric acid deposition velocities and fluxes were measured in a period in May 1993 and the data indicate deposition mainly governed by the atmospheric transport and fast adsorption at the surface. The measured ammonia fluxes and an estimate for the particulate ammonium flux are compared to a nine year mean value of the net throughfall from Norway Spruce at the location.

Nine Years of Measurements of Atmospheric Nitrogen and Sulphur Deposition to Danish Forest

Since 1985 measurements of gasses, aerosols, precipitation and throughfall have been carried out at three forest sites in Denmark with equal aged Norway Spruce plantations. The times series show a downward trend in the concentration of sulphur dioxide. Particulate sulphate, ammonia and particulate ammonium and the total nitrate seem to have a more constant concentration level. The wet deposition measurements show a decreasing trend in the content of acid (protons), sulphate, ammonium and nitrate, though for the nitrogen compounds it is only a slight fall. A decrease in concentrations of protons and sulphate is also seen in the throughfall measurements, in throughfall the nitrogen compounds hardly seem to decrease.

Deposition of Gases and Particles in the PBL: Evaluation of the Influence of a Vertical Resolution in Atmospheric Transport Models

The deposition processes determines the lifetime of a variety of gases and particles in the atmosphere. Computed depositions depend highly on the parameterisation and model concept. A vertical resolution of the tested model influences not only directly the description of dry and wet deposition processes, but also indirectly, as the chemical reactions are affected by the vertical distribution of gases and particles in the PBL. This indirect influence is due to the common assumption of full mixing in the PBL in models of no or only coarse resolution in the vertical. The effect of the vertical model resolution on estimations of concentrations and depositions of gases and particles is the subject of this paper.