Atmospheric behaviour of ammonia and ammonium

1.4.1 Scope of this thesis

A few models for ammonia and ammonium exist. Russell et al. (1983) made a multi-layer Lagrangian transport model describing the transport and formation of ammonium nitrate aerosol for California. They did not take reactions of ammonia and sulphuric acid into account, nor wet deposition. Their model was mainly used to compute diurnal variations of ammonium nitrate aerosol, ammonia and nitric acid concentrations, which were compared with measurements for one day.

De Leeuw et al. (1986) developed a multi-layer Lagrangian transport model with detailed chemistry including the reaction with sulphuric acid containing aerosol and wet deposition. Their model was especially designed to describe episodes with high aerosol concentrations.

More complicated models as discussed above have the advantage of giving more insight in detailed (chemical) mechanisms. But the results obtained with such models are often difficult to verify as components are involved which are not measured, or not at all levels the model gives results for. Moreover, more complicated models require relatively more computing time, which makes them less suitable to compute long-term average concentrations. At this moment no good information is available on seasonal variations in the ammonia emission. Moreover, much Information on the variation both in time and space of other processes is also lacking. This means that some processes involved can be described in detail, whereas others cannot. Such models suffer then from an imbalance in the treatment of different processes, and the results of the models will be no better than is dictated by the weakest link in the chain. It should be mentioned here that such models still can be very valuable to study mechanisms and possible interactions between different processes and components. But because of the lacking information on e.g. short-time variation in emission no realistic short-term concentrations can be obtained.

Some information on the processes involved is available, but sometimes only on a yearly basis. It seems therefore most appropriate to integrate the existing knowledge by using a relatively simple model for the computation of yearly averaged concentrations. A first attempt to do this was made by Fisher (1984). He tried to compute deposition fields for ammonium in precipitation and the total deposition (sum of dry and wet deposition) of NHx (sum of ammonia and ammonium) for Europe. But he had to conclude that his approach would remain limited until the main rates of removal of nitrogen compounds were better known.

The scope of this thesis is:

a. To evaluate the existing knowledge of all processes involved.
b. To acquire additional crucial information on some processes from own field measurements.
c. To integrate this knowledge by applying a model for the computation of long-term average (ground level) concentrations, with emphasis on the European scale. This model has been developed especially for ammonia and ammonium.
d. To show some new applications of models: computation of historical import/export balances and concentration patterns.

1.4.2. On the presentation

This thesis consists mainly of separate articles (chapters 3 to 6) which were submitted to Atmospheric Environment. This has some consequences:
a. Some information is presented in more than one article or part of this thesis.
b. A list of references appears after each chapter.
c. The articles have not been written at the same time. As a result some information was not yet known at the time of preparation of some articles, although this does not lead to any serious inconsequences.
In the following a preview is given of the contents of the different chapters.

Chapter 2

In this chapter some information is presented on the geographical distribution of the yearly averaged ammonia emission in Europe, which is essential to understand the model results.

Chapter 3

In this chapter field measurements are described. These measurements were performed for the following purposes:
a. To verify the estimated emission density
b. To get an indication on the vertical concentration profiles of ammonia, ammonium and related components In an area where emission occurs. This information is needed to model the horizontal transport.
c. To get an indication of the overall conversion rate of ammonia to ammonium.

Chapter 4

In this chapter is described how correction factors can be computed which can be used in a simple Lagrangian long-range transport model to describe the effects of turbulent mixing on concentration and deposition patterns. By using these correction factors more realistic results can be obtained.

Chapter 5

This chapter forms the basic part of the thesis and describes the evaluation and integration of the present knowledge, the setup of the model, numerical aspects of the model and comparison of model results with measurements. Although the model in principle would allow to vary most parameters as a function of time and space (e.g. dry deposition velocity, reaction rate, mixing height etc.) this was not done in practice in view of all uncertainties in the parameter values. Therefore, also no processes like escape into the reservoir layer were taken into account.

Chapter 6

In this chapter it is shown that it is not only possible to use a model to describe the present situation, but also to give estimates of concentrations in the past. These estimates make it possible to know where trends in concentrations measured in the past can be expected.

Chapter 7

In this chapter the conclusions of this study are summarized and discussed.