The influence of heterogeneous processes onto the urban atmosphere

A contribution to the subproject SATURN

Authors: Silvan Perego, Martin Junier

LPA-IGE/DGR Ecole polytechnique fédérale de Lausanne, CH-1015 Lausanne

    Aim of the research
     

    For many years summer smog has been recognized as an important problem to human health and to nature. It has been shown, that numerical computer models are a useful tool for investigating this complex phenomenon. Since high pollutant impacts mostly occur in clear sky conditions, most air pollution models neglected the influence of clouds and of aerosols in a first stage. It has become clear in the past years, however, that aerosols are important constituents of summer smog. Like clouds, they have an important influence on the radiation balance in the polluted atmosphere.

     

    Figure 1 shows the expected effects of clouds and aerosols: They affect the optical properties of the atmosphere. The radiation balance, therefore, is modified. This changes the photochemistry and via the heat budget, the wind field.

    By modifying the radiation balance, aerosols, therefore, can influence meteorological parameters and local climate. Their ability to change the properties of clouds is another interesting impact.

    Only a fully coupled meteorology/atmospheric chemistry model is able to simulate all these back-coupling effects. To do this, a model must include a radiation module which is able to consider multi-scattering effects, as well as aerosol and cloud modules and all the necessary links. The aim of this Saturn activity is to setup a modeling environment which includes all necessary features, and to use this utility to carry out a study on a Saturn field phase campaign and try to derive the importance of backcoupling effects.

    Activities during this year

In a first step, we have studied literature to find out which methods have already been developed and to judge the importance of their relative effects. While radiation effects and aerosols were judged to be very important, aqueous chemistry seemed to be of less importance to our problem. We decided therefore to include a multi-scattering radiation code and an aerosol module in a first step.

For the radiation module we started with the TUV-program of Madronich. Its normal version is based on a two-stream approach. Although more precise formulations exist, we found, that for a three-dimensional model, a more complicate and more computing time consuming approach would not be suitable.

Although Madronich's tuv code is a well structured and well functioning code, some adaptations were necessary in our case. To achieve higher performance, the code had to be optimized. We did this by classifying all tasks into 4 categories:
    1. Tasks which need to be executed at program start.
    2. Tasks which need to be executed once per time step.
    3. Tasks which need to be executed once per time step on each model layer.
    4. Tasks which need to executed in every time step in every grid cell.
Then we reorganized the TUV and rewrote parts of it according to our scheme. The original version furthermore was not able to account for inclined terrain. We, therefore, reanalyzed the two-stream formulas for inclined soil surfaces and added this feature to the code.

For calculating the aerosols we found the ISORROPIA-Code by Nenes et al. to be the most suitable to our needs. ISORROPIA is a thermodynamic equilibrium aerosol model witch is computationally efficient enough to be included in a three dimensional model. Furthermore we work with a code from Spyros Pandis (Carngie Mellon university) and Staphan Musarra (Sonoma Technology Inc) to calculate the number and size distributions of the aerosols. These particles distributions are needed by the radiation module to calculate the influences of the aerosols on the radiative budget.

The whole aerosol module is currently being coupled to an up-to-date atmospheric chemistry module in order to perform the first calculations.
 

Principal results
    Madronich's tuv code with our performance enhancements showed to be enough rapid, efficient and precise to be fully included into a combined meteorological/atmospheric chemistry model. While this module was mostly used to calculate photodissociation constants, first tests showed, that it also works perfectly well as a module for the meteorological radiation energy balances. Using the same radiation parameterization for both the meteorological and the atmospheric chemistry part of the model should enhance its consistency.

    Other calculations showed, that inclined terrain can have a significant impact to the photodissociation constants.
     

    Aim for the coming year

The following activities are planed for the next year:
    References
Nenes A. et al.; ISORROPIA: A New Thermodynamic Equilibrium Model for Multiphase Multicomponent Inorganic Aerosols, Aquatic Geochemisry 4 (1998) 123-152