Petersen, AC; (1999) Convection and Chemistry in the Atmospheric Boundary Layer. Doctoral thesis , Utrecht University.

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Abstract

The earth’s troposphere is the lowest layer of the atmosphere and has a thickness of about 10 km. It is the layer that contains most of the mass (80%) of the atmosphere. All weather phenomena that we experience have their origin in the troposphere. It is the stage for some well-known environmental problems: climate change, ozone smog, and acidification. These problems are related to the trace amount of gases that are emitted into the troposphere from anthropogenic sources. Although these emissions do not significantly change the composition of the atmosphere (78% stays N2, and 21% stays O2) the trace amounts of the emitted gases, such as CO2, CH4, NO, NO2, nonmethane hydrocarbons (NMHC), and SO2, either have an impact on the radiative balance in the atmosphere, or are reactive and can be transformed into other gases and aerosols that, in their turn, can have an impact on climate, acidification, plant stress, or human health—all depending on the type of the species and the concentration. Clearly, society has an interest to obtain knowledge on the tropospheric chemical composition and on the physical and chemical processes that control the fate of emitted gases in the atmosphere. Of course atmospheric physics and chemistry is an interesting topic for academic research for its own sake. Much of the atmospheric chemistry of, especially, short-lived trace gases (with lifetimes of several hours) takes place predominantly in the atmospheric boundary layer. The atmospheric boundary layer is defined as the layer of the atmosphere which is directly influenced by the earth’s surface. Since many of the relevant chemical reactions take place during daytime under the influence of sunlight, the so-called “convective atmospheric boundary layer” (CABL) is an important layer in the troposphere. It is in this layer that—under clear-sky conditions and significant heating of the surface—reactive gases are emitted and become involved in photochemical reactions. The turbulent mixing of chemically reactive gases in the CABL is the subject of this dissertation. First we introduce here the turbulent motions that characterize the CABL. Thermal convection is a common physical phenomenon occurring in the boundary layer, and is driven by the transfer (flux) of heat from the surface to the air above it. This flux results from a temperature difference between the surface and the overlying atmosphere (the temperature difference is typically caused by solar heating of the land surface or a relatively warm sea surface). The heat flux to the atmosphere leads to an increase in temperature, expansion, and associated with that to a decrease of the density and to buoyancy forces which cause an upward acceleration of air mass. A very well-known example of thermal convection is that over land on sunny days with little or no wind (so that surface friction is not the most important source of atmospheric turbulence). The solar energy absorbed at the surface is transferred to the atmosphere as heat in turbulent buoyant plumes. The CABL that then develops typically reaches a height of the order of 1 to 2 km over land in the middle of the day and is fully turbulent. In this dissertation, the CABL is studied as a medium for turbulent transport and chemical transformation of reactive gases. We are specifically interested in the time evolution of the concentrations of these gases in the CABL, and in their exchange between the CABL and the earth’s surface and biosphere. Gas emissions to the atmosphere take place through both natural processes and human activities. The focus is on ozone chemistry under different chemical conditions. In summer strong anthropogenic emissions of ozone precursors can lead to photochemical smog formation (i.e., high concentrations of ozone). The chemical destruction of ozone precursors, like hydrocarbons and nitrogen oxides, is specifically studied—both for anthropogenically perturbed and for background conditions. The goal of this dissertation is to gain increased understanding of processes that determine atmospheric chemistry and transport on regional and global scales. Two related matters are also addressed: first, how we can incorporate chemistry in a conceptually ‘simple’ dynamic model of the CABL, and second, what the impact of this description is in the context of large-scale atmospheric chemistry models. We will focus on one of the unresolved topics in atmospheric chemistry modeling, that deal with the time evolution of moderately fast and fast reacting species in the CABL. Here “moderately fast” and “fast” are defined relative to the turbulent mixing timescale (which is of the order of 10 min). As an introduction to the topic, we will first describe the turbulent nature of the CABL. For this purpose we exploit the similarities between convection in the atmospheric boundary layer and Rayleigh-Benard ´ convection in the limit of fully developed turbulence. Subsequently, we will elaborate on the central problem of the turbulent mixing of moderately fast and fast reacting chemical species in the CABL. After that, the tools that are used in this study—i.e., the models—will be introduced. Finally, we will formulate the research questions and outline the structure of this dissertation. Most references are to early articles on a certain topic, review articles, and text books, and are not intended to be exhaustive.

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