Subject tropospheric chemistry

The measurement of OH itself has presented a major challenge to atmospheric chemists. Even though measurements have been reported in the literature that tend to be in qualitative agreement with model calculations, the accuracy of the data continues to be the subject of much scientific debate. Currently, at least seven independent groups in the U.S. and Europe are addressing this problem. Thus, direct comparison of measured OH levels with model-predicted values continues to be one of the challenging frontiers of global tropospheric chemistry. 

The aqueous-phase and gas-phase chemistries of HO, are sufficiently closely coupled that the chemistry shown in Tables 8.11 and 8.12 can affect gas-phase concentrations as well. For example, including the aqueous-phase chemistry in models of tropospheric ozone formation alters predicted 03 concentrations, although whether the perturbation is significant is subject to some controversy (e.g., see Lelieveld and Crutzen, 1990 Jonson and Isaksen, 1993 Walcek et al., 1997 Liang and Jacob, 1997). 

The chemistry of tropospheric ozone production is one of the most extensively studied phenomena of environmental chemistry. However, even this subject has its controversial aspects, particularly with regard to how best to meet US Environmental Protection Agency mandated air quality standards for O3 and NO2. The essential reactions have already been presented in Section 12.4 and will only be discussed briefly here. 

In this book air chemistry is defined as a branch of atmospheric science dealing with the atmospheric part of the biogeochemical cycle of different constituents. In other words this means that we will deal mainly with the atmospheric pathways of those components that are involved in the mass flow between the atmosphere and biosphere, as well as in chemical interactions between the air and the other media of our environment (soils, oceans etc.). It follows from this definition that, on the one hand, our discussion will be restricted to the troposphere and the stratosphere4 and, on the other hand, the photochemistry of the upper layers, the subject matter of the aeronomy (e.g. Nicolet, 1964), will be omitted. This separation of the (photo) chemistry of the lower (troposphere and stratosphere) and upper atmosphere makes it possible to give a more compact treatment of our problem, including the global anthropogenic effects due to the increase of air pollution. 

Compounds are emitted into the atmosphere in both gaseous and particulate forms. In addition, gas-phase oxidation of a number of compounds can lead to products that condense into the particulate phase. The subject of tropospheric aerosols—their sources, chemistry, and removal—is a significant one and beyond the scope of the present chapter. 

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