Vertical atmospheric layering is included in a previously described, zonally averaged global multimedia distribution model. This model is used to simulate the fate of α‐hexachlorocyclohexane (α‐HCH), the main ingredient of the technical HCH pesticide mixture, for the 50 years of its large‐scale use (1947−1997). Worldwide historic emission estimates are compiled, assigned to 10 climate zones, and used as input in model calculations. The performance of the model is evaluated by comparing calculated and measured concentrations in the atmosphere and seawater. A major focus is on the arctic and northern temperate environment, and emphasis is also placed on absolute levels, time trends, latitudinal profiles, and air‐water exchange. In general, simulated and observed concentrations agree within one order of magnitude. Deviations are explained by the zonal averaging characteristics of the model and uncertainties associated with the environmental degradation rates of α‐HCH. In both model results and observations, the dramatic decrease in the global emission of α‐HCH is reflected in rapidly decreasing concentrations in the atmosphere and seawater, except in the Arctic Ocean, which apparently is the last refuge for α‐HCH in the global environment. The model thus provides an illustration of the response characteristics of the global system to the reduced emissions of α‐HCH. Its application to other chemicals with reduced emission is discussed.
Abstract-Vertical atmospheric layering is included in a previously described, zonally averaged global multimedia distribution model. This model is used to simulate the fate of ␣-hexachlorocyclohexane (␣-HCH), the main ingredient of the technical HCH pesticide mixture, for the 50 years of its large-scale use . Worldwide historic emission estimates are compiled, assigned to 10 climate zones, and used as input in model calculations. The performance of the model is evaluated by comparing calculated and measured concentrations in the atmosphere and seawater. A major focus is on the arctic and northern temperate environment, and emphasis is also placed on absolute levels, time trends, latitudinal profiles, and air-water exchange. In general, simulated and observed concentrations agree within one order of magnitude. Deviations are explained by the zonal averaging characteristics of the model and uncertainties associated with the environmental degradation rates of ␣-HCH. In both model results and observations, the dramatic decrease in the global emission of ␣-HCH is reflected in rapidly decreasing concentrations in the atmosphere and seawater, except in the Arctic Ocean, which apparently is the last refuge for ␣-HCH in the global environment. The model thus provides an illustration of the response characteristics of the global system to the reduced emissions of ␣-HCH. Its application to other chemicals with reduced emission is discussed.
The ozone production in the troposphere has been studied by means of a zonally averaged model which consists of a two‐dimensional transport model, a description of the emissions, wet and dry deposition, and chemical processes of importance for the ozone production in the troposphere. The transport model describes a closed circulation in the meridional plane below 10 hPa and has a resolution and a numerical solution which compares favorable with earlier two‐dimensional studies. The transport model also takes into account the fast vertical mixing in convective clouds and in frontal circulation. The production of nitrogen oxides by lightning has been coupled to the convection parameterization by assuming that the nitrogen oxides are transported vertically in the thunder clouds and released at the altitudes where boundary layer air entrained in the convective cells is released. Comparisons with observations indicate that the model is able to reproduce the seasonal variation of ozone in the meridional plane quite realistically. The calculated distributions of the chemical species which determine tropospheric ozone also compare well with measurements. The model estimated an annually averaged production of ozone in the troposphere over the northern hemisphere of 16.6×1010 molecules/cm2/s and over the southern hemisphere of 5.1×1010 molecules/cm2s. The annually and globally averaged dry deposition is 14.9×1010 molecules/cm2/s, and the corresponding injection from the stratosphere is 4.1×1010 molecules/cm2/s. A 50% reduction of the man‐made emissions from the industrialized society of nitrogen oxides resulted in a reduction in the ozone production of 2.9×1010 molecules/cm2/s in the lower troposphere over the northern hemisphere during the period of maximum photochemical production, June–August. The corresponding production decrease due to a 50% reduction of the emissions of volatile organic compounds and carbon monoxide from the same source, however, was 1.6×1010 molecules/cm2/s. Elsewhere, the effects of reductions are less significant due to smaller influence of man‐made emissions.
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