OBSERVATIONS OF CLOUD CHEMISTRY DURING LONG-RANGE TRANSPORT OF POWER PLANT PLUMES

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Abstract

Measurements of the chemical composition of cloud water have been made as part of a programme to study the chemical development of power plant plumes in trajectories over the North Sea.

During a two-day study (28–29 January 1981), the conditions were anticyclonic with light winds advecting the plume from the NE coast of England towards Denmark. The mixing layer overland was capped by stratocumulus beneath a very strong subsidence inversion, which resulted in the plume being entirely trapped within the layer. Low level acceleration occurred as the plume travelled towards the coast, accompanied by a shallowing of the mixing layer. This led to the unusual situation whereby the plume was confined to a shallow (400 m) stratocumulus-filled boundary layer throughout most of its travel. The light winds enabled approximately Lagrangian sampling of the plume after about 5 and 22 h travel (˜ 100 and 650 km from source). The very shallow boundary layer constrained the dilution of the plume to such an extent that even though ambient O3 was consumed within the plume by the reaction with NO, the NO2/NOx ratio was still <0.5 along the plume centre line after 22 h travel.

The measurements have been compared with the predictions of a reactive plume model involving both gas phase and solution phase chemistry. The model predicts oxidation rates for SO2 in the ambient air outside the plume to be substantially higher than those within the plume, at values of 0.5–1.0 and ˜ 0.04%h−1 respectively. This leads to the conclusion that nearly all the sulphate in the plume arose from entrainment of sulphate produced in cloud droplets outside the plume. The absence of an effective oxidation mechanism in solution for the conversion of NOx to HNO3 suggests that nitrate in the cloud water was derived from the gas phase oxidation of NOx. HCl was found to be the major contributor to cloud water acidity in the plume on this occasion. The resultant acidity suppressed the solubility of SO2 and this together with the low oxidant levels inhibited the production of sulphate in solution within the plume. The HCl contribution to acidity had declined markedly after 22 h travel and this loss corresponds to a dry deposition velocity of 13 mm s−1.

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