The Deposition of acidifying components on wet surfaces
The contribution of dew to acid deposition has been investigated by means of measurements as well as modelling. Frequency and duration of the occurrence of dew have been calculated by a dew model. The model is based on an analysis of the energy balance of a surface and only requires the input of a limited number of synoptic data. Model results proved to be in fairly good agreement with measurements of dew using an automatic sequential dew sampler. The number of dewy nights and hours was calculated for the year 1987 and amounted to about 225 nights and about 1600 hours. Thus dew is a frequently occurring phenomenon at night.
Dew has been sampled in different -urban and rural- locations and the chemical composition of these samples was compared with the composition of rain.
Contrary to rain, a strong influence of local air pollutant concentrations on dew compostion was observed. The mass fluxes of the main components and some trace elements in dew are small in comparison with rain. Furthermore, also in contrast with rain the unstable and reactive anions sullphite, nitrite and organic substances are practically always present in dew in high concentrations. Dissolved sulphur dioxide appeared to be mainly present as hydroxymethane sulphonic acid. Contrary to the deposition of species from the gas phase, it was established that aerosol fluxes deposited on the lower side. The deposition velocity of sulphur dioxide has been calculated from the measured concentrations of sulphur components in air and dew and shows a strong correspondence with the amount of dew.
KeywordsSulphur Dioxide Deposition Velocity Urban Site Deposition Flux Sulphur Component
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