Deposition of trace substances via cloud droplets in the Atlantic Rain Forest of the Serra Do Mar, São Paulo State, SE Brazil

The Atlantic Rain Forest of the Serra do Mar close to Cubatão, São Paulo State, SE Brazil, is severely affected by the emissions of a big industrial complex. Measurements of trace substance concentrations in air as well as in rain were carried out over a period of 6 years to investigate the interrelation of pollution and vegetation damage. Due to the local atmospheric circulation, orographic clouds occur very frequently at the top of the Serra do Mar, where vegetation damage also is very high. Therefore, additional information was required about trace substance deposition via cloud water. During three field experiments, various samples of cloud water and of the interstitial aerosol were taken by help of a sampling device especially modified for this purpose, and were analysed for the major anions and cations (hydrogen ions, ammonium, chloride, sodium, nitrate and sulphate). The trace substance concentrations found in cloud water were in the same range as for polluted sites in Europe. Over all samples, about 90–100% of the trace substances—both gaseous and particulate matter—found in the atmosphere before a cloud event were found in the cloud water. A resistance model using meteorological input data (wind speed, atmospheric liquid water content) was adapted to the characteristics of the Atlantic Rain Forest to estimate the cloud water deposition to vegetation. The results from cloud water analyses and from modelled cloud water deposition were combined to investigate the ion deposition to the vegetation. A rough estimate of the annual deposition showed, that the deposition via rain is in the order of one magnitude (factor 6–40) higher than that via cloud. The high amount of water deposition via precipitation overcompensates the higher trace substance concentrations in cloud water. Furthermore the trace substance deposition to vegetation via cloud water in the Atlantic Rain Forest is in the order of one magnitude lower than for typical German spruce forests due to the higher capture efficiency of the needles compared to that one of the leaves in the Serra do Mar.