Chemistry of cloud water at an upland site in mid-Wales

Weekly samples of cloud water and bulk precipitation were collected at an upland site in mid-Wales and analysed for pH, Na, K, Ca, Mg, NH4, NO3, SO4, Cl and dissolved organic carbon (DOC). Cloud water chemistry was dominated by seasalt ions which comprised ∼77% of the total ion sum and approximately one half of the sulphate was of non-seasalt origin. Cloud water samples were often black or grey in colour due to a suspension of fine particulate material. Solute concentrations in cloud water were large relative to those in bulk precipitation. Ratios of ion concentrations in cloud water relative to rain water were: 11 for Na, Mg and Cl, 8.5 for NH4, 7.5 for SO4 and NO3, 6.5 for K and 2.5 for DOC. These large ratios were observed primarily because the collectors were located close to the cloud base, although collector artifacts also contribute. In the absence of rain, cloud liquid water content was estimated as 0.17 g m−3 and sulphate and nitrate aerosol concentrations as 1.3 μg S m−3 and 0.8 μg N m−3, respectively. These estimates lic within the range of values reported for other studies in the UK. The annual deposition of sulphur and nitrogen from cloud water to a 10-m high forest canopy at the site was estimated as 0.85 g S m−2 year−1 and 0.53 g N m−2 year−1, respectively, giving values of total wet deposition to the forest of 2.25 g S m−2 year−1 and 1.39 g N m−2 year−1. These estimates indicate that the presence of a forest canopy enhances wet deposition inputs of nitrogen and sulphur by ∼41% relative to the inputs to moorland vegetation.