Determination of Fog-Droplet Deposition Velocity from a Simple Weighing Method

Fog water deposition can represent an important part of the atmospheric water, nutrient and pollutant inputs in specific areas such as mountainous or coastal regions (Shimadera et al., 2011). In order to determine the potential of fog water deposition on plants, a field experiment has been performed in the northeast of France to determine fog droplet deposition velocity on different types of plants. The main objective is to improve deposition models by enabling them to accurately account for water inputs from fog or low clouds at ground level. The flux of deposited fog water was estimated by exposing plants to fog and weighing them with a precision balance. Contrary to other flux measurement methods, the weighing method is simple to set up. Three plant types (small conifers, grass and cabbages) plus bare soil were used as impaction and deposition surfaces. A Particulate Volume Monitor (PVM-100) provided the liquid water content (LWC) to calculate fog droplet deposition velocities, and a Fog Monitor (FM-120), the characterization of the droplet size distribution. Two fog events with different features (visibility, LWC and droplet number) were compared with regard to deposition velocity. When wind speed was below 4 m s–1, mean fog droplet deposition velocities ranged from less than 2.2 cm s–1 on bare soil to 40 cm s–1 on cypress. Thus, the impaction of fog droplets can be an important part of fog water deposition on plants.