Particle Dry Deposition to Water Surfaces: Processes and Consequences

Algal blooms (increased abundance of phytoplankton) are an increasingly common phenomenon which has been causally linked to increased fluxes of nutrient (particularly nitrogenous) compounds to aquatic ecosystems. These blooms have implications for water quality and human health in addition to ecosystem productivity, health and ecological diversity. Anthropogenic emissions of nitrogen to the atmosphere are estimated to be comparable to, or greater than, biogenic emissions but are considerably more concentrated in space. Although fluvial pathways typically dominate the annually averaged nitrogen flux to coastal waters, atmosphere–surface exchange represents a significant component of the total flux and may be particularly critical during the summertime when both the riverine input and ambient nutrient concentrations are often at a minimum. In this chapter, we present an overview of the physical and chemical processes which dictate the quantity (and direction) of atmosphere–surface fluxes of trace chemicals to (and above) water surfaces with particular emphasis on the role of particles. Dry deposition (transfer to the surface in the absence of precipitation) of particles is determined by meteorological conditions, atmospheric concentrations, surface type/condition and the specific chemical and physical properties of the particle. Dry deposition can be conceptualized as a three-step process: (1) the gas or particle is moved toward the surface by thermally or mechanically driven eddies; (2) it is transferred by diffusion across a thin layer close to the surface where turbulence is absent; and (3) the gas or particle is captured by the surface. In the case of larger particles a second parallel pathway exists; particles are drawn towards the surface by gravity. Atmospheric particles determine dry deposition fluxes not only by serving as a conduit for transfer but also because of their action as sources or sinks of trace gases. The example given here is the transfer of nitric acid to sea salt particles as a result of heterogeneous chemistry acting as a competing sink to surface removal. To illustrate the importance of current uncertainties in our understanding of dry deposition processes and to highlight the role of some of the key parameters in determining the transfer rate (the deposition velocity) a simple model of particle dry deposition is presented. The model describes the calculation of the rate at which a particle of a given size and chemical composition will be moved towards the surface under given environmental conditions. Observational and experimental techniques for measuring dry deposition fluxes are also reviewed. The techniques used for gases are largely reliant on use of highly temporally resolved sampling (e.g., concentrations sampled 10 times per second) or highly accurate and precise measurements of concentrations, either in the vertical to resolve the gradient to or from the surface or conditionally sampled by the direction of transfer (to or from the surface). These stringent measurement requirements represent significant barriers to application to measurement of particle dry deposition fluxes although, as discussed, innovative solutions are now becoming available. In the final section, we examine meteorological controls on deposition to the coastal zone. This region of the world’s oceans and seas is most significantly impacted by human activities. More than half of the world’s population lives within 100 km of a coast and hence the overwhelming majority of anthropogenic fluxes to aquatic systems occur in the coastal zone. We discuss the particular challenges that arise from efforts to simulate and measure fluxes close to the coastline. These arise in part from the complexity of atmospheric flow in this region where energy and chemical fluxes are highly inhomogeneous in space and time and thermally generated atmospheric circulations are commonplace.