Pii: S0025-326x(00)00112-0

Algal blooms (increased abundance of phytoplankton) are an increasingly common phenomenon which has been causally linked to increased ̄uxes 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 ̄uvial pathways typically dominate the annually averaged nitrogen ̄ux to coastal waters, atmosphere±surface exchange represents a signi®cant component of the total ̄ux 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 ̄uxes 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 speci®c 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 di€usion 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 ̄uxes 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 ̄uxes 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 signi®cant barriers to application to measurement of particle dry deposition ̄uxes although, as discussed, innovative solutions are now becoming available. In the ®nal section, we examine meteorological controls on deposition to the coastal zone. This region of the worldÕs oceans and seas is most signi®cantly impacted by human activities. More than half of the worldÕs population lives within 100 km of a coast and hence the overwhelming majority of anthropogenic ̄uxes to aquatic systems occur in the coastal zone. We discuss the particular challenges that arise from e€orts to simulate and measure ̄uxes close to the coastline. These arise in part from the complexity of atmospheric ̄ow in this region where energy and chemical ̄uxes are highly inhomogeneous in space and time and thermally generated atmospheric circulations are commonplace. Ó 2000 Elsevier Science Ltd. All rights reserved.