Dry deposition is a natural process where atmospheric gases and tiny airborne particles are transferred directly from the air to surfaces on Earth without the aid of rain, snow, or fog. This constant, non-precipitative transfer acts as a significant sink, removing various substances and pollutants from the atmosphere. The process involves the adherence of these materials to surfaces such as vegetation, soil, buildings, and bodies of water. Dry deposition works alongside wet deposition as one of the primary ways atmospheric contaminants are removed.
The Physical Mechanism of Deposition
The movement of gases and particles from the turbulent atmosphere to a surface is conceptually modeled as a three-step physical process. The first step is aerodynamic transport, which involves turbulent mixing in the air to move the substance from the higher atmosphere down through the atmospheric surface layer. This transport is highly dependent on wind speed and atmospheric turbulence, where swirling air eddies push the material closer to the ground.
Once near the surface, the material must pass through a very thin, relatively stagnant layer of air known as the quasi-laminar sublayer. Within this layer, which is immediately adjacent to the surface, transport shifts from large-scale turbulence to molecular-level processes. Gases rely on molecular diffusion to cross this boundary, while very small particles are moved by Brownian motion.
The final step is the surface interaction, where the substance is taken up by the surface itself, such as via adsorption or absorption. For larger particles, a physical process called impaction becomes important, where the particle’s momentum causes it to collide with and stick to obstacles like leaves or needles. The speed of this entire process is quantified by the deposition velocity (\(V_d\)), a conceptual velocity that relates the flux of the deposited material to its concentration in the air.
Categorizing Deposited Materials
The materials involved in dry deposition fall into two main categories: gases and particulate matter, also known as aerosols. Gaseous pollutants include compounds like sulfur dioxide (\(\text{SO}_2\)), ozone (\(\text{O}_3\)), and ammonia (\(\text{NH}_3\)). The deposition of gases often relies on chemical reactions or solubility once the gas makes contact with the receiving surface, such as through plant stomata.
Particulate matter is a complex mixture of solid or liquid particles suspended in the air, with examples including dust, soot, sea salt, and sulfates. The mechanism by which these particles deposit is highly dependent on their physical size. Particles larger than ten micrometers in diameter are mainly removed by gravitational settling. Smaller particles, particularly those in the sub-micron range, rely more on impaction and Brownian diffusion to reach the surface.
Variables Controlling Deposition Rate
The rate at which dry deposition occurs is governed by a resistance model, which considers the obstacles the substance must overcome on its path to the surface. This model involves three main resistance components: aerodynamic resistance (\(R_a\)), quasi-laminar layer resistance (\(R_b\)), and surface resistance (\(R_c\) or \(R_s\)). Atmospheric factors largely control the aerodynamic resistance, where high wind speeds and increased air turbulence reduce \(R_a\), thereby accelerating the transport of material toward the ground.
The quasi-laminar layer resistance, \(R_b\), is influenced by the physical properties of the depositing material, such as its molecular weight and diffusivity. The surface resistance, \(R_c\), is the most variable component, determined by the characteristics of the receiving surface. For vegetation, \(R_c\) is closely linked to biological activity, such as the opening and closing of stomata, which allow gas exchange.
Surface properties like roughness, wetness, and temperature also play a significant role in controlling the deposition rate. A rough surface, such as a forest canopy, increases the surface area for collection and enhances turbulence, leading to faster deposition. Conversely, a dry surface may offer high resistance to the uptake of a soluble gas, while a wet surface or dew-covered leaf can significantly enhance deposition by dissolving the gas.
Impact on Ecosystems and Air Quality
The transfer of airborne material via dry deposition has significant consequences for natural ecosystems and air quality. A primary environmental concern is the contribution to acidification, which results from the deposition of sulfur and nitrogen compounds. When these acidic compounds settle onto soil and water bodies, they can lower the \(\text{pH}\), harming sensitive aquatic life and altering soil chemistry.
Reactive nitrogen compounds, such as ammonia and nitrogen oxides, also contribute to nutrient loading in ecosystems. This excess input of nitrogen can disrupt the natural balance of a habitat, leading to eutrophication in water bodies and allowing nitrogen-tolerant plant species to out-compete others. The resulting shift in species composition can reduce biodiversity and alter the fundamental function of the ecosystem.
Dry deposition of particulate matter directly impacts air quality and visibility. Fine particles like sulfates and soot scatter and absorb sunlight, contributing to regional haze. While the process removes these pollutants from the air, their accumulation on surfaces, particularly near emission sources, poses a continuous environmental challenge.