Aerosol dry deposition on vegetative canopies. Part I: Review of present knowledge

Abstract

This paper reviews the present knowledge of aerosol dry deposition, with respect to modelling and experimental aspects. In the first part, special attention is given to the existing mechanistic models, either with an analytical or a differential structure. Their predictions are compared against available measurements for grass and forest environments, obtained under controlled aerosol size and aerodynamic conditions. The observed differences are largely related to the parameterisation of the aerosol collection within the canopy. In the second part, existing experimental results are reviewed and a synthesis is provided through different inter-comparisons concerning: (1) the influence of atmospheric stability on fine particle deposition, (2) the evolution of coarse aerosol deposition with aerodynamic conditions and (3) the aerosol size dependence of deposition on grass and forest canopies. A wider compilation of measurements obtained on different canopies is finally proposed.

Introduction

Dry deposition of particles containing compounds such as sulphate, nitrate or radioactive substances has been studied worldwide over the last 30 years, and there remains a discrepancy between both measurement results and model predictions. Similar problems are encountered when considering biological hazards, for instance in connection with contamination by genetically modified organisms, spores or bacteria (such as the legionnella bacteria, among others). Despite its importance and the considerable research focused on this topic, our knowledge on aerosol dry deposition remains incomplete.

Together with wet deposition, dry deposition of aerosols is responsible for delivering to the biosphere atmospheric loads of various compounds: particles containing SO₄²⁻, NO₃⁻ and NH₄⁺ that contribute to potential acidification and eutrophication of the ecosystems, natural or anthropogenic radioactive particles, whose impact on the biosphere has to be estimated, base cations such as Na⁺, K⁺, Ca²⁺ and Mg²⁺ that influence the nutrient cycling in soil and ecosystems and toxic heavy metals, such as Pb, Cd and Zn (Ruijgrok et al., 1995).

Many models seek to explain the deposition process to complex surfaces by taking into account variables such as particle granulometry (size distribution), surface morphology and meteorology (Legg and Powell, 1979; Davidson et al., 1982; Slinn, 1982; Wiman and Agren, 1985; Giorgi, 1986; Peters and Eiden, 1992; Zhang et al., 2001). They can be designed for a unique type of surface or for various canopy configurations. The comparison of their predictions in the modelling framework of Slinn (1982) concludes that there are large differences among them (Ruijgrok et al., 1995).

Today, there is a large database of experimental results for various vegetation surfaces, obtained under different conditions (aerosol characteristics, meteorological situations) with several techniques (see the reviews of McMahon and Denison, 1979; Sehmel, 1980; Gallagher et al., 1997b; Zhang and Vet, 2006). As information describing the experimental conditions is not always reported, a comparison of different campaign results remains difficult. However, a relevant review by Gallagher et al. (1997b) suggests an apparently consistent trend in field measurements of the deposition velocity across the size spectrum. As these measurements have been obtained with different methods and under different aerodynamic conditions, there are still uncertainties about the size dependence of the deposition and about the influence of other parameters that describe the meteorological conditions and the canopy geometry. In addition, a conclusive comparison of the available data is far from being reached, due to the lack of measurement technique inter-comparison (Wesely et al., 1985; Hicks et al., 1989), which should provide the sources of artefact and the uncertainties for each technique (Gallagher et al., 2002).

In this paper, we propose to review the existing knowledge on dry deposition by emphasising aerosol dynamics. Thus, special attention is given to the mechanical processes which control the aerosol deposition. In the first part, the most commonly used mechanistic (or process-oriented) models are described and their predictions are compared on two typical canopies, which are grass and coniferous forests. This was done preserving the specific model characters (analytical formulation or differential equation system). In the second part, a comprehensive review of the aerosol dry deposition measurements is performed, from which a detailed comparison is inferred. This synthesis emphasises the influence of atmospheric stability on fine particle deposition, the dynamics of coarse aerosol deposition (diameter around 20 μm), the size dependence of the deposition and the influence of the canopy geometry. Finally, measurements obtained under different aerosol size, aerodynamic and canopy conditions are compiled.