Introduction

Earth's ecosystems are dynamic and can adapt to those environmental factors capable of causing injury, disease or mortality. These factors, termed stresses, can be of a biotic and/or abiotic nature. Among abiotic stresses, air pollution ranks as the most harmful (Fig. 8.1). Atmospheric particulate pollutants, especially primary pollutants (i.e. those released directly into the troposphere by natural or anthropogenic processes) include heavy metals and other elements. ‘Heavy metals’ (or trace or toxic metals) is a term applied to a large group of trace elements of both industrial and biological importance, with atomic densities greater than 6 g cm-3. Dust containing heavy metals is dispersed globally by atmospheric circulation and becomes a significant component of sediments, soils, and the hydrosphere. This has a major impact on Earth's ecosystems, owing to the rates and mechanisms by which atmospheric pollutants are (a) transferred to Earth's surface, (b) made available to receptor organisms, and (c) taken up by organisms. Some studies report that dust particles may contribute via dry deposition from 20% to as much as 90% of the total annual atmospheric deposition to forest canopies (Linbergh & Harris, 1981). Annually, wet and dry deposition contributes significant fractions of the total flux to forest floors; for Pb (70 to 100%), Zn and Cd (each ∼ 40%) and Mn (∼ 10%).