Dust particles, often referred to as particulate matter (PM), in the atmosphere arise from a wide variety of sources. Both the size and chemical composition vary widely in relation to the nature of the source and the history of the particles. Coarse particulates can be regarded as those with a diameter greater than 2.5 μm (e.g. PM10 – 10 μm), and fine particles less than 2.5 μm (PM2.5). Under humid conditions many particles attract water vapour and grow to form small droplets. The term 'aerosol' is often used for both solid particles and droplets suspended in air.
Particulate matter may be generated mechanically, for example by the wind, may be emitted directly to the atmosphere or may be formed by reactions in the atmosphere from precursor gases (e.g. Pruppacher & Klett, 1978). The three major sources, each forming their own size mode, are (e.g. Noble & Prather, 1996):
1. Chemical reactions of gases in the atmosphere.
These can result in the production of small particles of a few nm diameter (1 nm is one thousand millionth of a metre) which grow relatively quickly by coagulation. There are a number of formation paths:
- sulphates are formed from the atmospheric reaction of sulphur dioxide (SO2) derived from man-made or volcanic emissions, or from natural oceanic emissions of dimethylsulphide (DMS).
- nitrates are formed from the oxidation of nitrogen dioxide (NO2), which itself is mainly derived from the oxidation of nitric oxide (NO) released during fossil fuel combustion, to form nitric acid, which can then react to produce nitrates.
- ammonium containing aerosols result from reaction of ammonia (NH3) emissions with acidic gases.
- organic aerosols result from gas-phase oxidation of man-made and natural emissions of non-methane volatile organic compounds (NMVOCs), usually in the presence of sunlight to drive the oxidation process.
2. Combustion processes.
These include industrial and transport related combustion processes, which directly emit fine particles, typically in the size range 0.1 - 2.5 µm diameter (1 µm is one millionth of a metre):
- carbon containing particles (soot) are emitted from the combustion of carbon-based fuels (coal, oil, natural gas) by industry and vehicles
- particles containing heavy metals are emitted from a wide variety of industrial and transport sources. Concentrations of heavy metals in air as particulate matter (PM) do not currently exceed guidelines for human health in rural air, but there are few data to quantify the risks in urban areas, especially where resuspension from traffic may be an important secondary source (Derwent et al., 2009)
- fly-ash particles are emitted from high temperature combustion of coal
3. Mechanical generation.
This produces coarse particles (2.5 - 20 µm) which are distributed by wind turbulence:
- metal-containing particles come from frictional sources, such as wear of tyres and brakes (Thorpe and Harrison, 2008)
- mineral-containing particles are emitted from erosion of agricultural soils, volcanic eruptions, quarrying and building activities
- cement and fertiliser dusts come from factories and construction sites – some may also be emitted directly
- sea-salt particles are produced from bubbles bursting over the oceans and at the sea coast
Chemical reactions of acidic SO2 and NOx with alkaline NH3 also occur on the surface of existing particles. These reactions lead to substantial mixing between the different pollutants contained in particles. Hence, after a lifetime of several days in the atmosphere, an individual particle might contain both water-soluble components (e.g. sulphate, nitrate and ammonium) and insoluble components (e.g. carbon, heavy metals or soil).
The rates at which particles are removed from the atmosphere depend strongly on their size (e.g. Slinn, 1982). Large particles deposit rapidly near their source by gravitational settling, while very fine particles also deposit relatively fast because they are mobile. Particles of intermediate size range (0.1 - 2 µm) deposit slowly, and can therefore be transported over long distances between different countries and continents.
Impacts and Issues
Given the wide variety of pollutants and chemical compositions involved, dust particles play a role in many air pollution issues. These include:
- human health effects of particles; respiratory and heart problems caused by inhalation of small particles has been related to the mass concentration of particles below 10 µm diameter (PM10, e.g. Prescott et al., 1998)
- localised effects of dusts covering vegetation (Farmer, 1993)
- deposition of regional pollutants causing acidification and eutrophication
- deposition of heavy metals with toxic effects on plants, animals and humans
- transboundary transport of air pollutants as fine particles
- light scattering leading to the potential to offset global warming.
- climate change will affect PM concentrations in polluted environments by ±0.1-1μg m-3 over the coming decades. Wildfires fuelled by climate change could become an increasingly important PM source (RoTAP, 2012)
- light scattering leading to reductions in visibility
The trends in aerosol concentrations over time reflect the rates of emissions of the primary gaseous pollutants, as noted above.