Australia State of the Environment Report 2001 (Theme Report)
Lead Author: Dr Peter Manins, Environmental Consulting and Research Unit, CSIRO Atmospheric Research, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06746 9
Only one-quarter of the sulfur in the atmosphere is natural (Figure 8), and the rest is caused by human activity. Nature releases sulfur through decomposing marine algae and volcanic eruptions. Industry releases sulfur dioxide when coal and oil are burnt and sulfide ores smelted. Transport can be another source of sulfur dioxide. However, Australian fuels have a low sulfur content.
Figure 8: Annual emissions of sulfur dioxide (as millions of tonnes of sulfur, MtS) and the atmospheric sulfur dioxide cycle.
Source: CSIRO Atmospheric Research
Nitrogen oxides are generated by lightning and microbes, and by burning of fossil fuel and biomass.
Natural chemical processes in the lower atmosphere transform sulfur dioxide and oxides of nitrogen into trace levels of sulfuric and nitric acids, respectively. These chemicals can be further transformed into aerosols. The acids, the gases and their aerosol products are returned to the earth's surface in two main ways:
- through wet deposition in rain or snow (often called 'acid rain')
- through dry deposition, in which atmospheric particles and gases are deposited directly on water, soil, vegetation or other surfaces.
The natural emission, atmospheric transformation, transport and deposition of sulfur and nitrogen are integral and natural parts of the global nutrient cycle.
As acids and their precursors typically persist in the atmosphere for only a few days, human-produced emissions are usually deposited within some hundreds of kilometres of the source. Acid deposition (mainly in the form of 'acid rain') is high in regions surrounding urban and industrial centres in the mid-latitudes of the Northern Hemisphere, where it has caused substantial environmental problems. In Europe, long-range transport of acid gases is recognised as a serious transboundary pollution issue. Aerosols can also affect climate.
Australia does not experience significant long-range transport of acid pollutants from neighbouring countries.
Environmental damage from acid deposition can occur when the extra acid added to surface soil or water exceeds the capacity of these ecosystems to accommodate it. Acidification in lakes and streams may cause loss of aquatic life, whereas on land it may damage leaves and reduce growth of trees and other plants.