Sometimes all that I need is the air that I breathe ...
Department of the Environment and Heritage
GPO Box 787 Canberra ACT 2601 Australia
Telephone: +61 (0)2 6274 1111
Non-Dust Atmospheric Emissions From Minerals Processing
Environment Australia
2002
www.deh.gov.au/industry/industry-performance/ minerals/booklets/atmosphere/what-is.html

What is clean air?

The major constituents
The earth's atmosphere has evolved over geological time to its present composition, which, excluding water vapour, comprises a mixture of gases; approximately 78% nitrogen, 21% oxygen and 1% argon. The concentration of water vapour is highly variable but can reach up to 3%. This booklet is mainly concerned about gases present at concentrations much less than 1%.
The importance of trace constituents
With the exception of water vapour, the current proportions of gases are maintained by equilibrium processes which include biogenic and anthropogenic activity as well as geological processes which in many cases are intimately connected with biogenic processes. Scientists estimate that the current level of oxygen in the atmosphere was reached approximately 400 million years ago (Cloud, 1983). As the input and removal rates of the constituent gases change so too does the point at which equilibrium is established. Carbon dioxide is perhaps the most obvious example where this is occurring. Since the industrial revolution, the rate of input of carbon dioxide to the atmosphere has increased. If the input rate were to stabilise at its new higher level a new equilibrium would be established where concentrations would be higher than in the past and at the point where the removal rate matched the rate of input. The same principle applies for all the constituents of the atmosphere.
These gases are transparent to visible light. At sea-level light scattering from particle-free air would allow a theoretical visual range of approximately 320 km (Stern et al., 1984), which is well beyond the distance to horizon for most ground-based observers. Trace contamination can reduce visibility significantly. Small particles will scatter light and reduce visibility. The United States Environmental Protection Agency (US EPA, 1979) estimates that a density of 1 µg/m3 of particulate matter in the size range effective for scattering visible light (0.1 to 1 µm) along a sight line will reduce visual range by 30% from 320 km to 224 km and 10 µg/m3 will reduce visual range to between 30 and 40 km.
...
Not only do small changes in the composition of the atmosphere have significant effects on visibility and climate, but small changes in the concentrations of other trace gases can have implications for health and the viability of life. For example the World Health Organisation ( WHO Fact Sheet No 187 www.who.int/inf-fs/en/fact187.html , 2000) guideline value for 15-minute exposure to carbon monoxide is 100mg/m3 (87 ppm) and the 10-minute exposure guideline for sulphur dioxide is 0.5mg/m3 (0.175 ppm). These exposure levels are acceptable, but concentrations at ten times these levels are not.
Thus clean air is best defined by quantifying the trace levels of harmful polluting gases rather than focussing on the major constituents ...
Last updated: Sunday, 20-Jun-2004 05:52:11 EST
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State of the Environment Tasmania (2003)

The preparation of this SoE Report is one of the features of Tasmania's Resource Management and Planning System. SoE reporting provides a means to assess progress towards the sustainable development objectives www.rpdc.tas.gov.au/soer/copy/15/index.php defined under the system ...
A 'printer-friendly' version of the report, which contains the 'at a glance' pages from this website and the recommendations in full, can be downloaded from : www.rpdc.tas.gov.au/soer/file/66/index.php
Atmosphere Key Concepts
Structure of the Atmosphere
www.rpdc.tas.gov.au/soer/copy/38/index.php
Composition of Air Near the Earth's Surface
Gases Formula % by Volume
Nitrogen N2 78.08
Oxygen O2 20.95
Argon . Ar 0.93
Neon . Ne 0.001 8
Helium He 0.000 5
Hydrogen H2 0.000 05
Xenon Xe 0.000 009
Carbon dioxide CO2 variable (average 0.036)
Methane CH4 variable (average 0.000 1)
Ozone . O2 variable (polluted air average 0.000 004)
Carbon monoxide CO variable (polluted air average 0.000 02)
Sulfur dioxide SO2 variable (polluted air average 0.000 001)
Nitrogen dioxide NO2 variable (polluted air average 0.000 001)
Particles (dust etc.) variable (polluted air average 0.000 01)
Water vapour H2O variable (up to 4% in some areas)
Source: adapted from Crowder 1995 <source/523/index.php?PHPSESSID=4d6dd6e9890cd5187d5d0d971f1c37 3a&g

[tag: science]
[tag: environment]
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