As part of the State of the Environment reporting process, a qualitative assessment was made of ambient air quality in the eight state and territory capitals and a small number of major regional or industrial centres. Of the seven pollutants for which national health-based standards have been set, photochemical oxidants (as ozone) and particulate matter smaller than 10 micrometres (as PM10) were chosen as key pollutants potentially impacting on human health, reflecting the weight of scientific evidence.
The approach based the characterisation of an airshed from 1999 to 2009 on its worst performing monitoring station, rather than on the total number of exceedences across the airshed, since this is strongly influenced by the number of monitoring stations. Only data from monitoring stations established in accordance with an approved National Environment Protection Measure (NEPM) monitoring plan were considered. (In some cases, less than 10 years of NEPM monitoring data were available, and in one case—Perth—11 years of data were available.) Most large regional cities have only one NEPM monitoring station, and most monitor particles, but not ozone, since they lack the scale of industry and traffic likely to give rise to ozone as a secondary pollutant. In each state, the regional cities selected for analysis of PM10 and ozone (where this was monitored) were the worst performing in the state.
It is recognised that the 10-year goals set in the Ambient Air Quality NEPM for ozone and particles allow for one exceedence per year for ozone and five exceedences per year for particles. Nevertheless, given the nature of the health-based standards, any exceedence may have a potentially adverse impact and should therefore be taken into consideration, even if the goal is met.
Ozone levels were evaluated against the four-hour exposure standard rather than the one-hour, as the four-hour standard is more likely to give a better indication of the impact on the general population, rather than on sensitive individuals who are likely to be affected by acute (i.e. shorter term) events.
Procedure
For each year, monitoring data for ozone and PM10 from each of the selected stations were converted into air quality index (AQI) values. These were used as the basis for calculating the percentage of observations that fell in each of the five AQI-based qualitative categories (very good, good, fair, poor and very poor) commonly used by Australian environment protection agencies to report air quality.
Each of these yearly percentage distributions for each pollutant at each station was then assessed against the criteria set out in Table A, to assign a general AQI score to each pollutant. The results across the period were represented graphically to assist in identifying any trends. It must be emphasised that the criteria set out in Table A are essentially subjective in nature. In almost all cases, their application resulted in the most frequently occurring AQI category being selected to generalise the year as a whole. In a small number of years, the AQI distribution was bimodal, with the result being borderline between the very good and good categories.
Overall qualitative AQI scores for ozone and PM10 were then assigned to each city, based on the most frequently occurring scores during the decade. A summary of the results is presented in assessment summaries 3.4 to 3.7, and the complete set of graphs is available on the State of the Environment website.a
Table A Criteria for assigning annual AQI-based qualitative scores
Overall category |
Very good (%) |
Good (%)
|
Fair (%) |
Poor (%) |
Very poor (%) |
Very good |
>50 |
>20 |
<10 |
<10 |
<5 |
Good |
>20 |
>30 |
<20 |
<20 |
<10 |
Fair |
<10 |
<20 |
>30 |
>20 |
<10 |
Poor |
<10 |
<20 |
<20 |
>30 |
>20 |
Very poor |
<5 |
<10 |
<10 |
>20 |
>50 |
Supplementary rules
If the percentage very good is greater than 45 and is also greater than the percentage good, the assessment grade is
very good.
If the percentage good is greater than 75, then the percentage very good can be as low as zero and assessment
grade is good.