In this chapter, we continue to use the Air Quality Index (AQI) defined in the previous 2 state of the environment reports (Table 8), so that the state and trends of Australia’s air quality reported here can be directly compared across years. The AQI takes each pollutant measurement and uses the National Environment Protection Measures (NEPM) standard as the basis to make the comparison:
This means that, if a pollutant is measured at the limit set by the NEPM, the AQI calculation would yield a value of 100; if the measurement is at half the limit, the AQI would be 50.
New NEPM standards for ozone, nitrogen dioxide and sulfur dioxide were agreed on 15 April 2021. As all air quality data for this report were collected before this date, any assessments will be made against the previous NEPM standards.
The AQI also allows an easy comparison of levels of different pollutants across different locations. However, the use of different descriptions for the AQIs among the jurisdictions has been confusing (see Summer 2019–20 bushfires). The jurisdictions are now working towards unifying how they report air quality levels for clearer public messaging.
To make the air quality assessments in this report, data from every day between 1 January 2015 and 31 December 2019 for ozone, coarse particulate matter (PM10) and fine particulate matter (PM2.5) from each jurisdiction were evaluated against the relevant air quality limit. The total numbers of ‘hits’ for each AQI category were added, and the percentage of the total number of days for the AQI category was determined.
The overall assessment for each pollutant in each capital city is calculated by comparing the AQI category percentages with the rules in Table 9. For example, if a city has more than 50% of PM10 measurements in the ‘very good’ category but 7% in the ‘poor’ and 2% in the ‘very poor’ categories, the overall assessment will still be ‘very good’.
The trend in the assessment is calculated by comparing the results for 2015–19 with the results from the previous 2016 and 2011 state of the environment reports. For example, if the percentage of ‘very good’ AQIs has consistently increased, the trend will have improved.
Population-weighted annual PM2.5 maps
The United Nations Sustainable Development Goal (SDG) 11 is to make cities and human settlements inclusive, safe, resilient and sustainable. Relevant to air quality, SDG 11 has a target to reduce the adverse per capita environmental impact of cities by 2030. The key indicator for this target is the annual PM2.5 concentration, weighted by the population. As population-weighted PM2.5 has not been previously evaluated for Australia, we need to make an initial baseline assessment with which future state of the environment assessments can be compared. In this way, we will be able to establish whether the SDG 11 target will be met in Australia by the 2026 state of the environment report.
In this chapter, we mapped the annual mean levels of PM2.5 in cities (population weighted). The PM2.5 concentration data were taken from a satellite and land use regression model for 2018 (Knibbs et al. 2018, Knibbs 2020) at a resolution of 1 km. These concentration data, C, were weighted by the Australian population, P, resident in each of these 1 km × 1 km squares, summed across each Statistical Area Level 2 (SA2), i (ABS 2015). SA2 areas are defined as being at the suburb level containing 3,000–25,000 people, with an average of 10,000 people.
For example, divide an SA2 region into squares 2 km on a side. Square X has 200 residents and an annual PM2.5 concentration of 2 micrograms per cubic metre (μg/m3). The adjacent square Y has 800 residents and an annual PM2.5 concentration of 6 μg/m3. The population-weighted PM2.5 in this example SA2 region is
In producing the population-weighted maps, there were approximately 30–40 SA2 regions that had zero population – for example, in areas of wilderness. To avoid dividing the annual PM2.5 concentrations by zero population, the average PM2.5 across all the 1 km × 1 km grid cells in these SA2 regions was used instead.