About the air quality index


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1998-2008

In June 1998, there were two major changes in relation to air quality and how it is reported:

  1. national standards were set for how air quality is measured (using five criteria air pollutants)
  2. national reporting process was introduced.

The five pollutants to be measured under the 1998 national standards are ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide and air particles. 

In 2002, the then Environment Protection Authority began reporting on these pollutants to the national body. The old Regional Pollution Index (RPI) was updated in 2008 and was based on the five criteria pollutants (as per national standards) plus visibility (as per a standard set by NSW) at all sites in the air quality monitoring network.

1993-1998

In 1993, following a major review of monitoring needs, the NSW Government upgraded the monitoring network and extended it to the lower Hunter, Illawarra and western area of Sydney.

As a consequence of the expansion of the network, daily reporting of air quality was also reviewed, resulting in the reporting of a Regional Pollution Index (RPI). The RPI was produced for three regions in Sydney (Eastern Sydney, North Western Sydney and South Western Sydney) three sites in the lower Hunter (Newcastle, Wallsend and Beresfield) and three sites in the Illawarra (Wollongong, Kembla Grange and Albion Park).

Pre 1993

Sydney's air has been monitored for a range of pollutants since the 1960s. By the early 1980s, daily air quality reports were being released, based specifically on concentrations of ozone and fine particles. These Sydney Pollution Index (SPI) reports used a simple linear scale that reported pollution levels as low, medium or high. The SPI measured ozone and fine particles from 6am to 3pm. This reflected an emphasis on visible pollution and focused on daytime visibility as a measure of air quality.

At that time Sydney's population lived mainly east of Parramatta and in the south-western areas of Liverpool/Campbelltown. Monitoring was therefore concentrated in these areas. 

Air quality index (AQI) values

AQI values are derived from air quality data readings, which allows for more meaningful comparison of pollutants affecting air quality. The index is derived using the following formula:

 

AQ+ = pollutant data reading divided by the standard for that pollutant and multiplied by 100

 

Data readings are translated on to a linear scale based on relevant NEPM standards (for ozone, nitrogen dioxide, carbon monoxide, sulfur dioxide and PM10) and the OEH standard for visibility to derive the AQI values for the hourly AQI and daily AQI.

 

The AQI (air quality index) is updated hourly on OEH's air quality index webpage and provided to the electronic media and published in newspapers serving Sydney, the lower Hunter and Illawarra.

This information is also available by calling Environment Line:

  • 131 555 (local call cost throughout NSW)

  • (02) 9995 5555 (if calling from outside NSW).

More about the AQI and how it is calculated

Data readings

In order to provide more detailed information than is available in the AQI, measurements are also published as hourly data readings and daily data readings in scientific units rather than the derived values provided in the AQI.

Data readings are the actual scientific measurements for each air pollutant. The data readings are recorded in different units of measure, depending on the type of pollutant.

PollutantUnits used for air quality data
Ozone pphm (parts per hundred million)
Nitrogen dioxide pphm (parts per hundred million)
Visibility (as Bsp) 10-4 m-1
Carbon monoxide ppm (parts per million)
Sulfur dioxide pphm (parts per hundred million)
Particles µg/m3 (micrograms per cubic metre)

Averaging periods

Hourly

Data is collected from each site where these parameters are measured in the OEH network and reported as hourly AQI values and hourly data readings in scientific units

Daily

The air quality daily summary lists maximum pollutant values measured from 01.00 am to 12.00 midnight the previous day and presents them as a daily AQI values and as daily data readings in scientific units.

 

  HourlyDaily 
01.00am to 12.00 midnight
Ozone
Sulfur dioxide
Nitrogen dioxide 
Visibility
1-hour average Maximum 1-hour average of the 24 1-hour average between 01.00 am and 12.00 midnight
Ozone

Rolling 4-hour average: is an average of the previous 4 hours for each hour of the day. For example, for the hour ending 05:00, the 4-hour rolling average is calculated from the values for hours 02:00 to 05:00, for the hour ending 06:00 the next 4-hour rolling average is calculated as the average of values for hours 03:00 to 06:00.

(Note: The time of the average refers to the last hour of the averaging period - therefore averages for 0:01 to 03:00 includes data from the day prior to the reported day.)

Maximum rolling 4-hour average between 01.00 am and 12.00 midnight
Carbon monoxide

Rolling 8-hour average: is an average of the previous 8 hours for each hour of the day.

(Note: The time of the average refers to the last hour of the averaging period - therefore averages for 0:01 to 07:00 includes data from the day prior to the reported day.)

Maximum rolling 8-hour average between 01.00 am and 12.00 midnight

PM10
PM2.5

Rolling 24-hour average: is an average of the previous 24 hours for each hour of the day.

(Note: The time of the average refers to the last hour of the averaging period - therefore averages for 0:01 to 23:00 includes data from the day prior to the reported day.)

The arithmetic mean of the 24 1-hour averages between 01.00 am and 12.00 midnight
Note: 75% of the data must be available to calculate the 1-hour, 8-hour and 24-hour averages.

More about air quality sampling 

Standards and goals

A categorisation scheme is used to indicate how an AQI relates to relevant National Environment Protection Measure for Ambient Air (NEPM) for criteria pollutants or the NSW standard for visibility:

VERY GOOD GOOD FAIR POOR VERY POOR HAZARDOUS
0-33
34-66
67-99
100-149
150-199
200+

An AQI of 100 corresponds to the relevant NEPM standards for criteria pollutants or the relevant NSW standard for visibility. Hence, when the AQI is reported as POOR, VERY POOR or HAZARDOUS it indicates that the determining pollutant levels have reached or exceeded the relevant standard.

 

Standards/goals for AQI
PollutantAveraging periodMaximum concentrationGoal (maximum allowable exceedences)

Carbon monoxide

8 hours

9.0 ppm

1 day a year

Nitrogen dioxide

1 hour

1 year

0.12 ppm

0.03 ppm

1 day a year

none 

Ozone 

1 hour
4 hours

0.10 ppm
0.08 ppm

1 day a year

1 day a year

Sulfur dioxide

1 hour

1 day

1 year

0.20 ppm

0.08 ppm

0.02 ppm

1 day a year

1 day a year

None

PM10

1 day

50 µg/m3

5 days a year

Visibility (as Bsp)

1 hour

2.1 10-4 m-1

Not applicable

 PM2.5

1 day

1 year

25 µg/m3

8 µg/m3

Goal is to gather sufficient data nationally to facilitate a review of the standards for PM2.5.

Note:

ppm = parts per million by volume, i.e. parts of pollutant per million parts of air

PM10 = particles less than 10 micrometers in diameter

PM2.5 = particles less than 2.5 micrometers in diameter  

µg/m3 = micrograms per cubic metre, i.e. mass of pollutant per volume of air

Bsp = coefficient of light scattering due to particles. The lower the Bsp value, the lower the level of suspended particles and the better the visibility. The NSW OEH visibilty standard of 2.1 10-4 m-1 corresponds to a visual distance of approximately 9 kilometres.

See: Principal air pollutantsAbout the AQI and how it is calculated, Air quality sampling

 

Page last updated: 13 November 2015