Air quality data averaging periods

Air quality data readings are taken continuously and averaged to give hourly and daily air quality information.

Using our minute data, we report concentrations as hourly and daily averages. All averages are arithmetic means.

Hourly

Data are collected continuously at each monitoring site for the selected parameters and these are reported as hourly Air Quality Index (AQI) values and hourly data readings in scientific units

At some rural network sites instruments power up every 15 minutes to sample for 1 minute, and only continue measurements if TSP concentrations exceed 25 µg/m3. In this case, unlike the standard averaging method, an algorithm calculates hourly concentrations by accounting for the time between value changes.

Daily

The hourly readings, from 1:00am to 12 midnight, are summarised in daily lists, the daily Air Quality Index (AQI) values and the daily data readings in scientific units. The table below lists the ways in which data is presented.

Type of average used for hourly and daily web updates

Daily web updates made 

(using this type of average unless indicated otherwise)

Parameters updated 

1-hour average (AA1H)

Maximum of the 1-hour averages recorded for the day (i.e. between 01:00 and 24:00)

Ozone

Sulfur dioxide

Nitrogen dioxide

Visibility

 24-hour average (AA24H) Average of the 1-hour averages recorded for the day (i.e. between 01:00 and 24:00) PM10
PM2.5
Sulfur dioxide

Rolling 4-hour average (RA4H): 4-hour average calculated for each hour of the day, where the time of the RA4H (e.g. 05:00) refers to the last hour of the averaging period (02:00-05:00).  Note that the RA4H for 03:00 includes the hour 23:00 from the previous day up to 03:00 on the reported day.

Maximum of the rolling 4-hour averages recorded for the day (i.e. between 01:00 and 24:00)

Ozone

Rolling 8-hour average (RA8H): 8-hour average calculated for each hour of the day, where the time of the RA8H (e.g. 09:00) refers to the last hour of the averaging period (02:00-09:00). Note that the RA8H for 07:00 includes the hour 23:00 from the previous day up to 07:00 on the reported day.

Maximum of the rolling 8-hour averages recorded for the day (i.e. between 01:00 and 24:00)

Carbon monoxide

Rolling 24-hour average (RA24H): 24-hour average calculated for each hour of the day, where the time of the RA24H refers to the last hour of the averaging period. For example, the RA24H for 23:00 includes data from the previous day (23:00) up to the hour 23:00 from the reported day.

The arithmetic mean of 1-hour averages recorded for the day (i.e. between 01:00 and 24:00)

PM10

PM2.5

 

Fine particle averages – how air quality alerts are raised

Rolling 24-hour averages are calculated for PM10 and PM2.5 to compare against the National Environment Protection Measure (NEPM) air quality standards for fine particles. The NEPM fine particle standards are based on midnight-to-midnight daily averages.

The animated chart below shows the PM10 particle level average types that are calculated during a 24-hour period and the circumstances that will trigger an air quality alert against the NEPM standard of 50 µg/m3 for PM10.

The rolling 24-hour average window (cyan) moves forward one hour and calculates a new 24-hour rolling value (dark blue line) as an average of the previous 24 one-hourly values (light blue line). Solid green bars indicate 24-hour daily averages.

An air quality alert based on PM10 is raised whenever the dark blue rolling 24-hour average line goes above the red line for the NEPM standard.

On 9 January 2013 at 11 am, an air quality alert based on the rolling 24-hour average was raised for Warkworth in the Upper Hunter region when the dark blue rolling 24-hour average exceeded the NEPM standard.

Where a green bar for 24-hour daily average and the sliding cyan window for the rolling 24-hour average coincide, the rolling 24-hour average is the same as the 24-hour daily average.

In this animation, the green 24-hour average never exceeds the NEPM standard.

For PM2.5, the rolling 24-hour average and daily average are compared to the 24-hour NEPM PM2.5 standard of 25 µg/m3.

Animated chart showing the PM10 particle level averages calculated in 24 hour period.