Based on particles as PM10, the chart below shows the different types of PM10 averages that are calculated during a 24-hour period, and the circumstances that will trigger an air quality alert against the 24-hour NEPM standard for PM10 of 50 µg/m3 (red line). Alerts for PM2.5 are raised in a similar way by comparing against the 24-hour NEPM standard for PM2.5 of 25 µg/m3.
How air quality alerts are raised
Air quality alerts are raised when air pollutant concentrations exceed national standards. Find out how air quality alerts are raised for particles.
In the example below, the solid green bars indicate midnight-to-midnight 24-hour averages. Observed particle concentrations vary every hour (light blue line), and for the purposes of timely reporting, using midnight-to-midnight averages (green bars) would limit the time-sensitivity in reporting changing air quality (as it would take an entire calendar day for the reported value to update). We seek to provide live data to the public to help inform their health protection. Thus, our reporting for particles relies on a special type of 24h average called a 24-hour ‘rolling’ average (dark blue line).
The moving cyan window shows 24 one-hourly values (light blue line) that will be used to calculate a 24-hour rolling average (dark blue line). For the next hour, this window will move forward an hour and calculate a new 24-hour rolling value as an average of the previous 24 one-hourly values. For every hour of the day, a 24-hour rolling average is calculated and compared against the national standard (red line). An air quality alert for PM10 is raised if the dark blue PM10 24-hour rolling average line goes above the standard.
In this example for Warkworth in the Upper Hunter region, the midnight-to-midnight PM10 24-hour average (green bar) never exceeds the NEPM standard. However, on 9 January 2013, an air quality alert was raised at 11am for the Upper Hunter region when the dark blue PM10 24-hour rolling average exceeded the standard (red line). The alert was raised at 11 am, after the first hour that exceeded on that day. No further alerts were raised even though subsequent hours also exceeded the standard.
Alerts are raised only once per day per region, even if on the given day multiple parameters exceed their respective national standards or if multiple stations in a region exceed a national standard.
For example, on a day when both ozone and particles exceed the standards at multiple stations in a given region, only the first parameter and first station in a given region will trigger an alert, and the alert will be for the region to which the station belongs. This means that any further alerts will not be triggered on that day, even if any other parameter or additional stations in that region exceed national standards.
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