What is the Air Pollution Index?

Air quality in Armidale is monitored by the Armidale Dumaresq Council at the Civic Administration Building in Rusden Street. Data is collected continuously using a nephelometer sampling every 2 minutes. The data is averaged to get one-hour averages, and the maximum one-hour average each day (from 12 noon to 12 noon next day) is obtained. This maximum value is then scaled so that a nephelometer coefficient of 2.1 corresponds to an index of 50, to get the final value reported - the Air Pollution Index. This is equivalent to the standard index used by the NSW EPA to report air pollution.

The Air Pollution Index is categorised as LOW, MEDIUM, HIGH, as follows:

In Armidale an extra category, VERY HIGH, is used if the pollution index is 100 or higher.

NSW EPA Air Pollution Index (RPI)
In Sydney, three pollution indices are calculated - ozone, NO2 and fine particles. Average concentrations of each pollutant are calculated for each hour of the time period. The maximum of these hourly averages for each pollutant represents the index for that pollutant. The indices are scaled so that a value of 50 (high pollution) corresponds to the relevant standard or goal for the pollutant:

max 1-hour ozone concentration of 12 ppm,
max 1-hour NO2 concentration of 16 ppm
max 1-hour neph coefficient of 2.1

The highest of these three indices is called the Regional air pollution index (RPI). Thus

RPI = 50 * max(neph_coeff_max/2.1, Ozone_max/12, NO2_max/16)
where neph_coeff_max, Ozone_max and NO2_max are maxima of the hourly averages in the time period.

The index is calculated for three regions in Sydney  - East (including the CBD), North-west and South-west.  There are six monitoring stations in Sydney East -Rozelle (near the CBD), Randwick, Lydcombe, Earlwood, Woolooware and Lindfield.  The RPI is calculated separately for each of these 6 sites.  The worst of these 6 pollution indices is then used as the pollution index for the whole of Sydney East.
The latest individual values for each of the 3 Sydney sites, as well as sites in Newcastle and Wollongong for today can be found at http://www.epa.nsw.gov.au/air/aqupd.htm

In Armidale, ozone and NO2 are not measured, but are likely to be insignificant compared to the particle index in winter.

Nephelometer Coefficient - Bsp
The unscaled nephelometer measurement is called the nephelometer scattering coefficient, denoted by bsp, where 'sp' refers to scattering by particles. The value averaged over one-hour is usually less than 10, with the value 2.1 corresponding to a HIGH value on the Air Pollution Index.