Sampling air pollution

Learn how we sample air and measure pollutants for reliable air quality monitoring.

How air pollutants are measured

Technical information about the sampling methods and analytical techniques used in air monitoring programs is outlined below for each pollutant. Where applicable, Standards Australia methods for ambient air monitoring are used.

Ozone (O3)

Measurement technique: ultraviolet spectroscopy

Sample air is drawn into a cell where a beam of ultraviolet light is passed through it to an ultraviolet detector. Some of the light is absorbed by ozone in the sample, the amount being proportional to the number of molecules present. The decrease in intensity between the transmitted light and that of the source is used to determine the ozone concentration in the sample (Australian Standard 3580.6.1-2011).

Oxides of nitrogen (NO, NO2 & NOx)

Measurement technique: chemiluminescence

Sample air is drawn into a reaction chamber where nitric oxide (NO) in the sample reacts with a stream of ozone produced by an ultraviolet lamp in dried air. The reaction produces light in the wavelength range 600 nm to 3000 nm (chemiluminescence). The light intensity, which is measured by a photomultiplier tube is proportional to the concentration of nitric oxide. Total nitrogen oxides (NOx) concentration is measured in a separate sample stream by first reducing to nitric oxide (NO) using a selective converter. The concentration of nitrogen dioxide (NO2) reported is assumed to be the difference between total nitrogen oxides and nitric oxide (Australian Standard 3580.5.1-2011).

Ammonia (NH3)

Measurement technique: chemiluminescence

NH3 is measured with the same instrument as used for nitrogen oxides (NOx) by way of a modification which converts ammonia to nitrogen dioxide (NO). The nitric oxide (NO) and ozone then react to produce a characteristic luminescence. The ammonia analyser has three active channels or modes: Total Nitrogen (Nt = NO + NO2 + NH3), Total nitrogen oxides (NOx = NO + NO2), and nitric oxide (NO). The ammonia concentration is determined by subtracting the NOx mode signal from the Nt mode signal.

Suspended matter (nephelometer)

Measurement technique: integrating nephelometer

Air, having previously been heated to eliminate any water droplets (fog), is continuously drawn through a sample cell. A beam of light centred on wavelength 530 nm is used to illuminate the air stream. Suspended fine particles in the air cause some of this light to be scattered. A photomultiplier tube, mounted at right angles to the direction of the incident light, produces a signal proportional to the intensity of the scattered light. The method detects particles in the approximate size range 0.1 µm to 2.0 µm (Australian Standard 3580.12.1).

Particulate matter less than 2.5 µm (PM2.5)

Measurement technique: Beta Attenuation Monitor (BAM)

The BAM is a USEPA Federal Equivalent Method used for routine continuous PM2.5 monitoring. It draws air through a cyclonic inlet, accumulating PM2.5 particles onto a glass fibre filter tape. On saturation, the filter tape advances to provide a clean filter surface. High-energy beta radiation is passed through a defined spot on the filter and the intensity of radiation absorbed measured. Beta attenuation or the loss in beta signal through the filter is proportional to the mass of deposited particles. This, combined with the volume of air sample is used to calculate mass concentration (Australian Standard 3580.9.12:2013).  

Measurement technique: BGI low volume sampler

A USEPA Reference Method, this technique is used to meet NEPM regulatory monitoring requirements for PM2.5. Air is drawn through a cyclonic inlet that makes use of the aerodynamics of particles of different sizes to selectively capture those below 2.5 µm. A pre-condtioned Teflon filter paper, weighed before and after sampling provides 24 hour PM2.5 mass concentrations.

Particulate matter less than 10 µm (PM10)

Measurement technique: Tapered Element Oscillating Microbalance (TEOM)

The TEOM consists of a sensor unit which contains the sample inlet (PM10) and the TEOM microbalance for mass measurement. The TEOM microbalance consists of a filter which is held on the end of a tapered tube that oscillates upon particle impaction. As particles land on the filter, the filter mass change is detected as a frequency change in the oscillation of the tapered tube. The control unit of the TEOM houses the processing hardware and flow components. Combining the mass change with the flow rate through the system gives a measure of particulate concentration. The TEOM computes total mass accumulation on the filter as well as 30-minute, 1-hour and 8-hour mass concentration averages. It is therefore capable of providing continuous, real-time data (Australian Standard 3580.9.8-2008).

Sulfur dioxide (SO2)

Measurement technique: pulsed fluorescent spectrophotometry

A stream of sample air is drawn through a cell where it is exposed to pulsed ultraviolet light, resulting in excitation of sulfur dioxide molecules. These molecules subsequently re-emit light but at a different wavelength; they fluoresce. The intensity of the fluorescent light measured by a photomultiplier tube is proportional to the concentration of sulfur dioxide in the sample air (Australian Standard 3580.4.1-2008).

Carbon monoxide (CO)

Measurement technique: Infrared spectrometry

Sample air is drawn into a cell where a beam of infrared light is passed through it to a photodetector. Some of the light is absorbed by carbon monoxide in the sample, the amount being proportional to the number of molecules present. By comparing the light intensity received by the photodetector through the sample cell with that received through a similar cell containing reference gas, the concentration of carbon monoxide may be determined (Australian Standard 3580.7.1-2011).

Units and conversion factors

The units used by the EPA in reporting air quality data are based on common usage. Particle concentrations are generally expressed in micrograms per cubic metre (µg/m3) at 0ºC and gases in volumetric terms as parts per hundred million (pphm) or parts per million (ppm). One-hour suspended matter measurements (nephelometer) are reported as a coefficient of light scattering (bsp) units, while dust deposition rates are reported in grams per square metre per month.

The following table enables the reader to compare data in the air quality monitoring reports with those from other sources.

Pollutant

Units

Convert to

Multiply by

Ozone

pphm

µg/m3 (0°C)
µg/m3 (25°C)

21.4
19.6

Nitric oxide (NO)

pphm

µg/m3 (0°C)
µg/m3 (25°C)

13.4
12.3

Nitrogen dioxide (NO2)

pphm

µg/m3 (0°C)

20.5

Nitrogen oxides (NOx)

pphm

µg/m3 (25°C)

18.8

Sulfur dioxide (SO2)

pphm

µg/m3 (0°C)
µg/m3 (25°C)

28.6
26.2

Lead

µg/m3 (0°C)

µg/m3 (25°C)

0.92

Carbon monoxide

ppm

mg/m3 (0°C)
mg/m3 (25°C)

1.25
1.15

Calculation of averages and spread

Averages

The monthly averages as calculated in the quarterly air quality monitoring reports are the arithmetic means.

90-percentile

The 90-percentile values reported in the quarterly air quality monitoring reports are a true 90-percentile as they are based on the actual data sample, sorted in ascending order. The following algorithm is used to determine the 90-percentile values:

90-percentile = (v[i] * (i+1 - xi)) + (v[i+1] * (xi - i))

where:

v = sorted vector of observations v[1] <= v[2] <= v[3] <= ......... v[n]

xi = 1/3 + (p * (n + 1/3))

i = floor(xi) = the number xi rounded down to the nearest integer

p = percentile rank = 0.9

n = number of observations

Air, having previously been heated to eliminate any water droplets (fog), is continuously drawn through a sample cell. A beam of light centred on wavelength 530 nm is used to illuminate the air stream. Suspended fine particles in the air cause some of this light to be scattered. A photomultiplier tube, mounted at right angles to the direction of the incident light, produces a signal proportional to the intensity of the scattered light. The method detects particles in the approximate size range 0.1 µm to 2.0 µm (Australian Standard 3580.12.1:2001).

Page last updated: 17 March 2016