How we monitor air pollution

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).

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 (chemiluminescence) in the wavelength range 600–3000 nanometres. The light intensity, measured by a photomultiplier tube, is proportional to the concentration of nitric oxide. Total nitrogen oxides (NO_x) 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).

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).

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).

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).

History of particle monitoring methods

PM₁₀ particles were measured using two different methods; one using a high-volume sampler and the other, a tapered element oscillating microbalance (TEOM) measuring particles continuously. PM₁₀ measurements using high volume samplers was stopped in 2004.

In 1998, ambient air quality standards and goals for six common pollutants (carbon monoxide, lead, nitrogen dioxide, ozone, PM₁₀ particles and sulfur dioxide) were included in the NEPM for all states. NSW Office of Environment and Heritage (OEH) also measures and reports on visibility. In 2006, OEH stopped monitoring lead as levels became undetectable.

Measurement technique: Beta Attenuation Monitor (BAM)

The BAM is a USEPA Federal Equivalent Method used for routine continuous PM_2.5 monitoring. It draws air through a cyclonic inlet, accumulating PM_2.5 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). 

Measurement technique: BGI low volume sampler

A USEPA Reference Method, this technique is used to meet NEPM regulatory monitoring requirements for PM_2.5. 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-conditioned Teflon filter paper, weighed before and after sampling provides 24 hour PM_2.5 mass concentrations.

Ammonia (NH₃) is measured with the same instrument as used for nitrogen oxides (NO_x) by way of a modification which converts ammonia to nitric oxide (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 (N_t  = NO + NO₂ + NH₃), total nitrogen oxides (NOx = NO + NO₂), and nitric oxide (NO). The ammonia concentration is determined by subtracting the NO_x mode signal from the N_t mode signal.

To measure visibility or suspended matter, a sample of air, having been heated to eliminate any water droplets (fog), is continuously drawn through a sample cell. A beam of light of wavelength 530 nanometres is used to illuminate the air stream. Suspended fine particles in the air cause some of this light to be scattered. A photomultiplier tube, at right angles to the direction of the light coming in, produces a signal proportional to the intensity of the scattered light. The method detects particles about 0.1–2.0 micrometres (0.1 µm–2.0 µm) in diameter (Australian Standard 3580.12.1).