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Air quality sampling

• How are the pollutants measured?
• Units and conversion factors
• Calculation of averages and spread

How are the pollutants measured?

The following briefly outlines the sampling methods and analytical techniques used by the EPA and industry in their monitoring programs. Where applicable, Standards Australia (SA) methods for ambient air monitoring are adopted.

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).

Oxides of nitrogen (NO, NO₂ & NO_x)

Measurement technique: chemiluminescence

Sample air is drawn into a reaction chamber where nitric oxide 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. The light is detected by a photomultiplier tube, the intensity being proportional to the concentration of nitric oxide. The concentration of total nitrogen oxides is measured in a separate sample stream. They are first reduced to nitric oxide using a selective converter and its concentration determined as above. The concentration of nitrogen dioxide reported is assumed to be the difference between total nitrogen oxides and nitric oxide (Australian Standard 3580.5.1).

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).

Total suspended particulates (TSP), lead and other heavy metals

Measurement technique: high-volume sampler

Sample air is drawn at a flow rate of approximately 1 cubic metre per minute through a glass-fibre filter paper mounted beneath a protective hood to prevent material falling directly onto the paper. Sampling is carried out continuously for 24 hours on a one-day-in-six cycle. Particles in the approximate size range 0.1 µm to 50 µm are collected. Each sample is weighed in a temperature and humidity controlled environment and the mass of the unexposed filter paper subtracted to determine the mass of sample collected. These data, together with the total volume of air drawn in the 24-hour period, yield the concentration (Australian Standard 3580.9.3). Lead and other heavy metals are determined by chemically digesting the filter paper and the resulting solution analysed for metals using an atomic absorption spectrophotometer (Australian Standard 2800).

Particulate matter less than 10 µm (PM₁₀)

Measurement technique: size-selective inlet

Samples are collected and weighed as for TSP but using a high-volume sampler to which a size-selective inlet (SSI) has been attached. The SSI makes use of the aerodynamics of particles of different sizes to ensure that only those with a diameter less than 10 µm are collected (Australian Standard 3580.9.6).

Particulate matter less than 10 µm (PM₁₀) and less than 2.5 µm (PM_2.5)

Measurement technique: tapered element oscillating microbalance (TEOM)

The tapered element oscillating microbalance (TEOM) consists of two units. The sensor unit contains the sample inlet (PM10 or PM2.5) and the TEOM microbalance. The microbalance provides the sensitive mass measurement. The control unit houses the processing hardware and flow components. The filter is held on the end of a tapered tube. One end of the tube is free to oscillate while the other end is clamped. As particles land on the filter, the filter mass change is detected as a frequency change in the oscillation of the tube. Combining the mass change with the flow rate through the system provides a measure of the particulate concentration. The instrument computes the total mass accumulation on the filter as well as 30-minute, one-hour and eight-hour averages of the mass concentration. It is therefore capable of providing real time values (Australian Standard 3580.9.8).

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).

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).

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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.

 

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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

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