Glossary of air quality terms

Glossary of words, phrases and acronyms used in air quality monitoring and reporting.

Air quality terms


Acronym commonly used for the federal legislation, National Environment Protection (Ambient Air Quality) Measure. The AAQ NEPM sets a national framework to measure, monitor and report Australia's ambient (outdoor) air quality, as prescribed by the National Environment Protection Council (NEPC).

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Air pollutant

Air pollutants can be found anywhere, indoors or outdoors, and may be divided into three key groups: biological pollutants, air toxics and criteria air pollutants.

The AAQ NEPM requires ambient (outdoor) monitoring of 6 criteria air pollutants, including the 4 gases: ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2) and nitrogen dioxide (NO2). Airborne particles are monitored in 2 size fractions as PM10 and PM2.5 i.e. particles less than 10 micrometers (µm) and 2.5 µm in diameter respectively. Visibility (NEPH) is also measured and reported by NSW. Lead is not monitored due to low levels detected upon the phasing out of leaded petrol.

In NSW, the measurements of criteria air pollutants and NEPH are used for reporting the Air Quality Index (AQI) which is a simple but effective way to communicate how air quality compares across regions and pollutants.

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Air quality alerts

An air quality alert may be raised when pollutant concentrations reach levels which exceed national air quality standards for gaseous pollutants (ozone, carbon monoxide, nitrogen dioxide and sulfur dioxide), particles (PM10 and PM2.5) and/or the NSW adopted standard for visibility. Note that national standards for particles are based on midnight-to-midnight daily averages, while rolling 24-hour averages are used for AQI calculations and air quality alerts.

There are 2 types of air quality alerts:

  • hourly alerts:  An hourly air quality alert may be raised by any site within the NSW air quality monitoring network whenever national air quality standards and/or NSW adopted standard for visibility are exceeded (AQI>100). 
  • daily air quality forecast alerts: At 4pm every day, the air quality is forecast for Sydney for the next day. If it is forecast that the AQI will likely exceed 100 the next day, an air quality alert is raised.

In NSW, airborne dust events, smoke or ground-level ozone are the most common cause of hourly or forecast alerts. In the case of a forecasted alert for ozone, the predicted elevated levels of ozone may not occur until the afternoon as production of ozone is highly correlated with diurnal variability in meteorological conditions, such as temperature among other variables (ground-level ozone concentrations tend to increase when temperature goes up).

Air quality alerts of POOR and VERY POOR categories are likely to impact people particularly sensitive to the effects of air pollution. Extreme episodes of air pollution such as during prolonged bushfires (HAZARDOUS category) can affect everyone's health.

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Air Quality Index (AQI)

The AQI is a value derived from air quality data readings which allows for more meaningful comparison of pollutants affecting air quality. The index is derived using the following formula for each hour of the day:

 AQI pollutant = 
pollutant data reading   X 100
national standard

Computed AQI values correspond to various categories of air quality (either VERY GOOD, GOOD, FAIR, POOR, VERY POOR or HAZARDOUS) and are used to determine whether to raise air quality alerts. Alerts occur when AQI exceeds 100.

Of note for NSW, AQI values are calculated and reported (updated) for each hour of day (hourly AQI), and for each day (daily AQI). Both AQI calculations are done at different spatial scales:

  • Station AQI, determined by the maximum AQI across pollutants for each station
  • Regional AQI, determined by the maximum AQI across all stations in a region.

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Ammonia (NH3)

Ammonia is a colourless volatile gas with a pungent odour. It is emitted by both natural and human-derived sources such as: decaying organic matter like animal waste and landfills, agricultural products like fertilisers, industrial production, and in lower concentrations from combustion sources such as fires and automobiles employing catalytic converters. In the atmosphere ammonia contributes to secondary PM2.5 formation through reactions with acidic gases, where it exists in its ionic form ammonium (NH4+).

In NSW, ammonia is monitored at Stockton station in the Newcastle Local network. Ammonia concentrations are expressed in parts per hundred million (pphm) by volume.

Beta Attenuation Monitor (BAM)

This instrument works by collecting particles on a filter tape and measuring the reduction in beta rays travelling through the collected particles to determine particle mass (which is proportional to reduction in beta rays). Concentrations are reported as micrograms of particles per cubic metre of air (µg/m3), referenced to a temperature of 0°C and an absolute pressure of 101.325 kilopascals.

Current in NSW, we mostly use BAMs for PM2.5 monitoring.

Carbon monoxide (CO)

Carbon monoxide is an odourless, colourless gas produced by incomplete oxidation (burning). Although any combustion process will contribute CO, in cities, motor vehicles are by far the largest human source. Other sources include wildfires, and natural processes such as the oxidation, in the oceans and air, of methane produced from organic decomposition. The oxidation of ambient gases known as volatile organic compounds (VOCs) also contributes to the global concentrations of carbon monoxide.

Carbon monoxide enters the bloodstream through the lungs and inhibits transport of oxygen by blood, thereby reducing oxygen reaching the body's organs and tissues, especially the heart. People suffering from heart disease are most at risk and may experience chest pain from CO exposure particularly while exercising.

Carbon monoxide concentrations reported by NSW are expressed in parts per million (ppm), which is a mixing ratio describing the amount of CO gas in the air by volume. For example, 1 ppm of CO means that if counting a million (106) gas molecules, 1 of them would be CO and the rest (999,999 molecules) would be other gases.

Rolling 8-hour CO averages are used in deriving Air Quality Index (AQI).

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Daily air quality index (AQI)

The daily AQI data is available and updated for each hour of the day for each site and region. It is calculated as the daily maximum of the hourly AQI values for gaseous pollutants and midnight-to-midnight daily averages for PM10 and PM2.5.

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Data readings

Data readings are the actual scientific measurements, or concentrations, for an air pollutant.

The data readings for different pollutants are recorded in different units of measure, depending on the type of pollutant.

More about units used for reporting various pollutants

Data reading units

The data readings for different pollutants are recorded in different units, depending on the type of pollutant.

  • Gas concentrations are measured as mixing ratios, which is the number of molecules of the gas of interest compared to the total number of gas molecules present in the air sample. Because this measurement includes the impact of temperature and pressure on the amount of gas present in the air, it is independent of ambient temperature and pressure. On our website, gas concentrations (mixing ratios) are reported in parts per hundred million (pphm) for ozone, nitrogen dioxide and sulfur oxide. Carbon monoxide concentrations are reported in parts per million (ppm).
  • Particle concentrations are measured in micrograms per cubic metre of air, normalised to air at 0°C (273.15 K) and 1 atm (101.325 kPa). The normalisation is to account for differences in the temperature and pressure of air at different locations at different times of the year, when compared against standards.
  • Nephelometer readings are measured in unit length (m-1). Note that on our website, a reported NEPH value of 2.5 is actually equal to 2.5 x 10-4 m-1 (or 250 inverse megametres).

More about measurement units and conversion factors


Goals refer to the allowable number of days exceeding the national standards during a year. The national standards and goals for the 6 criteria air pollutants are set by the National Environment Protection Council (NEPC).

See standard

Indicative monitoring

Indicative monitoring is not formally defined in Australia. It is defined in the UK and this text uses terminologies based on the UK definitions.

Air quality instruments can be grouped into four classes: certified reference, certified equivalent, certified indicative, and indicative. The main difference is accuracy and repeatability of measurements and the cost of purchase and operation.

Indicative particle monitors are used in the NSW Rural Air Quality Network, including TSI DustTrak™ 8520 and TSI DustTrak™ 8534.

The TSI DustTrak™ 8520 instruments power up every 15 minutes to sample for 1 minute. They only continue measurements if concentrations exceed 25 µg/m3. This preserves battery power and reduces instrument wear and tear. The newer TSI EDT™ multi-channel monitors run continuously, use a heated inlet to remove the impact of fog on measurements and can potentially become certified indicative instruments.

Concentrations of particles are reported as micrograms of particles per cubic metre of air (µg/m3), referenced to a temperature of 0°C and an absolute pressure of 101.325 kilopascals.

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Micrograms per cubic metre

The concentration of particulate matter in air is expressed as the mass of particulate matter in a given volume of air. The unit of measurement used is commonly micrograms (µg - one millionth of a gram) of particulate matter in a cubic metre (1m3)of air.

For example, PM10 concentration of 8µg/m3 means that there is 8 X 10-6 grams of PM10 in a cubic metre of air.

NEPH (or visibility)

NEPH represents measurements reported by a nephelometer, as a measure of light scattering or reduction due to atmospheric particulate matter (PM). Scattering by PM impairs visibility, therefore this parameter is also referred to as visibility, as it indicates how visual range is affected by airborne particulate matter. The higher the NEPH, the lower the visibility, and lower the visual range. For example, during smoky conditions, affected stations would record high NEPH values.

In NSW, visibility (or NEPH) is reported in units of 10-4 m-1. This means that a NEPH value of 1.5 should be read as 1.5x10-4 m-1. On multiplying by 100, you can convert this number to the widely used unit for visibility, known as inverse megametres (Mm-1).

NSW has adopted a 1-hour visibility standard of 2.1x10-4 m-1 (210 Mm-1), which corresponds to a visual distance of approximately 9 kilometres. This means that NEPH > 2.1 will trigger “POOR” (or worse) air quality due to reduced visual range.  

Measurements of visibility (or NEPH) are used in the web reporting of Air Quality Index (AQI).

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The nephelometer measures light scattering due to particles in the air, also known as scattering potential (or bsp or σsp).

The nephelometer measurements of bsp are derived by exposing the air sample to an incident light of wavelength 525 nm which interacts strongly throughout the human range of visibility (with smog, fog, haze). Gaseous, and largely particulate components of the sampled air will cause the light to scatter. The instrument uses very sensitive, light-scattering sensors to determine bsp, and then calculates a visibility reduction index per unit length (m-1). Readings from this instrument are reported in NSW as NEPH or visibility in units of 10-4m-1 (or 100 inverse megametres).

Higher particulate concentrations mean more scattering, so bsp is a good measure of particulate pollution and is also used as a measure of atmospheric visibility. The higher the (NEPH) value of bsp, the higher the concentration of particles (and NEPH value), and the lower the visibility.

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Nitrogen dioxide (NO2)

Nitrogen dioxide is a brown gas and considered a major air pollutant due to its health impacts. It is often found at its highest concentrations near busy roads and can also be high indoors when un-flued gas appliances are used. It is a respiratory irritant which may worsen the symptoms of existing respiratory illness. Nitrogen dioxide makes people with asthma more susceptible to lung infections and asthma triggers like pollen and exercise.

Concentrations of NO2 reported on for NSW are expressed in parts per hundred million (pphm), which is a mixing ratio describing the amount of NO2 gas in the air (by volume). For example, 1 pphm of NO2 means that if counting a hundred million (108) gas molecules, 1 of them would be NO2 and the rest (99,999,999 molecules) would be other gases.

Measurements of NO2 concentrations and the AAQ NEPM stipulated national standard for NO2 are used in reporting the Air Quality Index (AQI).

Nitrogen oxide (NO)

Also called nitric oxide or nitrogen monoxide, NO is one of the principal oxides of nitrogen. It is formed by oxidation of nitrogen or ammonia present in the atmosphere. While nitric oxide forms in combustion systems and can be generated by lightning in thunderstorms, any emissions of NO are converted into NO2 through oxidation and titration with atmospheric ozone. Emissions of NO are converted mostly into NO2 over time, but usually during this process the pollutants are dispersed by normal wind processes.

In NSW, reported NO concentrations are in parts per hundred million (pphm), which is a mixing ratio describing the amount of NO gas in the air by volume. Measured concurrently with NO2 at most monitoring sites, but not used in reporting the Air Quality Index (AQI).

Oxides of nitrogen (NOx)

Oxides of nitrogen (NOx) describe the combined atmospheric concentrations of NO and NO2. NOX is formed during combustion and is composed predominantly of NO (90-95%) and a lesser amount of NO2 (5-10%) for petrol engines. Diesel engines can have much higher NO2/NOx ratios.

In NSW, concentrations of NOx are expressed in parts per hundred million (pphm), which is a mixing ratio describing the amount of NOx in the air by volume.

Ozone (O3)

Ground-level ozone is a colourless, gaseous secondary pollutant. It is formed by chemical reactions between volatile organic compounds (VOCs) and oxides of nitrogen in the presence of sunlight. Ozone is one of the key constituents of atmospheric photochemical smog, which has a harmful effect on human health.

As its formation is driven by availability of UV light and higher temperatures, its concentrations are higher in summer, and usually reach their maximum in the afternoons and early evenings.

Ozone is a lung irritant, affects lung function and can worsen asthma.  You may notice difficulty in breathing, coughing, and throat irritation if you are exercising outdoors when ozone levels are high.

Concentrations of ozone (O3) are expressed in parts per hundred million (pphm), which is a mixing ratio describing the amount of ozone gas in the air by volume. For example, 1 pphm of O3 means that if counting a hundred million (108) gas molecules, 1 of them would be O3 and the rest (99,999,999 molecules) would be other gases

The AAQ NEPM has set the national standard for ozone based on two data averaging periods, being 1-hour averages and rolling 4-hour averages based on 1 hour averages. The 1-hour and rolling four-hour averages for ozone are used in reporting the Air Quality Index (AQI).

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Particles are a suspension of solids or liquids in the air and reduce visual amenity and adversely impact health. The size of a particle determines its potential impact on human health. Larger particles are usually trapped in the nose and throat and swallowed. Particles smaller than 2.5µm in diameter may reach the lungs and cause irritation or other adverse health effects.

Particles are measured in various size fractions (PM2.5, PM10 and TSP). Concentrations of all size fractions are reported in micrograms of particles per cubic metre of air (µg/m3), referenced to a temperate of 0°C and an absolute pressure of 101.325 kilopascals.

Particles as PM10 and PM2.5

PM10 – particles less than 10 micrometers (µm) in diameter. Sources include sea salt, crushing or grinding operations and dust stirred up by winds over exposed soils or vehicles on roads. The AAQ NEPM has set the 24-hour national standard for PM10 as 50 µg/m3, defined as a calendar day 24-hour average.

PM2.5 – fine particles less than 2.5 µm in diameter. Sources include all types of combustion, including motor vehicles, power plants, residential wood burning, forest fires, agricultural burning, and some industrial processes. May also include sea salt. The AAQ NEPM has set the 24-hour national standard for PM2.5 as 25 µg/m3, defined as a calendar day 24-hour average.

Concentrations of PM10 and PM2.5 as reported on the website are expressed in micrograms of particles per cubic metre of air (µg/m3), referenced to a temperature of 0°C and an absolute pressure of 101.325 kilopascals. Of note, these are not hourly averages, but rolling 24-hour averages that are updated hourly. The national standards for particles are calendar day (24-hour) averages, whereas the web data is updated hourly, meaning that reporting 24-hour averages will not provide time-sensitive data to help track changing particulate levels. Thus, NSW has adopted the approach of reporting rolling 24-hour averages on an hourly basis.

The rolling 24-hour average concentrations for particles are used in calculating the Air Quality Index (AQI) values.

In NSW different monitors are used to measure particles. At most standard air quality monitoring sites, PM10 concentrations are measured using Tapered Element Oscillating Microbalance (TEOM) and PM2.5 using Beta Attenuation Monitor (BAM), TEOM incorporating a filter dynamic measurement system (FDMS) are used to simultaneously measure PM10 and PM2.5. For indicative monitoring of particles as TSP (Total Suspended Particles), PM10 and PM2.5 at NSW rural network stations, DustTrak aerosol monitors are used.

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See air pollutant

Regional AQI

See site AQI

Rolling average

It is an average of the previous N hours for each hour of the day. In the case of a rolling 4-hour average, the previous 4 hours leading to the hour in question are used.


  • for the hour ending 5:00, the 4-hour rolling average is calculated from the values for hours 1:00-2:00, 2:00-3:00, 3:00-4:00 and 4:00-5:00
  • the next 4-hour rolling average, i.e. for the hour ending 6:00, the 4-hour rolling average is calculated as the average of values for hours 2:00-3:00, 3:00-4:00, 4:00-5:00 to 5:00-6:00.

Rolling 4, 8 and 24-hour averages can include values from the previous day.

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SD1, Sigma theta

Standard deviation in wind direction. The variability of the wind direction can be used as an indicator of the amount of turbulence and therefore mixing of air. The term used is ‘sigma theta’, where sigma is a standard measure of variability and theta is a Greek letter commonly used to represent an angle in mathematics.

Site AQI

For each site, the highest AQI is used as the Site AQI. The highest Site AQI for each region is used as the Regional AQI for the region. This rule applies for both hourly and daily updated AQI data.

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Sulfur dioxide (SO2)

Sulfur dioxide is a respiratory irritant and may worsen existing respiratory illness. The main human activities producing sulfur dioxide are the smelting of mineral ores containing sulfur and the combustion of fossil fuels.

In NSW, sulfur dioxide concentrations are reported in parts per hundred million (pphm), which describes the amount of SO2 gas in the air by volume. For example, 1 pphm of SO2 means that if counting a hundred million (108) gas molecules, 1 of them would be SO2 and the rest (99,999,999 molecules) would be other gases.  

Measurements of SO2 are used in NSW for reporting of Air Quality Index (AQI).


The standards for ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide and air particles are set by the National Environment Protection Council. Standards which apply for the Australian (and NSW) context are described at Standards and goals.

Tapered Element Oscillating Microbalance (TEOM)

Continually measures the concentration of airborne particles. It does this by collecting and weighing the particles using a very sensitive balance. In NSW TEOMs are used for PM10 measurements.


A type of TEOM instrument which incorporates a filter dynamic measurement sytem (FDMS) unit. The FDMS unit measures the sampled air particle streams alternately using regular and chilled sample flows, to better estimate the loss of volatile components during measurement relative to regular TEOM units. This approach is important for measurements of PM2.5 for which TEOM-FDMS is a USEPA approved method. It is also an approved method for the measurement of PM10. At selected NSW stations, TEOM-FDMS is used for the continuous, simultaneous monitoring of PM2.5 and PM10.

Total Suspended Particles (TSP)

Total suspended particulates are solid particles and liquid droplets 100 micrometres or less in diameter. They come from natural and human-made sources (e.g. pollen, bushfires, motor vehicle emissions). PM10 and PM2.5, the smaller components of TSP, are associated with adverse health effects ranging from respiratory problems to premature death of people with heart and lung disease.

Visibility (or NEPH)

Represents visibility reduction as measured by the nephelometer (also referred to as NEPH). The higher the NEPH measurement, the lower the visual range is and poorer the visibility conditions. New South Wales has adopted a visibility standard of 2.1x10-4 m-1 (210 Mm-1, inverse megametres) to represent a visual range of around 9 kilometres.

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Wind direction

Represents the direction the wind is blowing from, according to the compass. As an example, wind blowing from the west, while travelling eastwards, is called a westerly wind. Wind direction in the NSW Air Quality Monitoring Network is measured concurrently with wind speed using an ultrasonic instrument capable of measuring horizontal wind speed and wind direction. Data is reported in degrees and represents a vector average across an hour. Wind direction measurements at most sites are made at 10 m above ground level.

Wind speed

Represents the rate or speed at which air is moving. It is measured using an ultrasonic device in the NSW Air Quality Monitoring Network. Measurements are made at 10 m above ground level, and data reported in the units of meters per second (m/s).