Air quality data guidance

1. Reference monitoring stations

Reference monitoring stations contain equipment which is highly accurate and measure the air quality approximately up to every 15 minutes. They are regularly calibrated and the data from them is ratified on an annual basis, using a full calendar years worth of data, usually in April of the following year. The data that is uploaded hourly is screened, during which erroneous data will be automatically deleted before being presented. All remaining data is marked as provisional and is subject to change following ratification. 

As reference monitoring stations are reasonably large and expensive to purchase and maintain, there are a limited number of them installed across Westminster.

Data from the reference monitoring stations can be used for a variety of purposes including understanding real time air quality, examining short and long term trends and for research purposes. Examples of ways the data can be used includes:

• Compliance reporting under the Air Quality Standards Regulations 2010
• Comparison with air quality objectives as laid out in the Air Quality Strategy
• Providing the public with information through air quality bulletins
• Forecasting future air quality levels
• Policy development for human health and ecosystem protection
• The European Monitoring & Evaluation Programme (EMEP)
• The UK Local Air Quality Management regime under Part IV of the Environment Act 1995
• National Indicators on environmental quality

(Reference: Automatic Urban and Rural Network (AURN) - Defra, UK)

Reference air quality monitoring stations and the pollutants monitored

Site Name	Management	Site Type	Pollutants Monitored
Covent Garden	Westminster City Council (part sponsored by Capco)	Urban background	NO2 PM2.5
Elizabeth Bridge	Westminster City Council	Roadside	NO2, PM2.5
Horseferry Road	Defra	Urban background	NO2, PM10, PM2.5, O3, Heavy Metals
Marylebone Road	Imperial College	Kerbside	NO2, PM10, PM2.5, SO2, O3, CO2, CO
Oxford Street (Selfridges)	Westminster City Council	Kerbside	NO2, PM2.5,
Waterloo Place	The Crown Estate	Roadside	NO2, PM1, PM10, PM2.5, TSP

Pollutants monitored

• NO₂, nitrogen dioxide
• PM_2.5, Particulate Matter of less than 2.5 micrometres in diameter
• PM₁₀, Particulate Matter of less than 10 micrometres in diameter
• SO₂, sulphur dioxide
• O₃, ozone
• CO₂, carbon dioxide
• CO, carbon monoxide
• TSP, Total Suspended Particles - particulate matter and black smoke ranging in size from less than 2.5 micrometres in diameter to 100 micrometres in diameter
• Heavy metals, arsenic, cadmium, cobalt, chromium, copper, iron, lead, manganese, nickel, selenium, vanadium, zinc in PM₁₀

With the exception of PM1 and TSP, there are accredited quality assurance and quality control procedures in place. The data is ratified by Imperial College London, with the exception of Horseferry Road which is ratified by Defra appointed consultants (Ricardo).

2. Diffusion tubes 

Passive diffusion tubes are small plastic tubes, usually attached to lamp columns or other street furniture. They usually remain in situ for one month before being sent for laboratory analysis. They provide information on air quality across a broader range of locations than can be facilitated by the continuous monitoring station network.

Passive diffusion tubes supplement data from the continuous monitoring stations and provide a better understanding of spatial trends. They are a relatively inexpensive means of monitoring Nitrogen Dioxide.

Once a year, a bias adjustment of the data is undertaken. This a mathematical formula which must be applied to the previous calendar year’s worth of diffusion tube data to ‘correct’ it.

This factor can be generated by us from co-location studies undertaken, the London wide environment programme or the Defra national adjustment. Any of these 3 factor can be used, provided the choice is justified.

Usually, the factor that will provide the worst case assessment would be selected. This means that the data reported on a month by month basis is subject to change.

The bias adjustment factor can amend the data by up to 1.5%. We undertake the adjustment and report on this each year in the Annual Status Report which is published on our website each year.

The data from passive diffusion tubes can be used for examining long term trends.

Some of the surveys reported in this tool are not bias adjusted and some data is collected bi-monthly, rather than monthly. You are advised to check the data prior to using it, to ensure it it suitable for your needs. 

3. Sensors

Air Quality sensors are a means of measuring air quality in real or near time and are relatively low cost (compared to a reference monitoring station). They can be installed on lamp columns or other street furniture and are powered either by solar or mains electricity. The pollutants measured by the sensor vary depending on the type installed, however they typically measure Nitrogen Dioxide and PM2.5. Some sensors also measure other pollutants or meteorological data.

Sensors can be deployed in areas where a reference monitoring station may be impracticable due to cost or space constraints, but where a greater number of pollutant information and/or a near real time feed of data is desirable.

The quality of the data provided by the sensor varies between the different sensor types. Generally, they are considered to provide an ‘indicative’ data set which means they are not as accurate as the reference monitoring stations.

The data from sensors can be used for a variety of purposes such as understanding indicative levels of real time air quality and provide indicative short term trends, which can assist with behaviour change and engagement projects.