Air quality - How do we measure it? (The science bit)
In South Ribble we measure particulate matter using zephyr air quality monitors and nitrogen oxides using diffusion tubes.
Zephyr monitors give us real time data South Ribble Public Air Quality Portal, while diffusion tubes are changed monthly and sent away to a laboratory to be analysed.
To use the live data portal South Ribble Public Air Quality Portal the following guidance may be useful.
The Zephyrs in South Ribble are numbered
- 1445 - Turpin Green Lane,
- 910 - Golden Hill Lane and
- 913 - Lostock Hall.
There is the option to toggle between the colour coded map and data using the map or data tabs in the top left-hand corner.
The data tabs will give you readings for
- NO - nitrogen monoxide
- NO2 - nitrogen dioxide
- O3 - ozone
- NOx - is a collective term used to refer to NO and NO2
- PM1, PM2.5 and PM10 - refer to different sizes of particulate matter.
You can change the time periods for the data in the graphs along the top bar above the graphs (24 hours, 48 hours, week or month).
The map can be used to view:
- NO2 levels,
- PM2.5 levels or the
- Air Quality Index (AQI).
The colour key on the right-hand side of the screen will explain the colouring. The data is updated hourly (if you loose colours just refresh the portal).
The 'info' and 'about' tabs in the top right-hand corner explain more about Air Quality and link back to this website.
Particulate Matter
Particulate matter is measured by size in micrometres (μm) which is one-millionth of a metre, or one-thousandth of a millimetre. The sizes that are measured and reported on are 10μm, 2.5μm and 1μm in diameter known as PM10, PM2.5 and PM1. For a sense of scale, a grain of sand measures about 90μm and a grain of salt is about 40μm, so measurements are taken on particles 4 to 40 times smaller than a grain of salt.
Concentrations of particulate matter are measured in cubic meter of air volume (m3), the total is written as a measurement of micrograms (one-millionth of a gram) per cubic meter (μg/m3). A set amount of air is drawn through a filter, the particulate matter that is left on the filter is weighed and gives a mass of PM that was in that volume of air. A value of 1 μg/m3 means one cubic metre of air contains one microgram of pollutant.
Global advisory levels for PM levels are set by the World Health Organisation. The UK Government says that concentrations must not exceed.
- PM10 an annual average value of 40 μg/m3
- PM10 a running 24-hour average of 50 μg/m3 more than 35 times in a single year
- PM2.5 an annual average of 25 μg/m3
The running 24-hour average is there to ensure that we are not exposed to high concentrations of pollutants for short periods of time and the annual average protects us from exposure over a long time period.
The number of pollution incidents varies year on year. Spikes in pollution can be seen around bonfire night, especially if the weather is cloudy, and as a result of construction work, dust blown over from the Sahara or periods of hot, sunny weather with little wind.
Nitrogen Oxides
Nitrogen dioxide levels are measured using diffusion tubes that are supplied and analysed by a laboratory. These small plastic tubes are attached in a vertical position and fixed to a lamp column or drainpipe. They have a cap at the top end that contains a filter that absorbs nitrogen dioxide. When analysed in the lab, it can tell us how much nitrogen dioxide was in the air during the monitoring period (usually about a month).
The UK government says that levels of nitrogen dioxide must not exceed 40 μg/m3.
PM and NOx are measured in 3 locations:
- kerbside - within one metre of the edge of a busy road
- roadside - between 1m from the edge of the busy road and the back of the pavement, usually 5m from the road
- urban background - sites in an urban area, away from emission sources and used to measure the roadside levels against.