The combined nowcast is made by merging together the four pollutant nowcasts (nitrogen dioxide(NO2),ozone(O3), PM10 particles(PM10) and PM2.5 particles(PM2.5)), taking the highest of the air pollution index at each location. For example, if the forecast or measurement for ozone(O3) is Moderate and for PM10 particles(PM10) it is Low, the overall index assigned will be Moderate. There is no provision within the index to take into account the possible effects of a mixture of pollutants (COMEAP, 2011). London is affected by different types of pollution episodes. For instance ozone(O3) will likely dominate the index during summer time smog and PM2.5 particles(PM2.5) will likely dominate during winter time smog and during episodes where particle pollution comes to us from mainland Europe.
Measurements from monitoring stations are only able to report air quality at that particular place. The nowcast combines these measurements with our detailed model to show a prediction of what air quality is like across the whole of Greater London.
The four pollutants chosen are ones which are known to have an effect on health within London and are able to be predicted with this model. Levels of carbon monoxide (CO) are now well below EU Limit Values and have not been mapped, However this pollutant remains important in the formation of ozone(O3) across the northern-hemisphere and indoors, exposure to CO from faulty heating and cooking appliances can be dangerous. Concentrations of sulphur dioxide (SO2) are too unpredictable to create a map using this method. Each pollutant has different sources and behaviour, so try selecting each pollutant in turn to get a full picture of air quality at your chosen location.
Often air pollution levels in most areas of London are sufficiently small to be classified as low according to the air quality index. When this index was created these levels were considered unlikely to cause any adverse health effects. There is currently debate about whether there is any safe level for these pollutants. Much of the pollution in London is associated with traffic and zooming in to show individual roads will usually reveal locations where pollution levels are higher.
Based on comparison of nowcast and measurements from monitoring sites we expect the nowcast to be a good visualisation of measured data to within half an index value for most locations. The 20m resolution of the model is not able to fully represent pollution concentrations within a few metres of roads and very localised sources of PM10 particles (PM10). We will be monitoring the performance of the nowcast and expect to be adjusting and improving it over time. A current indication of nowcast accuracy is available here.
Some of the pollution created by cars reacts with ozone(O3) and causes it to change into other chemicals. This means that ozone(O3) is on average lower in the middle of London and close to busy roads.