A link to data from over 1500 sites across the UK that monitor air quality.
They are organised into networks that gather a particular kind of information, using a particular method.
There are two major types of network
Automatic
non-automatic networks
For each monitoring site you can select a range of statistics which are relevant to the particular monitoring method. Basic statistics are annual mean, maximum and minimum values, with daily means, monthly means and a range of percentile values available where these are meaningful.
Barnet-specific data can be found here.