Nitrogen dioxide is monitored using automatic (reference) analysers and non-automatic (passive diffusion tubes).  There are currently 169 monitoring sites with diffusion tubes across the district.

The diffusion tubes are exposed for one month and then they are collected and send to a laboratory and analysed to get a monthly average value of nitrogen dioxide.  The annual average nitrogen dioxide monitoring data from 2014-2021 for all sites in Bath and North East Somerset is provided below.  

Click on each site location to bring up the graph showing the annual average data for all the years that the monitoring was in place.


Where possible, monitoring is located close to building facades because that is where people spend the longest periods of time.  Where this is not possible, monitors are located on signposts or lampposts by the roadside somewhere on the footway.  Concentrations of nitrogen dioxide fall off rapidly as you move away from the roadside.  Concentrations "at monitor" are the monitored concentration at the location where it is collected e.g. roadside.  For diffusion tubes located in roadside locations, the concentration at the building facade have been calculated using the distance adjustment tool on the LAQM helpdesk website (monitored concentration at closest facade).  For monitors located close to the facade these values will be the same.  You can toggle the graph between the two values.

Diffusion tubes are subject to interference from weather that does not affect the automatic analysers, so to counter this, a bias adjustment factor is applied.  This is established from study sites where diffusion tubes have been co-located with automatic (reference) monitors.  2021 data will be included when it the final QA/QC has been carried out.

A site denoted as triplicated is where 3 tubes are exposed at one location.  This is to prevent data loss e.g. interference from a spider nesting in the tube.  At these locations the average across the 3 tubes is given.

The diffusion tube network is reviewed annually so data is not available for all sites for all years.

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