Non-Automatic Hydrocarbon Network
The UK Non-Automatic Hydrocarbon Network measures ambient benzene concentrations at various sites around the United Kingdom.
What is measured?
Benzene is monitored to assess compliance with UK Objectives (between 3.25 and 16.25 µgm-3 depending on area and compliance date, expressed as a running annual mean), as well as with the corresponding EC Air Quality Directive Limit Value (5 µgm-3 annual average).
What is the purpose of the network?
Until 2007, 1,3-butadiene was also monitored in order to assess compliance with the UK Air Quality Strategy Objective (2.25 µgm-3 expressed as a running annual mean). However, the network was reviewed in 2007, and in view of the fact that
- 1,3-butadiene levels at all the sites were well below the Objective and
- levels at half of the sites were at or below the detection limit for the method used.
Defra took the decision to discontinue monitoring 1,3-butadiene.
As the Objectives and Limit Values for benzene relate to the annual average concentration, it is not necessary to use a monitoring method with short time resolution. Sampling is therefore undertaken using pumped samplers, located at monitoring stations operated within the Automatic Urban and Rural Network (AURN)
How is the network run?
Ambient air is pumped through sampling tubes containing a proprietary absorbent (Carbopack X), using purpose-built pump units that switch between two tubes to produce two nominally identical samples covering each fortnight. Every fortnight, the tubes are changed, and the instruments checked by Local Site Operators. Exposed tubes are sent to the network management unit for analysis.
The fortnightly pumped measurement method for benzene was developed specifically for this network; this following the requirement of the 2nd Daughter Directive that, in view of their inherently lower measurement uncertainty, measurements for reporting purposes be made by pumped sampling rather than by diffusive sampling. Previously, pumped sampling for benzene had been geared to short periods of a day or less. The combination of a suitable sorbent material and sound engineering in the pump control box has led to a very successful method.
The EU instructs CEN, the European Committee for Standardisation, to set out standard methods to be used to comply with Directives. The relevant CEN benzene standards (EN 14662, five parts) include the pumped method used in this network.
View data for sites for Non-Automatic Hydrocarbon Network network
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