Tunnel portal emissions and portal concentrations were estimated for a selection of tunnels on the Highway Agency’s road network. These estimates were based upon recorded traffic flows, speeds and fleet compositions and employed simple emission functions. Pollution profiles for nitrogen dioxide were measured in the vicinity of two tunnel ports, the Southwick and Bell Common tunnels. Measurements were undertaken using simple low cost diffusion tubes, with the results biased adjusted against continuous chemiluminescence analysers operated by neighbouring local authorities. Standard Palmes diffusion tubes were employed, in combination with the passive Ogawa samplers. Similar nitrogen dioxide measurements were recorded with the two measurement methods at the Southwick tunnel. However at the Bell Common tunnel the Ogawa samplers recorded significantly higher concentrations when compared to the Palmes tubes. It appears that the Ogawa samplers considerably over-estimated nitrogen dioxide concentrations at this site, characterised by relatively high nitrogen dioxide concentrations. Tunnel portal emissions may be visualised as horizontal jets, which are emitted at the tunnel exit, in part through the piston effect of the traffic travelling through the tunnel bore. Upon emission this jet plume is subject to interaction and sheer with the prevailing meteorology. The latter will also be influenced by topography in the immediate vicinity of the portal, where the majority of tunnel exits occur within cuttings. The measured pollution profiles were compared with the results from the tunnel portal air pollution model, GRAL, developed by the Technical University of Graz. From this relatively small study, it may be concluded that the GRAL model appears suitable for the assessment of tunnel portal air quality. Plume stiffness appears to be the most significant parameter in improving the agreement between measured and modelled concentrations.

Want to know more about this project?