Södra länken (The southern link) is a motorway which connects Essingeleden, Stockholm with Värmdöleden, Nacka and it partially runs underneath the city.
In total, it is approximately 6km long, with about 4.5km underground. It is the third largest urban, underground motorway in Europe, only exceeded by the M30 orbital motorway in Madrid and the Blanka tunnel complex in Prague. The tunnel contains 4 lanes in each direction at its widest point.
On both ends, a ventilation tower is installed, in order to guarantee continuous air interchange. The construction started in 1997 and it opened in 2004. The reason for its creation was a law from the 1990s which did not allow a conventional motorway so near the residential areas in the city, due to pollution and noise.
The choice of gas monitoring was an extraxtive analysis system. According to European regulations and standards, this system should contain chemiluminiscence instruments (Eco Physics CLD 700 AL). Originally, Carbon Monoxide was also monitored, but this has since been abandoned. The old systems have been updated to new ones, with the road authorities choosing extractive systems from Eco Physics.
Using the road-side traffic monitoring in downtown Hong Kong as a reference, Eco Physics’ latest model for Ambient Air monitoring, the nCLD AL2, was chosen. This decision was made due to the fact that besides the demanded specifications (lag time 3 seconds, resolution 0.4 ppb, 1Sigma < 0.2 ppb), this instrument is a two-channel ambient air monitor.
This allows the user to collect NO, NOx and NO2 values simultaneously, not alternatively. Additionally, all essential components for a chemiluminescence detector come with an attached PCB, which allows them to communicate with the mainframe and share their performance in real time. All this data is displayed on the diagnostic output screen and the service technician can easily locate any incident if maintenance is needed. Each nCLD AL2 is integrated into a rack system, which draws its sample from several measuring points in the tunnel (2-5 sample points). The sample lines are up to 500m in length and are equipped with an automated pump/valve system. In total, 42 sample points are spread over the tunnel system.
The maximum tolerated NO2 concentration is 400 µg/m3 (1h average), which is lower than in for instance the North link in Stockholm, where the value is 1000 µg/m3. In fact, for different reasons, the NO2 has increased over time, and for short periods the values will nowadays be considerable higher than 400 µg/m3 in different parts of the tunnel. This is because of the increase of diesel vehicles and the higher Ozone concentrations in the ambient air.