Södra länken (The southern link), is a motorway, which is connecting Essingeleden, Stockholm with Värmdöleden, Nacka and partially runs underneath the city. In total, Södra länken is approximately 6 km long, while about 4.5 km lay underground. Considering the distances, Södra länken therefore is the third largest urban, underground motorway 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. The yellow marked areas in Pic. 1 show the subterranean parts. On both ends, a ventilation tower is installed, in order to guarantee continous air interchange. The construction started in 1997 and its opening took place in 2004. The reason for its creation was a law implementation from the 1990s, which did not allow a conventional motorway near to the residential areas in the city, due to air pollutants and noise disturbance.
The choice, which style of gas monitoring should be implemented for a ventilation control system, favored an extraxtive analysis system. According to European regulations and standards, this systems should contain chemiluminiscence instruments (ECO PHYSICS CLD 700 AL). At the time, also Carbon Monoxide was monitored, but has been abandoned ever since. In 2018 the old systems were changed to new ones, and also this time the road authorities favored the extractive systems with ECO PHYSICS analyzers inside.
Considering for instance 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 sec., 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 alternating. 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 these 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 500 m of 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 conecentration is 400 µg/ m3 (1 h avarage), wich is lower than in for instance the later ”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.
Reasons given are the increase of diesel vehicles and the higher Ozon concentrations in the ambient air.