2025-05-13

The Light in the Tunnel

Left to its own, the air quality inside a road tunnel would eventually make the tunnel impossible to travel in. The visibility would be impaired by high levels of particulates, and it would be difficult to breathe. Even relatively short exposures to the potentially very high levels of air pollutants could also have long-term health effects.

This is the reason why there are regulations on how bad the air quality may get inside the tunnels, and why there are big fans to force clean air into the tunnel tubes while evacuating the potentially dangerous fumes. Authorities require monitoring of the air quality from a health and road safety perspective, while tunnel operators need air quality data to control the ventilation system.

What to Monitor

Vehicles with combustion engines give rise to a variety of air pollutants but from a health perspective, the key substances are CO, NO_X (alternatively NO₂, the key component of NO_X), and particulates. The latter are also of concern with respect to road safety due to visibility issues that can occur inside the tunnels.

Particulate concentrations can be monitored in terms of PM levels, but it could also be sufficient to measure visibility, requiring less complex instrumentation. The CO concentrations are often measured as is. NO_X concentrations can also be measured as is, but it works just as well to measure NO₂ only since the NO fraction of NO_X quickly is oxidized to NO₂ once emitted from the vehicles.

In addition to the air pollutant concentrations, it is also customary to measure air temperature and air flow, and then there is of course also pure traffic-related information such as vehicle numbers and speeds to monitor.

A Tough Environment

A road tunnel means a harsh operating environment for monitoring equipment. There are high temperatures, high dust concentrations, high and corrosive emissions from the passing traffic, and vibrations on top of that. Any air quality monitoring instrumentation placed in the tunnels must be able to withstand these tough conditions, and it must do it with a minimum of maintenance and potential repair work as the traffic can only be shut down or diverted for short periods during low-traffic hours, typically during night-time. Installing the wrong type of equipment can become very costly and cumbersome to handle for the tunnel operator.

The OPSIS Offer

The OPSIS DOAS gas analyser measures gas concentrations along an open light path. This approach has proven to work very well in tunnel environments. The light emitters and receivers made of stainless steel are very durable and can withstand both high temperatures, high dust levels and vibrations. They can also take high-pressure water cleaning of the tunnel walls. The more sensitive gas analyser is typically placed in an air-conditioned cabinet installed for example in a cross-passage between tunnel tubes. Light picked up by the receiver is sent to the analyser via a well-protected fibre optic cable. All in all, this forms a very reliable and durable monitoring system. The use of optical multiplexers allows more than one light path to be connected to the same gas analyser, thereby adding further cost effectiveness to the system.

OPSIS gas analysers are installed in road tunnels all around the world. Track records show very reliable operation, with a minimum of maintenance. Repeat orders from multiple tunnel operators confirm the quality and value-for-money of the systems.

Get in touch with OPSIS or any of its representatives if you want a hassle-free air quality monitoring solution also for your road tunnel!