High levels of air pollution are a common problem in both road and railroad tunnels. However, sources and emission processes differ significantly, as reflected by the physical and chemical properties of the two aerosols. As particle concentrations and properties affect exposure of and health effects for people on platforms and in vehicles, effective ways to reduce emissions and exposure are important.
This study aims to improve the knowledge of the differences between PM10 in the rail and road tunnel environments, their sources and the possibilities to address problems with high particulate levels.
Measurement campaigns were carried out at Arlanda Central, a railroad tunnel station below Arlanda airport and in Söderleden road tunnel, a road tunnel in central Stockholm.
Measurements included particle concentrations, size distributions, size resolved element content, NOx, and organic and elemental carbon. Traffic and meteorology were measured and/or collected from existing databases.
In Söderleden road tunnel, the campaign (non-intentionally) included both a period that was mainly wet and one that was dry. This gave the opportunity to study the differences in the importance of suspension to the contribution to particle levels.
The results show that the rail tunnel environment was characterized by relatively high concentration peaks of coarse particles and low levels of NOx and NO2. Some trains were linked to emissions of ultrafine particles. The composition of the airborne particles is dominated by iron, with smaller contributions from copper, zinc and other metals.
The road tunnel is characterized by high levels of ultra-fine particles, NOx and NO2 and, in dry condition, also high levels of coarse particles. As the traffic is more intense than in the rail tunnel, particle levels are more constantly high during busy traffic. In humid conditions the coarse particles were dominated by iron whereas particles below about 1 micron were dominated by sulphur. In dry conditions, increases in the typical mineral elements silicon, potassium, calcium and iron were substantial. Chlorine represents a significant percentage in both wet and dry conditions, which suggests a contribution from road salt. The iron is suggested to originate from brake were in wet conditions, and from both brake wear and road wear in dry conditions.
By comparing the data with train passages and information on train types, it was found that most of the high particulate levels recorded at Arlanda C are correlated to older trains with locomotives of type Rc with their wagons. These mainly have mechanical brakes and are also braked during longer time and distance before stopping at the station. The content of elemental carbon in the air of the railroad environment was unexpectedly high, about half of the content of the road tunnel, despite lack of combustion sources. This is considered to be due to wear of graphite from the train pantograph.
The main focus of action against high particle levels in railroad tunnels has been on ways to prevent exposure by separating trains from the platform or to vent contaminated air, while studies on the opportunities to prevent emissions are fewer
This study demonstrates the potential to reduce particulate emissions by identifying the types of trains and train individuals and their characteristics, technical systems that causes particle emissions, maintenance status and also how they are driven. In road tunnels abatement measures against coarse particles are linked to measures that reduce studded tire wear of road surface or reduce the suspension of deposited dust. This can be done by reducing the use of studded tires, improved pavements, effective dust control and cleaning, in addition to reducing traffic and speed.