Air pollution measurements in traffic tunnels

Our plain-English summary of Air pollution measurements in traffic tunnels, by R. De Fré, P. Bruynseraede, J.G. Kretzschmar (Environmental Health Perspectives, 1994).

The study

• Authors: R. De Fré, P. Bruynseraede, J.G. Kretzschmar
• Journal: Environmental Health Perspectives
• Published: 1994;102 Suppl 4(Suppl 4):31-7
• Cited by: 16 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Air pollution measurements during April 1991 are reported from the Craeybeckx highway tunnel in Antwerp, Belgium. The tunnel was used daily by an average of 45,000 vehicles, of which 60% were gasoline fueled passenger cars, 20% diesel cars, and 20% trucks. Of the gasoline cars, only 3% had three-way catalysts. Tunnel air concentrations of nitrogen oxides, sulphur dioxide, carbon dioxide, carbon monoxide, nonmethane hydrocarbons, volatile organic compounds, polycyclic aromatic hydrocarbons, and lead are presented. The traffic emissions in the tunnel are calculated by the carbon balance method, which uses the increase of the total carbon concentration in the tunnel air as the reference quantity. Division of the concentration of any pollutant by the total carbon concentration gives emission factors per kilogram of carbon. These emission factors can be converted directly to emissions relative to fuel consumption or per kilometer. The fraction of diesel used in the tunnel was derived from sulphur to carbon ratios in tunnel air. A calculation procedure with breakdown of emission factors according to vehicle categories was used to estimate countrywide emissions. The estimated emissions were compared to results from the Flanders Emissions Inventory [Emissie Inventaris Vlaamse Regio (EIVR)] and calculated emissions according to the emission factors proposed by the European Commissions CORINAIR Working Group. For NOx there is excellent agreement. For carbon monoxide and hydrocarbons, the tunnel data produced higher emissions than the CORINAIR model would predict but lower than the official EIVR statistics. The estimated lead emissions from traffic are found to be 22 to 29% of the lead in gasoline.

In plain English

This study measured air pollution in the northbound tube of the Craeybeckx highway tunnel in Antwerp, Belgium, in March and April 1991. About 45,000 vehicles a day passed through on working days. The authors used a 'carbon balance' method, comparing the rise in each pollutant with the rise in carbon in the tunnel air, to estimate traffic emissions and Belgian national emissions. For highway traffic at a speed of 120 km/hr, the estimated emissions per kilometer were 4.49 g of carbon monoxide and 4.01 g of nitrogen oxides. These were close to the European CORINAIR model's 3.78 g and 3.99 g. Nonmethane hydrocarbons were 1.33 g, almost twice the model's 0.68 g. Carbon monoxide, hydrocarbon and nitrogen dioxide emissions rose markedly in congested traffic. Only 22% of the lead expected from gasoline was recovered in this tunnel, and 28.6% and 24% in re-analysed 1989 data from two Brussels tunnels. Asbestos fibres were sampled occasionally, but the article reports no asbestos results. The authors concluded that the method is a fast way to assess on-road emissions, but its use for particle-bound pollutants still needs validation.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Field air-monitoring study in a road tunnel, using a carbon balance (mass balance) method to estimate vehicle emission factors and national emissions
• Who was studied: Air in the northbound tube of the 1600-m Craeybeckx highway tunnel, Antwerp, Belgium, measured March to April 1991. About 45,000 vehicles per day on working days and 23,000 per day at weekends. The traffic was assumed to be 60% gasoline cars, 20% diesel cars and 20% trucks, and only 3% of the gasoline cars had three-way catalysts.
• Exposure: Motor vehicle exhaust in tunnel air. The pollutants measured were nitrogen oxides, sulfur dioxide, carbon dioxide, carbon monoxide, nonmethane hydrocarbons, volatile organic compounds, polycyclic aromatic hydrocarbons, dust and lead. Asbestos fibres were sampled only occasionally, and no asbestos results are reported.
• Main results:
  • Estimated highway emissions, calculated for a traffic speed of 120 km/hr, from the tunnel data compared with the CORINAIR model: carbon monoxide 4.49 vs 3.78 g/km; nitrogen oxides 4.01 vs 3.99 g/km; nonmethane hydrocarbons 1.33 vs 0.68 g/km.
  • Compared with the Flemish Region Emissions Inventory, the tunnel-based estimates (kg/day per km of highway) were: carbon monoxide 184 vs 519; nitrogen oxides 165 vs 178; sulfur dioxide 8 vs 18.
  • Carbon monoxide, hydrocarbon and nitrogen dioxide emissions increased markedly when traffic in the tunnel became congested.
  • The share of carbon in the tunnel air coming from diesel fuel, estimated from sulfur-to-carbon ratios, averaged 56%, weighted by hourly traffic.
  • Only 22% of the expected lead was recovered in the Craeybeckx tunnel. In 1989 data from two Brussels tunnels, the lead recovered was 28.6% and 24% of the lead in the gasoline. The authors assumed that only 22 to 29% of the lead in gasoline is emitted, which contradicts the CORINAIR assumption of 75%.
  • If tunnel air were compared with Belgian ambient air quality standards, the nitrogen dioxide and sulfur dioxide standards would be exceeded.
• The authors' conclusion: The authors concluded that the carbon balance over a traffic tunnel offers a relatively fast way to assess on-the-road emissions for many cars under actual driving conditions. They found it useful for checking emission factors and inventories, but its applicability to aerosols still requires further validation.
• Limitations the authors state:
  • Nighttime estimates of the diesel fraction came out above 100%, from an error the authors could not identify. They state that this uncertainty shows inherent weaknesses in the carbon balance method.
  • The tunnel measurements represent only two traffic modes, do not include cold starts, and overrepresent high-speed driving.
  • The method assumes no significant deposition of pollutants in the tunnel. However, sulfur dioxide was washed out in wet weather and soot was visibly deposited on the walls.
  • The lead result needs clarification before the method can be applied with confidence to particle-related pollutants.

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