Evidence of primary air pollutant reduction and photochemical pollution enhancement after opening a road tunnel to the public

Measurements of local air pollution and road traffic were taken during a period of one month before and after the opening of a road tunnel to the public in nearby Sachseln, a small city located in a rural region (Obwald) at the center of Switzerland. Two 5-day periods of time (before and after the tunnel opening) of equivalent weather and road traffic features were selected for assessing the tunnel impact on air quality. Despite the road traffic across Sachseln dropped by about 50% after the tunnel opening, its daily profile remained unchanged. After a further time split, statistical analyses allowed determining the modal concentrations of air pollutant at daytime and nighttime (background). Truck traffic was found to be the controlling variable of PM10 concentration. After the tunnel was opened, daytime concentration of PM10, PAH and NO(x) dropped by about 50%. On the contrary, the ambient O3 concentration increased by about 35%, probably as a result of reducing NO(x) in a VOC-limited regime for ozone formation.