Road traffic is a serious problem in the Chamonix valley: road traffic and air pollution worry the inhabitants. The big fire in the Mont-Blanc tunnel made it possible, in the framework of the POVA project (Alpine Valley Pollution), to undertake measurement campaigns with and without heavy-vehicle traffic through the valley, towards Italy (before and after the tunnel re-opening). Atmospheric prediction model ARPS 4.5.2 (Advanced Regional Prediction System), developed at the CAPS (Center for Analysis and Prediction of Storms) of the University of Oklahoma, enables to resolve the atmosphere dynamics above a complex terrain. This model is coupled to the TAPOM 1.5.2 atmospheric chemistry (Transport and Air POllution Model) code developed at the Air and Soil Pollution Laboratory of the Ecole Polytechnique Fédérale de Lausanne. The numerical codes MM5 and CHIMERE are used to compute large scale boundary forcing. Using 300-metre grid cells to calculate the dynamics and the reactive chemistry makes possible to accurately represent the dynamics in the valley (slope and valley winds) and to process chemistry at fine scale. Atmospheric chemistry indicators are computed for days of the field campaign. NOy, H 2 O 2 /HNO 3 , O 3 /NOz demonstrate a VOC controlled regime. A new indicator based on ozone chemistry contribution and computed from the chemistry code is suggested for the purpose of localising ozone production area in the valley. Various scenarios of traffic reduction demonstrate a significant increase of ozone concentration at night.