Vertical and horizontal concentration profiles from a tracer experiment in a heterogeneous urban area

An atmospheric tracer dispersion study was conducted in a heterogeneous district of the city of Nantes, France, in May 2010 and June 2012 during the measurement campaigns FluxSAP. Vertical (0–100 m) and horizontal (≈ 300 m) profiles were measured at distances of 20 to 1200 m from the tracer gas release location. The vertical profiles show vertical mixing occurring progressively with distance: the 0–100 m air column is homogenous beyond 600 m, whereas at shorter distances a maximum of concentration is observed, either at ground level for distances less than 200 m, or at altitude for intermediate distances. Our measurements are compared with the Briggs-urban model which may be used in emergency situations: the model gives coherent results with our measurements in terms of vertical and horizontal dispersion parameters but we observe concentration differences up to an order of magnitude between model and measurements. Based on our results we propose new relationships for the horizontal and vertical dispersion parameters for distances from 0 to 1 km. For emission–measurement distances above 1 km, differences appear between the trends deduced from our measurements and those of the Briggs-urban model: it will be necessary to acquire data at greater distances, 2000 m and more, to verify and validate these trends.