Regional transport and dilution during high‐pollution episodes in southern France: Summary of findings from the Field Experiment to Constraint Models of Atmospheric Pollution and Emissions Transport (ESCOMPTE)

Abstract

In the French Mediterranean basin the large city of Marseille and its industrialized suburbs (oil plants in the Fos-Berre area) are major pollutant sources that cause frequent and hazardous pollution episodes, especially in summer when intense solar heating enhances the photochemical activity and when the sea breeze circulation redistributes pollutants farther north in the countryside. This paper summarizes the findings of 5 years of research on the sea breeze in southern France and related mesoscale transport and dilution of pollutants within the Field Experiment to Constraint Models of Atmospheric Pollution and Emissions Transport (ESCOMPTE) program held in June and July 2001. This paper provides an overview of the experimental and numerical challenges identified before the ESCOMPTE field experiment and summarizes the key findings made in observation, simulation, and theory. We specifically address the role of large-scale atmospheric circulation to local ozone vertical distribution and the mesoscale processes driving horizontal advection of pollutants and vertical transport and mixing via entrainment at the top of the sea breeze or at the front and venting along the sloped terrain. The crucial importance of the interactions between processes of various spatial and temporal scales is thus highlighted. The advances in numerical modeling and forecasting of sea breeze events and ozone pollution episodes in southern France are also underlined. Finally, we conclude and point out some open research questions needing further investigation

North-western Mediterranean sea-breeze circulation in a regional climate system model

“…They also allowed the analysis of the contribution of the sea-breeze to the regional transport of humidity (Bastin et al 2005a and pollutants Menut et al 2005) and the evaluation of existing sea-breeze scaling laws with the large body of observations collected during the campaign . A review of the main results from this campaign can be found in Drobinski et al (2007). Such literature enables a critical evaluation of the HyMeX/MED-CORDEX simulations described hereafter.…”

“…Two major flow regimes alternate during summertime in this region, the Mistral and Tramontane northerly-northwesterly winds and the sea breeze induced by the sea land temperature contrast which advects moisture and pollutants as far as 100 km inland thus affecting the inhabitants leaving in the countryside (e.g. Bastin et al 2007;Drobinski et al 2007). A large number of data were collected (meteorological data from surface weather stations, radiosondes and constant volume balloons, wind profiles from lidar, radar and sodar profilers, … ; see Cros et al 2004 for more details) and meso-scale numerical simulations performed.…”

Mass tracking for chemical analysis: the causes of ozone formation in southern Ontario during BAQS-Met 2007

“…The surface temperature during the passage of Marseille's sea-breeze front has been found to be oscillatory Drobinski et al, 2007). Cold air transported inland due to the sea breeze results in temporarily increased stability over the land (isentrope inclination allows the seabreeze flow to run along the slope, and adiabatic cooling associated with rising air retards the flow).…”

“…High-resolution modelling studies have suggested that sea and lake breezes interact with Marseille's VOC emissions to create high ozone episodes (Lasry et al, 2005). Sophisticated urban heat island modelling for the same region suggests that Marseille's urban heat island circulation is affected by the sea breeze: the urban circulation when the city was affected by sea breezes was suppressed relative to instances of synoptic flows originating over the land (Lemonsu et al, 2006a;Drobinski et al, 2007). Work by the same group (Lemonsu et al, 2006b) also suggested the presence of "deep" and "shallow" sea-breeze circulations, the former inhibited by topography, the latter enhanced by topography.…”

“…Relatively low NO 2 and high O 3 in the Marseille marine boundary layer (indicative of a more photochemically aged airmass) suggest that the sea-breeze circulation acts as an efficient photochemical reactor . Pollutant concentrations in Marseille are highest just upwind of the sea-breeze front (the front's propagation speed is lower than the wind speed, and the vertical mass flux at the front is less than the horizontal flux within the sea-breeze, allowing pollutants to accumulate just behind the front, with some upward transport; (Drobinski et al, 2007). The accurate simulation of the fine-scale features of sea-breeze mesoscale transport in the Marseille area is crucial in order to predict both ozone peaks and ozone plumes (Pirovano et al, 2007).…”