F. Gheusi

Isotopic Composition of Gaseous Elemental Mercury in the Free Troposphere of the Pic du Midi Observatory, France

Understanding the sources and transformations of mercury (Hg) in the free troposphere is a critical aspect of global Hg research. Here we present one year of observations of atmospheric Hg speciation and gaseous elemental Hg (GEM) isotopic composition at the high-altitude Pic du Midi Observatory (2860 m above sea level) in France. Biweekly integrated GEM from February 2012 to January 2013 revealed significant variations in δ(202)HgGEM (-0.04‰ to 0.52‰) but not in Δ(199)HgGEM (-0.17‰ to -0.27‰) or Δ(200)HgGEM (-0.10‰ to 0.05‰). δ(202)HgGEM was negatively correlated with CO and reflected air mass origins from Europe (high CO, low δ(202)HgGEM) and from the Atlantic Ocean (low CO, high δ(202)HgGEM). We suggest that the δ(202)HgGEM variations represent mixing of recent low δ(202)HgGEM European anthropogenic emissions with high δ(202)HgGEM northern hemispheric background GEM. In addition, Atlantic Ocean free troposphere air masses showed a positive correlation between δ(202)HgGEM and gaseous oxidized Hg (GOM) concentrations, indicative of mass-dependent Hg isotope fractionation during GEM oxidation. On the basis of atmospheric δ(202)HgGEM and speciated Hg observations, we suggest that the oceanic free troposphere is a reservoir within which GEM is readily oxidized to GOM.

Low Tropospheric Layers Over Reunion Island in Lidar-Derived Observations and a High-Resolution Model

In November and December 2008, ground-based mobile lidar (GBML) measurements were carried out on Reunion Island (Indian Ocean,

Ozone et pollution atmosphérique à grande échelle (2) La campagne de mesures Pic 2005

21^{\circ }07^{\prime }\hbox {S}, 55^{\circ }32^{\prime }\hbox {E}

Tropospheric ozone over Equatorial Africa: regional aspects from the MOZAIC data

21∘07′S,55∘32′E, 700 km east of Madagascar) with an ultraviolet (355 nm) aerosol-backscatter lidar. Complex substructures were identified within the planetary boundary layer (PBL). A 500-m-resolution non-hydrostatic model was used to simulate the dynamics of the lower troposphere for two observation periods characteristic of the two main weather regimes in this season: the “trade-wind” regime and the “breeze” regime. The model captured the observed structures with a high degree of realism compared to the GBML. A complete diurnal cycle of the PBL along the south coast of the island during a “trade-wind” day was observed and simulated. The PBL depth was found to be anti-correlated with the wind speed. The model showed that the PBL along the coast behaved as a shallow-water flow in hydraulic theory. As the flow accelerated in response to lateral constriction, conversion of potential into kinetic energy forced the PBL top downwards. This favoured rapid transport of concentrated surface emissions within the contracted surface layer, with a possible impact on air quality. GBML observations were also conducted during the early morning of a “breeze” day on the western slope of the Maïdo mountain (2,200 m), at the top of which a new atmospheric observatory has been in operation since 2012. Both model and GBML revealed two superposed layers. The upper layer, higher than approximately 1,600 m above mean sea level, corresponded to free tropospheric air driven by the trade winds. Below, westerly counterflow advection of humid marine air occurred as a result of wake vortices in the lee of the island. The model suggests that free-tropospheric conditions prevail at the observatory from the second half of the night to mid-morning.

Numerical simulations of local circulations and pollution transport over Reunion Island

Ozone as an environmental concern extends beyond the questions usually covered by the media – stratospheric ozone depletion over the Antartic and urban pollution peaks. Strong exposures to this pollutant are frequent even far from pollution sources, and the background tropospheric content of ozone has been grown fivefold over the last century. In response to this concern at the French national scale, formerly independent monitoring stations have been coordinated since 2004 in a structured network: Paes (French acronym for atmospheric pollution at synoptic scale). The data are put in free access online.

CORSiCA: a Mediterranean atmospheric and oceanographic observatory in Corsica within the framework of HyMeX and ChArMEx

The « Pic 2005 » field experiment took place in June and July 2005 around the Pic du Midi (2 875m asl, in the central Pyrenees). Its main goal was to investigate the spatial and temporal structure of the ozone field around that summit, which is a permanent measurement site of the Paes network (La Meteorologie, no 58). The article provides a summary of the scientific context, a description of the observation network deployed for the campaign, and an illustrative panel of the collected observations and main lines of research.