MASS BALANCE OF FREE TROPOSPHERIC AEROSOL AT THE PUY DE DÔME (FRANCE) IN WINTER

The size-segregated chemical composition of aerosols was investigated during winters 2000 and 2001 at Puy de Dome (1465 m above sea level, France), a site most of the time located in the free troposphere. Aerosols have been sampled using low-pressure cascade impactors (Electrical Low Pressure Impactor (ELPI) and Small Deposition Impactor (SDI) 13 and 12 stages) and analyzed for inorganic (Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3-, and SO42-) and organic (HCOO-, CH3COO-, and C2O42-) ions, organic and elemental carbon (OC and EC), insoluble dust, and total mass. Under cloudy conditions, the sampling includes interstitial aerosol as well as the residue of evaporated cloud droplets. Aerosols (and residues of cloud droplets) were sampled in different air masses, which can be classified into three different categories according to their aerosol load and composition: background (BG), anthropogenic (ANT), and specific events (EV) that include advection of Saharan dust and upward transport from the polluted boundary layer to the site. On the basis of the presence or absence of coarse sea-salt particles, a further classification permits us to distinguish air masses that have or have not been exposed to the ocean. A closed mass balance is achieved on submicron ranges (mean departure of 18.5%) for the three main air mass categories, providing a reliable description of main aerosol types in the west European free troposphere. The total aerosol mass at 50% relative humidity is close to 2.7 +/- 0.6 mg m(-3) in BG, 5.3 +/- 1.0 mg m(-3) in ANT, and 15 to 22 mg m(-3) in EV air masses. The aerosol mass distribution generally exhibits two submicron modes (Acc1 at 0.2 +/- 0.1 mm and Acc2 at 0.5 +/- 0.2 mm geometric mean diameter (calculated for every impactor stage) and a supermicron mode (2 +/- 1 mm). Aerosols exhibit a high degree of external mixing with carbonaceous (EC and OC) and ionic species associated with Acc1 and Acc2. Concentrations of light carboxylates and mineral dust never exceed 4% of the total content of analyzed compounds, except for a Saharan dust event during which the contribution of insoluble dust reaches 26% of the total aerosol mass. Depending on the sampled air mass, bulk water-soluble inorganic species and carbonaceous material account for 25-70% and 15-60% of the total mass, respectively. The OC fraction is higher in air masses with low aerosol load (53%, 32%, and 22% for BG, ANT, and EV, respectively). Conversely, the EC fraction is enhanced from 4% in BG to 10% in ANT and 14% in EV. The inorganic fraction is more abundant in EV (55%) and ANT (60%) than in BG (40%) air masses as a result of enhanced nitrate and, to a lesser extent, sulfate and ammonium levels.