GAW Report No. 205 - IGAC Project

CHAPTER 6 - EUROPEB) Particulate matter pollutionBesides from air quality network measurements, climatological information about aerosolloads over the Paris region is available from optical measurements. Aerosol optical density (AOD) ismeasured by a sunphotometer part of the AERONET network 20 km SW of Paris town centre.During clear sky days, AOD is mostly comprised between 0.1 and 0.4, with a median value of 0.17[Chazette et al., 2005]. The average single scattering ratio (at 532 nm) of 0.9 is typical for urbanaerosol [Raut and Chazette, 2007]. From comparisons of aerosol backscatter lidar measurementsat the same site and model simulations, it has been inferred that secondary organic aerosol wasprobably underestimated in the CHIMERE model simulations [Hodzic et al., 2004].Regional / continental origin of aerosols in the Paris regionHourly concentrations of inorganic salts (ions) and carbonaceous material in fine aerosols(aerodynamic diameter, A.D. < 2.5µm) have been determined from fast measurements performedfor a 3-week period during the spring of 2007 at an urban site within the city of Paris [Sciare et al.,2010, submitted]. The sum of these two chemical components (ions and carbonaceous aerosols)has shown to account for most of the fine aerosol mass (PM 2.5 ) in this area. This time-resolveddataset allowed investigation of the factors controlling the levels of PM 2.5 and showed that pollutedperiods with PM 2.5 >40µg/m 3 (during periods I and III in Figure 14) were characterized by air massesof continental (European) origin and a chemical composition made up of 75% of ions. By contrast,clean marine air masses show the lowest PM 2.5 concentrations (typically of about 10µg/m 3 , period IIin Figure 14) with carbonaceous aerosols contributing most of the mass (typically 75%). The ratherstable levels of carbonaceous aerosols observed during this study suggest that the region of Parisis a major contributor to this fraction.Figure 14 - Time resolved PM measurements in downtown Paris in May/June 2007. The periods I and III correspond to apredominance of inorganic ions of mainly continental origin in Paris downtown PM2.5, the period II corresponds to apredominance of carbonaceous aerosol of mainly local origin[Sciare et al., 2010]By contrast, long-range transport from Europe is proposed as the main contributor for ionsmeasured in Paris during the springtime. Further studies need to address if these results are validon a longer climatological time scale.206