WP3: Megacity plume case study

Overview and background

The major objective of this WP is to provide new experimental data to better quantify sources of primary and secondary aerosol in and around a large agglomeration and to document its evolution in the megacity plume; this will be done through organizing dedicated field campaigns in and around the Paris agglomeration. Greater Paris has been chosen for such a campaign because it is a major and dense pollution source (more than 10 million inhabitants), surrounded by rural areas and relatively flat terrain. A particular focus will be put on organic carbon, for which secondary formation, but also primary emissions are still not well quantified. Emission inventories for black carbon (BC) and organic carbon (OC) are much more uncertain than those for gaseous species. For example, European scale model simulations suggest a possible underestimation of elemental carbon emissions by about a factor of two (Schaap et al., 2004). Moreover, emission inventories for primary organic aerosol (POA) are probably misused in chemistry-transport models, because POA evaporation during dilution of emissions is not considered (Robinson et al., 2007). In addition, secondary organic aerosol (SOA) is underestimated in many models (Volkamer et al., 2006). Recently, the use of factor analysis models in conjunction with high time resolution measurements of aerosol chemistry with an Aerosol Mass Spectrometer (AMS) have opened new opportunities for a more detailed source apportionment, with discrimination of a hydrocarbon-like organic aerosol (HOA) and different components of an oxidized organic aerosol (OOA) (Lanz et al., 2007). In addition, carbon-14 analysis has been shown to be a powerful tool in discriminating between the fossil and biogenic fractions of elemental and organic carbon (Szidat et al., 2006). The application of these methods to detailed aerosol and precursor gas measurements, as planned in this study, will allow for a substantial improvement in our understanding of anthropogenic carbon-containing aerosols.

Methodology and advancement beyond the state-of-the-art

Specific measurement campaigns will be set up in the Paris region during 2009: a ground based segment with observations at an urban and a suburban site during one summer and winter month will allow for documenting the aerosol composition and properties, and their variability, near primary emission sources. An airborne segment with dedicated flights with the French ATR-42 aircraft in the Paris plume during one summer month will permit documenting the evolution of the megacity plume and especially the build-up of secondary organic and inorganic aerosol species from precursor gases. These measurements will combine a very large suite of state of the art instruments, including several new techniques, capable of tracing chemical and physical aerosol properties, and related precursor gas concentrations. Conjunct airborne and ground based measurements of the chemical SOA composition and of oxidised VOC will offer the opportunity to document gas phase aerosol interaction at various stages of the plume development. In addition, the modification of optical and hygroscopic parameters during plume aging will be addressed. The data will also be compared to the aged aerosol found at Puy de Dome, where the aerosol measurements will be performed by CNRS-LAMP at no cost for MEGAPOLI. The experiment will build on the experience of the recent MILAGRO campaign performed in Mexico-City (Doran et al., 2007). The Paris experiment will be substantially smaller than the MILAGRO experiment, however, due to the application of several new techniques (e.g. the carbon-14 analysis of both EC and OC, the application of the Lanz (2007) method, the inclusion of both organic aerosol mass spectra and elemental spectra at high time resolutions) it will go beyond the MILAGRO campaign in the area of source apportionments.

Data from these campaigns will allow for a detailed assessment, for one case study, of how megacity emissions impact on air quality, regional scale atmospheric composition and regional climate (in conjunction with WPs 4, 5, 6, and 7). In addition to the observations funded within MEGAPOLI, we will strive to obtain national funding to support further measurements, e.g., with the mobile laboratory of PSI. Furthermore, we have had expressions of interest from several other scientific groups for participating in the campaign on institute or national funding, for instance from Prof. Stephan Borrmann, director of the Particle Chemistry Department at the MPIC.

FP7 EC MEGAPOLI, 2008-2011