Abstract
Abstract. A detailed characterization of air quality in the megacity of Paris (France) during two 1-month intensive campaigns and from additional 1-year observations revealed that about 70 % of the urban background fine particulate matter (PM) is transported on average into the megacity from upwind regions. This dominant influence of regional sources was confirmed by in situ measurements during short intensive and longer-term campaigns, aerosol optical depth (AOD) measurements from ENVISAT, and modeling results from PMCAMx and CHIMERE chemistry transport models. While advection of sulfate is well documented for other megacities, there was surprisingly high contribution from long-range transport for both nitrate and organic aerosol. The origin of organic PM was investigated by comprehensive analysis of aerosol mass spectrometer (AMS), radiocarbon and tracer measurements during two intensive campaigns. Primary fossil fuel combustion emissions constituted less than 20 % in winter and 40 % in summer of carbonaceous fine PM, unexpectedly small for a megacity. Cooking activities and, during winter, residential wood burning are the major primary organic PM sources. This analysis suggests that the major part of secondary organic aerosol is of modern origin, i.e., from biogenic precursors and from wood burning. Black carbon concentrations are on the lower end of values encountered in megacities worldwide, but still represent an issue for air quality. These comparatively low air pollution levels are due to a combination of low emissions per inhabitant, flat terrain, and a meteorology that is in general not conducive to local pollution build-up. This revised picture of a megacity only being partially responsible for its own average and peak PM levels has important implications for air pollution regulation policies.
Highlights
Megacities, defined as agglomerations with a population above 10 million inhabitants (Gurjar and Lelieveld, 2005) often concentrate in a small area a substantial part of a country’s population, economic activities, and air pollutant emissions
Additional data sets used in this study are described in the Supplement: satellite data sets (aerosol optical depth (AOD) from the Advanced Along Track Scanning Radiometer (AATSR), NO2 columns from the SCIAMACHY instrument, both on the ENVISAT platform) in Sect
elemental carbon (EC) and organic carbon (OC) were determined by a thermal-optical method using a Sunset Laboratory Carbon Analyzer (Sunset Lab., OR, USA) and the EUSAAR 2 protocol defined by Cavalli et al (2010)
Summary
Megacities, defined as agglomerations with a population above 10 million inhabitants (Gurjar and Lelieveld, 2005) often concentrate in a small area a substantial part of a country’s population, economic activities, and air pollutant emissions. As a secondary aerosol component, sulfate needs some time to be formed from SO2 emissions by gaseousor aqueous-phase oxidation pathways (Seinfeld and Pandis, 2006), important SO2 emissions of industrial origin are often located outside of a megacity Another important secondary inorganic aerosol component, and for organic aerosol (OA; both of primary and secondary origin), only few explicit estimations of the local versus advected contributions to their burden in megacities are available. Our major aim is to quantify the contribution of local versus advected sources of fine aerosol in a post-industrial megacity, with a special focus on organic aerosol sources This will be addressed by combining observations from both the MEGAPOLI and PARTICULES campaigns, and considering results from chemical transport modeling and satellite observations. We will place results found for Paris in a larger context of particulate matter origin in other selected megacities
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