Particulate carbon in precipitation at European background sites
Introduction
Aerosol present in the atmosphere contains a significant fraction of carbonaceous matter, which is generally classified in two major components: elemental carbon (EC) and organic carbon (OC). EC is a primary species produced by the incomplete burning of fossil fuels and biomass. It is emitted directly into the atmosphere in the form of fine particles and has a chemical structure resembling that of graphite. Due to its high specific absorption of solar radiation, EC can have direct and indirect climatic effects, such as the reduction of the amount of solar radiation reaching the ground, enhancing the cloud absorption and the reduction of the albedo of snow and ice due to deposition of particles containing EC, whose balance can contribute to the warming of the atmosphere (Chuang et al., 2002; Hansen & Nazarenko, 2004; Jacobson, 2001).
The OC component in the aerosols has both a primary and secondary origin. Primary OC may be produced from combustion processes and emitted mainly as submicron particles, or from mechanical processes that release into the atmosphere plant spores and pollen, vegetation debris, microorganisms, soil organic matter and marine aerosol, predominantly as coarse particles. Secondary OC is produced from gas to particle conversion of volatile organic compounds, either as a result of the condensation of low vapor pressure compounds, when concentrations exceed saturation levels, or from physical or chemical adsorption of gaseous species on aerosol particle surfaces (Seinfeld & Pankow, 2003). Studies on the aerosol chemical composition revealed that OC consists of a complex mixture of organic compounds, which is not yet completely resolved, and include, among others, aliphatic hydrocarbons, aromatic hydrocarbons, aldehydes, ketones, alcohols and carboxylic acids (e.g. Alves, Carvalho, & Pio, 2002). OC particles are very efficient at scattering light, and therefore exert a direct aerosol climate forcing and contribute to visibility reduction (Malm & Day, 2000; Molnár et al., 1999). Water soluble organic compounds often represent a significant contribution to the OC aerosol mass, making this fraction an important source of cloud condensation nuclei and thus possibly influencing indirect climate forcing (Corrigan & Novakov, 1999; Krivácsy et al., 2001).
Carbonaceous matter in atmospheric aerosols has been determined at many locations around the world, including sites in urban, rural, coastal, marine and polar areas (e.g. Castro, Pio, Harrison, & Smith, 1999; Hitzenberger et al., 1999; Krivácsy et al., 2001; Putaud et al., 2000; Sharma, Lavoué, Cachier, Barrie, & Gong, 2004, see Putaud et al., 2004 for a recent review focused on Europe). Most of the measurements were restricted to short-time campaigns, especially at background and remote regions, and there is a lack of information on seasonal variability in those conditions. This shortness of data motivated the European CARBOSOL project (A Study of the Present and Retrospective State of the Organic Versus Inorganic Aerosol over Europe) during which aerosol was studied over a time span of two years at a set of 6 rural/background sites arranged over a west-east transect from the Azores (Atlantic Ocean) to Hungary (Pio et al., 2007). This recent project provided a comprehensive aerosol data set for various conditions in Europe (marine versus continental, rural versus forested, boundary layer versus free troposphere, and winter versus summer) (Legrand & Puxbaum, 2007).
Current understanding on the atmospheric cycle of these particles is still largely incomplete since one of its key aspects, the wet deposition, which is considered to be an important process at removing particulate carbon from the atmosphere (Ducret & Cachier, 1992), is still poorly documented. To date, only a small number of studies were dedicated to the measurement of particulate carbon in precipitation. This paucity of our present knowledge on the spatial variability of wet deposition represents an important limitation to validate global models that simulate transport and concentration of atmospheric particulate carbon (Cooke, Ramaswamy, & Kasibhatla, 2002; Cooke & Wilson, 1996; Liousse et al., 1996) and consequently render difficult any accurate simulations of the anthropogenic forcing of aerosol on climate.
In this context, the present work aims to increase the current data set on the spatial variability of particulate EC and OC concentrations in precipitation to gain new insights on the atmospheric cycle of these carbonaceous species. In the frame of the CARBOSOL project, precipitations were collected at several sites (or at nearby locations) where atmospheric aerosol sampling was performed in parallel. Precipitation samples were analyzed for an extended array of organic and inorganic species, but this paper mainly focuses on particulate EC and OC. Simultaneous data on dissolved organic carbon (DOC), and inorganic ions (in particular sulfate) are used here as a supporting information to explore the processes that determine the scavenging of carbon particles from the atmosphere by wet deposition.
Section snippets
Experimental
Precipitation was collected at five background sites distributed along a west-east European transect, from the Azores islands (North Atlantic Ocean) to the Hungarian plains, in central Europe (Fig. 1). They include a marine background site at the Azores, two rural surface background sites (Aveiro in Portugal and K-Puszta in Hungary), and two mountain sites (Schauinsland in Germany and Sonnblick in Austria). The main characteristics of these sampling sites are detailed in Pio et al. (2007).
Precipitation data
Table 1 summarizes the characteristics of precipitation events that were documented at the different sites. Mean daily precipitation rates were slightly higher at the mountain sites (Schauinsland and Sonnblick) than at low land sites (Azores and Aveiro). The number of documented events ranged from 9 at the Azores to 42 at Schauinsland. The representativeness of the documented precipitations with respect to all precipitations varies from 1% at the Azores to 45% at K-Puszta. In a large extent
Conclusions
The information gathered from this study increase our knowledge on particulate carbon content of precipitation. The lowest particulate TC levels in precipitation were found at the coastal site of Aveiro, at the remote site of the Azores islands, and at the elevated site of Sonnblick. On the other hand, the highest levels were found at the continental low land site of K-Puszta. EC was found to be a minor contributor to total particulate carbon in rain and snow samples. This was particularly true
Acknowledgments
The authors are indebted to many members of the CARBOSOL team who contributed to sampling, sample handling and transfer. In addition, the authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and the Portuguese Meteorological Institute for supplying observational data. This work was supported by the European Commission under Grant number EVK2-CT-2001-00113 and by the Research Unit CESAM (Centre for Environmental
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