Abstract
Abstract. The quantification of sources of carbonaceous aerosol is important to understand their atmospheric concentrations and regulating processes and to study possible effects on climate and air quality, in addition to develop mitigation strategies. In the framework of the European Integrated Project on Aerosol Cloud Climate Interactions (EUCAARI) fine (Dp < 2.5 μm) and coarse (2.5 μm < Dp <10 μm) aerosol particles were sampled from February to June (wet season) and from August to September (dry season) 2008 in the central Amazon basin. The mass of fine particles averaged 2.4 μg m−3 during the wet season and 4.2 μg m−3 during the dry season. The average coarse aerosol mass concentration during wet and dry periods was 7.9 and 7.6 μg m−3, respectively. The overall chemical composition of fine and coarse mass did not show any seasonality with the largest fraction of fine and coarse aerosol mass explained by organic carbon (OC); the average OC to mass ratio was 0.4 and 0.6 in fine and coarse aerosol modes, respectively. The mass absorbing cross section of soot was determined by comparison of elemental carbon and light absorption coefficient measurements and it was equal to 4.7 m2 g−1 at 637 nm. Carbon aerosol sources were identified by Positive Matrix Factorization (PMF) analysis of thermograms: 44% of fine total carbon mass was assigned to biomass burning, 43% to secondary organic aerosol (SOA), and 13% to volatile species that are difficult to apportion. In the coarse mode, primary biogenic aerosol particles (PBAP) dominated the carbonaceous aerosol mass. The results confirmed the importance of PBAP in forested areas. The source apportionment results were employed to evaluate the ability of global chemistry transport models to simulate carbonaceous aerosol sources in a regional tropical background site. The comparison showed an overestimation of elemental carbon (EC) by the TM5 model during the dry season and OC both during the dry and wet periods. The overestimation was likely due to the overestimation of biomass burning emission inventories and SOA production over tropical areas.
Highlights
Carbonaceous aerosols play a major role in aerosol climate impact, through a direct and an indirect effect on incoming solar radiation
Soot has a graphite-like structure and is generated by incomplete combustion of organic material; this component of ambient aerosol is referred to as elemental carbon (EC) and equivalent black carbon (EBC), we should keep in mind that the use of one of these terms implies a different empirical definition (Bond and Bergstrom, 2006)
The fine aerosol mode ratio of 1.7 was chosen to the value determined for submicron particles by AMS measurements (Chen et al, 2009), while the ratio 1.4 was used for coarse particles because coarse organic carbon (OC) is expected to be mainly primary biogenic
Summary
Carbonaceous aerosols play a major role in aerosol climate impact, through a direct and an indirect effect on incoming solar radiation. Carbonaceous aerosols are treated here as composed by organic carbon (OC) and a strongly light-absorbing carbonaceous component that, consistently with the IPCC report (IPCC, 2001) we will call soot. Soot has a graphite-like structure and is generated by incomplete combustion of organic material; this component of ambient aerosol is referred to as elemental carbon (EC) and equivalent black carbon (EBC), we should keep in mind that the use of one of these terms implies a different empirical definition (Bond and Bergstrom, 2006). EC is measured by thermal-optical techniques and corresponds to the more refractive component of carbonaceous aerosols. EBC definition is based on aerosol optical properties and it is quantified by the intensity of light attenuation
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