Abstract #49 Carbonaceous Aerosol in La Gran Sabana, Canaima National Park, Venezuela Pacheco M. and Sanhueza E. IVIC. Laboratorio de Química Atmosférica. Apartado Caracas 1020-A Venezuela. SUMARY Due to potential importance in the carbon cycle and global climate, increasing attention has been given to study atmospheric black carbon (BC). Very few estimates of the relative contribution of BC to the different compartments of the carbon cycle are available to date. We selected a remote place in Venezuela, La Gran Sabana, Canaima National Park, located nearby the northern border of the Amazon Forest, to study carbonaceous atmospheric aerosol. Size fractionated particles were collected using a cascade impactor. Total carbon (TC) and BC were quantified by an optimized two steps thermal method. The mean TC was 2.41  1.29  g/m3, which is in the same order of magnitude as those found in remote continental atmospheres. The results show that carbon aerosol is mainly present in the fine fraction. The BC concentration was 0.53  0.14  g/m3; 33% and 67 % were in course and fine particle size, respectively. No correlation between nss-K + and BC was found, suggesting that BC is not coming from local biomass burning. Comparing with the Amazon forest, relatively low OC concentration, estimate from difference TC-BC, was found in La Gran Sabana site, likely due to a low secondary production of organic particles. Introduction Carbonaceous aerosols have a direct radiative forcing because they can scatter and absorb solar and infrared radiation in the atmosphere. Recent field measurements have confirmed that organic aerosols may be efficient cloud nuclei and consequently play an important role for the indirect climate effect as well (Penner et al., 2001). The radiatively important properties of atmospheric aerosols (both direct and indirect) are determined at the most fundamental level by the aerosol composition and size distribution. Atmospheric aerosol particles are known to contain organic carbon material in variable amounts, depending on their location. The data set of TC and BC are limited. Very few measurement has been done at continental pristine areas. Since local effects of biomass burning does not seem to affect aerosol composition of this region (Sanhueza et el., 2001), La Gran Sabana results could represent the boundary layer carbonaceous aerosol baseline. In this work, we present results of TC and BC from aerosol at La Gran Sabana, Canaima National Park, Venezuela. Sampling Aerosol were collected at the Parupa Scientific Station and Yuruany, Fig 1, which are located in the eastern part of the park and are mainly surrounded by grass savannas. At Parupa, particles were collected using a Hi-Vol sampler equipped with a five stages cascade impactor (CI). Pre-filter Fiber glass filter (BC) Pre-filter Quartz Filter (BC) Diameter 1/2 “ Diameter 1/4 “ Flow adjuster Hi-Vol engine Top to adaptation Teflon filter (Ions) Figure 2. Aerosol sampling system Also, Low-Vol samples were collected in Parupa and Yuruani using a Hi-Vol motor adapted to collect three samples at the same time, Fig 2. Two holders with two stacked-filters unit were used to collect fine particles for the analysis of TC and BC. The third holder, with one stage, was used for the analysis of water-soluble ions. We used pre-heat glass fiber filters and quartz filters for TC and BC collection. Teflon filters were used for water soluble ions measurements. Analytical methods Two Analytical methods were used; 1) Determination of TC and BC by Elemental Analysis (EA) and 2)Determination of evolved gases (i.e. CO 2 by NDIR) using two ovens connected in series, Fig 3. The BC quantification was made using a two step thermal method; pre combustion were necessary when AE was used. The evolved gases method was developed in our lab and this allow us to measure TC and BC consecutively. Figure 3. TC and BC analyzer system Site Description La Gran Sabana is part of Canaima National Park, a pristine environment located in the Southeastern part of Venezuela, Figure1. Flat topped mountains, locally known as Tepuys, emerge sharply from the rolling hill terrain showing highly differentiated vegetation at their summit. The western park is mostly covered by tropical moist forest, while the eastern part is a mosaic of forest, shrubs and grass savannas. The average annual precipitation is 1515mm, with a significant amount of rainfall also during “dry” season. The average temperature are: 20,3  0,9 o C minimum, 22,5  0,7 o C average and 23,0  0,9 o C as maximum. Figure 1. Sites of Sampling Results and Discussion Analytic techniques comparison: Good correlation between the commercial equipment (EA) and home made apparatus was found, Graphic 1. An advantage of the home made apparatus is the possibility to do thermal carbon behavior (i.e. different carbon compounds at different temperature). Graphic 1. Measurement of TC in aerosol using a commercial AE instrument and the home made equipment Size Distribution TC and BC show similar size distribution, Graphic 2. Both are mainly in the fine fractions and exhibit a bimodal shape. About 63% TC aerosol is in fine fraction, DMM <1.5  m, suggesting a secondary origin. 56% of BC is in the fine mode, DMM <0.49  m, which is in agreement with its production during incomplete combustions Table 1. TC and BC concentrations, ug/m 3, in aerosol fine fractions at Parupa Comparisons between the fine particles sampling methods The TC and BC concentrations obtained using different sampling methods, Table 1, did not show any significant difference, therefore, the Hi-Vol adaptation can be successively use to collect three simultaneous samples (i.e., total, fine), at a lower cost. Organic aerosol are an important constituent of Parupa aerosol. Table 1 shows that TC concentrations were major than nss-SO 4 Graphic 2. TC and BC size distribution · TC and BC average concentrations: The TC and BC (Table 2) values in the aerosol samples are in the same order of magnitude for the two places measured, which corroborates the homogeneity of the La Gran Sabana atmosphere (Sanhueza et al., (2001). The TC and BC concentration values are similar to those found in rural and remote marine areas. This confirms the assumption that La Gran Sabana is a pristine zone and that the BC concentrations can be used as a baseline for continental areas. Table 2. TC and BC concentrations at different places Acknowledgements: These studies on biogeochemical cycles in tropical ecosystems were supported by the Venezuelan National Science Council (FONACIT) through the Grant No G Conclusions The TC is an important component of La Gran Sabana atmospheric aerosol (e.g. higher concentrations than nss- SO 4 ). Since, this aerosol compounds are mainly from natural sources, they could have important implications for the atmospheric chemistry of continental pristine regions. Determinations of the principal constituents of carbonaceous aerosol should be the next step, in order to asses its possible implications (e.g., possible CCN ) TSP = Total suspeded particles References Andreae M., Andreae T., Ferek R., y Raemdonick H. (1984). “Long-range transport of soot carbon in the marine atmosphere”. The Science of the Total Environment, 36, Cachier H., Pierre M., y Buat P. (1989). “Determination of atmospheric soot carbon with a simple termal method”. Tellus, 41B, Magda Claeys, Bim Graham, Gyorgy Vas, Wu Wang, Reinhilde Vermeylen, Vlada Pashynska, Jan Cafmeyer, Pascal Guyon, Meinrat O. Andreae, Paulo Artaxo, Willy Maenhaut (2004). “Formation of Secondary Organic Aerosols Through Photooxidation of Isoprene”. Science, 303, IPCC (2001) Penner J., Andreae M., Annegarn H., Barrie L., Feichter J., Hegg D., Jayaraman A., Leaitch R., Murphy D., Nganga J. y Pitari. “Aerosols, their direct and indirect effects”, Capítulo 5 en: Climatic Change, The IPCC Scientifics Basis. Cambridge University Press Krivàcsy Z., Hoffer A., Sàrvàn Z., Temesi D., Baltensperger U., Nyeki S., Weingarther E., Kleefeld S., y Jennings S. (2001). “Role of organic and black carbon in the chemical composition of atmospheric aerosol at European background sites”. Atmosferic Environments 35, Sanhueza E., Holzinger R., Donoso L., Santana M., Fernández E. y Johnny R. (2001). “Compuestos orgánicos volátiles en la atmósfera de La Gran Sabana. I: Concentraciones y química atmosférica”. Interciencia. 26, CI