Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 5 2005 10.5194/acpd-5-8027-2005 http://www.atmos-chem-phys-discuss.net/5/8027/2005/ http://www.atmos-chem-phys-discuss.net/5/8027/2005/acpd-5-8027-2005.html http://www.atmos-chem-phys-discuss.net/5/8027/2005/acpd-5-8027-2005.pdf 8027 8054 2005-09-05 Chemical transformations in organic aerosol from biomass burning A. Hoffer A. Gelencsér M. Blazsó P. Guyon P. Artaxo M. O. Andreae Max Planck Institute for Chemistry, Mainz, Germany Air Chemistry Group of the Hungarian Academy of Sciences, Veszprém, Hungary Institute of Materials and Environmental Chemistry CRC, Hungarian Academy of Sciences, Budapest, Hungary Instituto de Fisica, Universidade de Sao Paulo, Sao Paulo, Brazil Fine aerosol particles were collected separately during daytime and nighttime at a tropical pasture site in Rondônia, Brazil, during the burning and dry-to-wet transition period in 2002. Total carbon (TC) and water-soluble organic carbon (WSOC) were measured by evolved gas analysis (EGA). Based on the thermochemical properties of the fine aerosol, the relative amounts of the low and higher molecular weight compounds were estimated. It was found that the thermally refractory (possibly higher molecular weight) compounds dominated the TC composition. Their contribution to TC was higher in the daytime samples than in the nighttime ones. The relative share of WSOC also showed a statistically significant diel variation and so did its refractory fraction. Anhydrosugars and phenolic acids were determined by GC-MS and their diel variation was studied. Based on the decrease of their relative concentrations between the biomass burning and transition periods and their distinctly different diel variations, we suggest that the phenolic acids may undergo chemical transformations in the aerosol phase, possibly towards more refractory compounds (humic-like substances, HULIS), as has been suggested previously. These conclusions are supported by the results of the thermally assisted hydrolysis and methylation gas chromatography-mass spectrometry of the same filter samples.