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Water soluble organic compounds (WSOC) in aerosol samples collected in the Amazon Basin in a pe- riod encompassing the middle/late dry season and the begin- ning of the wet season, were investigated by H NMR spec- troscopy. HiVol filter samples (PM2.5 and PM>2.5) and size-segregated samples from multistage impactor were sub- jected to H NMR characterization. The H NMR methodol- ogy, recently developed for the analysis of organic aerosol samples, has been improved by exploiting chemical methy- lation of carboxylic groups with diazomethane, which al- lows the direct determination of the carboxylic acid content of WSOC. The content of carboxylic carbons for the dif- ferent periods and sizes ranged from 12% to 20% of total measured carbon depending on the season and aerosol size, with higher contents for the fine particles in the transition and wet periods with respect to the dry period. A compre- hensive picture is presented of WSOC functional groups in aerosol samples representative of the biomass burning pe- riod, as well as of transition and semi-clean atmospheric con- ditions. A difference in composition between fine (PM2.5) and coarse (PM>2.5) size fractions emerged from the NMR data, the former showing higher alkylic content, the latter being largely dominated by R-O-H (or R-O-R') functional groups. Very small particles (<0.14µm), however, present higher alkyl-chain content and less oxygenated carbons than larger fine particles (0.42-1.2 µm). More limited variations were found between the average compositions in the differ- ent periods of the campaign.

Functional group analysis by H NMR/chemical derivatization for the characterization of organic aerosol from the SMOCC field campaign