As part of the European contribution to the Large‐Scale Atmosphere‐Biosphere Experiment in Amazonia (LBA‐EUSTACH), aerosols were sampled at representative pasture and primary rainforest sites in Rondônia, Brazil, during the 1999 “burning season” and dry‐to‐wet season transition (September‐October). Water‐soluble organic compounds (WSOCs) within the samples were characterized using a combination of 1H Nuclear Magnetic Resonance (NMR) spectroscopy for chemical functional group analysis, and Gas Chromatography‐Mass Spectrometry (GC‐MS) for identification and quantification of individual low‐molecular‐weight compounds. The 1H NMR analysis indicates that WSOCs are predominantly aliphatic or oxygenated aliphatic compounds (alcohols, carboxylic acids, etc.), with a minor content of aromatic rings carrying carboxylic and phenolic groups. Levoglucosan (1,6‐anhydro‐β‐D‐glucose), a well‐known cellulose combustion product, was the most abundant individual compound identified by GC‐MS (0.04–6.90 μg m−3), accounting for 1–6% of the total carbon (TC) and 2–8% of the water‐soluble organic carbon (WSOC). Other anhydrosugars, produced by hemicellulose breakdown, were detected in much smaller amounts, in addition to series of acids, hydroxyacids, oxoacids, and polyalcohols (altogether 2–5% of TC, 3–6% of WSOC). Most correlated well with organic carbon, black carbon, and potassium, indicating biomass burning to be the major source. A series of sugar alcohols (mannitol, arabitol, erythritol) and sugars (glucose, fructose, mannose, galactose, sucrose, trehalose) were identified as part of the natural background aerosol and are probably derived from airborne microbes and other biogenic material. The bulk of the WSOCs (86–91% WSOC) eluded analysis by GC‐MS and may be predominantly high‐molecular weight in nature.
