Aerosol particles (PM2.5) were collected during the day (n = 6) and nighttime (n = 9) from a tropical pasture site in Rondonia, Brazil during an intensive biomass burning period (16-26 September, 2002). Higher normalized (by K+, levoglucosan, or apparent elemental carbon, ECa) mass concentrations of SO42- and CH3SO3- in daytime suggest their photochemical production, while the opposite trend for NO3- suggests its transfer to the aerosol phase at lower temperatures and higher humidities, as well as possibly production through hydrolysis of N2O5 on aqueous aerosol particles. About 4.2-7.5% of OC (5-13% of water-soluble organic carbon (WSOC)) could be characterized at the molecular level using GC-MS and GC-FID. Among the detected organic compound classes, abundances of anhydrosugars and aromatics were higher in night samples, but sugars/sugar alcohols, diacids, oxoacids and alpha-dicarbonyls were more abundant in day samples. Consecutive day and night samples showed that delta C-13 values of total carbon (TC) were lower in daytime samples, which can be interpreted as resulting from higher contributions of refractory TC depleted in C-13 due to predominantly flaming combustion. The delta N-15 values of total nitrogen (TN) ranged from +23.5 parts per thousand to +25.7 parts per thousand, however, there was no trend in day and night samples. Higher values of delta C-13 and delta N-15 for biomass burning particles than those of unburned vegetation reflect positive isotopic enrichment either during the formation of particles or after the emission of particles in the atmosphere. (C) 2009 Elsevier Ltd. All rights reserved.