This paper investigates the physical and chemical characteristics of aerosols at ground level at a site heavily impacted by biomass burning. The site is located near Porto Velho, Rondônia, in the southwestern part of the Brazilian Amazon rainforest, and was selected for the deployment of a large suite of instruments, among them an Aerosol Chemical Speciation Monitor. Our measurements were made during the South American Biomass Burning Analysis (SAMBBA) field experiment, which consisted of a combination of aircraft and ground-based measurements over Brazil, aimed to investigate the impacts of biomass burning emissions on climate, air quality, and numerical weather prediction over South America. The campaign took place during the dry season and the transition to the wet season in September/October 2012. During most of the campaign, the site was impacted by regional biomass burning pollution (average CO mixing ratio of 0.6 ppm), occasionally superimposed by intense (up to 2 ppm of CO), freshly emitted biomass burning plumes. Aerosol number concentrations ranged from ∼ 1000 cm −3 to peaks of up to 35 000 cm −3 (during biomass burning (BB) events, corresponding to an average submicron mass mean concentrations of 13.7 µg m −3 and peak concentrations close to 100 µg m −3. Organic aerosol strongly dominated the sub-micron non-refractory composition, with an average concentration of 11.4 µg m −3. The inorganic species, NH 4 , SO 4 , NO 3 , and Cl, were observed, on average, at concentrations of 0.44, 0.34, 0.19, and 0.01 µg m −3 , respectively. Equivalent black carbon (BC e) ranged from 0.2 to 5.5 µg m −3 , with an average concentration of 1.3 µg m −3. During BB peaks, organics accounted for over 90 % of total mass (sub-micron non-refractory plus BC e), among the highest values described in the literature.