Overview of the inorganic and organic composition of size‐segregated aerosol in Rondônia, Brazil, from the biomass‐burning period to the onset of the wet season

Abstract

The aerosol characterization experiment performed within the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia–Smoke, Aerosols, Clouds, Rainfall and Climate (LBA‐SMOCC) field experiment carried out in Rondônia, Brazil, in the period from September to November 2002 provides a unique data set of size‐resolved chemical composition of boundary layer aerosol over the Amazon Basin from the intense biomass‐burning period to the onset of the wet season. Three main periods were clearly distinguished on the basis of the PM 10 concentration trend during the experiment: (1) dry period, with average PM 10 well above 50 μ g m−3; (2) transition period, during which the 24‐hour‐averaged PM 10 never exceeded 40 μ g m−3 and never dropped below 10 μ g m−3; (3) and wet period, characterized by 48‐hour‐averaged concentrations of PM 10 below 12 μ g m−3 and sometimes as low as 2 μ g m−3. The trend of PM 10 reflects that of CO concentration and can be directly linked to the decreasing intensity of the biomass‐burning activities from September through November, because of the progressive onset of the wet season. Two prominent aerosol modes, in the submicron and supermicron size ranges, were detected throughout the experiment. Dry period size distributions are dominated by the fine mode, while the fine and coarse modes show almost the same concentrations during the wet period. The supermicron fraction of the aerosol is composed mainly of primary particles of crustal or biological origin, whereas submicron particles are produced in high concentrations only during the biomass‐burning periods and are mainly composed of organic material, mostly water‐soluble, and ∼10% of soluble inorganic salts, with sulphate as the major anion. Size‐resolved average aerosol chemical compositions are reported for the dry, transition, and wet periods. However, significant variations in the aerosol composition and concentrations were observed within each period, which can be classified into two categories: (1) diurnal oscillations, caused by the diurnal cycle of the boundary layer and the different combustion phase active during day (flaming) or night (smouldering); and (2) day‐to‐day variations, due to alternating phases of relatively wet and dry conditions. In a second part of the study, three subperiods representative of the conditions occurring in the dry, transition, and wet periods were isolated to follow the evolution of the aerosol chemical composition as a function of changes in rainfall rate and in the strength of the sources of particulate matter. The chemical data set provided by the SMOCC field experiment will be useful to characterize the aerosol hygroscopic properties and the ability of the particles to act as cloud condensation nuclei.