Optical and physical properties of aerosols in the boundary layer and free troposphere over the Amazon Basin during the biomass burning season
1Max Planck Institute for Chemistry, Mainz, Germany
2Institute of Physics, University of São Paulo, São Paulo, Brazil
3University of Puerto Rico, USA
4Institute of Nuclear Energy Research, São Paulo, Brazil
Abstract. As part of the Large Scale Biosphere-Atmosphere Experiment in Amazonia – Smoke, Aerosols, Clouds, Rainfall and Climate (LBA-SMOCC) campaign, detailed surface and airborne aerosol measurements were performed over the Amazon Basin during the dry to wet season from 16 September to 14 November 2002. Optical and physical properties of aerosols at the surface, boundary layer (BL) and free troposphere (FT) during the dry season are discussed in this article. Carbon monoxide (CO) is used as a tracer for biomass burning emissions. At the surface, good correlation among the light scattering coefficient (σs at 550 nm), PM2.5, and CO indicates that biomass burning is the main source of aerosols. Accumulation of haze during some of the large-scale biomass burning events led to high mass loadings (PM2.5=200 µgm−3), σs (1400 Mm−1), aerosol optical depth at 500 nm (3.0), and CO (3000 ppb). A few rainy episodes reduced the aerosol mass loading, number concentration (CN) and CO concentration by two orders of magnitude. The correlation analysis between σs and aerosol optical thickness shows that most of the optically active aerosols are confined to a layer with a scale height of 1660 m during the burning season. The average mass scattering and absorption efficiencies (532 nm) for small particles (diameter Dp<1.5 µm) at surface level are found to be 5.3 and 0.42 m2 g−1, respectively, when relating the aerosol optical properties to PM2.5 aerosols. The observed mean single scattering albedo (ωo at ~540 nm) for submicron aerosols at the surface (0.92±0.02) is significantly higher than reported previously. The scattering efficiency (dσs/dCN) of particles increases 2–10 times from the surface to the FT, most probably due to the combined affects of coagulation and condensation.