Microphysics of Amazonian aerosol under background conditions and the impact from the urban pollution and biomass burning

Abstract:

The Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) campaign took place from January 2014 to December 2015 in the vicinity of Manaus, Brazil. One main objective of GoAmazon 2014/5 is to investigate the aerosol lifecycle under background conditions and the impact from the Manaus pollution plume and biomass burning. Here we present the diurnal variation of aerosol properties, including aerosol size distribution and CCN spectrum, observed at the T0a background site (Amazon Tall Tower Observatory, 150 km upwind of Manaus) and the T3 site (70 km downwind of Manaus). Also shown are vertical distributions of aerosol observed onboard the DOE Gulfstream-1 research aircraft. During the wet season, aerosol under background conditions often exhibited a bimodal size distribution with an average concentration of ~320 cm^-3. The vertical profile of aerosol size distribution showed high concentrations of Aitken mode particles in the free troposphere, suggesting particle sources at high altitudes. The sources and sinks of the boundary layer aerosol particles under the wet season background conditions are examined. During the dry season, background aerosol concentration increased by a factor of ~5 to ~1500 cm^-3, due to a combination of regional biomass burning emissions and other factors. Background aerosol size distribution was typically unimodal with the mode diameter between 100 and 200 nm. Nucleation and Aiken mode particle concentrations exhibited strong enhancements in the Manaus plume. As the plume traveled downwind, particle growth and higher CCN activation fraction were observed, and are attributed to condensation of secondary species and coagulation. The impact of the Manaus urban plume on aerosol size distribution, CCN spectrum, and optical properties are examined, and the results from wet and dry seasons are compared.