Saharan dust in Brazil and Suriname during the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia (LBA) ‐ Cooperative LBA Regional Experiment (CLAIRE) in March 1998

Abstract

Advection of Saharan dust was observed via chemical and optical measurements during March 1998 in Brazil and Suriname during the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia (LBA)‐Cooperative LBA Airborne Regional Experiment (CLAIRE)‐98 experiment. In Brazil the dust outbreak produced an increase of a factor of 3 in the daily mean mass concentration (up to 26±7 μg m−3) of particles smaller than 10 μm equivalent aerodynamic diameter (EAD), and in the daily mean aerosol particle scattering coefficient σs (up to 26±8 Mm−1 STP, ambient humidity). Background levels of aerosol scattering (ambient) were σs ∼ 10 Mm−1. The effect of dust advection was evident for all major crustal elements (Al, Si, Ca, Ti, Mn, and Fe), as well as the sea‐salt elements (Na, Cl, and S), as the dust layer was transported at low altitude (below 800 hPa). Coarse P and organic carbon (OC) concentrations were not influenced by the occurrence of dust, and were mainly emitted by the rain forest. The dry scattering mass efficiency of dust (particles smaller than 10 μm EAD) was estimated to be between 0.65 (±0.06) and 0.89 (±0.08) m2 g−1. Airborne profiles of aerosol scattering showed two distinct types of vertical structure in the dust layer over Suriname, either vertically uniform (15, 26 March), or plume‐like (25 March). Dust layers extended generally up to 700 hPa, while scattering layers occasionally encountered at higher altitudes resulted from smoke emitted by biomass burning in Venezuela and Colombia. Observations in South America were supported by measurements in Israel and Tenerife (Canary Islands), where the dust outbreaks were also detected.