Unexpectedly high optical thickness of aerosols in the center of Valle del Cauca

Valle del Cauca concentrates an important fraction of the large national industry, as evidenced by its position as the first Department in industrial emissions of greenhouse gases in Colombia. To this must be added the fact that the geographic valley of the Cauca River (VRC) produces almost 80% of Colombia's sugar cane, and that pre-harvest burning is still used in 34% of the harvested area. This research is part of the CACIQUE project and measurement campaign (CAuca river valley sugarCane pre-harvest burning aIr QUality Effects), which seeks to quantify and attribute sources of pollution to the air quality measured in Palmira, center of the VRC. In this extended summary we report measurements of aerosols, in situ in the surface layer, and of the atmospheric column by solar photometry at the Palmira Headquarters of the National University of Colombia (3,512 N, 76,308 W). Instrumentation included a beta analyzer and high and low volume PM2.5 and PM10 samplers, a cascade impactor for particle size mass distribution, and a nephelometer. Solar photometry measurements show that the aerosol optical thickness (AOT) at 500 nm measured in Palmira (~ 0.22) is almost ~ 2 higher than in Bogotá (~ 0.14), and only marginally lower than that measured in the Carboniferous Zone of the Cesar (~ 0.28). Even more unexpected is the fact that this high AOT appears inconsistent with the comparatively low concentrations measured at the surface. Angstrom exponents, derived from photometric measurements, are in the range 0.2-2, which is indicative of a mixture of 1) predominantly fine particulate matter, which would include secondary aerosols (as revealed by our analysis of PM2.5), and soot from biomass burning, and 2) coarse material, including dust. In general, more than three quarters of the attenuation by aerosols in the atmospheric column is attributable to fine material. The mean volumetric particle size distributions in the atmospheric column were comparable to the mass distributions in the surface layer, with modal diameters of ~ 0.3 μm and ~ 6 μm in the column, and ~ 0.9 μm and ~ 5.2 μm in the column. surface layer.