EFFECT OF DRY-SEASON BIOMASS BURNING ON AMAZON BASIN AEROSOL CONCENTRATIONS AND OPTICAL-PROPERTIES, 1992-1994

Aerosol concentrations and properties have been derived from a network of ground-based Sun-sky radiometer measurements in Brazil's Amazon ba sin region since 1992. The measurements characterize the background ae rosol environment and aerosol emissions from biomass burning at eight selected sites. The duration and frequency of the measurements provide the foundation of an aerosol climatology based on direct sun measurem ents of aerosol optical thickness and retrievals of size distribution from solar aureole measurements. The aerosol optical thickness measure ments clearly illustrate that for sites located within regions of biom ass burning the duration of smoke above background levels often exceed s 2 months and frequently at levels an order of magnitude above backgr ound. The aerosol optical thickness range during preburning conditions was 0.11 to 0.27 at 440 nm. Under these conditions, stratospheric aer osols from Pinatubo constituted a significant part of the signal in 19 93 but were about 50% less in 1994. During the burning season; smoke e levated the aerosol optical thickness above 1.0 for seasonally average d values measured at 440 nm at sites located in active source regions in Mate Grosso, Rondonia, and Tocantins states. The measurement sites are located in the cerrado and forest conversion areas. Analysis of th e size distribution of the particles indicated that the increase in ae rosol optical thickness was associated with an increase of an accumula tion and coarse particle modes. The asymmetry factor ''g'', computed f rom the phase function, showed considerable spectral dependence betwee n the preburning and burning seasonal phases. The 1020-nm channel was reduced from 0.66 to similar to 0.53, while at 440 nm little seasonal phase variation was noted. Conditions of burning were sufficiently str ong that the atmospheric conditions associated with the climatological definition of a dry season was subdivided into (1) preburning, (2) tr ansition to burning, (3) burning, and (4) transition to wet season pha ses for most sites. Averages and frequency distributions were used to characterize each seasonal phase by site. Changes in total column wate r vapor amount, also retrieved from direct sun measurements, did not h ave an apparent effect on the optical properties of the aerosols.