Ácidos dicarboxílicos no MP2,5 em Manaus: fontes e quantificação

Abstract

The fine particulate material (PM2.5) is formed by several chemical compounds, whose largest mass fraction is composed of organic carbon. Among these, dicarboxylic acids (DCA) stand out because they can change the density of cloud condensation nuclei and the radiation and rainfall indices. However, little is known about them in PM2.5 in Manaus. In this sense, we analyzed the DCA (C2-C9) present in PM2.5 collected on quartz filters during the dry season of 2017 and 2018, in the day and night periods in Manaus, using gas chromatography coupled to a mass spectrometer (GC-MS). It was also analyzed the metals (ICP-OES), soluble ions (CI), black carbon (BC) (Reflectance), and levoglucosan (LEV) (GC-MS) present in the filter, to study sources and formation. The DCA was in greater concentration at night, with the night/2017 being the period with the greatest concentration. In descending order, the majority of compounds in the two years were C2>C3>C4. C2 was more concentrated at night. The occurrence of the linear regression between C2 and other acids (C3-C9) at night indicated the occurrence of nocturnal secondary formation in the aqueous phase, showing that the atmosphere is active even in the dark. Differences in the NH4+/SO42- ratio between the years indicated a probable difference in acidity in the particles. The PM2.5 night/2018 was the one with the highest ratio value pointing to slightly more basic particles. This variation may have caused lower concentrations of DCA at night/2018. The secondary formation of DCA was one of the main sources of DCA for the particle. Among the primary sources, there was the influence of burning fossil fuels and other associated sources (BC as a marker) in both years, with emphasis on PM2.5 collected in 2017. Another primary source was biomass burning (indicated by LEV), with emphasis on PM2.5 of 2018, due to the achievement of a positive linear regression between the concentration of C9 and the number of fire outbreaks in the metropolitan region of Manaus (R2: 0.720), as well as linear regression values between the total concentration of DCA and LEV (R2: 0.353) only for that year. Statistical and chemical evidence of the influence of the formation of Fe-oxalate and Cu-oxalate complexes were obtained in both years, however with greater strength for PM2.5 collected in 2018. The more significant presence of these complexes in 2018 may have influenced the sulfate concentration in the particles and, consequently, the difference in the acidity of the aerosol observed between the periods studied. Therefore, it was possible to know the organic composition in terms of DCA of particles collected in Manaus and observe the interaction of this organic aerosol with dissolved metals in the particle and with soluble ions, as well as the relationship of DCA and primary sources of particles in an urban environment related to burning (fossil fuel and biomass). The DCA collected in Manaus were influenced mainly by fossil burning, with changes in composition, formation, and degree of acidity when this organic aerosol was under a greater influence of biomass burning observed in 2018.