Optical properties and spectral dependence of aerosol light absorption over the Brazilian Pantanal

Characterizing optical properties of aerosols, particularly the absorption processes, is fundamental for understanding the role of these particles in ecosystems as well as the climate, in general. Currently, changes in precipitation regimes in the southern Amazon basin have resulted in a considerable increase in biomass burning, whereby large amounts of aerosols and gases are emitted into the atmosphere. These increased emissions can impact the ecosystem, modifying mass and energy flows at the surface. This study, motivated by the need for more aerosol observations was the result of a long-term campaign carried out in the Brazilian Pantanal, which provided continuous in-situ measurements of aerosol optical properties between January 2017 and December 2019. From these data, optical properties of aerosols were quantified and analyzed seasonally. Corrected estimates for absorption coefficient (σabs) and Angstrom absorption exponent (αabs) were utilized to characterize the spectral dependence of absorption by aerosols. With additional measurements of the scattering coefficient (σscat), it was possible to evaluate the single scattering albedo (ω0). The mean values for σabs, σscat, ω0 at 525 nm and αabs 370–880 nm during the wet season were 0.66 ± 0.58 Mm−1, 6.16 ± 5.75 Mm−1, 0.90 ± 0.06 and 1.43 ± 0.49, respectively. These values, within measurement uncertainties, were similar to results found in the central Amazon. During the dry season, absorption and scattering increased approximately 88% and 86%, respectively, providing strong evidence for the contribution of local and regional emissions from biomass burning. The values of ω0 did not reveal significant seasonal differences, however, a sharp reduction was observed, medians below 0.8, at the beginning of the burning period, which was associated with more recent local burnings and, therefore, greater absorption. The mean values evaluated for different spectral intervals reveal a strong contribution of brown carbon (BrC) during the dry season, with medians of 1.78 and 0.87 for the intervals of 370–590 nm and 590–880 nm, respectively. This work also quantified BC concentrations for the dry and wet seasons, obtaining mean values of 0.75 ± 0.83 and 0.12 ± 0.09 μgm−3, respectively. In general, a striking similarity was encountered for aerosol optical properties between the Pantanal and the Central Amazon during the wet season. The results presented here characterize the background values for aerosol optical properties in the Pantanal, and the additional effects resulting from biomass burning emissions, therefore providing essential information to assess the effects these particles have on the ecosystem.