A study of the aging processes of Manaus plume impact on the optical properties over the Amazon rainforest using the WRF-Chem model

Abstract:

As aerosol particles age while travelling in a plume, several processes such as SOA production, black carbon (BC) deposition, particle growth, and the lensing effect change the optical properties of the aerosol population. Understanding how natural biogenic aerosol optical properties (AOP) vary when they are influenced by urban emissions is key to the understanding of its impact on the radiation budget over forested areas. In this study, we use WRF-Chem model to assess the spatial variability of AOP over forested and urban areas in the central Amazon region during the GoAmazon 2014/5 experiment. Simulations with and without anthropogenic emissions were performed for seven days during the Amazonian wet season in order to better understand the impact of biogenic-anthropogenic interactions on SOA formation and AOP. The optical properties analyzed includes single scattering albedo (SSA), scattering and absorption Ångström exponent and aerosol radiative forcing. During the Manaus plume aging process, it was observed an increase in the SSA in the plume from 0.89 (above Manaus) to 0.98 (140 km away from Manaus) as a result of SOA production and BC deposition. Additionally, when anthropogenic emissions are switched ON and the plume is 4 hours old, incoming solar radiation (at surface) shows a significative decrease (750 to 600 W/m^-2), a reduction of 20% of surface solar flux and a significant increase in the diffuse radiation flux, that affects Net Primary Productivity significantly in central Amazonia. It has also been observed that there is a strong correlation between the difference in BC concentration and the difference in temperature between simulations with and without anthropogenic emissions (ON - OFF). When the BC difference is maximum, the difference in temperature is also at a maximum of 0.7 °C. This happens when the plume is 4 hours old, when the distance between the center of the plume and Manaus is around 50 Km. At this point, the difference of SSA starts growing faster and the difference in BC starts to fall. This is likely induced by plume dilution, which reduces the differences between temperatures. These results shows that an urban plume interacting with an atmosphere dominated by natural biogenic aerosols has a significant impact on the radiation balance as well as the surface temperature.