Relationship between Land Use and Spatial Variability of Atmospheric Brown Carbon and Black Carbon Aerosols in Amazonia

Abstract

The aerosol radiative effect is an important source of uncertainty in estimating the anthropogenic impact of global climate change. One of the main open questions is the role of radiation absorption by aerosols and its relation to land use worldwide, particularly in the Amazon Rainforest. Using AERONET (Aerosol Robotic Network) long-term measurements of aerosol optical depth (AOD) at a wavelength of 500 nm and absorption AOD (AAOD) at wavelengths of 440, 675, and 870 nm, we estimated the fraction and seasonality of the black carbon (BC) and brown carbon (BrC) contributions to absorption at 440 nm. This was conducted at six Amazonian sites, from central Amazon (Manaus and the Amazon Tall Tower Observatory—ATTO) to the deforestation arc (Rio Branco, Cuiabá, Ji-Paraná, and Alta Floresta). In addition, land use and cover data from the MapBiomas collection 6.0 was used to access the land transformation from forest to agricultural areas on each site. The results showed, for the first time, important geographical and seasonal variability in the aerosol optical properties, particularly the BC and BrC contributions. We observed a clear separation between dry and wet seasons, with BrC consistently accounting for an average of approximately 12% of the aerosol AAOD at 440 nm in the deforestation arc. In central Amazon, the contribution of BrC was approximately 25%. A direct relationship between the reduction in forests and the increase in the area dedicated to agriculture was detected. Moreover, places with lower fractions of forest had a smaller fraction of BrC, and regions with higher fractions of agricultural areas presented higher fractions of BC. Therefore, significant changes in AOD and AAOD are likely related to land-use transformations and biomass burning emissions, mainly during the dry season. The effects of land use change could introduce differences in the radiative balance in the different Amazonian regions. The analyses presented in this study allow a better understanding of the role of aerosol emissions from the Amazon Rainforest that could have global impacts.

Evaluation of MODIS Dark Target AOD Product with 3 and 10 km Resolution in Amazonia

“…The time series of the AERONET AOD and the MODIS DT 3 km and 10 km AOD are shown in Figure 2. All sites present an evident seasonality, which is already expected due to the impact of emissions from fires that occur in the dry season on the region [31,32,40]. In general, the maximum AOD values in all sites are directly related to the regional fire events.…”

“…The time series of the AERONET AOD and the MODIS DT 3 km and 10 km AO shown in Figure 2. All sites present an evident seasonality, which is already expecte to the impact of emissions from fires that occur in the dry season on the region [31,3 In general, the maximum AOD values in all sites are directly related to the regiona events. The high loads of aerosols emitted by the regional biomass burning that occ central and northern Brazil, in the region locally known as the deforestation arc [31], extensive areas over South America and can impact locations relatively distant from origin emission [29,[41][42][43][44][45].…”

“…All sites present an evident seasonality, which is already expecte to the impact of emissions from fires that occur in the dry season on the region [31,3 In general, the maximum AOD values in all sites are directly related to the regiona events. The high loads of aerosols emitted by the regional biomass burning that occ central and northern Brazil, in the region locally known as the deforestation arc [31], extensive areas over South America and can impact locations relatively distant from origin emission [29,[41][42][43][44][45]. A systematic evaluation of the MODIS DT AODs for each site was performed using linear regression models, which are shown in Figure 3, where the punctual measurements available on each site are represented.…”

“…In this study, we prioritize the use of level 2.0 data. For the AOD analysis, we focused on the measurements taken on 500 nm, since this wavelength is commonly used because of its proximity to the MODIS estimates at 550 nm [31].…”

“…In general, the maximum AOD values in all sites are directly related to the regional fire events. The high loads of aerosols emitted by the regional biomass burning that occurs in central and northern Brazil, in the region locally known as the deforestation arc [31], cover extensive areas over South America and can impact locations relatively distant from their origin emission [29,[41][42][43][44][45].…”