Abstract
Abstract. Intra-seasonal variability of smoke aerosol optical depth (AOD) and downwelling solar irradiance at the surface during the 2002 biomass burning season in South America was modeled using the Coupled Chemistry-Aerosol-Tracers Transport model with the Brazilian developments on the Regional Atmospheric Modeling System (CCATT-BRAMS). Measurements of total and fine mode fraction (FMF) AOD from the AErosol RObotic NETwork (AERONET) and solar irradiance at the surface from the Solar Radiation Network (SolRad-NET) were used to evaluate model results. In general, the major features associated with AOD evolution over the southern part of the Amazon basin and cerrado ecosystem are captured by the model. The main discrepancies were found for high aerosol loading events. In the northeastern portion of the Amazon basin the model systematically underestimated total AOD, as expected, since smoke contribution is not dominant as it is in the southern portion and emissions other than smoke were not considered in the simulation. Better agreement was obtained comparing the model results with observed FMF AOD, which pointed out the relevance of coarse mode aerosol emission in that region. Likewise, major discrepancies over cerrado during high AOD events were found to be associated with coarse mode aerosol omission in our model. The issue of high aerosol loading events in the southern part of the Amazon was related to difficulties in predicting the smoke AOD field, which was discussed in the context of emissions shortcomings. The Cuiabá cerrado site was the only one where the highest quality AERONET data were unavailable for both total and FMF AOD. Thus, lower quality data were used. Root-mean-square error (RMSE) between the model and observed FMF AOD decreased from 0.34 to 0.19 when extreme AOD events (FMF AOD550 nm ≥ 1.0) and Cuiabá were excluded from the analysis. Downward surface solar irradiance comparisons also followed similar trends when extreme AOD were excluded. This highlights the need to improve modelling of the regional smoke plume in order to enhance the accuracy of the radiative energy budget. An aerosol optical model based on the mean intensive properties of smoke from the southern part of the Amazon basin produced a radiative flux perturbation efficiency (RFPE) of −158 Wm−2/AOD550 nm at noon. This value falls between −154 Wm−2/AOD550 nm and −187 Wm−2/AOD550 nm, the range obtained when spatially varying optical models were considered. The 24 h average surface radiative flux perturbation over the biomass burning season varied from −55 Wm−2 close to smoke sources in the southern part of the Amazon basin and cerrado to −10 Wm−2 in remote regions of the southeast Brazilian coast.
Highlights
The 3BEM smoke particles emissions are based on a database of fire pixel counts and burned area derived from the combination of remote-sensing fire products from Geostationary Operational Environmental Satellite-Wildfire Automated Biomass Burning Algorithm (GOES WF ABBA product; Prins et al, 1998), the Brazilian National Institute for Space Research (INPE) fire product, which is based on the Advanced Very High Resolution Radiometer (AVHRR) aboard the NOAA polar orbiting satellites series (Setzer and Pereira, 1991), and the Moderate Resolution Imaging Spectroradiometer (MODIS) fire product (Giglio et al, 2003)
Likewise for Belterra and Balbina sites, which are located in the northeastern Amazon basin (Fig. 3b). Over these two sites the model results are systematically lower when compared with AErosol RObotic NETwork (AERONET) total AOD550nm (Fig. 3a), which point out the greater relative importance of coarse mode to the total aerosol optical depth (AOD) during the biomass burning season in comparison with the southern part of the Amazon basin
Over Cuiabathese high AOD events supposedly include a large contribution of coarse mode aerosols, which are not taken into account in the model
Summary
Biomass burning is a major source of carbonaceous aerosols to the regional atmosphere, yet characterization of observations and model representation of smoke aerosol emissions and spatial and temporal variability of abundance and intrinsic optical properties are still uncertain (Kinne et al, 2006; Reid et al, 2005; Longo et al, 2010). The present paper describes results of a modeling effort aiming to simulate the South American regional smoke plume produced during the 2002 biomass burning season. The presented results focus on the simulation of the AOD of the regional smoke plume and the associated radiative flux perturbation (RFP) of the solar radiation at the surface. 4. The first results that are presented and discussed are comparisons between model-calculated and observed seasonal variability of aerosol optical depth, followed by an evaluation of modeled downwelling solar fluxes at the surface.
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