South American regional smoke plume in recent years: main sources and impact on solar radiation focusing on the Pantanal 2020 biomass burning season

The 2020 biomass burning season in Brazil was marked by an unprecedented amount of fire counts across the Pantanal biome, which led to high levels of air pollution within the biome and downwind areas. Large amount of fire counts was also detected in the Amazon Forest during 2020 compared with the recent years. However, the contribution of Pantanal fire emissions to the regional smoke plume was speculated to rival the contribution of fire emissions from Amazon. Aiming to contextualise the 2020 biomass burning season focusing on the unprecedented role of Pantanal, the present study’s main goal 5 is to analyse the recent biomass burning seasons in Brazil looking at the fire counts, the regional smoke plume and its impact on surface solar radiation (SSR). The focus is on the biomes most affected by the recent biomass burning events, Amazon forest, Cerrado and especially Pantanal. To characterise the regional smoke plume we analysed aerosol optical depth, single scattering albedo and its impact on the solar radiation reaching the surface. The influence of interannual variability of the wind at 850 hPa on the transport of the regional smoke plumes was also explored. In 2020, the regional smoke plume covered an 10 area well above 6 million km2, the largest area in the last six years, but equivalent to the observed in a more remote past, as in 2007 and 2010. However, from the point of view of Pantanal, 2020 was an unprecedented year, not due to the amount of smoke over the biome, but regarding the biome contribution to the regional smoke plume. The number of fire counts was 3.4 times higher than the mean value considering the period from 2003 to 2020. The entire biome was continuously covered by a thick smoke layer from September to October, which resulted in a monthly mean deficit of surface solar radiation up to 15 300 Wm-2. Additionally, the 2020 regional smoke plume presented higher optical absorption when compared with the recent years plumes, which could be related to the Pantanal larger fire emission. However, current knowledge on optical and radiative properties of smoke aerosols from Pantanal is limited compared to the one resultant from Amazon and Cerrado fire emissions, which prevent a definitive conclusion. 1 https://doi.org/10.5194/acp-2021-1086 Preprint. Discussion started: 11 February 2022 c © Author(s) 2022. CC BY 4.0 License.


Introduction
In South America, the regional biomass burning smoke plume is the most important signature of anthropogenic activities from the point of view of injection of pollutants into the atmosphere at continental scale during the dry season (Prins and Menzel, 1992, Artaxo et al., 1998, Freitas et al., 2004, Longo et al., 2007. Its geographical dimension and pollutants loading, along with its climate and air quality effects, have been comprehensively studied during the last decades (Freitas et al., 2004, Ignotti et al., 2010, Artaxo et al., 2013, Rosario et al., 2013, Chen et al., 2013, Sena et al., 2015, Moreira et al. 2017 variability, not shown here, closely follows the variability of fire counts. During 2019, in the Amazon, the fire count peaked in August instead of September, and that was the case for Cerrado in 2016. While 2016 and 2018 were less active years from 120 the point of view of fire counts in both Cerrado and the Amazon, the years of 2015, 2017, 2019 and 2020 were very active.
In Cerrado, the intraseasonal variability and the amount of fire counts were quite similar in these active years, except for 2015 which presented a more active October when compared with the typical feature. For the Amazon, the major difference between 2020 and the previous five years was the high fire activity in August, characterised by a level similar to that of September, the climatological peak month. It should be noted that among the analyzed years, 2017 presented the highest fire count in Cerrado 125 and Amazonia at the peak of the BB season. However, when one looks at the Pantanal biome scenario, 2020 was indeed an extreme BB season, with almost three and four times the fire counts typically observed in August and September, respectively.
On top of that, the previous year, 2019, was already a record breaking year, when considering the previous 4 years. The 2020 extreme fire counts scenario for Pantanal is clearly seen in aerosol mean loading over the biome, with twice the values observed in the earlier years. The result of the observed high amount of aerosol over Pantanal can also be evaluated from the 130 unprecedented low amount of solar radiation at the surface in October, when compared to the previous 5 years. Mean aerosol loadings for Amazon and Cerrado in the 2020 BB season were among the highest values observed in recent years but they were not substantially, or even, higher than the highly polluted previous years. For instance, in September of 2017, the aerosol loading over Amazon was higher than in 2020, and for Cerrado, the September 2015 mean AOD was similar to that of 2020.
At this point, it is worth mentioning that the difference in the total area occupied by each biome must be taken into account.

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It is also worth pointing out that there is a strong gradient in fire counts and smoke distribution across both the Amazon and Cerrado biomes, being the southern portion of the former and the westernmost portion of the latter mostly affected, following what is called the deforestation belt (Fearnside et al., 2009, Braghiere et al., 2020. Over these specific portions of the biomes, aerosol loadings are usually much higher. Regarding the amount of solar energy at the surface, the monthly mean value for the Amazon, at the peak of 2020 BB season, is among the lowest values, only comparable to the value observed in 2017 BB 140 season, and consistent with the AOD behavior. An interesting aspect to point out is that, for the Cerrado biome, the years of 2017 and 2020 presented the highest amount of solar radiation at the surface, nevertheless fire counts were among the highest observed values for these particular years. To summarize, this multiyear analysis across the 3 biomes definitely revealed that Pantanal signature is the main highlight aspect in the context of 2020 regional smoke plume, since the number of fire counts and AOD dramatically increased in this biome, compared to the previous years, especially in September and October. For 145 instance, in September, fire counts rose from about 2900 counts in 2019 to more than 8100 in 2020. By contrast, the amount of solar energy reaching the ground in Pantanal was significantly reduced, from about 730 Wm-2 in October 2018 to around 600 Wm-2 in the same month of 2020. To complement this biomes-based interannual and intraseasonal analysis, available data from the AERONET sun photometer stations distributed across the BB regional plume typical corridor were analyzed