3.1. Variations in PM2.5 and BC concentrations
The variations in PM
2.5 concentration for the respective analysis periods are shown in
Figure 2 and complemented in
Table 1. PM
2.5 varied from approximately 7 μg.m
-3 in P4 to more than 80 μg.m
-3 in P5, the average for the entire period being approximately 36 μg.m
-3 (36.62 ± 31.69 μg.m
-3). The variations in the concentration of BC and dust are also evident in
Figure 2, it is possible to observe the same pattern of variation in PM
2.5, with a minimum in P4 and a maximum in P5. These variations can be justified based on the meteorological patterns shown in
Figure 3. Although the total analysis period is considered dry, precipitation was recorded in P4, P6, and P7.
Figure 3a also shows the relationship between accumulated precipitation and the aerosol load in the atmospheric column, in this case, it is possible to observe a different pattern of AOD about PM
2.5 measurements on the surface. The total fraction of AOD is practically due to fine AOD, a characteristic of BB emissions. In this case, it is possible to observe the increase in AOD between periods P1 to P4 with a consequent reduction reaching the minimum AOD in P6, with values close to 0.4, which are commonly found in the rainy season in this region [
39].
The variation in surface concentrations has different dynamics than the aerosol load in the atmospheric column because wet deposition can act differently at each specific scale. In this study, we hypothesize that the cleaning that occurs in the atmospheric column due to wet deposition in precipitation episodes decreases the aerosol load, decreasing AOD values. However, precipitation is not enough to extinguish the burning spots, which continue to emit gases and particles that concentrate primarily in atmospheric regions closer to the surface, that is, the concentration of PM2.5, in the dry period, will directly depend on the local burning and the influences of wind intensity and direction.
At a comparative level, the work of Santos et al. [
20], carried out on the same website and with the same techniques, found maximums varying between 14 and 20 µg.m
-3 for the concentration of PM
2.5 during the dry period of 2012. Our results show an increase of approximately 4 times for the maximum concentrations of fine aerosols in the Pantanal. The work of Artaxo et al. [
40], south of the Amazon biome, in a region directly affected by BB emissions, found maximums varying between 300 and 350 µg.m
-3 in the 2010 drought. The study by Santanna et al. [
34], with measurements close to the urban area of Cuiabá, found maximum values that varied between 50 and 60 µg.m
-3 in August 2004.
On average, PM
2.5 concentrations for the central Amazon during the dry period vary between 3.4 [
40] and 4.8 µg.m
-3 [
15], for the south of the Amazon, in the arc of deforestation, this average goes to 33 [
40]. The average found in our study approximates the PM
2.5 concentrations of regions highly impacted by changes in land cover due to deforestation [
40]. We emphasize that the averages found in our study still have a low sample value, and the average value may decrease with more observations, however, the analyses show a high concentration of PM
2.5 in the dry period of 2022 in the Pantanal.
For BC concentrations, our results also show higher concentrations than Santos et al. [
20] in the Pantanal. For the 2012 dry period, BC maximums varied between 1.6 and 1.8 µg.m
-3, while our results show maximums of 3.7 and 2.8, in periods P5 and P6, respectively. Recent work by Palácios et al. [
22] also over the north of the Pantanal, with aethalometer measurements, found daily values above 3 µg.m
-3 for the dry periods of 2017 and 2019, years in which the Pantanal was also heavily impacted by local fires [
22]. The mean for BC in our results was 1.83±1.65 µg.m
-3 while Palácios et al. [
22] obtained 1.01±0.95, and 0.90±0.81 µg.m
-3, for the years 2017 and 2019, respectively. These differences can be justified by the differences in the methods of obtaining BC.
Table 2 shows a comparison of our results for PM
2.5 and BC with other averages obtained for the Amazon and in urban areas.
Table 2 shows the differences in magnitude of PM
2.5 and BC concentrations and their respective variations, in studies in Brazil compared to our results.
All information in
Table 2 was extracted for the fine fraction of aerosols in the period considered dry, that is, with the influences of BB emissions. The results show large variations between values that can be explained by several factors, such as the method of obtaining PM
2.5 and BC, characteristic local emission, and the sampling period. Regarding the sampling period, we highlight that for El Niño years, the central and northern region of Brazil is influenced by a precipitation deficit, causing an increase in the dry period resulting in larger burned areas and more emissions of aerosols [
41]. The study by Palácios et al. [
41] showed that for El Niño conditions there is a significant increase in the aerosol load south of the Amazon basin. The increase in BC concentrations, found in this study, may have a direct influence on the local microclimate, feeding positive feedback on temperature maximums [
32]. The study by Curado et al. [
32], in the Pantanal, showed a positive correlation between BC and temperature maximums with consequences for carbon capture.
3.2. Meteorological influences
Figure 3ab shows the average meteorological conditions for each period of analysis, although there is no direct relationship, it is possible to observe some relationships between AOD, PM
2.5, and BC concentrations. AOD values are above 0.4 in the initial five periods of AOD
total and period three recorded the highest AOD
fine average of 0.93. We reinforce that AOD
fine is linked to the amount of fine optically active aerosols in the atmosphere, therefore a good correlation with PM
2.5 is expected. However, there were differences in the patterns for the periods analyzed. Between P1 and P2, the behaviors of AOD and PM
2.5 were similar, but, from P3 onwards, surface concentrations are reduced, while AOD
fine continues to rise, air temperature, and relative humidity patterns follow the measurements of the surface. The air temperature gradually increases from P1 and P2 and then decreases at P3. These results are in line with the analysis by Curado et al. [
32] who found a positive correlation between BC concentration and air temperature maxima.
Figure 3b also shows the average behavior of the radiation net Rn. The behavior of Rn has no relationship with AOD or surface concentrations, because this interaction is highly complex [
41]. Blocking direct solar radiation, through scattering or absorption, can negatively influence Rn, however, retention of this radiation in the atmosphere can increase the amount of long-wave radiation, influencing Rn positively [
32]. In general, more measurements in a specific experimental design need to be developed in the Pantanal so that these feedback processes are better understood. In this study we justify the main variations in surface concentrations based on the number of burning spots that occurred during the analysis period. FO is a recurring problem in the Pantanal, due to the environmental impact and the damage to health they can cause [
42]. During the dry season, fires are constant as shown in
Figure 4, and with the contribution of wind intensity and direction (
Figure 5), concentrations can still increase due to transport in the atmosphere [
22].
Our analysis shows that P1 obtained the highest value for FRP, reaching 16,711 W, however, the distance from BAPP, FO (
Figure 4), wind direction (
Figure 5) and PM
2.5 concentration (
Figure 2) suggest low BB influence on collected samples. The largest PM
2.5 peaks occur in periods two and five, with the value of FRP 3561 and 8606 W, respectively. Considering the position of the FO and the wind speed and direction, the samples collected in these periods may have been directly influenced by regional forest fires. Furthermore, period five presented the highest number of FO (
Figure 4) and at the same time the maximum dust concentration of 3.7 μg.m
-3 (
Figure 2), the highest concentration of chemical elements, including the maximum concentration of Pb of 25 ng.m
-3. When analyzing the correlations between surface concentrations (PM
2.5 and BC) with the other variables used in this study, no statistically significant correlation was found. We suggest that by continuing the sampling campaign, a more significant number of samples can achieve such a correlation. This is also suggested for evaluating the effects of meteorological parameters.
3.3. Elemental concentration and Enrichment factor
Figure 6 shows the most significant elemental concentrations for the study period. Comparing again with the study by Santos et al. [
20] it is possible to notice the increase in S, K, Fe, and Si. The concentrations of Fe and Si are close to the 2012 measurements, however, the maximum concentrations of S and K presented values three times higher. Elemental concentrations follow the same pattern as PM2.5 and BC concentrations with small variations for Si and Fe. For Si, concentrations were the same in periods P6 and P7, while Fe suffered an approximate reduction of 50% for the respective periods. The study by Santanna et al. [
34] explains that the increase in Al, Si, and Fe concentrations in the dry period is generally associated with soil conditions and wind speed since during this period the soil is arid and there is a characteristic increase in the average wind speed. which also explains the high sodium concentrations.
Although this study did not perform a factor analysis, the dominant elemental concentrations show a strong relationship with emissions characteristic of biomass burning with contributions from soil resuspension [
15,
20,
34,
40]. The concentrations of Fe, S, and K can be a good indication of the contribution of BB, the classic study by Andreae [
43] highlights that the concentrations of K can be used as an indicator of fires in the flame phase, as it is a characteristic marker from BB. However, Urban et al. [
44] highlight the limitations of using K as a marker when studying soil resuspension emissions in fertilized areas.
For the heavy metals Pb and Cd, the averages found in our study were 4.28 and 8.56 ng.m
-3. In the case of Pb, the maximum concentration reaches 24.93 ng.m
-3, this peak concentration occurs in period 5, a value six times higher than the average concentration of this element (
Table 1). The presence of Pb may be related to anthropogenic factors, especially mining activities that contaminate the soil with heavy metals and increase the suspension of contaminated soil [
45]. The dust compound, calculated from the concentration of the elements Al, Si, Ca, Ti, and Fe (Equation (2)), is the main constituent of natural atmospheric aerosols. Annually, the Amazon receives a substantial load of dust from the African continent, which transports tons across the Pacific Ocean [
45]. In the Pantanal, land use by agricultural activities, mining, and vehicle traffic is responsible for the suspension of dust, which, depending on the composition of the soil, can pose health risks [
20,
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39,
40,
41,
42]. This work calculated the maximum dust concentration at 3.7 μg.m
-3, while in the study by Morais [
47] during the dry season at the Amazon Tall Tower Observatory (ATTO), the maximum concentration was 1.4 μg.m
-3, reinforcing that area with a greater concentration of human activities promote greater soil suspension.
Using reference measurements from Arana and Artaxo [
15], it was possible to determine the enrichment factor (Ef) of 17 elements, as shown in
Figure 7. Ef’s values close to 1.0 have a strong natural component while elements with values high levels of Ef have a strong anthropic influence. The results in
Figure 7 show that for all evaluated elements there is an anthropogenic contribution. For the fine fraction of aerosols, BB emissions are responsible for the high values of K, S, Mg, and Zn [
48,
49], in this case, we highlight the Ef of K which was approximately 10 times higher than the natural concentration. As for soil resuspension emissions, we have increases in Al, Si, Ti, Fe, Ni, and Cu, which varied in elevations of 1.6 for Si and 12.8 for Cu (times higher). The results also show that Pb was the most anthropic element in the study with Ef above 200, although its average concentration is low (
Table 1) its Ef shows that the region is undergoing considerable anthropic transformation with possible implications for the environment and the population health. As previously mentioned, these transformations can be directly associated with mining activities that contaminate the soil with heavy metals and increase the suspension of contaminated soil [
45].