The elevated NO2 concentrations were observed throughout the study period. Higher NO2 concentrations can be attributed to greater vehicular influence/fires, characterized by a “heavy” fleet. The notched boxplot analysis showed differences between the biomes about NO2 and O3 concentrations, with exceptions in some periods of the year. The highest concentration of NO2 is found in the Pantanal in the dry season, in contrast, the Atlantic Forest and Cerrado biomes, had higher concentrations than in Pantanal in the rainy season. There was variability for the O3 concentrations with higher concentrations in Cerrado and greater variability in the dry months in all biomes. There are differences in the amount of burned area in the three biomes, where, regardless of the time of year, there is a higher occurrence in the Pantanal, followed by the Cerrado and Atlantic Forest (Silva-Junior et al. 2020; Oliveira-Júnior et al. 2020). The amount of rainfall and temperature showed different patterns in the dry and rainy seasons. The highest total rainfall records occur in the Atlantic Forest in the dry season. In the rainy season, there is less difference between the biomes. The temperature tends to be lower in the Atlantic Forest and similar in the Cerrado and Pantanal (Fig. 4).
The rainfall and air temperature have been reported as the agents that regulate the pollutants due to the dispersion and photolysis of polluting agents (Zhang, Liu and Zhao, 2018; Borge et al. 2019; Souza et al. 2022). The increase in air temperature results in an increase in the O3 concentration, on the contrary, the humidity which adversely affects the O3 concentration. Notably, the average annual burned area in the Pantanal is higher than in the Cerrado, which, in turn, is higher than in the Atlantic Forest (Table 1). Studies have reported a significant increase in fire foci in the Cerrado, associated with the dry season and reduced rainfall in the rainy season, followed by changes in land use and land cover in the region (Silva Junior et al. 2021; Oliveira-Júnior et al. 2020). We found the most fire in the Pantanal (wetlands), biome with lower population and highway density. The industrial emissions and combustion of coal in thermoelectric plants, fossil fuels used in transport and fire have been reported as contributors to air pollution in MS state (Silva-Júnior et al. 2021; Souza et al. 2022) polluting agents. Another important factor is the traffic load and population density in the state of MS, which is higher in Cerrado, reflect higher O3 in this biome.
In general, the levels of NO2 and O3 concentrations in the cold-dry seasons (from April to September – dry period) for the three biomes had more variability and higher values. The dry period in MS State is characterized by cold, dry air temperatures due low levels of solar radiation that was supposed to reduce the rate of photochemical processes and at the end, reduce the dispersion rates and transportation of pollutants. Some studies have been reported that NO2 has a longer life expectancy during winter compared to spring seasons (Duncan et al. 2010; Matandirotya and Burger, 2021). In addition, the high precipitation received during the rainy period means that the life expectancy of tropospheric NO2 is reduced as it quickly reacts with water to form nitric acid (Matandirotya and Burger, 2021). During monsoon periods, minimal levels of O3 were observed due to insufficient solar radiation (clouds interaction with radiation) and pollutant washout, as well as O3 consumption by HOx radicals. The sharp rate of increase in ozone concentration was from minimum (from April to June) to maximum (August-September) and therefore one can better understand the seasonal difference between temperatures and rainfall for the duration of sunshine duration (Ali et al. 2012) and pollutant washout. The air begins to cool in winter and the ozone-laden air becomes denser and descends to lower altitudes, resulting in a lower ozone peak (Ganguly et al. 2010). The maximum tropospheric NO2 level was observed in the winter and the minimum in the monsoon months over the biomes. The maximum observation found in winter is due to local thermal activities and population density/fires (Kalita et al. 2011).
Some authors have observed an inverse relationship between O3 and NOx (Pancholi et al. 2018), since the formation of O3 is initiated by the photolysis of NO2, as NOx reacts quickly with O3, thus generating NO2 (Pancholi et al. 2018; Souza and Ozonur, 2019). In MS State there is a delay in the response of higher concentrations of O3 and NO2 in relation to mild temperatures, which does not occur with fire foci. On colder days, the photochemical formation of O3 is inhibited by the lower availability of solar radiation (Souza and Ozonur, 2019; Souza et al. 2020a).
The level of suspended particulate matter (PM) is high, leading to regular fog and haze of smoke that causes pollution related problems such as lung cancer; traffic disturbance on roads, as well as reduced visibility in winter (Gunaseelan et al. 2014). For the seasonal variation in the biomes, the maximum rainfall occurs in the summer with the minimum in the winter in the three biomes. Planetary boundary layer (PBL) height may be the reason for low rainfall during winter (Ganguly et al. 2010). The maximum temperature observed occurs in the summer and in the monsoon season. In these stations, due to the intense solar radiation, the photochemical reaction of O3 is high. The maximum concentration of NO2 observed in winter and after the monsoon season is due to excessive consumption of heating fuels. In the monsoon season, the wind brings dry, cold and dense air masses, resulting in the scattering and dilution effects that produce NO2 through NO oxidation and lead to a vertical transport of O3. The maximum rainfall was observed in the monsoon season, when atmospheric stability is high, which delays the photochemical process and the reduction of the ozone layer by the deposition of water droplets. Thus, the O3 concentration is strongly dependent on humidity (Sharma et al. 2013). The high values of NO2 were observed in September with the contribution to emissions from the burning of NOx from the soil (Yienger and Levy, 1995) due to fires to clean areas and the increase of the temperature in MS State. The transport of cleaner air from the north (Lower Chaco) results in the loss of NO2 through reaction with OH radicals. Due to the higher moisture content, the wet deposition rate of NO2 may also be faster in June-September (Sheel et al. 2010).
The difference in total rainfall in each biome, especially in the dry season, may have contributed to the behavior of NO2 and O3, since rain tends to reduce the concentrations of these gases, as described above. The seasonality of rainfall and temperature influence the planting schedule and the conversion of LULC into MS through the use of fire, which contributes to the increase in the concentration of these gases. The Cerrado is the biome with the high population density, highest highway density, and a large proportion of the biome composed of pasture. These indicators are effective, together with temperature, they are effective for the highest concentration of polluting gases, especially O3, as the PCA demonstrated. In the Cerrado the concentrations of O3 were noticeably higher. The area burned in the Cerrado was intermediate, between the Pantanal and the Atlantic Forest, and this also contributes to atmospheric pollution.
In the Pantanal, the concentration of NO2 was higher than in the other biomes, despite the low population density and few roads. The Pantanal is the largest wetland in the world and has greenhouse gas emissions and PM with a diameter of less than 2.5µm (PM2.5ìm) to the atmosphere (Cerri et al. 2009). However, the use of fire for the conversion of agricultural areas, due to the magnitude of the burned areas, effectively contributes to the higher concentration of NO2 in the dry period, when the highest amount of fire occurs (Oliveira-Júnior et al. 2020).
The categories of land use and land cover (Table 5) associated with the occurrence of fire in the state, indicate these differences and determine the need for more detailed analysis in each biome. In Atlantic Forest and Cerrado, the change in land use and land cover indicates a reduction in pasture area and an increase in agriculture. In the Cerrado, there is still an increase in the area of a mosaic of agricultural and pasture. In the Pantanal, grassland areas increased significantly between 2005 and 2020, and a reduction in the Wetland area, which is closely linked to agricultural activities and fire in the biome (Leal Filho et al. 2021; Marengo et al. 2021; Abreu et al. 2022). In addition, the increase in urban infrastructure was effective in all biomes, especially in the Pantanal (Abreu et al. 2022). The growth of commercial activities is associated with air pollution (Chen and Kan, 2008).
Surprisingly, the positive correlations between NO2 and O3 corroborate the visual analysis (Fig. 2 and Fig. 3) in which the concentrations of these pollutants have simultaneous or close peaks, contrary to what was observed in Delhi, India (Taiwari et al. 2015), Milan, Italy (Zoran et al. 2020) and Hong Kong, China (Hossain et al. 2021), in which an increase in O3 concentration induces a reduction in NO2 concentration. The concentration of ozone depends on the absorption of solar radiation, the absolute concentrations of NOX, VOCS, and the ratio of NOX to VOCS (Sheel et al, 2010). During the day, the O3 concentration increased due to the reactions of photolysis of NO2 and photo-oxidation of VOCs, CO, hydrocarbons, and other O3 precursors. Many studies have shown that the concentration of ozone increases with the increasing intensity of solar radiation and temperature on clear days.
The main components confirm the opposite relationship between O3 and NO2 concentrations since NO2 is the precursor of O3 (Tiwari et al. 2015). However, the concentrations in the MS state appear to be high enough that the highest concentrations of these pollutants are close. The components also indicate an opposite arrangement between rainfall and pollutants and a direct arrangement between fire foci and pollutants. Silva-Junior et al. (2020) found large carbon emissions, especially in the Cerrado and Amazonia, associated with the occurrence of fire. The burning of fossil fuels and biological materials does not exclusively generate carbon compounds as a product. The formation and reduction of NO2 in the combustion of oxy-fuel involving lignite (NO + HO2 ↔ NO2 + OH), with the HO2 mechanism being the main NO2 former (Ndibe et al. 2013). According to Távora Maia and Bozelli (2022), the state of MS is a major contributor to GHG emissions, especially in areas dominated by the Pantanal and Cerrado. Also, emissions related to traffic activities are responsible for the largest share of atmospheric pollutants concentration in urban areas and, in Brazil, road transport is responsible for most of the movement of cargo and the fleet grows 12% per year (Vasques and Hoinaski, 2021).
In the formation, movement and dispersion of ozone, local climatic conditions play an important role. Variations in local climatic conditions such as solar radiation, wind speed, direction, precipitation, and relative humidity can influence the level of ozone in the atmosphere. Humidity is the precursor to the occurrence of rainfalls. In moderate rainfalls, the concentration of NO2 and ozone increases, and in heavy rainfalls it can be reduced, and its concentrations are influenced by NOX precursors and weather conditions (temperature and solar radiation), it is an important absorber of infrared and UV radiation and primary source of the most important oxidant in the atmosphere, the hydroxyl radical OH, which is highly reactive with organic and inorganic compounds, ozone is associated with lung and respiratory diseases, as well as mortality and its negative effect on vegetation, ecosystems (Fuhrer and Booker, 2003; Schaub et al. 2005; Ito et al. 2005; Ebi and McGregor, 2008).
The association of meteorological variables, such as solar radiation, temperature, cloudiness, precipitation, wind speed, horizontal transport, and PBL height, with peculiar factors of the topography and circulation of a given region, can considerably influence the concentrations and dispersion of atmospheric pollutants, through the determination of photochemical reaction rates (Oliveira-Júnior et al. 2020).
Among the meteorological phenomena, radiation and cloud cover are the ones that most influence the vertical distribution of O3. The climate of the biome region is a combination of several factors, the most important of which is the availability of solar energy. Its climate is predominantly hot and humid, with average annual temperatures ranging between 24ºC and 26ºC, that is, the average thermal amplitude is around 2ºC, which characterizes a spatial and seasonal homogeneity of the local temperature (Reis et al, 2022; Souza et al. 2022, 2021). According to Souza et al. (2021) and Pobocikova et al. (2021), the seasonality of the rainfall regime in the region is defined as follows: the dry period occurs from May to September, and the month of October is considered a month of transition to the rainy season that extends from November to March, and April is the transition month to the dry season.
It is worth remembering that in the Region, the largest wetland in the world is located, the Pantanal, which is crossed by the Paraguay River and its tributaries, representing a large river network. Because it is located in the tropics, energy exchanges between the continental surface and the atmosphere are quite intense. Another meteorological forcing that modulates the climate in the Region is convection. These convective systems lead to an intense spatial and temporal variability not only in the hydrological cycle of biomes, which indicates the interaction of meteorological conditions with the vertical distribution of O3.
The high intensity of UV radiation, combined with high humidity in the tropical atmosphere, results in an increase in the amount of OH radicals from the photolysis of O3. At the same time, the wetland, as a wetland, emits large amounts of Non-Methane Hydrocarbons (NMHC), with large-scale biogenic activity, and CO from the burning of biomass, which, in turn, can be oxidized to produce O3 with great efficiency (Kirchhoff and Rasmussen, 1990).
In these environments, the O3 concentration tends to follow the intensity of solar radiation, resulting in a high concentration of O3 during the day. In these cycles, the increase in the level of ozone concentration during the daytime period is attributed to the combined effect of the photochemical production of ozone in the mixed layer and the transport coming from the upper layers, which is favored at noon by the convective activity and, consequently, associated subsidence movements, both mechanisms being activated by solar radiation.
The emission and atmospheric transport of particles due to fires is a growing public health problem that mainly affects vulnerable communities and the most sensitive people, such as children, people with lung and/or heart diseases, and workers prone to occupational diseases and populations socially vulnerable (Souza et al. 2012; Souza et al. 2013). A significant and positive relationship was found between ozone concentrations during the fire period and hospitalizations for asthma (Souza et al. 2017) in areas close to a forest fire.