**3. Detection of biomass burning events**

Wildfires generate large amounts of suspended particles in the atmosphere and increase the levels of carbon monoxide. The presence of these particles reduces both visibility and solar radiation reaching the Earth's surface. Besides, they act as cloud condensation nuclei, modifying the climate and the air composition and being harmful to human health [35]. On 8 November 2019, a dense feather of smoke was detached from Australia's coasts due to the intense fires that affected the region. These smoke layers were dragged by the winds to South America, entering Argentine territory on 14 November and remaining in suspension until the next day. In the particular case of the fires in Australia, a large amount of soot not only affected the entire surrounding region, devastating forests, and wildlife: the effects were seen around the planet, with measurements of the transport of aerosols at thousands of kilometers from the emission sources. An immediate effect of the accumulated soot from such a biomass burning was the alteration of river courses and the drinking water production in Eastern Australia [36].

The Australian fires started in September 2019 and intensified in November, given the drought conditions that affected the region. It was the second warmest summer registered, having a rainfall regime below the Australian summer average [36]. These major fires produced dense smoke plumes, detected by the Suomi NPP (National Polar-orbiting Partnership) VIIRS (Visible Infrared Imaging Radiometer Suite) satellites. The images are presented in **Figures 7** and **8** for 8 November, and in **Figures 9** and **10**, for 9 November. The figures show Australia's east coast, the most affected area. **Figures 7** and **9** show the Earth's surface's natural-looking satellite images, called True Color RGB images (I1-M4-M3). Meteorological clouds can be distinguished in white and smoke layers in translucent gray tones. **Figures 8** and **10** show another combination of spectral bands (M11-I2-I1), which allows observing, in shades of blue, the smoke plumes and, in reddish shades, the scars left on the surface of the Earth by fires (burned surface) [37]. These smoke plumes crossed the Pacific Ocean, reaching the American continent and Argentine territory in mid-November 2019.

Measurements from sensors onboard satellites and ground-based platforms were used to analyze the November biomass burning aerosols intrusion event from Australia. Within the satellite measurements, the data from the OMPS sensor (Ozone Mapping Profiler Suite) [38, 39] onboard the Suomi NPP satellite were analyzed for the study of the space–time variability of the Aerosol Index (AI Aerosol Index). AOD (Aerosol Optical Depth) measurements at 550 nm were retrieved from the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument onboard the TERRA satellite [40, 41]. Additionally, the total columnar CO (carbon monoxide) content and the AI were sensed by the TROPOMI (TROPOspheric Monitoring Instrument) instrument onboard the Sentinel-5P satellite [42, 43]. AI is a qualitative index that indicates aerosol's presence at the higher layers of the atmosphere, absorbing or reflecting UV radiation. The main types of aerosols detected with this index are desert dust, biomass burning, and volcanic ash plumes. An advantage of AI is that it can be calculated for clear and (partially) cloudy ground pixels.

*Lidar Observations in South America. Part II - Troposphere DOI: http://dx.doi.org/10.5772/intechopen.95451*

#### **Figure 7.**

*Satellite image of Australia's east coast from 8 November 2019. VIIRS - Suomi NPP sensor (combination: I1-M4-M3).*

#### **Figure 8.** *Satellite image of Australia's east coast from 8 November 2019. VIIRS - Suomi NPP sensor (combination: M11-I2-I1).*

#### **Figure 9.**

*Satellite image of Australia's east coast from 9 November 2019. VIIRS - Suomi NPP sensor (combination: I1-M4-M3).*

#### **Figure 10.** *Satellite image of Australia's east coast from 9 November 2019. VIIRS - Suomi NPP sensor (combination: M11-I2-I1).*

The ground-based aerosol optical depth (AOD) data is obtained from the AERONET/NASA sun photometer network measurements at Buenos Aires, CEILPA-BA (34.555 W, 58.506 S, 26 m), Córdoba, and Pilar (31.667 W, 63.883 S, 333 m) stations, at Level 1.5 (data where the clouds have been extracted automatically) [44, 45].

The AOD is the aerosol vertical column integrated extinction at a given wavelength. This dimensionless quantity indicates how much aerosols attenuate the solar radiation as it passes through the atmosphere. Another value, the Ångström coefficient (or exponent), shows the AOD spectral dependence, and it is related to the root mean square distribution of the aerosol radii. It is calculated as the slope of the linear fit of the spectral AOD in a particular wavelength interval in a log–log scale graph. By relating the AOD at 440 nm and the Ångström coefficient, it is possible to classify the aerosol type using the classification table of Reference [46].

Measurements from the lidar instrument installed at CITEDEF were analyzed to determine the height of the aerosol layers. The normalized aerosol backscattering coefficient was calculated at 532 nm [47–49]. This system allows measuring the atmosphere's profiles from a few meters to several kilometers, exceeding the tropopause height up to the lower stratosphere. **Table 1** summarizes the variables analyzed, the sensors, and the platform employed.

**Figure 11** shows the AI's space–time evolution measured by the OMPS sensor from 8 to 13 November 2019. The images show how a high AI value (greater than 5) smoke plume emerges from Australia's coasts on day 8 November 2019. This plume advances over the Pacific Ocean and reaches the coast of South America on 13 November 2019.

On 14 November 2019, the Australian smoke arrived over Argentine territory for the first time, through Neuquén province and covering the country's entire central region. **Figure 12** shows the AI coverage map (OMPS), indicating aerosols' presence throughout the area.

**Figure 13** shows the TROPOMI sensor AI measurement for 14 November. The AI retrieved from OMPS and TROPOMI show similar values, around 1, in almost the entire territory and, in particular, values between 2 and 3, in the province of Entre Ríos. Both overlapping measurements are plotted in **Figure 14** to compare the AI measurements coverages with the two sensors. The TROPOMI measurement is taken as the basis, and the polygon (red outline) from the OMPS AI coverage is superimposed. It can be seen that the presence of aerosols in the upper layers of the atmosphere is the same for both sensors. It is known that biomass burning is one of the primary sources of CO release to the atmosphere. **Figure 15** shows the regional


#### **Table 1.**

*Instruments and variables used to analyze the November biomass burning aerosols intrusion event.*

#### **Figure 11.**

*Spatial–temporal evolution of the smoke plume through the OMPS sensor analysis of the Aerosol Index (AI), from 8 to 13 November 2019.*

*Lidar Observations in South America. Part II - Troposphere DOI: http://dx.doi.org/10.5772/intechopen.95451*

#### **Figure 13.**

*Aerosol Index (AI), measured with the TROPOMI sensor for 14 November 2019.*

#### **Figure 14.**

*Comparison of the Aerosol Index (AI) coverages, calculated with the TROPOMI sensor (in color palette) and the OMPS sensor (polygon in red), for 14 November 2019.*

total columnar abundance of CO measured by the TROPOMI instrument [50]. The values observed throughout the region are not high.

A slight increase of CO measurements was observed in the same area where AI values were high, as shown in **Figure 16**, where the CO measurement overlaps the AI coverage (OMPS - red polygon). CO measurements reached a maximum of 0.04 mol/m2 over the province of Entre Ríos.

A slight increase of CO measurements was observed in the same area where AI values were high, as shown in **Figure 16**, where the CO measurement overlaps the AI coverage (OMPS - red polygon). CO measurements reached a maximum of 0.04 mol/m2 over the province of Entre Ríos.

Another interesting measurement to analyze the aerosols' presence is the AOD, which indicates at which level the aerosols prevent the sunlight from passing through the atmosphere. Aerosols scatter and absorb sunlight, resulting in reduced visibility. An AOD of less than 0.1 is characteristic of a clean atmosphere, with a very low number of suspended particles and maximum visibility. The AOD increases due to the increase of suspended particles, and this causes visibility loss.

**Figure 17** shows the AOD measurement at 550 nm from the MODIS - TERRA sensor for 14 November 2019. Maximum values of 0.6 are observed in the southern part of the province of Entre Ríos. Values about 0.3 in the vicinity of the City of Buenos Aires and 0.2 in the vicinity of the City of Córdoba are also observed.

This satellite measurement can be contrasted with the AERONET/NASA sun photometer measurements available at Buenos Aires and Córdoba. **Figure 18** shows the AOD temporal evolution at 440 nm for 14 and 15 November 2019 for the Buenos Aires station and 15 November 2019 for the Córdoba station. At both stations, the values are higher than 0.1 along the two days.

**Figure 15.** *CO in the total column from TROPOMI-Sentinel-5P for 14 November 2019.*

*Lidar Observations in South America. Part II - Troposphere DOI: http://dx.doi.org/10.5772/intechopen.95451*

#### **Figure 16.**

*CO in the total column from TROPOMI-Sentinel-5P and the coverage of the AI (OMPS - red polygon).*

**Figure 17.** *AOD at 550 nm from MODIS-TERRA for 14 November 2019 (logarithmic scale).*

#### *Remote Sensing*

The Ångström coefficient calculation can be used to provide additional information related to the size of the aerosols. The higher this coefficient is, the smaller the particle size. Values less than 1 suggest the domain of coarse particles (e.g., dust), and values greater than 1 suggest the domain of fine particles (e.g., smoke). **Figure 19** shows the Ångström coefficient from MODIS-TERRA product, where maximum values of 1.8 are observed in the southern part of Entre Ríos (in light blue), and 1.1 in the surroundings of Buenos Aires and south of Santa Fe (in green).

#### **Figure 18.**

*Temporal evolution of the AOD at 440 nm during 14 and 15 November 2019 (BA: CEILAP-BA station (blue); CO: Pilar station (red)).*

**Figure 19.** *Ångström coefficient (Blue) of MODIS-TERRA for 14 November 2019.*

#### *Lidar Observations in South America. Part II - Troposphere DOI: http://dx.doi.org/10.5772/intechopen.95451*

The Ångström coefficient was calculated from the AERONET/NASA sun photometers' AOD data at 870, 670, 500, and 440 nm. **Figure 20** shows the AOD at 440 nm versus the calculated Ångström coefficient. This style of graphs allows classifying the type of aerosols suspended in the atmosphere. From such an analysis, it can be extracted that most of the particles are originated from biomass burning, therefore being of the types Biomass Burning and Contaminated Continental, according to the classification table in Ref. [46].

Measurements with the lidar system made it possible to determine the aerosol layers' heights over the City of Buenos Aires. **Figures 21** and **22** show the normalized aerosol backscattering spatial–temporal evolution at 532 nm. On the horizontal axis is UTC's time (Local Time is UTC-3 h); on the vertical axis is the height in kilometers (the tropopause is about 13 km). The color palette shows the intensity of the signal. The blue color represents a clean, molecular atmosphere (without suspended particles), and the red color indicates particulate material. For both days, numerous well-defined layers of aerosols were observed above the atmospheric boundary layer at various heights and with different intensities, up to 13 km.

This work analyzed one event of the arrival of smoke plumes from Australia's intense fires to the Argentinean territory in November 2019. The study determined that, during that period, the AOD values were about 0.25, on average, and the Ångström coefficients were about 1.2. The aerosol layers were found above the atmospheric boundary layer, between 2 km and 13 km of altitude, in the vicinity of the City of Buenos Aires. The CO values were slightly increased without presenting significant risk values for human health. The importance of conducting this type of study is to show that in such aerosols transport events, particles can be transported for hundreds of kilometers from their origin and affect the climate, air quality, and visibility of other areas very distant from the emission source. Satellite measurements, in combination with sun photometers and lidar systems measurements, have allowed an essential synergy for the detection and spatial–temporal monitoring of the smoke columns that, generated thousands of kilometers away, arrived in Argentine territory. Together, these measurements help understand wildfires' environmental impacts in short and long time series, as they provide relevant data for climate and particle dispersion models [51].

#### **Figure 20.**

*Ångström coefficient versus AOD (440 nm) for 14 and 15 November 2019 (BA: CEILAP-BA station (blue); CO: Pilar station (red)).*

**Figure 21.**

*Spatial–temporal evolution of the normalized aerosols backscattering at 532 nm for 14 November 2019.*

**Figure 22.** *Spatial–temporal evolution of the normalized aerosols backscattering at 532 nm for 15 November 2019.*
