Analysis of NO 2 Pollution over Bangladesh between the Two COVID-19 Caused Lockdowns in 2020 and 2021 Using Sentinel-5P Products †

: Due to the COVID-19 pandemic, all countries around the world have imposed nationwide lockdowns to control the spreading of the virus. During the lockdown period, many countries saw a drastic drop in air pollution. In Bangladesh, there were two nationwide lockdowns. The ﬁrst lockdown was imposed on 26 March–30 May in 2020 and the second lockdown was imposed on 3 April until the study period of 31 May in 2021. This study aimed to analyze the NO 2 pollution over Bangladesh during the two periods of lockdown. Tropospheric NO 2 column spatial conﬁguration was measured over Bangladesh using Sentinel-5P data. A map of the monthly average concentration of tropospheric NO 2 in 2020 and 2021 over Bangladesh was produced using the HARP toolkit and Python. Then, the map was compared with same period Sentinel-5P product’s map for the same period in 2019. It was found that during the ﬁrst lockdown in Bangladesh between 26 March and 30 May 2020, NO 2 concentration drastically decreased in April but increased in May. However, during the second lockdown from 3 April to 31 May in 2021, the NO 2 concentration was found to be much higher. Most of the pollution occurred in the Dhaka district. During the second lockdown, the restrictions were much lighter than those during the ﬁrst one, which impacted the NO 2 concentration. This kind of study can be essential for the authorities to look closely at air quality and use sentinel data to improve air quality monitoring in the future.


Introduction
The COVID-19 pandemic has caused significant changes in society, the economy, and the lifestyles of people worldwide. To contain the spread of the SARS-2 virus, countries all around the world have been imposing different types of measures such as wearing masks, maintaining physical distancing, and vaccines. Initially, lockdowns were the most used strategy for most of the countries around the world to stop the transmission of the COVID-19 virus. The temporary or total closure of international borders, educational institutions, and non-essential businesses and restrictions on citizen mobility, have been some of the measures taken during lockdowns [1]. These lockdowns have led to some positive impacts on the environment, especially in improving air quality. A number of studies have proved that lockdowns have temporarily improved the air quality around the globe [1][2][3][4]. Several papers have also suggested a significant reduction of air pollution in Bangladesh [5,6]. Most of these papers investigated multiple air pollutants (CO 2 , O 3 , NO 2 , SO 2 , PM 2.5 , PM 10 , etc.). Nitrogen dioxides (NO 2 ) are one of the important air constituents. The main sources of NO 2 gases are industry, power plants, residential heating, and vehicle exhausts in the form of nitric oxide (NO) [7]. This reddish-brown gas is produced from NO conversion through the oxidation process. NO 2 is a great indicator of human-made Eng. Proc. 2021, 11, 30 2 of 6 combustion activities and is a precursor of ozone (O 3 ) and aerosols. Respiratory diseases and asthma can be caused by NO 2 . About 4 million cases of pediatric asthma annually result from exposure to NO 2 [8]. Moreover, it can lead to environmental deterioration by producing acid rain [9].
The objective of this study was to collect and analyze the air quality data of Bangladesh during the lockdown periods (March to May) in 2019, 2020, and 2021. The analysis of 2019 was the baseline to compare the trends of NO 2 found during the lockdown periods in 2020 and 2021.

Study Area
Our study was focused on Bangladesh. Bangladesh is located in South Asia at 20 • 34 and 26 • 38 N and 88 • 01 and 92 • 42 E [6]. Bangladesh is a densely populated country with a density of 1265 people per km 2 [10]. Geographically, Bangladesh, a low-lying riverine country, is most vulnerable to climate change [6]. Bangladesh was the most polluted country in 2019 according to the world air quality report by IQAir [10]. There is a lack of air pollution source inventory in Bangladesh [11]. The industrial sector of Bangladesh has been experiencing some expansion, and vehicular and industrial emissions are considered the main sources of air pollution in Bangladesh [12].

Data
The Sentinel-5 Precursor is an earth orbiting single satellite system dedicated to providing information and services on air quality, climate, and the ozone layer [13]. The satellite mission was launched to continue the data between previous missions such as SCIAMACHY, GOME-2, OMI, and the upcoming Sentinel-5 [14]. Sentinel-5P consists of the TROPOspheric Monitoring Instrument (TROPOMI) [9]. TROPOMI is a passive-sensing hyperspectral imager that allows acquisitions of 8-band imagery covering the domain of UV, visible to near-infrared, and shortwave infrared [9,14]. TROPOMI has a spatial resolution of 7 × 3.5 km 2 , which is higher than that of all of its predecessors. The high spatial resolution of Sentinel-5P provides new potential for monitoring air pollution sources [9].
The data used for the study were downloaded from NASA's Earthdata website [4]. The data were downloaded using the subset data option available on the website. Data were downloaded only for the extent of Bangladesh. The data came in netCDF format, which stores multidimensional scientific information, including dimensions, variables/parameters, attributes, and coordinates [9]. A total of 276 offline L2 NO 2 products were used for the study. A total of 93, 90, and 93 products were used, respectively, for 2019, 2020, and 2021, covering 26-31 March and the full month of April and May for each year.

Methodology
The analysis was carried out following the methodology presented in the RUS Copernicus training in "Monitoring Pollution with Sentinel-5p", a case study in Italy during 2019-2020 [15], using the Python language with the HARP atmospheric toolbox provided by ESA. The NO 2 Level 2 product had been converted to Level 3 using the HARP tool in Python. The Sentinel-5P NO 2 Level 2 product was resampled to the spatial resolution of 0.01 × 0.01 degrees covering the extent of Bangladesh between 20 and 27 degrees of latitude. The data were filtered to a tropospheric NO 2 column density value quality over 75. This process was done to avoid errors due to cloud cover. During the conversion of the Level 2 product, tropospheric NO 2 column density was derived from the main product, and the unit was converted to Pmolec/cm 2 from the default unit of mol/m 2 . Then, 6-day mosaics of average tropospheric NO 2 column density were produced for the month of March for each year. Monthly mosaics of average tropospheric NO 2 column density were produced in April and May for each year. The tropospheric NO 2 column density data for the time series analysis of specific cities (Dhaka and Chattogram, Bangladesh) were extracted from the Level 3 product using the Python programming language. The data were then plotted in a graph using WPS Office.

NO 2 Concentration Distribution over Bangladesh
This study focused on the NO 2 pollution over Bangladesh during the two lockdown periods in 2020 and 2021. It was found that the most NO 2 pollution occurs in Dhaka and its surrounding districts, such as Narayanganj and Gazipur. Most of the industries are situated in these districts, which could be the reason for the pollution. Additionally, Chattogram district also faced NO 2 pollution because of having an industrial zone. The comparative analysis of maps produced from the monthly average mosaic of the Sentinel-5P product for tropospheric NO 2 vertical column density showed various trends in each month of the lockdown. In March 2020, when the first lockdown was announced by the government of Bangladesh, the density of NO 2 declined sharply in other cities of Bangladesh compared to the pollution of the same period in 2019. However, in 2021, there was no lockdown in March, so it was expected and found that the concentration of NO 2 was high all over the country. The concentration of NO 2 was much higher than that of the same period in 2019. Figure 1 shows the distribution of NO 2 over the country between 26-31 March of each of the three studied years. for the month of March for each year. Monthly mosaics of average tropospheric NO2 column density were produced in April and May for each year. The tropospheric NO2 column density data for the time series analysis of specific cities (Dhaka and Chattogram, Bangladesh) were extracted from the Level 3 product using the Python programming language. The data were then plotted in a graph using WPS Office.

NO2 Concentration Distribution over Bangladesh
This study focused on the NO2 pollution over Bangladesh during the two lockdown periods in 2020 and 2021. It was found that the most NO2 pollution occurs in Dhaka and its surrounding districts, such as Narayanganj and Gazipur. Most of the industries are situated in these districts, which could be the reason for the pollution. Additionally, Chattogram district also faced NO2 pollution because of having an industrial zone. The comparative analysis of maps produced from the monthly average mosaic of the Sentinel-5P product for tropospheric NO2 vertical column density showed various trends in each month of the lockdown. In March 2020, when the first lockdown was announced by the government of Bangladesh, the density of NO2 declined sharply in other cities of Bangladesh compared to the pollution of the same period in 2019. However, in 2021, there was no lockdown in March, so it was expected and found that the concentration of NO2 was high all over the country. The concentration of NO2 was much higher than that of the same period in 2019. Figure 1 shows the distribution of NO2 over the country between 26-31 March of each of the three studied years. In April 2020, the government strengthened the strictness of the lockdown and we can observe that there was a significant drop in NO2 concentration in 2020. Compared to the concentration in 2019, the concentration of NO2 dropped drastically all over the country, including in Dhaka and Chattogram city. In 2021, the lockdown was declared on 1 April, but the lockdown was not implemented strictly. This resulted in no such difference in NO2 pollution in the major cities such as Dhaka, Chattogram, and Rajshahi. In April 2020, the government strengthened the strictness of the lockdown and we can observe that there was a significant drop in NO 2 concentration in 2020. Compared to the concentration in 2019, the concentration of NO 2 dropped drastically all over the country, including in Dhaka and Chattogram city. In 2021, the lockdown was declared on 1 April, but the lockdown was not implemented strictly. This resulted in no such difference in NO 2 pollution in the major cities such as Dhaka, Chattogram, and Rajshahi. Rather, the concentration of NO 2 was higher compared to that in April 2019. Figure 2 shows the comparative cartography of NO 2 concentration over Bangladesh. Rather, the concentration of NO2 was higher compared to that in April 2019. Figure 2 shows the comparative cartography of NO2 concentration over Bangladesh. In May 2020, the strictness of lockdown was compromised by the government. Transports such as private cars and buses began to return to the road and industries started their working process slowly. This was reflected in the NO2 concentration across the country. The concentration of NO2 increased compared to that in April 2020 but it was still less polluted compared to the same period of NO2 pollution in 2019. In 2021, the lockdown was compromised and public transports started taking half of its capacity. This was reflected in the NO2 pollution throughout the county. In May 2020, the strictness of lockdown was compromised by the government. Transports such as private cars and buses began to return to the road and industries started their working process slowly. This was reflected in the NO 2 concentration across the country. The concentration of NO 2 increased compared to that in April 2020 but it was still less polluted compared to the same period of NO 2 pollution in 2019. In 2021, the lockdown was compromised and public transports started taking half of its capacity. This was reflected in the NO 2 pollution throughout the county.

NO 2 Concentration over Selected Cities: Dhaka and Chattogram
Dhaka and Chattogram districts were found to be hotspots of NO 2 pollution in Bangladesh as can be seen from Figures 2-4. We had analyzed the concentration of NO 2 in these two specific cities to understand the difference and impact of lockdown in detail. The time series of weekly average tropospheric NO 2 vertical column number density de-rived from the Sentinel-5P product shows that Dhaka city underwent a significant drop of NO 2 concentration from 26 March to 5 May 2020 compared to 2019. After 5 May, the concentration increased in the next week and it continued to increase until the end of the study period. In contrast, the concentration of NO 2 was higher than that in 2019 and 2020 from 26 March until 26 May. After 26 May, there was a drop of NO 2 concentration until the study period. Figure 3 shows the time series of NO 2 pollution over the Dhaka district.

NO2 Concentration over Selected Cities: Dhaka and Chattogram
Dhaka and Chattogram districts were found to be hotspots of NO2 pollution in Bangladesh as can be seen from Figures 2-4. We had analyzed the concentration of NO2 in these two specific cities to understand the difference and impact of lockdown in detail. The time series of weekly average tropospheric NO2 vertical column number density derived from the Sentinel-5P product shows that Dhaka city underwent a significant drop of NO2 concentration from 26 March to 5 May 2020 compared to 2019. After 5 May, the concentration increased in the next week and it continued to increase until the end of the study period. In contrast, the concentration of NO2 was higher than that in 2019 and 2020 from 26 March until 26 May. After 26 May, there was a drop of NO2 concentration until the study period. Figure 3 shows the time series of NO2 pollution over the Dhaka district.   in these two specific cities to understand the difference and impact of lockdown in detail. The time series of weekly average tropospheric NO2 vertical column number density derived from the Sentinel-5P product shows that Dhaka city underwent a significant drop of NO2 concentration from 26 March to 5 May 2020 compared to 2019. After 5 May, the concentration increased in the next week and it continued to increase until the end of the study period. In contrast, the concentration of NO2 was higher than that in 2019 and 2020 from 26 March until 26 May. After 26 May, there was a drop of NO2 concentration until the study period. Figure 3 shows the time series of NO2 pollution over the Dhaka district. Chattogram district showed a different trend compared to Dhaka district. The concentration of NO2 was observed to decline in 2020 compared to 2019 until the sixth week of the study period, which was 29 April-5 May. After that period, the concentration fluctuated compared with 2019.  Chattogram district showed a different trend compared to Dhaka district. The concentration of NO 2 was observed to decline in 2020 compared to 2019 until the sixth week of the study period, which was 29 April-5 May. After that period, the concentration fluctuated compared with 2019.
In 2021, the concentration was very high in both 2019 and 2020 throughout the study period. There was not much of a decline observed in NO 2 concentration in 2021 compared to that in 2019 and 2020 in Chattogram. Figure 4 shows the time series of weekly average NO 2 concentration in Chattogram city.

Conclusions
This study was carried out to examine the variations in NO 2 concentrations during the lockdowns in the years 2020 and 2021. At the time of the strict lockdown in April 2020, there was a significant variation in the concentration of NO 2 compared with 2019. NO 2 levels had dropped dramatically across the country, including in Dhaka and Chattogram. However, Eng. Proc. 2021, 11, 30 6 of 6 when the lockdown was loosened by the government and people started their daily lives as before, the concentration of NO 2 again became higher than in the lockdown period.
Overall, it has been found that the strict lockdown imposed in 2020 showed a decrease in NO 2 pollution. However, during the second lockdown in 2021, the lockdown was only partially implemented and thus did not impact much on the NO 2 pollution.
Continuous data collection is quite difficult for developing countries such as Bangladesh as it requires a lot of effort and costs. The Sentinel-5P mission provides continuous and various types of data that can be useful for monitoring air quality and air pollution in Bangladesh. In the future, Sentinel-5P and remote sensing analysis can be implemented effectively to monitor and control air pollution in Bangladesh.