








key: cord-350919-by4zwniq
authors: Chowdhuri, Indrajit; Pal, Subodh Chandra; Saha, Asish; Chakrabortty, Rabin; Ghosh, Manoranjan; Roy, Paramita
title: Significant decrease of lightning activities during COVID-19 lockdown period over Kolkata megacity in India
date: 2020-07-28
journal: Sci Total Environ
DOI: 10.1016/j.scitotenv.2020.141321
sha: 
doc_id: 350919
cord_uid: by4zwniq

Abstract The outbreak of COVID-19 has now created the largest pandemic and the World health organization (WHO) has declared social distancing as the key precaution to confront such type of infections. Most of the countries have taken protective measures by the nationwide lockdown. The purpose of this study is to understand the effect of lockdown on air pollutants and to analyze pre-monsoon (April and May) cloud-to-ground and inter-cloud lightning activity in relation to air pollutants i.e. suspended Particulate matter (PM10), Nitrogen dioxides (NO2) Sulfur dioxide (SO2), Ozone (O3) and Aerosol concentration (AC) in a polluted tropical urban megacities like Kolkata. After the strict lockdown the pollutants rate has reduced by more than 40% from the pre-lockdown period in the Kolkata megacity. So, decreases of PM10, NO2, SO2, O3 and AC have a greater effect on cloud lightning flashes in the pre-monsoon period. In the previous year (2019), the pre-monsoon average result shows a strong positive relation between the lightning and air pollutants; PM10 (R2 = 0.63), NO2 (R2 = 0.63), SO2 (R2 = 0.76), O3 (R2 = 0.68) and AC (R2 = 0.83). The association was relatively low during the lock-down period (pre-monsoon 2020) and the R2 values were 0.62, 0.60, 0.71, 0.64 and 0.80 respectively. Another thing is that the pre-monsoon (2020) lightning strikes decreased by 49.16% compared to the average of previous years (2010 to 2019). The overall study shows that the reduction of surface pollution in the thunderstorm environment is strongly related to the reduction of lightning activity where PM10 and AC are the key pollutants in the Kolkata megacity.

J o u r n a l P r e -p r o o f governments of India and West Bengal have implemented strict measures, such as the closure of shops, supermarkets, malls, all industries, public transport, airports, etc. There is also some relaxation after lockdown 3.0 (after 3 May 2020), with the exception of the red zone of infection with COVID-19. Due to the social distancing prevention, some researchers (Kerimray et al., 2020; Li et al., 2020) reported air quality improvements associated with lockdown measures like consequent decrease of vehicle and industrial transit. The improvement of air quality and the reduction of air pollutants have a broad effect on large urban areas or megacities. The worldwide research has also proved that due the lockdown, air and water quality has also been improved (Yunus et al., 2020) . Sharma et al. (2020) analyzed data on air pollutants and assessed the impact of the lockdown on air quality in different parts of India. During the partial lockdown, the air quality of Sao Paulo in Brazil improved dramatically and the level of pollutants decreased by 77% (Nakada and Urban, 2020) . The National Capital Region (Delhi) in India is India's most polluted megacity, but the COVID-19 pandemic lockdown suddenly reduced the concentration of air pollutants and significantly improved air quality after three days of lockdown (Mahato et al., 2020) .

As a result, many researchers (Mahato et al., 2020; Mandal and Pal, 2020; Yunus et al., 2020) have carried out the COVID-19 pandemic lockdown on environmental quality, such as air, water and noise quality, in different parts of the world. However, due to the decline in air pollutants, some micro-climatic phenomena have also changed where the levels of air pollutants have changed dramatically, as lightning and thunderstorms are among them. J o u r n a l P r e -p r o o f positive charge of super-cooled cloud droplets and a negative charge of soft hail or snow pellets. The super-cooled cloud droplets move upward, positioned in the upper part of the cloud, and the soft hail in the middle or lower part of the cloud (Ogawa and Brook, 1964) .

Thus, the positive charge accumulated in the upper part of the thunderstorm cloud and the middle and lower parts of the cloud are accumulated by negative charges and their electrostatic discharge is generated by a massive lightning (Lhermitte and Williams, 1985) .

Many scientists have shown that the change in major weather conditions caused the urban effect (Orville et al., 2001; Soriano et al., 2001) . A large number of literature (Chaudhuri and Middey, 2013; Kar et al., 2009; Kar and Liou, 2014) has shown that there is a strong relationship between the air pollutant and the lightning in the highly polluted megacity of the tropical region. Different studies have shown that the increase in cloud-to-ground and intercloud lightning has an effect on nitrate oxide and O 3 in urban atmospheres (Pawar et al., 2012) . Air pollution, therefore, has a direct and indirect impact on the global climate and atmosphere. Concentration of aerosols in the atmosphere has an effect on rainfall and cloud formation and has worked as a change in weather conditions. Suspended particulate materials (PM 10 ) are the most important air pollutants and cloud lightning determinants (Naccarato et al., 2003) . Possible concentration of SO 2 in the atmosphere to increase lightning flashes in urban areas (Soriano and de Pablo, 2002) . Kar et al., (2009) reported that the major concentration of PM 10 and SO 2 in the atmosphere is responsible for the hike of lightning strikes. On the other hand, the concentration of NO 2 on the troposphere has a major effect on J o u r n a l P r e -p r o o f Although different studies have included a lockdown impact on improving air quality in urban areas, changes in air quality and air pollutants and the impact on the climate of microregions such as megacities have not yet been discussed. This means that the COVID-19 lockdown imposed has an indirect impact on the micro-climate phenomenon. The present study has been conducted considering the ambient air pollutants, i.e., PM 10 , NO 2 , SO 2 , O 3 and aerosol on lightning flashes during the pre-monsoon period (April and May). A higher amount of pre-monsoon lightning flash is reported over the Kolkata megacity area than the monsoon period (Chaudhuri and Middey, 2013) . Hence, the pre-monsoon season has been selected for this study because this time coincides with the COVID-19 lockdown period. The main objectives of this study are therefore to analyze the trend of ambient air pollutants (PM 10 , NO 2 , SO 2 , O 3 and aerosol) during the lockdown and pre-lockdown periods. And to compare the pre-monsoon cloud-lightning activities affecting these air pollutants during the COVID-19 lockdown period and some pre-lockdown periods in the Kolkata megacity area.

The location of the study is the Kolkata megacity, the capital city of West Bengal, as well as the largest megacity in Eastern India (Fig. 1) . Physiographically, it is situated over the Indo-Gangetic plain specifically the mature part of the Ganges delta. The average altitude of Kolkata and its environs is 6.05 m with salty marshy wetland topography; after the implementation of the wetland restoration project, it is found especially towards the eastern parts of the megacity. The climate is tropical wet and dry ('AW' type of Kopen classification), characterized by wet in summer and dry in winter. The mean annual temperature and rainfall are 26°C and 1,582 mm respectively. The megacity faces five seasons, i.e., Pre-monsoon (Apr-May), monsoon (June-Sep), post-monsoon (Oct-Nov), winter (Dec-Jan) and spring (Feb-Mar). During the pre-monsoon period, this area experienced several local storms, characterized by strong winds and afternoon rains with J o u r n a l P r e -p r o o f thunderstorms and lightning. The monthly average temperature of pre-monsoon season is 30°C to 38°C and rainfall ranges from 0 to 150mm. The rainfall mainly occurs due to the Nor-waster and severs tropical cyclone over the Bay of Bengal. The wind blows in the north, north-east, south and south-east direction during the pre-monsoon period. As per records of the census of India and Government of West Bengal, huge urbanization took place around the Kolkata and the population of megacity has touched 14.85 million. Kolkata is the main commercial hub of eastern India, and a number of macro and micro industries are located here. So pollution is the main problem in Kolkata, mainly air pollution, PM and other air pollutants are higher than other major cities in India. It caused respiratory diseases, such as lung cancer, (CPCB, 2020).

Most of the lightning studies were conducted in Kolkata during the pre-monsoon season (April-May). This study period is therefore 24 March to 20 May, which is the period of (Fig. 4) .

The Normal trend method used to compare both air pollutants and lightning flash counts for the period 2010 to 2020 (Pre-monsoon period).

The Mann-Kendall statistical S test calculated as following Eq.1 (Kendall, 1975; Mann, 1945 ).

(1)

Here, and are rank of observation in and time series. Mann (1945) and Kendall (1975) have reported that statistics S and the mean and variance are computed as Eq.3.

J o u r n a l P r e -p r o o f

Where, n is the number of observations, m is the number of groups of tied ranks and the notation t is extend of any given time. When the n >10 the standard normal variable (Z) is computed as following Eq.4.

The slope estimates of N datasets were computed by the following equations (Sen, 1968 ).

Where, and are the value of data at the time j and k ( > ), respectively. The median of slope or Sen's slope estimator of odd and even data is computed as Eq.6 and 7

Where, is median of data trend. Eq.6 applied if N is odd data and if N is even Eq.7 is used. When the median slope is statistically different than zero, then confidence interval of at specific probability (Da Silva et al., 2015; Gilbert, 1987 ) estimated as Eq.8 from the pre-lockdown to during lockdown period and the trend of daily PM 10 decreases by -1.00μg/m 3 during the lockdown period (Table 1 and 2). The PM 10 sources, such as vehicles and traffic, are very low on the roads and the industries have been tightly closed to maintain the social distance in Kolkata during the lockdown. Other important pollutants, i.e. NO 2 and SO 2 have also shown the significant reduction during the COVID-19 pandemic lockdown ( Fig. 2) . In the study area, average concentrations of NO 2 and SO 2 decreased by almost -68.38% and -40.38% respectively, and the trend of daily reduction is about -0.373 and -0.153μg/m 3 from the pre-lockdown to lockdown (Fig. 2 , Table 1 and 2) due to the emission from diesel, in smaller degree from gasoline vehicles, manufacturing industry and power plants have totally stopped. O 3 in the lower troposphere acts as a pollutant and potential for respiratory hazards. Therefore, the concentration of O 3 is also much below the permissible limit and also has reducing trend (-42.58%) from pre-lockdown to lockdown, with a daily trend of -0.571μg/m 3 /day (Fig. 2 , Table 1 and 2). The concentrations of aerosol extracted from daily MODIS data during the pre-locking period to the lock-down period also show the same trend as for other pollutants (-57.92%) from the pre-and during the lock-down period and a daily decrease of -1.099μg/m 3 /day (Fig. 2 , Table 1 Thereafter the above mentioned pollutants and aerosol concentration are minutely increases in the present phase of the lockdown period (Fig. 2) .

Here Table 1 Here Table 2 Here

Current research corresponded to pre-monsoon thunderstorm and lightning activity during the Previous studies have shown that Sulfuric acid (H 2 SO 4 ) particle is more active in the formation of new cloud condensation nuclei (CCN) as well as thunderstorm and lightning (Perry and Hobbs, 1994; Siingh et al., 2011; Thornton et al., 1997) . The reaction of SO 2 with hydroxyl radical (OH) formed H 2 SO 4 in the atmosphere. The formation of H 2 SO 4 is shown in equations 9, 10 and 11 as follows.

J o u r n a l P r e -p r o o f 2 + 2 → 2 + 3 (10) 3 + 2 → 2 4 (11)

Scatter plot (R 2 value of 2019 is 0.76 and 2020 is 0.71) revealed that higher SO 2 concentration is associated with high lightning and flashing in Kolkata, but the scatter distribution and ten-years trend showed a reduction in SO 2 and lightning both during the 2020 lockdown period (Fig. 3. 

The impact of the lock-down and its associated level of pollutants has been estimated in this study to determine the relationship between the level of pollutants and cloud flashes. For this purpose, the pre-monsoon season in the present and previous years has been selected purposively. The atmospheric disturbances have been found in this region and associated surrounding regions, popularly known as "Kalbaisakhi" or "Norwester". Pollution in urban areas has a significant impact on the generation of lightning flashes and their associated frequency and intensity. Although the Kolkata megacity was considered to be one of the major polluted megacity due to rapid commercial, industrial and transport activities, heavy lightning flashes with high vertical air currents are common in any pre-monsoon season.

Apart from the various meteorological components, the suspended particulate matter (SPM) has a major impact on surface lightning flashes during the pre-monsoon period. The global pandemic of the COVID-19 lockdown has changed India's air quality. All cities in India are witnessing a downward trend in pollutants. This is primarily due to fewer automobiles and roadside food vendors using coal stoves, which is the significant source of emissions in Indian cities. There is a significant decrease in the tendency of lightning flashes has been observed during the lockdown period of the Kolkata megacity and its associated regions (Fig.   3 ). However, the COVID-19 lockdown reduces the level of air pollutants by closing down man-made sources of emissions. On the other hand, major air pollutants, nitrate oxides and J o u r n a l P r e -p r o o f Aerosol concentration -6.86 *** -1.099 ***, **, and * are the significant at 1%, 5%, and 10% level of significance respectively.

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