On December 8^th, 2019, the novel SARS-CoV-2 corona-virus first reported in Wuhan, China. It causes COVID-19 disease, which spreads across the world. On January 9^th, 2020, coronavirus was discovered as a novel infectious agent of COVID-19 [1, 2]. It is a communicable disease that spreads very quickly from one person to another. To date, all over the world, the total reported cases are estimated to be 4,628,821, and 308,654 people died because of this pandemic. All world governmental bodies are worried about this crisis because its cure has not been discovered yet. Currently, prevention is the only way to avoid infection. For this, many countries announced the complete lockdown to prevent the spread of infection and succeeded. The Indian government also imposed total fives lockdown with various rules and regulations. Out of five lockdowns, the first lockdown implemented from March 25^th, 2020, to April 14^th, 2020, for 21 days was very strict; there were strict restrictions on most of the activities, like the transportation, market, industries, etc.

Rajasthan also implemented all rules and regulations of this lockdown. We know that industries and transportation are major sources of pollutants, like particulate matter (PM_2.5), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), carbon monoxide (CO), and ozone (O₃), which are directly impacting the air quality. It is a major concern of health risk and cause of large premature mortality [3, 4]. In research, it has been found that the air quality parameters of the most polluted cities (Delhi, Lucknow, Kolkata, Patna, and Bihar) and the least polluted cities (Amaravati, Panchkula, Shillong, and Chennai) of India have been changed during the lockdown period. The information regarding air quality parameters of the most and least polluted cities is provided in Table 1 [5]. In India, Rajasthan is also a very growing state with a lot of industrial development, power plants, and transportation. According to the WHO Global Air Pollution Database 2018, all four smart cities, i.e., Jaipur, Udaipur, Ajmer, and Kota, are listed in the polluted cities of the world. The major pollutants include CO, which is mainly introduced into the air by the burning of fossil fuels and vehicles exhaust, SO₂ from petroleum refineries and thermal power plants, NO₂ from thermal power plants and vehicle exhausts, ozone from industrial release and vehicle exhaust, and particulate matter consists of solid in the air in the form of dust, smoke, and vapour.

Recently, researchers have reported that the lockdown period dealt with social distancing and decreasing vehicle transport, significantly improving air quality. Tobias et al. [6] reported that NO₂ and particulate matter (PM₁₀) were reduced by 51% and 31%, respectively, in Barcelona (Spain) during the lockdown. Nakada and Urban [7] analyzed that concentrations of CO, NO, and NO₂ decreased by 64.8%, 77.3%, and 54.3%, respectively, during the partial lockdown in the Sao Paulo state of Brazil. Dantas et al. [8] also reported a significant reduction in NO₂, CO, and PM₁₀ levels during the COVID-19 partial lockdown in Rio de Janeiro, Brazil. It has been estimated that long-term or wide exposure to air pollution is correlated with respiratory diseases, such as bronchitis, aggravated asthma, respiratory allergies, heart disease, stroke, and death [9-11]. Particulate matters (PM_2.5 and PM₁₀) are considered as one of the most health risk factors and cause millions (~ 9%) of deaths around the world annually [11]. A higher concentration of NO₂ emitted from transportation and fuel combustion (outdoor anthropogenic sources) is another most important air pollutant that causes toxicity to human respiratory systems [12, 13]. A higher concentration of NO₂ causes severe illnesses, like cytokine-mediated lungs inflammation, diabetes, hypertension, cardiovascular diseases, and even death [11, 14]. Polluted air is associated with more SARS fatality and influenza [15, 16]. In situ study declared that SARS-CoV-2 shows stability in ambient aerosols, which can be considered a source of COVID-19 transmission [17]. Atmospheric aerosol works as a cofactor that can alter immune defence response system pathways and causes severe pneumonia [16, 18-20].

In India, indoor air pollution is also causing serious public health issues because most households depend on the use of solid fuel. The long periods of lockdown to control COVID-19 in India causes adverse health effects through household air pollution [21]. In this study, we have selected four major cities of Rajasthan, i.e., Jaipur, Ajmer, Udaipur, and Kota. The details of the selected cities are provided in Table 2 [22]. The main goal of this study is to analyze the impact of strict lockdown amid COVID19 on the air quality of the smart cities of Rajasthan, India, by comparing the particulate matter, carbon monoxide, nitrogen dioxide, sulfur dioxide, and ozone concentrations determined during the strict lockdown with values obtained before and during the lockdown.

Changes in concentrations of major pollutants during the pre-lockdown and during-lockdown period are depicted in Fig. (2). Indian government declared a strict lockdown in India on March 24^th, 2020, which was implemented from March 25^th, 2020, to April 14^th, 2020. During this period, i.e., 21 days, all four smart cities of Rajasthan observed substantial diminution of the pollutants. The present article assessed air quality during the strict lockdown (March 25^th, 2020 to April 14^th, 2020). Results showed the highest reduction in PM_2.5 found in Ajmer (53.07%), followed by Jaipur (38.32%), Kota (28.39%), and Udaipur (15.37%), respectively. PM₁₀ was also found to be reduced in Ajmer (45.56%), followed by Jaipur (36.18%), Udaipur (29.40%), and Kota (19.69%), respectively. Whereas, concerning NO_2, the highest reduction was found in Udaipur (66.07%), followed by Jaipur (62.90%), Kota (56.83%), and Ajmer (54.36%), respectively. In the case of SO₂, the highest reduction was found in Udaipur (28.70%), and the lowest was found in Jaipur (11.71%).

Fig. (2 A,B). Comparative analysis of different air quality parameters (PM2.5, PM10, NO2, CO, SO2, and ozone) of smart cities of Rajasthan, i.e., Jaipur, Udaipur, Ajmer, and Kota before and during the period of lockdown.

However, increasing O₃ concentration was recorded at 12.82% in Kota and 6.08% in Ajmer, but Udaipur and Jaipur showed negative results -6.08 and -13.59%, respectively. Overall the result concluded that the air quality index of the cities was found more satisfactory during the lockdown period than the pre-lockdown period by comparing with standard AQI. In India, a high number of people were infected due to coronavirus. During the pre-lockdown (from March 1^st to 24^th, 2020), a total of 994 active cases and 18 deaths were registered. During the lockdown period (March 25^th, 2020, to April 14^th, 2020), a total of 30462 active cases and 823 deaths were registered. It was reported that active cases were increased during lockdown conditions (Table 4). The average death rate percentage was found to be 1.81% during the pre-lockdown and 2.70% during the lockdown phase. The relationship between active cases and deaths may specify the severity of any pandemic.

It has been investigated that meteorological conditions, such as air, temperature, wind direction, and wind speed, are known to greatly impact urban air pollutant’s level via some physical and chemical processes, like accumulation or dispersion and multiphase reactions of aerosol formation and growth [24]. Therefore, it is reasonable to suppose that changes in urban air pollutant concentrations during the COVID-19 lockdown may partially depend on meteorological conditions during that period [25]. Movements of air affect the fate of air pollutants. Concentrations of pollutants build up by non-dispersion of air pollutants due to calm air. However, when strong, turbulent winds blow, the concentration of air pollutants decreases. To estimate air quality, some important factors need to be measured, which can help in understanding the chemical reactions which occur in atmospheres, such as wind speed and direction, temperature, humidity, rainfall, and solar radiation [26].

In India, Rajasthan is also a growing state with the establishment of various kinds of industries and high pressure of transportation. Moreover, WHO also listed all four smart cities, i.e., Jaipur, Udaipur, Ajmer, and Kota, as the most polluted cities of the world based on the global air pollution database. The air quality of all these smart cities was measured by the data provided by the four automatic monitoring stations. The impact of lockdown on air quality in different parts of the world indicated how human beings drastically affect the environment. Therefore, the results of the present article will possibly help us in rethinking the kind of future we are establishing. The article may also help explain how industries and transportation negatively affect the environment and how we can restore the environment and provide a good quality ecosystem to future generations. Currently, on the one hand, the outrageous viruses have affected our lives, and on the other hand, the process of environmental restoration is also going on. Hence, global concern for air pollution has led to drawing significant attention towards analyzing air pollution during the pandemic. In the metropolitan area of São Paulo (2100 km²), the NO₂ concentration reduction is visualized by satellite measurements [5, 27, 28]. However, Sharma et al. [29] reported an increase of 17% in O₃ concentration in India. In China, about 25% carbon and 30% NO₂ emission have been reduced during the period of lockdown [30-33]. Overall, pollutant concentrations decreased in all analyzed areas.

This study can be helpful in evaluating our present role in environmental restoration or environmental destruction. It will also be helpful in updating the present plan toward the assurance and conservation of nature. The main focus of this study is to evaluate the air quality by comparing the concentration of particulate matter PM_2.5, PM₁₀, NO₂, CO, SO_2, and ozone in smart cities of Rajasthan, i.e., Jaipur, Udaipur, Ajmer, and Kota before and during the period of lockdown. CO levels showed the highest significant reduction in Udaipur (50.76%), followed by Jaipur (19.96%), Ajmer (17.11%), and Kota (5.51%), due to the ban on transport and driving. The levels of PM_2.5, PM₁₀, NO₂, and SO_2, were also found to be substantially decreased for each smart city.

Ozone concentration was recorded greater than before lockdown due to decreased nitrogen oxides level. Although, more specific studies are required to explore the interactions between SARS-CoV-2 and air pollutants to understand their impacts on human health. Future studies should be investigated to examine the role of indoor air pollution (biomass and tobacco smoke) during COVID-19. Biology and atmospheric chemistry should be implicated in a more holistic way to the disease management and mitigation to address the current pandemic condition and future viral epidemics. We should not ignore our surrounding environment, which is causing not only chronic diseases but also severe infectious diseases. To establish a healthy future, we need to reduce air pollution from all sources, such as road traffic and heat generation, by strengthening public health policies.