Effect of the COVID-19 Lockdown on Ambient Air Quality in Major Cities of Nepal

Background. The Nepalese government announced a nationwide lockdown beginning on March 24, 2020 as an attempt to restrain the spread of COVID-19. The prohibition in flight operations and movement of vehicles, factory shutdowns and restriction in people's movement due to the lockdown led to a significant reduction in the amounts of pollutants degrading air quality in many countries. Objectives. The present study aimed to analyze changes in particulate matter (PM) emissions and the air quality index (AQI) of six cities in Nepal i.e., Damak, Simara, Kathmandu, Pokhara, Nepalgunj and Surkhet due to the nationwide lockdown in response to the COVID-19 outbreak. Methods. Daily PM concentrations of each of the six study cities from January 24 to September 21, 2020 were obtained from the World Air Quality Index project (https://aqicn.org) and analyzed using R Studio software. The drop percentage was calculated to determine the change in PM2.5 and PM10 concentration during different time periods. Independent sample Mann–Whitney U tests were performed to test the significance of differences in mean concentration for each site during the lockdown period (24 March–24 July 2020) and its corresponding period in 2019. Similarly, the significance of differences in mean concentrations between the lockdown period and the period immediately before lockdown (23 January–23 March) was also examined using the same test. Results. During the lockdown period, in overall Nepal, AQIPM2.5 and AQIPM10 were within the moderate zone for the maximum number of days. As a result of the lockdown, the highest immediate and final drop of PM2.5 was observed in Damak (26.37%) and Nepalgunj (80.86%), respectively. Similarly, the highest immediate drop of PM10 was observed in Surkhet (37.22%) and finally in Nepalgunj (81.14%). Analysis with the Mann–Whitney U test indicated that for both PM types, all sites showed a statistically significant (p < 0.05) difference in mean concentrations during lockdown and the corresponding period in 2019. Conclusions. The present study explored the positive association between vehicular movement and PM emissions, highlighting the need for alternative fuel sources to improve air quality and human health. Competing Interests. The authors declare no competing financial interests.


Introduction
As the SARS-CoV-2, a novel coronavirus causing COVID-19 rapidly spread, the Director General of World Health Organization (WHO) declared the outbreak to be a Public Health Emergency of International Concern on 30 January 2020. 1 The disease being air-borne and highly infectious, 2 its outbreak ushered many countries to go into lockdown and restrict transportation, unnecessary gatherings and travel in order to contain virus spread. 3 The government of Nepal also imposed a complete nationwide lockdown from 24 March 2020 after a second coronavirus case was confirmed, which restricted domestic and international flights, vehicular movement except for essential purposes, prohibited gatherings, suspended school and colleges and shut down factories, industries and brick kilns. 4 Concerned about the suffering economy, the government lifted the nationwide lockdown on 22 July 2020 after almost 4 months. The country then underwent episodes of regional lockdowns in response to increased infection rates in some regions.
Although the efficacy of lockdown in Nepal to suppress the pandemic was minimal, it played an important role in improving air quality. [5][6][7] Lockdown-induced improvement in Research pollutants.
The major pollutants affecting the atmosphere throughout the world are ozone, particulate matter (PM), lead (Pb), carbon monoxide (CO), nitrogen dioxide (NO 2 ), sulfur dioxide (SO 2 ), and other toxic compounds. 14 In Nepal, the National Ambient Air Quality Standard (NAAQS) has defined nine variables, i.e., PM 2.5 , PM 10, ozone (O 3 ), NO 2 , SO 2 , total suspended particulates (TSP), carbon monoxide (CO), benzene (C 6 H 6 ), and lead (Pb) to characterize the air quality of a region. However, PM 2.5 and PM 10 are of primary concern as they are thought to be key contributors of air pollution. 15,16 Particulate matter is a collective term describing small solid and liquid particles which are present in the atmosphere over relatively brief (minutes) to extended periods of time (days to weeks). 17 The major sources of PM include emissions from power plants, industries, automobiles, construction sites, unpaved roads, fields, smokestacks and fires. 18 Particles with a diameter greater than 10 μm generally do not get passed through nasal hairs and defense mechanisms of the upper respiratory system and are not of public health concern. 17 However, PM less than 10 μm (PM 2.5 and PM 10 ) is of highest importance as it poses the most serious risks to human health. 15, 19,20 Epidemiological studies have shown positive associations between exposure to PM and frequency of certain illnesses and mortality. Exposure to fine PM is associated with a number of cardiovascular outcomes such as hypertension, atherosclerosis, arrhythmias, myocardial ischemia, heart attacks, heart failure and strokes. 20 Karki et al. (2016) reported 11 300 inpatient admissions due to respiratory problems and a 3.7% rise in mortality and 1% rise in respiratory hospitalization per 10 µg/ m 3 rise in PM 2.5 . Similarly, a 10 µg/ m 3 increase in PM 10 was found to increase risk of hospitalization by 1.70% for respiratory and 2.29% for cardiovascular admissions. 19 A significant increase in the number of automobiles has created an alarming air pollution problem in Nepal, especially in densely populated cities like the Kathmandu valley. 22 In fiscal year 2018-19, 318 477 new vehicles were registered, of which 78% were motorcycles and 6% were cars/vans. 23 Similarly, within the same fiscal year, 436 new industries were registered, of which 38% were energy based, construction, manufacturing and mineral industries whose contributions to emissions are considered to be the major cause for the degradation of air quality. 24 Nepal was ranked 8th in PM 2.5 concentrations in the regional ranking of Central and South Asia, and the capital city of Nepal, Kathmandu was ranked as the sixth most polluted capital city. 25 In 2020, the Environmental Performance Index of Nepal ranked it in the 178 th position in global air quality among 180 countries. 26 Amidst the context of worsening air pollution, but continuous failure of most of governing bodies worldwide to address this environmental issue, the COVID-19 lockdown experience could be very beneficial for designing efficient pollution control measures. Studies have begun to quantify the efficiency of such lockdowns for controlling air pollution. Khan et al. (2020) studied the variation of air quality in major cities of Pakistan before and after the lockdown period. 11 That study compared air quality in major cities before the lockdown from March 1 to March 23 and then during the lockdown from March 24 to April 15 and showed a 49% reduction in the amount of NO 2 in Lahore, 45% in Peshawar and 56% in the twincities. In Nepal, Paudel et al. (2020) compared the concentration of PM 2.5 in Ratnapark, Kathmandu during the lockdown period in 2020 with that of the corresponding period in 2019 to determine the impact of the lockdown on air quality. 7 The results showed that the number of days exceeding the NAAQS of Nepal in 2019 for PM 2.5 was higher than in April 2020.
Although many studies have reported improvement in ambient air quality due to the COVID-19 lockdown, it is unknown whether these improvements have occurred across the whole country because these studies were restricted to a specific place or region (e.g., Gautam et al. in Kathmandu and Dhobi in Western Terai) 5, 6 . In an attempt to represent the air quality of the entire country, the present study included six major cities from six out of seven provinces of Nepal to study the change in PM 2.5 and PM 10 concentrations due to lockdown Baral

Methods
For the present study, 6 different cities from 6 out of 7 provinces of Nepal were selected. No city from the 7 th province was included due to the unavailability of data from that area. A detailed description of study sites with geographical coordinates of the air quality monitoring center in selected cities is given in Table 1 and their location in Figure 1.

Analysis
For the analysis of the effect of the lockdown imposed by the governing authorities on air quality, PM 2.5 and PM 10 were selected. The threshold for the emission of PM of the WHO is given in Table 2.
The government of Nepal has set and enforced NAAQS guidelines and has a legal obligation to maintain PM emission standards. The details of the threshold maintained by the Nepal Government for PM emissions is given in Table 3.

Data source
We received the data from the World Air Quality Index project, China, which is a non-profit project started in 2007. 29 Its mission is to promote air pollution awareness for citizens and provide unified and worldwide air quality information. The project provides transparent air quality information for more than 130 countries, covering more than 30 000 stations in 2 000 major cities, via these two websites: aqicn.org

Data collection and interpretation
Data were obtained from the website aqicn.org. 29 To clearly visualize how PM concentrations fluctuated before, during and after the lockdown, data was requested from 24 January 2020 to 21 September 2020 which covers the two months preceding and succeeding the lockdown period.

Equation 1
where I p is the index for pollutant p, C p is the truncated concentration of pollutant p, BP Hi is the concentration breakpoint greater than or equal to C p , BP Lo is the concentration breakpoint that is less than or equal to C p , I Hi is the AQI value corresponding to BP Hi , and I Lo is the AQI value corresponding to BP Lo .
After calculating the AQI value, it was categorized by level of concern. The USEPA has designated six AQI categories (Table 4), with each category indicating a level of health concern. 33

Results
The AQI PM2. 5 Figure 2 -(a), (b), (c), (d) Research study as shown in Figure 2. Figure 2 presents the total number of days with available daily PM 2.5 and PM 10 data and number of days which exceeded the USEPA 'moderate category' , i.e., AQI>100, for different cities in Nepal. In January and February, the AQI PM2.5 level at Damak, Simara, Kathmandu, Pokhara, Nepalgunj and Surkhet exceeded the USEPA standard (moderate level, AQI>100) for the maximum number of days, while after the lockdown, i.e., March 24 2020 onwards, the number of days during which the moderate zone was surpassed began to decrease.
The AQI PM10 value exceeded the USEPA 'moderate category' (AQI>100) for the maximum number of days in Damak and Simara. But after the lockdown, the AQI PM10 of these two cities remained within the moderate zone for the maximum number of days. In other cities except Damak and Simara, the AQI PM10 remained within the moderate category during the study period.

Particulate matter
The nationwide lockdown decreased anthropogenic activities, which resulted in a significant drop in PM concentrations (PM 2.5 , PM 10 ) in all cities of Nepal ( Figure 3). After the lockdown was implemented on March 24 2020, a drastic drop was recorded in mean and daily concentrations of PM 2.5 and PM 10 in all cities of Nepal as shown in Figure 3 and summarized in Table 5. Similarly, during and after the lockdown the daily concentrations of PM 2.5 and PM 10 remained within the WHO limit (25 µg/m 3 and 50 µg/m 3 , respectively) for the maximum number of days compared to before the lockdown.
The daily concentration of PM 2.5 and PM 10 in Damak exceeded the WHO limit 25 µg/m 3 and 50 µg/ m 3 , respectively, for the maximum number of days before the lockdown, and during and after the lockdown remained within the WHO limit Baral, Thapa

Table 5 -Variation in PM 2.5 and PM 10 Concentrations Before, During and after Lockdown
Research (Figure 3a). In Simara, Kathmandu, Pokhara, Nepalgunj and Surkhet, a similar pattern was observed with a drastic drop in mean and maximum daily concentration of PM 2.5 and PM 10 and daily emissions lower than the WHO limit during and after the lockdown. Data on PM emissions after lockdown in Damak, Simara and Surkhet could not be calculated due to unavailability of this data.

Air mass trajectory
An analysis of air mass trajectory was performed for sixteen different time periods. Figure  The present study observed that Damak is mostly influenced by a western air mass from Syria during 15-21 March 2019 and during 15-21 March 2020, the air mass originated from the Mediterranean Sea (Figures  4a-b). Similarly, during 22-28 March 2019 the air mass of Damak originated from the Arabian Sea and eastern India, while during the corresponding time in 2020 the air mass originated from the Arabian Sea and the Bay of Bengal (Figures 4e-f). The air mass of Damak during 15-21 July 2019 originated from the Indian Ocean and the Gulf of Mannar and during the equivalent time in 2020 the air mass originated from the Indian Ocean (Figures 4i-j). This pattern was similar to the period of 22-28 July 2019 and the corresponding period of 2020 as the air mass was chiefly received from the Indian Ocean and the Bay of Bengal (Figures 4m-n).
Similarly, the sources of air mass reaching to the other cities of Simara, Kathmandu, Pokhara, Nepalgunj, Surkhet before and during the lockdown period and the corresponding period in 2019 are summarized in Table 6 and Figure 4.

Immediate and final drop in particulate matter concentrations
Due to the nationwide lockdown, a significant drop was observed in PM 2.5 and PM 10 concentrations (µg/ m 3 ) in almost all cities in Nepal.

Mann Whitney U-test
Analysis with the Mann-Whitney U test indicated that both PM types showed statistically significant (p < 0.05) differences in mean concentration across all sites during lockdown and the corresponding period in 2019. In addition, a significant difference in mean concentrations was observed between the lockdown period and the period immediately before the lockdown. Due to unavailability of data, differences between mean PM concentrations during the pre-lockdown period in 2020 and the corresponding period of 2019 could not be tested.

Discussion
The COVID-19 lockdown resulted in a significant decrease in PM concentrations in cities across Nepal, largely caused by the reduction of anthropogenic activities and traffic. Our results are in agreement with a study done by Gautam et al. (2020) which reported a decrease in AQI PM10 and the number of days exceeding the USEPA standard after March 2020 in Kathmandu. 6 In Pakistan, the AQI PM2.5 was observed to improve to normal from the unhealthy to moderate range during lockdown due to termination of economic activities, such as the closure of factories in hotspot zones to prevent further COVID-19 outbreaks. 11 The results of these studies show that for the maximum number of days, the overall AQI in Nepal remained within the moderate category (AQI<100) during lockdown, highlighting the significant improvements in air quality during this time period.
Before enforcement of the nationwide lockdown on March 24 2020, daily PM 2.5 and PM 10 levels in all cities of Nepal exceeded the WHO limit (25 μg/ m 3 and 50 μg/m 3 , respectively, for PM 2.5 and PM 10 ) and the USEPA's moderate category (AQI>100) for the maximum number of days. 33  After the lockdown was imposed, concentrations of PM 2.5 and PM 10 immediately started to decrease in most cities in the present study, but this trend lasted for only a few days after which concentrations rose again, exceeding the WHO limit until mid-April. The period during    40 We could not assess the air quality for any city from Province number 7.

Conclusions
Air pollution is one of the prime factors behind increasing human morbidity and mortality. To the best of our knowledge, the present study is the first investigation to be published analyzing the effect of the COVID-19 lockdown on PM emissions in Nepal. These emissions showed a significant reduction after the lockdown began. More specifically, for the maximum number of days in Nepalgunj, AQI PM2. 5 and AQI PM10 remained within the USEPA standard and also recorded the lowest mean PM emissions during lockdown. Similarly, the highest final drop percentage was observed in Nepalgunj, which showed the biggest improvement in air quality. It shows positive correlation between the number of vehicles and PM emissions, as Nepalgunj had the least number of vehicles. The lower the number of vehicles, the lower the emissions. This shows the importance of regulating the number of vehicles operating in a particular space at one time. This can be best achieved by encouraging people to use public transportation and limiting the use of private vehicles. Similarly, use of energy sources such as compressed natural gas, solar energy and electricity should be promoted as alternatives to fossil fuels to reduce emissions.