The minute solid particles or liquid droplets suspended in the atmosphere are known as an aerosol. Atmospheric aerosols are mainly produced by the mechanical disintegration processes occurring over land (e.g., lift of dust) and ocean (e.g., sea-spray) and by chemical reactions occurring in the atmosphere [1, 2]. Natural aerosols such as dust, fly ash, soot, pollen grains, and carbon particle occur naturally as they mix in the atmosphere by different natural phenomena, thereby covering the majority of the aerosol in the atmosphere, on the other hand, anthropogenic aerosols are located considerably in a polluted area due to human activities, including fossil fuel combustion, biomass burning, industrial smoke, power plants, coal mining, and other several activities, overlaying approximately 10% of the entire aerosols . These different aerosol groups often clump together to form a hybrid of both natural and anthropogenic. Aerosols can make their way into the atmosphere almost everywhere globally, depending on the season and weather conditions .
Aerosol scatters and absorb sunlight (size range 0.1-1 micrometres are most effective) [4, 5] and change the cloud's characteristics . They directly affect limiting visibility and causing a risk to human health, including premature deaths, allergies, respiratory problems such as Influenza, Pneumonia, and harmful cardiovascular effects such as heart attacks and strokes [3, 7, 8]. On examining the long-term records of aerosol particles and lung cancer incidence, Tie et al.  reported that the mortality due to lung cancer is closely correlated to the levels of aerosol particles present in the atmosphere near the surface. High aerosol pollution causes wide-ranging consequences for human health, natural ecosystems, visibility, weather, radiative forcing, and tropospheric oxidation (self-cleaning) capacity . In addition, aerosols influence the melting of snow and ice in the Himalayas and around the Tibetan Plateau, which ultimately plays a crucial role in climate change .
Aerosol Optical Depth (AOD) is an optical measure of light extinction (absorption and scattering) of the solar beam due to the number of total aerosols in the entire atmospheric column and provides an indirect measure of aerosols in the atmosphere . As aerosol deposition is continually rising due to industrialization and urbanization, high population density, or biomass burning in different regions of Nepal, aerosols studies, particularly AOD, has risen rapidly in recent years . In the past, several works (e.g., Acharya et al. , Bhattarai et al.  Jha et al. , Regmi et al. , Regmi et al. , Chapagain , Carrico et al. ) were executed in Kathmandu, Pokhara and other different sites over Nepal to measure and study of particulate matter such as total suspended particulate matter (TSP), polycyclic aromatic hydrocarbons (PAH), Black Carbon (BC), AOD and some gaseous pollutants.
The COVID-19 pandemic has impacted each thing of human lifestyles and the worldwide economy. Depending on the extent of its impact, the authorities of different nations were adopting distinctive ranges of involvement, such as tour restrictions, shutting down factories, industrial activities, and other social activities to manipulate the unfold of the relatively contagious virus. Several studies, such as Sharma et al. , Tobias et al. , Abdullah et al. , Acharya et al. , Chauhan and Singh , Bao & Zhang , over the different regions of the globe; India, Spain, Malaysia, Europe, USA, China, has claimed that lockdown had contributed significantly in improving air quality due to decrement in emission. Ranjan et al.  aimed to study the effect of the Covid-19 pandemic on aerosol optical depth, concluding a huge reduction in AOD, about 6% to 45 %, over the different regions of India. Acharya et al.  revealed that lockdown reduced AOD by ~ 20% at maximum places in Europe and USA. Raza et al.  reported that AOD was reduced in Pakistan during the lockdown period in 2020 in comparison to the previous years. From the worldwide study of AOD during the pandemic period, Sanap  recorded a decrease in AOD from mid-March to April 2020; however, a drastic reduction in AOD was observed in May. Furthermore, despite many impacts of lockdown on people's social life and the national economy, several findings [e.g., Shrestha et al. , Gautam et al. , Baral & Thapa  observed that lockdown had temporarily better environmental conditions in Nepal.
In this particular paper, we present results on the implications of observed diurnal and monthly averaged variation of aerosol optical depth due to anthropogenic aerosols over the two sites of Nepal by dividing the study into two scenarios, i.e., before the lockdown (1 January 2020–23 March 2020) and during the lockdown (24 March 2020–31 May 2020). We emphasized the variation of anthropogenic aerosol pollution in the most polluted and the least polluted regions. This work is also based on the observational findings utilizing Total Precipitable Water (Tpw) and visibility correlation with their respective AOD wavelength during the study period. This paper is organized as follows: in Sect. 2, we describe the methodology, in Sect. 3, we discuss the results, and finally, we conclude our study in Sect. 4.