BIBECHANA Vol. 20, No. 1, April 2023, 1–9 ISSN 2091-0762 (Print), 2382-5340 (Online) Journal homepage: http://nepjol.info/index.php/BIBECHANA Publisher:Dept. of Phys., Mahendra Morang A. M. Campus (Tribhuvan University)Biratnagar Study of aerosol optical properties in Lumbini, Nepal Santosh Sapkota1, Sabin Gautam2, Santosh Pokhrel1 Aayush Gautam3, Kailash Basnet1 , Roshan Kumar Mishra4, Jeevan Regmi1,∗ 1Prithvi Narayan Campus, Tribhuvan University 2Pokhara Astronomical Society 3Birendra Multiple Campus, Tribhuvan University 4University of Maryland, Baltimore County ∗Corresponding author. Email: jsregmi28@gmail.com Abstract The mixture of different sized particles (fine and coarse) with air composition forms aerosols. Increased economic activities, vehicles, and rapid urbanization made Lumbini one of the heav- ily polluted regions in Nepal. Data are extracted from AERONET websites between 2013 to 2019 with standard deviation. We are mainly focused on understanding variations in aerosol optical properties: aerosol optical depth (AOD), angstrom parameter (α and β), visibility, single-scattering albedo (SSA), refractive index (real and imaginary), and asymmetry param- eter (AP) in the Lumbini region. The maximum value of AOD (675nm) in Lumbini occurred mostly during post-monsoon season (0.61 ± 0.38) whereas; the values of AOD were found to be lower during the monsoon season (0.18 ± 0.12). Most of the AOD values except monsoon season are found to be greater than 0.4, indicating a higher level of pollution in the study area. There is a weak correlation between precipitable water and AOD, maximum correlation (0.04) occurs at the lowest value of AOD at 440nm while the minimum (0.01) at the highest value of AOD at 1020nm. The turbidity coefficient (β) has an adverse effect on visibility. The aver- age Visibility over Lumbini was found to be 8.76 ± 4.95 km during the period of 2013-2019. Single-scattering albedo (SSA) accretions occur at wavelengths between 440 and 675 nm, but the pattern changes from 675 nm to 1020 nm. All parameters were found to be distinct and seasonal fluctuations among this station are mainly due to the different aerosols availability such as biomass burning, mixed aerosols, and anthropogenic aerosols over the Lumbini site. Keywords Aerosol optical depth, Angstrom exponent , Lumbini. Article information Manuscript received: October 09, 2022; Accepted: March 18, 2023 DOI https://doi.org/10.3126/bibechana.v20i1.48825 This work is licensed under the Creative Commons CC BY-NC License. https://creativecommons. org/licenses/by-nc/4.0/ 1 Introduction Aerosols are atmospheric molecules that are ei- ther produced by anthropogenic factors (fossil fuel, burning, biomass, transportation, etc.) or by nat- ural factors (volcanic, sea salt, desert, etc.). It has a variety of potential effects on our planet’s radia- tion balance. Aerosols can absorb and scatter solar 1 http://nepjol.info/index.php/BIBECHANA jsregmi28@gmail.com https://doi.org/10.3126/bibechana.v20i1.48825 https://creativecommons.org/licenses/by-nc/4.0/ https://creativecommons.org/licenses/by-nc/4.0/ Santosh Sapkota et al./ BIBECHANA 20 (2023) 1-9 2 energy, which has an impact on cloud formation by changing the microphysics and lifetime of the clouds [1]. Though a lot of information about our climate system is known, there are more parameters and associated factors that make a deep understand- ing of the climate system uncertain [2]. Aerosols in the atmosphere are responsible for changing the climate system, reducing visibility and crop produc- tivity, and harming human health [3]. The size and structure of aerosols vary from location to location and are influenced by pollution levels that make their optical and microphysical properties compli- cated [4]. Studies on aerosols conducted in the past have also shown that aerosols produced by human activity can change regional temperature, the hy- drological cycle, clouds, and precipitation [5]. The main difficulty in the current climatic study is to quantify regional-scale variations in aerosol concen- tration, composition, and size with high enough temporal and geographical resolution [6], which ne- cessitates methods that integrate observations with atmospheric models. The combination of anthropogenic sources, and biomass burning over the IGP region, which also af- fects the air quality in Lumbini, is the main source of aerosol loading [7], which results in the creation of atmospheric black carbon (ABC) [8–10]. Thus, emitted particles get trapped under conducive me- teorological conditions which results in the air pol- lution of the region [11]. Several parameters, including Aerosol Optical Depth (AOD), Angstrom Exponent (α), Turbidity Coefficient (β), Precipitable Water (PW), Single Scattering Albedo (SSA), visibility and Asymme- try Parameter (AP) has been examined in this ar- ticle. SSA represents the scattering ability of the aerosol; higher SSA values denote a more scattering aerosol, while lower SSA values indicate absorbing aerosol. By analyzing SSA values, we can predict whether the aerosols have cooling or warming ef- fect [12]. For purely scattering aerosol, the value of SSA is 1 and for pure absorbing particles, it is zero. The Value of the SSA varies with the aerosol types also. For example; the value of SSA for biomass burning ranges from 0.88-0.94, for desert dust it ranges from 0.92-0.99 and it is 0.89-0.98 for urban industrial aerosol [13]. The real and imaginary parts, which make up the refractive index, are adversely influence by chemical composition [14]. The refractive index de- scribes how atmospheric particles scatter and ab- sorb light [15]. The real part’s higher value indi- cates the domination of scattering type of aerosol particles, whereas the imaginary part’s higher value shows the domination of absorbing type of aerosol particles. Various kinds of aerosols, the refractive index’s real value has a range of possible values. The value of the imaginary part varies from 0.003-0.014 for urban industrial aerosols, 0.00093-0.021 for biomass burning, and 0.007-0.0029 for desert dust, and the value of real part ranges from 1.40-1.47 for urban industrial aerosols, 1.47-1.52 for biomass burning, and 1.36-1.56 for desert dust [16]. The Asymmetry Parameter (ASY), which is af- fected by particle size and composition, has a value between -1 (backscattered) and 1 (forward scat- tered) [17]. The dependence of aerosol loading on a particular size is explained by the relationship of AOD and Angstrom exponents. The Turbid- ity Coefficient (β) reflects aerosol loading, whereas the Angstrom exponent (α) denotes aerosol size dis- tribution. The variation in the size of aerosol has guided by its source. The aerosol optical depth (AOD) is an extinc- tion coefficient of aerosols in the vertical direction, and it is used to describe turbidity, estimate the aerosol content of the atmosphere, evaluate the de- gree of atmospheric environmental pollution, and can be measured to assess the aerosol load on the atmosphere. Moreover, the amount of water that comes back to the earth’s surface in the form of precipitation when the cloud condenses formed by evaporation from the surface of the earth is known as precipitable water. It is the main component of the greenhouse because it is capable of absorb- ing and reemitting infrared radiation. This work mainly focuses on understanding the aerosol opti- cal properties over Lumbini on monthly variation and also describes the seasonal variation. 2 Data set and Methodology This research is focused on the various aerosol prop- erties over Lumbini, Nepal (27.67N, 83.50E, 150m elevation). Lumbini is known as the birthplace of the Buddha, which is located within the Nepal terai region, called the land of the subtropical chain of forests marshes, and grasses. Lumbini is one of Asia’s most polluted regions in the Terai region [18]. The major sources of air pollution are gases pro- duced by different industries, brick-kiln, fire, and smoke. Besides this, Lumbini is bordered by India and lies in the IGP region, so the transportation of various types of air pollutants from the northern part of India leads to an increase in the air pollu- tion over Lumbini [19]. Since Lumbini represents the characteristics of the Terai region of Nepal bor- dered to India especially, IGP region and a histor- ical Pilgrimage site of Nepal, we are motivated to study the various aspects of aerosol optical proper- ties. Aeronet Robotics network (AERONET) (aeronet.gsfc.nasa.gov) has installed the Cimel Sun- Photometer in Lumbini International Research In- stitute (LIRI). We have used AERONET level 2, Santosh Sapkota et al./ BIBECHANA 20 (2023) 1-9 3 version 3 data for our analysis which are quality assured data of Lumbini site over the period of 7 years from January 2013 to December 2019 [20]. There are eight distinct ways to measure AOD using a solar photometer, with wavelengths ranging from 340 nm to 1020nm [21]. The relationship be- tween AOD and angstrom exponent (α) is given by the power-law equation [22] τ = βλ−α (1) The wavelength in microns associated with AOD value is denoted by λ . Angstrom’s turbidity coeffi- cient is denoted by β. Equation (1) can be written in the logarithmic format as: ln τ(λ) = lnβ − α lnλ (2) Ångström parameters ( α and β) were obtained from equation (2). From the spectral AOD (λ), the angstrom exponent (AE) α can be further defined as: α = −d ln τAOD(λ) d lnλ = ln τAODλ2 τAODλ1 ln λ2 λ1 (3) The visibility (V) of the air in km is given by the following formula [23]. β = 0.5α ( 3.912 V − 0.01162 ) (0.02472V (V − 5) + 1.132) (4) where V denotes the visibility in the atmosphere which is inversely proportional to extinction coeffi- cient, [24]. The ratio of scattering coefficient to the extinc- tion coefficient gives the single scattering albedo (SSA), can be expressed as: ω = σscatt σ (5) The asymmetry parameter can be calculated as, g = 1 2 ∫ π 0 cos θ′P (θ), sin θ,dθ (6) where,θ angle between incident and scattering di- rection. P (θ) represents the angular distribution of scattering light. Figure 1: Map showing the geographical location of Lumbini, Nepal. 3 Results and Discussion To acquire a qualitative knowledge of atmospheric aerosols and aerosol size, the AOD and Angstrom exponent (α) are often used. The parameter with low and high values indicates coarse and fine par- ticle dominance, respectively. Figure 2 shows the monthly average aerosol optical properties, namely, AOD, Angstrom exponent (AE) (440-870nm), and PW from January to December at Lumbini with standard deviations from 2013 to 2019.The monthly average is obtained by averaging daily mean data over seven years. The highest AOD (675nm) was found in the monsoon season in June (0.61 ± 0.38). Due to fog, haze, and clouds, the AOD is higher during the win- ter and post-monsoon seasons [25]. In contrast to the pre-monsoon and summer seasons, it was de- termined that the angstrom exponent (α) was at its maximum during the post-monsoon and winter sea- sons due to the influence of anthropogenic sources. The value of α was found to be greater than 1.0 in all months of the year, representing the domi- nance of fine mode aerosols due to combustion re- lated anthropogenic sources. The lowest AOD was found in the monsoon season in July (0.21± 0.14) due to the increment in the level of precipitable water that helps to wash out the aerosols in the atmosphere. The amount of precipitable water was shown to rise to start in January (1.23 ± 0.23), reach its peak in July (5.91 ± 0.36), and then gradually decline. In the Lumbini site, the monsoon represents the rainy season and is responsible for 75 % of the annual precipitable water. The variation of AOD, α, and precipitable water was found to be similar to [26], which agrees with our results. The short wavelength dominates the fine-mode Santosh Sapkota et al./ BIBECHANA 20 (2023) 1-9 4 Figure 2: AERONET station Lumbini monthly averages of AOD (440nm), Angstrom exponent (440- 870nm), and Precipitable water. Figure 3: Monthly average AOD observed between January 2013 and December 2019 using AERONET sun/sky radiometers at 1020, 870, and 440 nm. particles at wavelengths below 870 nm, whereas the long wavelength dominates the coarse-mode parti- cles at wavelengths over 870 nm. AODs first show considerable seasonal and wavelength dependence. As wavelength rises, AODs eventually decrease, as shown in Fig. 3. The abundance of smaller size par- ticles is indicated by higher AODs at lower wave- lengths. AODs with wavelengths of (1020 nm, 870 nm, and 440 nm) and precipitable water (cm) at the Lumbini station have very weak correlation coeffi- cients of 0.01, 0.02, and 0.04 correspondingly, indi- cating the weak correlation represented in Fig. 4. The fact that there is a weak correlation between AOD’s of various wavelengths and precipitable wa- ter suggests the amount of aerosol or other particu- late matter in the atmosphere decreases as precip- itable water increases. The correlation coefficient was found to be higher at the lowest wavelength 440 nm with perceptible water in comparison with 870 and 1020nm. The linear regression functions of the daily AOD concentration (y) with the percep- tible water(x) were y = −0.01x + 0.34(R2 = 0.01) at 1020nm y = −0.02x+ 0.43(R2 = 0.02) at 870nm, and y = −0.06x+ 0.99(R2 = 0.04) at 440nm. The Turbidity Coefficient (β) is strongly linked with visibility impairment. A measure of the quan- tity of aerosol loading in the atmosphere is the tur- bidity coefficient. The relationship between visi- bility and the turbidity coefficient (α) is inverse (Fig.5), indicating that visibility is affected by aerosol loading to some extent. The average visi- bility for the given period of time was found to be 8.76 ± 4.95 km which is significantly low and in- dicates Lumbini having a hazy atmosphere most of the time. But few monsoon days are fairly clean with the highest visibility in July (20 km) and Au- gust (19 km). Because more precipitable water washes out the aerosol load in the atmosphere dur- ing the monsoon season, visibility was found to be at its maximum. Winter, post-monsoon, and pre- Santosh Sapkota et al./ BIBECHANA 20 (2023) 1-9 5 Figure 4: Correlation between Precipitable Water and AODs (1020, 870 and 440nm) of Lumbini locations of Nepal. Figure 5: Relation between visibility and Turbidity coefficient (β) over Lumbini from 2013 to 2019. monsoon seasons have the lowest visibility due to Fog, haze, cold waves, and smoke from industries. At the Lumbini station, the turbidity coefficient value was determined to be higher than 0.15, in- dicating a turbid atmosphere at all seasons. Single Scattering albedo (SSA), a metric of light extinction, is another important optical property of aerosols to make sure the aerosol’s scattering and absorption properties, which are strongly depen- dent on wavelength and have significant character- istics over Lumbini. Single scattering albedo (SSA) is calculated using the particle scattering coefficient to total extinction coefficient ratio [27]. The chem- ical nature and size distribution of aerosols have a significant impact on SSA [28]. Pure absorption has a value of zero, while scattering has a value of one [29,30]. The reported SSA estimates for desert dust (0.78-0.94), urban and mixed aerosols (0.83-0.98), and biomass burning (0.92-0.99) [28]. The average SSA shows high wavelength dependence, increasing from 0.90 ± 0.03 at 440nm to 0.92 ± 0.03 at 675nm and dropping to 0.90 ± 0.04 at 1020nm (Fig.6). The monsoon and post-monsoon seasons, respec- tively, have seen the highest and lowest SSA lev- els. The highest SSA value indicates that scattering aerosols predominate, whereas the lowest SSA value indicates that light-absorbing aerosols predominate. Our values are supported by earlier research [29] that indicates the summer and post-monsoon sea- sons experienced the largest and least SSA, respec- tively. Over Lumbini, post-monsoon had the high- est concentration of light-absorbing aerosols com- pared to other seasons [30]. Similar trend of spectral variation of SSA was observed in Pokhara [31]. The real and imaginary parts of the refractive index at the Lumbini location are depicted in Fig. 7. At 440 nm, the average real Santosh Sapkota et al./ BIBECHANA 20 (2023) 1-9 6 Figure 6: Seasonal variation of SSA over Lumbini from 2013 to 2019. Figure 7: Seasonal variation of the aerosol real and imaginary part of the refractive index over Lumbini from 2013 to 2019. part of the refractive index is 1.47 ± 0.04; at 870 nm, it is 1.49 ± 0.04; and at 1020 nm, it is 1.48 ± 0.04. The real part of the refractive index reaches its maximum and minimum values during the post- monsoon and monsoon seasons, respectively, which agrees with the possible range of values as given by [27] for various types of aerosol particles and previous result for the Lumbini region [26]. The hygroscopic growth of aerosol and relative humid- ity are responsible for the decrease in the value of the refractive index (real part) during the monsoon season. Additionally, the average imaginary part of the refractive index declines from 0.01 ± 0.006 at 440 nm to 0.008 ± 0.004 at 675 nm and then re- mains almost constant at 870 nm and 1020 nm. In contrast to the monsoon and pre-monsoon seasons, the post-monsoon and winter seasons were shown to have greater values for the imaginary part of the refractive index. The value of the asymmetry parameter has found to be wavelength dependent. As the wave- length rises, the value of the asymmetry parameter (g) falls (Fig. 8). The average value of the asym- metry parameter (g) decreases from 0.72 ± 0.02 at 440 nm to 0.64 ± 0.03 at 1020nm. The value grad- ually declines from 440 nm to 1020 nm during the winter, post-monsoon, and monsoon seasons while during pre-monsoon the value increases from 870nm to 1020 nm. 4 Conclusion The key aerosol optical properties like AOD, AE, PW, SSA, AP and refractive index over Lumbini site was analyzed from 2013 to 2019 us- ing AERONET data. It revealed some interesting features. The months of December and July were found to have the highest monthly average AOD (675nm). The alpha value was found to be more than 1.0, indicating that fine mode aerosols pre- dominated throughout the duration of a year. 75 % of the water vapour that is observable through- out the year is precipitable during the monsoon season. AOD values are found to be spectral de- pendent. It is observed that AOD value decreases with the increase in wavelength. AODs of short wavelengths can give higher precision in measuring the size of aerosols. This implies that the aerosols’ size is dispersed in the atmosphere. The Precip- itable Water observed higher during the periods when the AOD is also higher indicating the possi- bility of the hygroscopic growth of aerosols and the dust storm from IGP region. The visibility and the turbidity coefficient (β) have an inverse relation- Santosh Sapkota et al./ BIBECHANA 20 (2023) 1-9 7 Figure 8: Seasonal variation of the asymmetry parameter over Lumbini from 2013 to 2019. ship. Due to more precipitable water during sum- mer, the visibility over Lumbini was greater during the period of highest precipitable water. The av- erage SSA value of different years for Lumbini in- creases progressively from 440 nm to 675 nm and shows a different pattern from 675nm to 1020nm. 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Atmosphere, 11(8):874, 2020. https://doi.org/10.1364/AO.55.006199 Introduction Data set and Methodology Results and Discussion Conclusion