Chemical Composition and Source Characterization of Hailstones in Dhaka, Bangladesh

A comprehensive analysis on the chemical composition and source apportionment of hailstone samples were conducted in Dhaka, Bangladesh. pH, electrical conductivity (EC), total dissolved solids (TDS), water soluble ions (Na, K, Ca, Mg, Cl, 4 SO − , 3 NO − , 3 HCO − ) and trace metals (Zn, Fe, Cu, Mn) of hailstone were determined. The result revealed that the average pH, EC, TDS were 6.95 ± 0.54, 356.3 ± 150.6 μS∙cm and 17.5 ± 2.89 mg∙L, respectively. The water soluble ions followed the order: Ca > Cl > 4 SO − > 3 HCO − > Na > Mg > K > 3 NO − . The concentrations of trace metals ranged in order with Zn > Fe > Cu, while the concentration of Mn was below detection limit. Sodium adsorption ratio (SAR) was 0.20 ± 0.09 meqL which indicates it is benign to plants and safe for irrigation. The order of neutralization factor (calculated with average concentrations) found in hailstone was NFCa(1.16) > NFMg (0.36) > NFK(0.32) which were originated from earth crust. Notable correlation was found in between soil tracers Ca and Mg (r = 0.87), indicating their common source dust. Enrichment factor analysis revealed that Ca, Mg and K are mainly from crust, whereas 3 NO − and 4 SO − are mainly attributable to anthropogenic origins. Further source contribution analysis revealed that anthropogenic actions accounted for 99.2% of total NO3 and 89.6% of total 4 SO − , while 99.2% of total Ca and 95% Mg were from crustal source.

heterogeneous nucleation mechanisms at a below average atmospheric temperature [1]. According to radar observations, in large areas of Switzerland hail takes place on average once per convective season per km 2 [2]. But the internal Alpine valleys show an exceptional appearance [2]. Radar-based hail detection is beneficial because of the near-continuous exploration of the region, the near-real-time accessibility of data, and good parallel spatial conclusion (at least near the radar station) [3]. The signal of hail movements can vary greatly in adjacent areas [4] and their intensity depends on the reviewed time period [5]. In Europe, usually large hail is detected in environments characterized by high border layer moisture, a high raising condensation level and vertical lapse rates [6], and by mixture of high-CAPE (convective available potential energy) moderate-shear or moderate-CAPE high-shear atmosphere [7]. Formation of hailstorms depends on three main ingredients: an inconsistent atmosphere, lower tropospheric humidity and a lifting process, i.e. Thunderstorm trigger [8]. Relative significance of these components depends on regional elements, such as terrain barriers, thermotopographic wind systems, or warm water surfaces [9]. Severe hailstorms regularly cause damage to buildings, automobiles, crops in a large extent and resulting in huge economic and assured losses [10]. Determining the composition of hailstone may provide valuable documentation on the relative contribution of different origins of pollutants to hailstone chemistry and enhance comprehension of the local and regional diffusion of pollutants. But knowledge on chemical composition and their source apportionment in hail is still limited. Previous experiments with hail in developed nations (EU, USA, Canada) have given information only about climatology, size distribution, and suppression of hailstones. Santoyo et al., 2002 provide information only about the concentration of anions found in hailstones [1]. There is also lack of research on how atmospheric pollution control the chemistry of hail. Our recent study has revealed that polluted air mass can cause enrichment of anthropogenic species in dew water and fog water at Indo-gangetic plain (IGP), which comprise of India, Pakistan, Nepal and Bangladesh [11] [16] [32]. So we decided to study the physiochemical properties and sources of chemical species present in hailstones at one of the major city of IGP (Dhaka, Bangladesh).
In the present work, we have collected hailstone samples on February, 2016 in Dhaka, capital of Bangladesh. We have studied different physical properties (pH, electrical conductivity and total dissolved solid) and measure the concentration of water soluble ions and trace metals. The sources of chemical species present in hailstones were characterized by enrichment factor, correlation co-efficient and percent source contribution calculation. Journal of Geoscience and Environment Protection lated city in the world accompanied by 15 million people [12]. The environment of Dhaka can be contaminated from particular types of regional or long-range anthropogenic sources [13]. It has experienced with unrestrained growth and few managing restraints, resulting in vigorous traffic blockage with a mix of transport medium such as cars, buses, trucks, bicycle rickshaws, bay-taxis, and all struggles for coexistence on the roadways. Due to the abundance and mismanagement of emission sources, atmospheric condition of Dhaka is getting worse day by day.

Meteorology of Dhaka, Bangladesh
The climate of Bangladesh is described by high temperature, excessive humidity, and clearly noticeable seasonal dissimilarities in precipitation [14]. The weather of Bangladesh is divided into four seasons: pre-monsoon (March-May), monsoon (June-September), post-monsoon (October-November), and winter (December-February) [14]. In this study, hailstones were collected during the winter season.

Sampling Procedure and Handling
A severe hailstorm occurred in 24th February, 2016 at the Dhaka University area. Dhaka University has an urban campus and located in the southern part of Dhaka metropolitan area. It is one of the busiest spots of Dhaka city and congested with traffic, shopping outlet, street food shop and restaurant. We put aluminum foil on five different locations around the campus. As soon as hailstones dropped on the aluminum foil, they were collected using gloves. Hailstones samples of four representative events were collected at four different locations in the University of Dhaka ( Figure 1). Hailstones were rinsed with de-ionized water before storing them in clean PET (Polyethylene terephthalate) bottles. The samples were immediately preserved in refrigeration to avoid reduction of substance by vaporization and also to prevent pollution.

Analytical Procedure
The hailstone samples were first divided into two parts. One part was used for measuring pH, EC and TDS. The other part of each sample was used to determine concentration of water soluble ions and trace metals. pH, EC and TDS were measured by a pH meter (pH211, Hanna Instruments), conductivity meter (CM-5S, DKK-TOA Corporation) and TDS meter (hold), respectively. Another part of the sample was divided into two for trace metals and water soluble ions analysis. One part of the sample was mixed with 0.2% nitric acid (1% V/V) and kept alone. Concentration of water soluble cations (Na + , K + , Ca 2+ , Mg 2+ ) and trace metals (Fe, Zn, Cu, Mn) were measured by flame atomic absorption spectrophotometer (model: A Analyst 800, Perkin Elmer). Concentrations of water-soluble anions (Cl − , 2 4 SO − , 3 NO − ) were determined by ion chromatography (model: 881 compact, IC pro, Metrohm, Germany). Sample blank was prepared using the chemicals used for analysis (without sample) and analyzed using the same procedure. Concentration of different ions from blanks was subtracted from the sample to ignore contamination error in the measurement. Concentration of 3 HCO − was evaluated as follows from pH [11]:

pH, EC and TDS
The melted hail samples were examined for EC, pH and TDS and the results were summarized in table 1. pH values ranged from 6.16 to 7.08 with an average value of 6.95 ± 0.54, which is almost neutral. It must be highlighted that the acidity of hailstone was mainly neutralized by soil derived basic compound. The average value of EC was 70.5 ± 65.9 µS•cm −1 varied from 155 µS•cm −1 to 37 µS•cm −1 . Average TDS value was 17.5 ± 2.89 mg•L −1 varied from 15 mg•L −1 to 20 mg•L −1 . We compared our measured EC, pH and TDS with rain, fog and dew water in Table 1. The average pH was found higher than rain and dew water. Average EC and TDS were found lower than fog and dew water.

Chemical Composition
The average concentration of cations (Ca 2+ , Na + , Mg 2+ , K + ), anions (Cl − , HCO − ) and trace metals (Zn, Fe, Cu, Mn) for hailstone in Dhaka and comparison with rain, fog and dew water in Bangladesh were summarized in concentration in hailstone samples were higher than rain water but much lower than fog and dew water.

Sodium Adsorption Ratio (SAR)
The SAR value is noted as a water quality parameter for irrigation. From the following equation SAR is calculated: SAR Na Ca Mg 2 Here, Ca 2+ , Na + and Mg 2+ were calculated in meq•L −1 of calcium, sodium and magnesium ions, respectively [18]. FAO (1985) suggests that water having SAR value less than 3.0 is acceptable for irrigation [19].

Neutralization Factor
By determining the neutralization factor, contribution of basic substances to the neutralization procedure is determined, via the equation [22]: [ ] Here, neutralization factor of X is to be determined. The order of neutraliza-

Enrichment Factors
The enrichment factors (EFs) for individual ions were used to identify their origin in hailstone as general crust, sea salt and anthropogenic source. Na is considered as a reference element for marine sources. Ca is used as reference element for soil origin [23]. For continental crust, Fe, Al and Ca are standard lithophilic elements used as reference elements [21]. EF seawater and EF crust of major cations and anions were determined by the following calculation, taking Na and Ca as reference elements [24]: Here, X is the concentration of individual element. (X/Na) sea is the ratio of elemental concentration to the Na concentration from seawater composition [25], and (X/Ca) crust is the ratio of the elemental concentration to the Ca concentration from crustal composition [26]. An EF value much less than or much higher than 1 is considered to be diluted or enriched respectively relative to the reference source [27].
The EF of major ions of hailstone samples collected at Dhaka is shown in Table 2. Cl − has an average EF crust value of 0.48 ± 0.18 indicating its anthropogenic source. Further, average EF crust and EF seawater value of K + and Mg + suggested it crustal origin with a small amount of marine origin. The smaller EF crust and EFseawater value of K + indicated primarily crustal source, with preferential contribution from sea sources [24]. 3 NO − is enriched comparable to crust source (average EF = 167.9 ± 127.3) and sea source (average EF = 44000 ± 31777.9) which suggested the contribution of 3 NO − from mainly anthropogenic source. Relatively small average EF seawater value of 2 4 SO − indicates a small portion of 2 4 SO − was from marine source. Again, relatively high EF crust value of 2 4 SO − suggests that the main sources of 2 4 SO − are anthropogenic activities.

Correlation Coefficient Analysis among Hailstone Chemical Species
Correlation analysis was done with r values < 0.01 to investigate the association and likely common sources between the chemical species in hailstone samples [28]. It can be inferred that the correlation coefficient data set is statically small but still give useful information for the common sources of these elements (Table 3). Hence a strong correlation was found between soil tracers Ca 2+ and Mg 2+ (r = 0.87), suggesting a common source of these elements which was soil dust [29]. Further, correlation between (Mg 2+ , 3 NO − ) and (Ca 2+ , Cl − ) were 0.81 and 0.67, respectively was observed probably due to the interaction between the acids (HNO 3 and HCl) with basic compounds (Ca 2+ and Mg 2+ ) as they move to upper atmosphere from soil by air [30]. Similarly, a strong correlation was found SO − originated from marine source [30]. Fe and Zn are strongly correlated (r = 0.99) with each other meaning they are coming from the same origin. Significant inconsistent and irregular correlation suggested that these elements were originated from different distinct local sources.

Source Contributions for Different Ionic Species
The marine, crustal and anthropogenic sources of ionic components in hailstone samples were calculated by using the following equation [30]: %SSF 100 X Na X Na = Here, X is the concentration of individual element. SSF, CF and AF represents the sea salt fraction, crustal fraction and anthropogenic fraction, respectively (Table 4).  Crustal fraction dominated for Ca 2+ (99.2%) and Mg 2+ (95.0%) in hailstone. Similar observations have been made of calcium and magnesium in dew water sample in a previous study in Bangladesh [11]. Ca 2+ and Mg 2+ could be obtained from the dissolution of both primary and secondary minerals (CaCO 3 , Ca-CO 3 ·MgCO 3 , and CaSO 4 •2H 2 O) commonly present in aerosol particles [31]. Coarse fraction of potassium can be coming from soil but fine particles are due to biomass burning, coal combustion and wood burning [21]. For this reason K + was considered as terrestrial source because it is hard to distinguish between soil and anthropogenic fraction in hailstone samples [22]. Cl − mainly originated from marine (54.2%) and a portion from anthropogenic sources (45.4%). About 99.23% and 90.26%

Conclusion
Ionic and some elemental composition of Hailstone samples in Dhaka city were analyzed in the present study. The average value of pH was 6.95, which was almost neutral. Average TDS value was 17.5 mg•L −1 revealed that hailstone contained very less dissolved solids. Average sodium adsorption ratio (SAR) was 0.20 ± 0.09 meq•L −1 , indicates that it is suitable for irrigation. Ca 2+ had the highest concentrations (163.42 ± 81.9 µeq•L −1 ) which acted as a significant neutralization factor in hailstone samples. Enrichment factor analysis revealed that Cl − , 3 NO − , 2 4 SO − were mainly from anthropogenic source and Ca 2+ , Mg 2+ and K + were from Crustal source. Source contribution analysis further confirmed that anthropogenic activities are the dominant source of 3 NO − and 2 4 SO − , while Ca 2+ and Mg 2+ were from crustal source.