3.1 Mass concentration analysis
The PM1 mass concentration has been measured for 24 h during the study period of July 2018 to July 2019 at four different locations of Bengaluru city as mentioned earlier. A total of 18 samplings were made during the study period. In general, it is observed that the mass concentration values were found to vary from 21.27- 68.64 µg m-3. The average mass concentration in winter, summer, monsoon and post monsoon seasons were observed to be 47.62, 40.25, 32.05 and 38.76 µg m-3 respectively. The lowest mass concentration of 20.16 µg m-3 was recorded in the month of June 2019 which is monsoon season and highest mass concentration of 68.64 µg m-3 was recorded in December 2018 which is winter at BAS location. The highest value 68.64 µg m-3 of PM1 mass concentration observed during winter season was consequently higher than PM2.5 National Ambient Air Quality Standard (NAAQS) 24 h standard limit of 60 µg m-3 (data.gov.in) [15]. The observed PM1 mass concentration is being compared with PM2.5 NAAQS 24 h standard limit since PM1 limit value has not been legislated as referred by E Koulouri et al. [16].
Washout of atmospheric particles due to rain during monsoon season might be the reason for observed lower mass concentration. Highest mass concentration observed in winter may be due to the temperature inversion in ambient air. It is to be noted that the mass concentration values are having dependency on environment sampling location.
The dependence of PM1 mass concentration with temperature, Relative Humidity (RH) and wind speed is shown in Fig. 2. The mass concentration with temperature, wind speed and RH showed a negative correlation, the extent of correlation is listed in Table 1a. The negative correlation between mass concentration and temperature is because of ventilation of particulates outside the atmosphere due to breaking of aerosol layer with increase in temperature as a result of sunrise Fig. 2a. Also, it has been observed that mass concentration with RH follows almost similar trend on some days and opposite trend on other days of sampling. Increase in mass concentration with RH indicates stacking up of particulates one above the other up to a certain threshold height which results in positive correlation. Once the particle growth exceeds a threshold limit then due to gravity settling all the particles settles down resulting in negative correlation Fig. 2a between mass concentration and RH as studied by GE Shaw et al. [17].
3.2 Morphological analysis of PM1
The PTFE filter paper used for the collection of PM1 samples has been used for the morphological study with the help of Scanning Electron Microscope (SEM). SEM image gives specific source of PM1 whether it is due to primary formation or secondary formation as suggested by Mirjana et al. [18]. Direct evidence of the composition and morphology of aerosol particles can be provided by single particle analysis as suggested by Geng et al. Ro et al. Utsunomiya et al. [19-21]. Particulates with different shapes such as isometric, platelets and fibres exist in the atmosphere and morphological properties often lead to the assumption of particle sphericity for most of the applications. An added complication is that particles with irregular morphology can collapse into a spherical shape upon humidification. As the particulates are smaller in size, they get into the alveolar duct and alveoli in lungs and damages the epithelial cells and morphology may induce significant pro-inflammatory responses in airway epithelial cells as observed by Huang et al. [22]. The morphology relates to chemical composition as well as toxicity if the compounds are organic as identified by Borgie et al. [23].
In the present study, we have also used EDAX technique to identify elemental composition. As suggested by Atar Singh et al. [24] EDAX gives almost accurate elemental composition of PM1. But, sometimes two peaks resulting from two different elements may overlap obscuring each other and making identification difficult. However, there are methods to overcome this limitation. Any unambiguous peak assignment may be impossible. The detection limit of EDAX is typically about 0.2-0.5% as discussed in Seifollah and Shokrollah [25].
3.2.1 Morphological analysis of PM1 at BAS location
A total of five samplings were carried out in BAS location during the study period along with SEM-EDAX analysis in July month. From Fig. 3a it is clear that the particulates are regular (spherical) and irregular in shape with smooth and rough surface area. Elemental composition in PM1 is expressed in terms of weight% which indicates the relative concentration of the element in the sample. The elemental composition is found to be Carbon with 30.72 weight% and Oxygen with 1.21 weight% with traces of Phosphorus.
The morphology of PM1 was carried out in December month. As shown in Fig. 3b the particulates were found to have irregular shape with rough surface area and some of them were biological particles indicating the source of PM1 from natural activities. The elemental composition was found to be Carbon with 46.81 weight%, Nitrogen with 17.88 weight%, Oxygen with 34.25 weight% with traces of Sulphur.
In the month of January two samplings were carried out. Morphological analysis of one sample showed particulates with irregular shape and rough surface area Fig. 4a. The elemental composition was found to be Carbon 50.93 weight%, Nitrogen 13.32 weight%, Oxygen 34.59 weight% with traces of Sulphur and Potassium.
Other sample was used for microorganism analysis as the size of bacteria or virus varies from 100 nm to few micrometre sizes as suggested by Ya-fen Wang et al. [26]. The sample filter paper was placed on LB-Agar solution in an autoclaved plate which acts like a growth medium for bacteria or virus. Same was stored in incubator overnight at a temperature of 37 °C and SEM of the same filter paper was carried out. The morphology of collected sample showed the presence of rod-shaped particles which confirms the presence of bacteria Bacillus Fig. 4b as observed in the work of Christine G et al. [27]. The elemental composition was found to be Carbon 37.30 weight %, Nitrogen 2.04 weight %, Oxygen 7.54 weight % and traces of Phosphorus and Sulphur.
The sampling carried out in the month of February was again used to study the presence of microorganism with SEM image technique. The SEM image of this sample Fig. 4c confirms the presence of microorganism with rod shaped morphology that is bacteria Bacillus. The elemental composition is found to be Carbon 44.29 weight%, Nitrogen 4.51 weight%, Oxygen 11.61 weight%, Sodium 7.52 weight% with traces of Silicon, Phosphorus, Sulphur.
3.2.2 Morphological analysis of PM1 at HOS location
A total of four samplings were carried out at HOS location during the study period and following are the observations:
The SEM image morphology for the location HOS Fig. 5a shows the presence of spherical and irregular shaped particles with smooth and rough surface area. The elemental compositions are as follows Carbon, Oxygen with 29.89 weight % and 1.65 weight% respectively with traces of Phosphorus.
Particulates with different morphologies has been observed for the location HOS during Diwali festival (5-6 November 2018). On 5th November 2018, the particulates were spherical and irregular in shape with smooth and rough surface area Fig.5b. The elemental composition on previous day of Diwali was Carbon 58.64 weight%, Oxygen 28.77 weight%, Nitrogen 11.66 weight% and traces of sulphur. Whereas on 6th November 2018, along with spherical and irregular shape, the particulates are observed to have plate like, rod like structures with smooth and rough surface area Fig. 6a with elemental composition of Carbon 50.30 weight%, Oxygen 41.89 weight%, Nitrogen 5.44 weight% and traces of Aluminium, sulphur and potassium.
The morphology of collected samples at HOS location in December Fig. 6b shows spherical shaped particulates with smooth and rough surface area. Rod shaped particulates were found which may be Bacillus pumilus as observed in the study made by Flavien Pillet et al. [28]. The elemental composition was found to be Carbon 56.17 weight%, Nitrogen 12.09 weight%, Oxygen 30.92 weight% with traces of Potassium and Sulphur.
3.2.3 Morphological analysis of PM1 at DOM location
Seven samplings were carried out in DOM location during the study period. Following are the observations:
The samplings were carried out in December and January months and on both days of sampling, the particulates were found to have irregular shape with rough surface area Fig. 7a -7b. The elemental composition during these two samplings were Carbon 56.85 weight%, Nitrogen 15.31 weight%, Oxygen 26.98 weight% with traces of Sulphur and Potassium and Carbon 52.36 weight%, Nitrogen 19.81 weight%, Oxygen 27 weight% with traces of Sulphur and Potassium respectively.
Two samplings made during the months of March and April were again used for the study of presence of microorganism. The SEM images of these samples Fig. 8a - 8b once again confirms the presence of microorganism with rod shaped morphology that is bacteria Bacillus along with some particles having cubical shape with smooth surface area which may be Sodium Chloride.
The elemental composition is found to be Carbon 36.63 weight%, Nitrogen 2.10 weight%, Oxygen 6.50 weight%, Sodium 5.32 weight% and Chlorine 3.89 weight% and Carbon 47.31 weight%, Oxygen 25.05 weight%, Nitrogen 10.32 weight%, Sodium 3.12 weight% and traces of Silicon, Phosphorus, Sulphur, Chlorine and Calcium respectively.
The morphology of sampling collected in the month of May showed the presence of particulates in ambient air with spherical & irregular shape having rough and smooth surface area Fig. 8c. The elemental composition was found to be Carbon 31.31 weight%, Oxygen 2.62 weight% with traces of Sulphur.
The morphology of sampling made during the months of May and June shows that the particulates are regular and irregular in shape with smooth and rough surface area Fig. 8d-9a and elemental composition is found to be Carbon 29.37 weight%, Oxygen 2.68 weight% with traces of Sulphur and Carbon 33.91 weight%, Oxygen 3.63 weight% and traces of Nitrogen, Sodium and Sulphur respectively.
3.2.4 Morphological analysis of PM1 at DCH location
A total of two samples were carried out in the study period for this location and following are the observation:
The morphology of sampling carried out in the month of June and July shows that the particulates are regular and irregular in shape with smooth and rough surface area Fig. 9b-9c. The elemental composition is found to be Carbon 29.61 weight%, Oxygen 1.06 weight% and traces of Aluminium and Sulphur and Carbon 29.33 weight%, Oxygen 1.50 weight% with traces of Sulphur.
The morphology of particulate matter during the study period is found to be spherical in shape with smooth surface area which shows that the source of PM1 in ambient air is mainly from anthropogenic activities such as vehicular traffic, industrial activities, bio mass burning. The spherical nature of PM1 indicates that the source is from vehicular emission and industrial activities with carbon dioxide as the major pollutant. However, particulates having irregular shape with rough surface area identify the source of PM1 as due to dust particles and resuspension of road dust. The EDAX analysis made for all the samples observed a following standard deviation in the elemental composition of Carbon, Nitrogen and Oxygen is 10.77 weight%, 12.02 weight% and 12.77 weight% respectively.