Scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX) and X-ray diffraction (XRD) systems were used to demonstrate the overgrowth of soot to fractal like structure and its subsequent coalescence with crystal shaped silicate particles. Sample was obtained from a very clean area of Delhi at a height of 16 m from ground with the help of a five stage cascade impactor in the winters of 2006. Impactor collects particles in five different size ranges (i.e. ≥10.9, 10.9 - 5.4, 5.4 - 1.6, 1.6 - 0.7 and ≤0.7 μm). In the present investigation only the particles collected in the size range 1.6 - 0.7 μm (D50 = 0.980 μm) have been considered. It has clearly been observed that the soot particles tend to grow or rather agglomerate in a fractal like structure. During this process they incorporate other chemically and structurally different particles (crystal silicate in the present investigation) to make multi phase and multi chemical amorphous aggregates. These aggregates are formed during/after its collection on the sampling substrate and may be as many as hundred times more than the expected size interval (D50 or cut off range).
The term “soot” is used for carbonaceous particles that are produced by the combustion of fossil fuels or vegetation, and have characteristic morphology, size and microstructure [
Characterization of soot has been carried out in different parts of world by various techniques. Some of the important studies are as follows: Samson [
All the above mentioned studies suggest the importance of characterizing the atmospheric soot particle. However, in Indian context no study so far has been done on the characterization of soot. In the present study analysis of the atmospheric soot of Delhi, one of the most polluted cities in world has been carried out by SEMEDX and XRD techniques. In this paper we have tried to demonstrate how the soot particles grow or rather overgrows after its deposition on to the sampling substrate.
The study area JNU is in Delhi. Delhi, the capital city of India and one of the most polluted cities in the world, has a population of over 14 million along with 4.8 millions of registered vehicles, three coal based thermal power plants and 125,000 industrial unites [
A five-stage cascade particulate separator (CPS-105, Kimoto Electric Co. Ltd. Japan) was used for the collection of aerosols. The sampling was done for 24 hours at an average flow rate of 600 L/min. CPS-105 collects particles in five different size ranges viz. ≥10.9, 10.9 - 5.4, 5.4 - 1.6, 1.6 - 0.7 and ≤0.7 μm. In the present study only size range 1.6 - 0.7 μm was considered. Possibility of small error could not be ruled out during the deposition, thus the particles deposited may be considered to vary between 0.5 μm to 2.0 μm.
Samples were collected on Whatman EPM-2000, glass micro fiber filters. Filters were kept in vacuum desiccators for 24 hours to remove any moisture content before mounting them on the air sampler. After the sampling the filter papers were immediately transferred to vacuum desiccators to again de-moisturize them in the same manner.
The sample (dry filter paper) was cut in 1 mm2 size. A very thin film of gold and palladium was deposited on the surface of the sample to make them electrically conductive using vacuum coating unit. This extremely fine coating was done through the evaporation of Au-Pd plate under inert atmosphere (argon environment). The SEMEDX analyses were carried out with the help of a computer controlled field emission SEM (JEOL JSM-6330F, JEOL Ltd., Akishima Tokyo 196-8558 Japan) equipped with a EVEX-EDX detection system, Princeton, Gama Tech Instruments, NJ, 8540, US. In the present investigation, the SEM was used in its most common mode, the emissive mode. In the present investigation the current density was restricted to 5.0 keV to reduce the contribution of silicon substrate and minimize the loss of regrown aggregates.
XRD analysis was carried out using a Philips, X’Pert PRO X-ray diffraction system, PANalytical, Holland, with a curved position sensitive detector (PSD) which gives a resolution of 0.03 mm over a range of 10˚ - 80˚ 2θ using standard techniques [
In order to check the morphology and have an evidence of growth of soot and its association with other chemically different particles the electron micrographs of aerosols in the size range of 0.7 μm to 1.6 μm with EDX-spectrum and XRD pattern are provided in Figures 1-3. Here, it is important to mention that the occurrence of gold (Au) and palladium (Pd) in EDX-spectrum is due to the fact that Au-Pd was used for a very fine coating of all the samples to make them electrically conductive (refer Section 2.2). These two elements (Au-Pd) have not been included in the discussion.
As evident from the EDX spectra (
Now with the help of Figures 1(a) and (b) it can be said that the circles and the square at right hand corner show the crystal silicate particles while bunches of amorphous particles are soot. Bunches of amorphous soot have been generated in many phases viz.
1) Initially fine soot particles have (<1 μm) after depositing on collection substrate coagulated or rather grown in a fractal like structure [16,18,24,25].
2) During the growth process soot kept incorporating other substances [
3) Additional agglomeration during aging (during or after sampling) may also be observed which finally resulted, the aerosol to convert into a big particulate mass comprising particles of different chemistry and structure.
To confirm the above mentioned statement number (2) and (3) rectangular portion of
From Figures 1(a) and 3(a) it is clear that bunches of soot are much larger than 10 μm size, whilst it should strictly be between the size ranges of 0.5 to 2.0 μm. The two white smaller and larger circular spots, in Figures 1(a), 3(a) and (b) represent the max and min size ranges of particles that can be deposited on the filter. This manifests that the soot particles have grown after impacting on sampling substrate. This result is in the close agreement with the results of Wittmaack et al. [
us to underestimate the size of the aerosols and hamper size classification in the instrument. It is pertinent to mention that during sampling rh and temperature varied between 45% - 98.5% and 5.1 - 15.4 respectively. This is why we are getting soot aggregates (Figures 1(a) and 3(a)) having diameter many-many times what is expected cut off size of the impactor. Therefore we can say that this piling up or rather agglomeration of soot keep on taking place even after sampling i.e. during desiccation, storage etc. This has some serious health implications as well, because the same phenomenon can also occur inside human body, fine soot particles can penetrate deep inside our body and afterwards coagulate to coarse particles. Replenishment and flushing out of coarse particles from human body is very difficult than the fine particles. The degree of impairment is escalated when it is incorporated with some hazardous materials. The importance of this study is increased due to the fact that Delhi is highly polluted, having a huge amount of suspended soot and a large number of people (traffic police personnel and road side vendors) are continuously exposed to soot particles.
Characterization of soot particles in the fine size range of 0.5 to 2.0 μm was carried out using the SEM-EDX and XRD techniques. It was found that soot which is ubiquitous in fine size range in Delhi’s ambient air keep growing during sampling by the instrument or even after its collection on substrate. They also agglomerate with other chemically and structurally different particles. Since this growth is also humidity dependent thus during high level of rh soot particles can even grow to 100 times more than the respective expected cut off size of the impactor. Because of the aforesaid characteristics, it can be inferred that soot keeps incorporating other hazardous particles.
This study has been sponsored and funded by the Department of Science and Technology (DST), Government of India, New Delhi in the form of Young Scientist Project (SR/FTP/ES-19/2004) to Arun Srivastava. We deeply acknowledge Prof. Pulickel M. Ajayan, Rensselaer Polytechnic Institute (RPI), Troy, NY for his permission, and Anchal Srivastava and Ray Dove for carrying out SEM-EDX at RPI. Special thanks to S. Venkatesan, SES, JNU for his help during XRD analysis.