Finally, the specific pollutants in the air that are responsible for the discolouration of the white marble of Taj Mahal have been identified. Particulate carbon and fine dust particles that are deposited on the marble are responsible for its browning.
Carbon is of two types — black carbon and light absorbing organic carbon or brown carbon. The results from a study were published a few days ago in the journal Environmental Science & Technology.
Besides studying air samples collected from the area, the authors used marble samples on the building to collect the pollutants. They also undertook computer modelling to study the colour change brought about by reflectance of the particles.
Both organic carbon and dust particles have the ability to preferentially absorb light in the blue region of the spectrum. The absorption of blue light by these pollutants in turn gives the marble surface a brown hue.
“There is one group of organic carbon which absorbs light in the blue region of the spectrum and this is called brown carbon. Discolouration is because of what is happening to reflectance, and reflectance is in turn influenced by these particles,” said Prof. S.N. Tripathi from the Department of Civil Engineering and Centre for Environmental Science and Engineering, Indian Institute of Technology (IIT), Kanpur. He is one of the authors of the paper.
Role of dust particles
The ability of fine dust particles to produce the brown hue is a well known in North India. According to him, it is the presence of haematite in the dust that is responsible for the brown hue.
“If haematite is not present in the dust then the dust would be only scattering in nature,” he said. Haematite is the ingredient that absorbs the blue wavelength of the spectrum.
Though the absorption of blue light by individual dust particles may be smaller than that by brown carbon, the copious amount of dust of two micron size found in the particulate matter makes the overall absorption much higher than that by brown carbon.
The study revealed that particles larger than two micron in size accounted for nearly 70 per cent of the deposited particle surface area. These relatively coarse particles are by default the dust particles.
Pure dust particles per se do not have the ability to stick to surfaces. “But what we see is a potpourri of particles. The organic carbon is very sticky,” Prof. Tripathy said. Unlike the dust particles, carbon particles are in the 100 nanometre to 1 micron size. Burning of biomass like wood and dung, burning of trash and crop residue are the primary sources of brown and black carbon.
On studying the marble samples, the researchers found that black carbon produces a greyish discolouration, while brown carbon and dust produce yellowish-brown hues.
A combination of these two result in darker shades of yellow-brown. The sample targets were in place only for a brief period of two months.
“We found the colour of surrogate marbles matched well with model results. Modelling showed the combined effect of dust and carbon in discolouration of the marble samples,” he said.