Author: G V Pavan Kumar

As a scientist, professor, writer & podcaster of physics & its history & philosophy......I am interested in the historical and philosophical evolution of ideas and tools in the physical sciences and technology. I research the intellectual history of past scientists, innovators, and people driven by curiosity, and I write about them from an Indian and Asian perspective. My motivation is to humanize science.

Polluted air…..why do we care ?

Delhi
Delhi airport : A peek from the widow of the flight at around 9.30 in the morning

If you are an Indian researcher, you cannot escape a visit to Delhi. For the last few years, I have been visiting Delhi for various research related reasons: conference, grant meeting etc. A few days ago I had an opportunity to visit Delhi for a half-a-day meeting.

For me, Delhi embodies a rich feeling of delicious north Indian (street) food, extreme temperatures (by Indian standards), loud taxi music, an assorted flavor of Hindi dialects, and of course, national politics. Of late, Delhi also has gained a lot of attention in another matter: pollution. In winters, for several years now, smog (smoke + fog) has been a major problem, and has drastically perturbed the lives of Delhi citizens. This problem is not confined to Delhi. Various parts of India are not doing great either.

Anyway, as soon I landed in Delhi, it was foggy (see picture), and the visibility was poor. Clearly, there was something in the air, and it was not pleasant.  I wondered about all the kids who travel to school in such an air, and the possible effects on their health. There were several questions running through my mind:  Why is the polluted air the way it is ? How does one quantify pollution? What are the effective methods to detect pollution, and how can it be contained effectively? I knew some scientific aspects of air pollution, but I was curious about how at all air quality was measured and quantified. Below are some facts related to pollution and some interesting connection to light scattering.

There are several reasons for air pollution. In India, some of the major reasons include crop and biomass burning, emission from automobiles and industries, dust etc. There are mainly 8 kinds of pollutants: PM10, PM2.5, NO2, SO2, CO, O3, NH3, and Pb. A majority of the problems are caused by the so-called particulate matter. These tiny objects which can cause severe harm to human beings can be mainly classified as PM10 and PM2.5, where PM stands for particulate matter and the numbers 10 and 2.5 represents the size of such particles in microns. To give you a comparison, the width of our hair is approximately 100 microns in thickness. So imagine a particle which is thinner than your hair entering your respiratory system. This inhalation causes severe trouble to your lungs and the worst part is that it can cause irreversible damage to the inner walls of your respiratory tracts. Even more disturbing fact is that smaller the size of the particle deeper is the penetration in to our system, and greater harm it does to human well-being.

So, how to detect these small particulate matter? There are several ways to detect these tiny objects of which I found two methods to be interesting and effective.

First one is based on light scattering. Generally, the instrument used to monitor air quality using light scattering is called as nephlometer (In Greek nephos means cloud). This is a powerful and compact instrument that can continuously detect and monitor density of particulate matter. The measurement is based on Mie scattering (named after Gustav Mie, more about him in future), where the size of the scatterer is generally comparable to the wavelength of light. It assumes that the scattering particles are spherical in nature and isotropic in composition. It works on the basic principle that when you shine light through smog (at the ground level), the intensity of the scattered light carries characteristic signature of the size of the particle and its concentration. More specifically, the intensity of the scattered light depends on two important ratios. One is the ratio of particle size to wavelength of light and second is the ratio of refractive index of the particle to its surrounding medium. By calibrating the instrument for known particles and concentrations, the unknown size and concentration of the pollutant can be determined. (If you are interested to learn more, see this old research paper). As mentioned earlier, the measurement assumes the scatterer to be spherical and isotropic, which is not the usual case in the air. So corrections due to variation in shape and compositions have to be taken into consideration in this measurement. However, one of the major advantages of this measurement is that it is quick and portable, and hence a lot of air quality measurements are based on these instruments.

Alternatively, if one needs very accurate measurement of particle size, the instrument to use is Tapered Element oscillating microbalance (TEOM).  In this a tiny piece of tapered glass acts like a tuning fork. This tuning fork vibrates at a specific frequency which can be measured with reasonable accuracy. As one may guess, if something is moving, the speed of movement can be affected by adding weight on the moving object. In this case, the vibrating piece changes its frequency as soon as a small particle is in contact with it. The difference in the frequency is now related to the mass of the particle. Thus by using simple physics, one can obtain a powerful instrument to monitor air quality. Apart from the above-mentioned methods, there are various approaches to monitor air-pollution. Each of them have pros and cons, and are utilized depending on the situation.

Coming back to my Delhi trip, I finished my work, and headed towards the airport in a taxi. I casually asked the driver whether he was worried about the pollution in his city. He did mention that it was a concern, but after a brief pause he grinned and said – “odd-even phir sae shooru ho ra hain, business badega” (odd-even is starting gain, business will go up (note: odd-even was eventually stalled this time)). I grinned back at the driver, and remembered a quote of Charles Kettering : “The only difference between a problem and a solution is that people understand the solution”.

Colourful Sky in Leonardo’s Eye

There are very few people in human history who have combined arts and science like Leonardo da Vinici. He was a polymath: a great painter, inventor, sculptor, scientist and as you will see – a keen observer of nature, and many more. One of the great aspects of Leonardo is that he recorded his observation as texts, which gives us a deep and direct insight into his thinking.

All of us have been captured by the beauty of sun lit sky. It has made us gaze and wonder about its colour. Of course, it has inspired a countless number of artist and scientist to ask the question : what is the origin of colours of a sun-lit sky ? Leonardo himself was fascinated by this question, and led him to view this question both as a painter and as a scientist. Thanks to the great work of J.P. Richter, who has translated the Literary works of Lenardo da Vinici into english (available free online), we obtain a direct peek into the mind of Leonardo which is an everlasting treasure trove: more you dig more you get.

 

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The title page of the translated book

In the collected works, what has caught the attention of scientists is a chapter titled “Aerial Perspective”. In there, Leonardo is trying to converse with his fellow painters on how to create perspective in paintings.  While doing so, he makes some vital observation and proposes hypotheses, and further discusses about some experiments to test them. Leonardo was a keen observer. His approach to art was heavily influenced by an analytical way of looking at the problem at hand. In his writings, he appeals to painters to pay close attention to angles and perspectives in the geometry. In order to attain precision he gives elaborate explanation based on his observation. Below is an example where he explains how to represent the atmosphere in paintings.

“Why the atmosphere must be represented as paler towards the lower portion? Because the atmosphere is dense near the earth, and the higher it is the rarer it becomes. When the sun is in the East if you look towards the West and a little way to the South and North, you will see that this dense atmosphere receives more light from the sun than the rarer; because the rays meet with greater resistance.”

 

It is remarkable how his efforts to create a painting inspired him to go deeper and hypothesize a physical phenomenon. Below sentences reveals a connection he makes between colour of the sky and the presence of “insensible atoms”.

“ I say that the blueness we see in the atmosphere is not intrinsic colour, but is caused by warm vapour evaporated in minute and insensible atoms on which the solar rays fall, rendering them luminous against the infinite darkness of the fiery sphere which lies beyond and includes it”

Although, now we know that light scattering from molecules (not atoms) as the reason for colourful sky, we need to really appreciate Leonardo’s quantum leap of thought. Remember, his texts are dated around late 1400s or early 1500 AD, where the presence of atoms and molecules were not yet verified. As a person with scientific aptitude, Leonardo not only hypothesized, but also tested them with experiments. Below he refers to a beautiful experiment with smoke and the perception of colour arising due to the background.

“Again as an illustration of the colour of the atmosphere I will mention the smoke of old and dry wood, which, as it comes out of chimney, appears to turn very blue, when seen between the eye and the dark distance. But as it rises, and comes between the eye and the bright atmosphere, it at once shows of an ashy grey colour; and this happens because it no longer has darkness beyond it, but this bright and luminous space.”

To me this is nothing but a first rate example of looking at nature through a scientific eye, and adapting this view as means to a certain end. It is a tribute to Leonardo who paid such meticulous attention to details, and attempted to explain an unexplained physical phenomenon – all in the name of getting a painting right !  This is also a wonderful example of how aesthetics and science combined in the mind of Leonardo, which further led to some breathtaking work. After all, science and arts are two aspects of human expression, and Leonardo combined them effectively.

There is another lesson we can learn from such endeavours: observations play a key role. Sometimes, when a student is working in a laboratory, she or he may wonder why one should keep records of ones observations. Well, to them I say – look at Leonardo, he took an important step to write down his observations and this served not only as a template for further exploration, but also clarified his thoughts about the phenomenon he was interested in. Writing this way serves two purposes: one is to record the observation at the moment of exploration and other is to seed new thoughts and questions that can be derived out of these recordings.  You can surely get a lot out of this approach – give it a try.

Coming back to the sky – what is the exact origin of its colour? It took almost 400 years after Leonardo’s observations for someone to come up with an ‘accurate’ answer. And that person was Lord Rayleigh (actual name: John William Strutt). In his remarkable research paper published in 1899, Rayleigh explained the blue of the sky as due to molecular scattering. The opening paragraph of this paper is historic and reproduced below- 

“This subject has been treated in papers published many years ago. I resume it in order to examine more closely than hitherto the attenuation undergone by the primary light on its passage through a medium containing small particles, as dependent upon the number and size of the particles. Closely connected with this is the interesting question whether the light from the sky can be explained by diffraction from the molecules of air themselves, or whether it is necessary to appeal to suspended particles composed of foreign matter, solid or liquid. It will appear, I think, that even in the absence of foreign particles we should still have a blue sky.”

The final statement is significant as it recognizes that light scattering can occur purely due to molecules in air, even after discounting the contribution of suspended particles. Rayleigh gave his famous formula in 1871, which drew an inverse relationship between the intensity of scattered light from a very small particle (compared to wavelength of light) and the fourth power of the wavelength of light. In other words, smaller the wavelength of light (violet-blue in case of visible light), more will it be scattered from the molecules in the sky, and hence the blue colour.

One may wonder why not a violet coloured sky. After all, if the inverse relationship between scattering intensity and wavelength holds good, then according to visible colour distribution (VIBGYOR), violet should be the dominant colour of the sky. The answer to this puzzle is a complex one. Mainly because what we perceive as blue is due to a combination of at least three concomitant effects: Rayleigh scattering, human perception and the background in which the scattered light from molecules are observed. Although the colour of the sky is beautiful to perceive from earth, the intricate understanding of the optical processes in atmosphere of planets, including earth, is still a work in progress.

The foundations laid by Leonardo opened a new line of thought, and Rayleigh put forth an important explanation that forms the basis for a majority of studies on light scattering since 1900s. We will revisit Rayleigh and his work many times in this blog; meanwhile enjoy watching the sky with your scientific eye – after all sky is no limit for science!

Beginning of scattering

Why a new blog, now ?

Well, I have been pondering for sometime to write about my research on light scattering. One of the main motivations of why I became a student of science was to understand  scattering of light. Be it the blue of the sky, the flying comets in space or a glowing molecule in a biological cell, light scattering has something to say on everything in this universe. This ubiquity of a physical process is worth exploring, and has lead me to take this intellectual journey.  In an essence, I was, and continue to be fascinated by the beautiful concept of how light scatters off matter of various scales, be it the size of galaxy or be it a tiny little atom.   Over the past 15 years or so, light scattering has been integral to everything I have been working on, and I think it is high time that I share the beauty of this subject through the posts in this blog. I intend to do this by exploring various aspects of light scattering – from fundamental theory to mind boggling applications.

History has a role – The field of light scattering itself has a very rich history, dating back to observations of Leonardo Da Vinci, spectacular opti’k’s  by Newton, marvelous explanations  by Rayleigh, creative experiments by Raman, connections by Tyndall, conditions by Kerker and so on….in fact the list is endless, and the story is compelling, which has to be said. As we take this journey, we will visit the masters, pick their brains, ask questions, and pester them for answers. This process, I promise, will be rewarding, and will hopefully keep you interested…

Lab stories – Another reason for starting this blog is to emphasize the experimental techniques in area of light-scattering research. Most of the times, the hard-fought battles of experimental laboratory researchers in unveiling the truth and beauties of nature goes unrecognized and under-represented, especially among general audience of science. Although, Einsteins and Maxwells of the world, deservingly get a lot of attention and applause for their theories, people like Bloembergen and Askins don’t get their share for the spectacular experiments they have performed. This blog, in way, is to compensate for such discrepancies.

Unity in diversity – An ulterior motive behind this venture is also to explore new aspects of light scattering in physics, which is evolving and taking new shape as I write. In this discussion about new physical concepts of light and matter, I wish to highlight its relevance and connection to various branches of science and technology. We will evidence how sub-branches of sciences progressed due to ideas and applications from light scattering. In this context, let me give you two examples: soft-matter physics and radiative transfer in astrophysics. A tremendous amount of information about nature of soft-matter and interstellar matter has been derived from light-scattering theories and experiments. As you may notice, the scales of these problems are tremendously different, but the underlying physics is essentially the same. In this blog I intend to showcase this unity of concepts in a diversity of problems in science and technology, how light scattering takes a center stage in this play.

Ultimately, what excites me to share this story is that I get to be a student all over again. This is a happy place to be for a professor: it keeps you grounded and engaged. After all, in the process of every learning and expression their is a concomitant enlightenment and scattering of thought. This is my intellectual kick to scatter forward…..