 All right, good morning, everybody. Welcome back. OK, so we're on lecture five out of six lectures for a quality module. And by and large, for every session, what I do is I let the students know what is it that is going to get covered. And so by the time they're done with this hour-long lecture, in this case, one and a half hour lectures, there's a sense of what will get accomplished. And then when you walk out from the lecture hall, you have a sense of accomplished something. You have a sense of having learned something. Some things might be new. Some things might be old and get reinforced. And some new questions might emerge. There's some work to be done, some reading material, some home assignments, et cetera. So the point is that by the end of the class, the students are walking out excited about new things that they have learned and that there's work to be done. So as I was saying, lecture five of air quality. And one of the things I always find a good idea, and it works very well, is to actually let the students know what would be the focus of today's lecture and that they can actually walk away from the lecture, having a sense of accomplishment and having a sense of new openings for action. So today's learning objective then is that one would be able to explain. I just say explain. It's when I say explain, I actually mean that you could explain to someone. You could explain to another student. You can explain to somebody in your family. You could explain to somebody on the street as to, what is air quality in terms of the particulate matter and how do you actually measure the size and concentration of particulate matter. We've discussed this over the last few lectures that we discussed concentration first. When we looked at sulphur dioxide and we looked at how much sulphur could be burnt in a room and we had said that the volume of the room is going to then determine how diluted that sulphur dioxide will get and whether that will be acceptable for breathing or not based on the three hour standard. Then in the last lecture, I think we focused a lot on the particulate matter, not the gaseous pollutants, but particulate matter and we said not all particles are created equal and therefore it's important to get a sense of the size because different sizes will either first be inhalable or not inhalable and second that they would have different chemistry and they would, because just by virtue of the fact that they come from different sources so some of them could come from just dust which is crustal in nature, geological in nature, benign, doesn't have a health effect whereas particles which are coming from chemical reactions or combustion can actually have a larger toxicity. So these particular size particles and where they deposit in the lungs is also going to determine the extent of the problem and it had turned out that the size of the particle, there's a certain range of size of particles which are both toxic and have a high chance of deposition in the alveoli. So therefore it's important to be able to deal with the size and the concentration. When we talked about size again, last time we had said PM10, we said 10 micrometers and above usually would get stopped by a normal adult human nose under normal breathing conditions whereas for anything less than 10 micrometers, it is considered respirable. So you will be able to breathe it in. Now most of it you will breathe it out also. It's just that there's a certain range of particle sizes which there's a good 70 to 80% chance that it will actually deposit in the alveoli which is the worst place because that's where all the blood exchange is happening and if there is some toxic effects then that's a direct contact in some sense. Okay, so let's continue with this then. We had talked about if students were to just ask to do a measure of the total. And I've just added this word since the last slide, last time I showed you the slide. Total, it's important to say total because we are not differentiating based on size. We're saying anything that enters this tube and that will get stopped by this filter is what we are measuring. So we're not distinguishing between PM10, PM2.5 or any other sizes, it's a total mass. And this is pretty straightforward and we discussed this last time as well. We also discussed the possibility of maybe giving some of these to your students and ask them to make their own estimates of what the pollution levels are. This is simple, you just need a filter paper which is pre-wade and then filter cartridge a flow meter like a rotometer and a vacuum pump and something to stop watch to be able to do the timing. And then you just need to do the mass of the filter before, mass of the filter after that gives you delta M which is the increase in the mass. Of course it's desiccated to eliminate the aspects of humidity. And then Q you have measured using the rotometer. One of the things I really emphasize a lot is whether the flow meter is calibrated. Most of the times the flow meters that are available they have a certain calibration but I just feel much more comfortable if I have calibrated it myself. And it is a routine practice in our lab that before any student starts any studies of any kind the instruments have to be calibrated and there needs to be a degree of confidence that if I'm saying 10 liters per minute I actually mean 10 liters per minute no more, no less. Very critical for all the measurements because a 10% or a 20% error in the flow meter over here would cause a huge change in the concentration which is not such a good idea. Especially if you're looking at meeting standards especially if you're looking at health effect of people. So it's important to kind of deal with it as a almost I would go to the extent of saying like a medical instrument. It doesn't have the importance is no less than that of a medical instrument. And that's the relationship I usually want to establish with students to say look when you are dealing with these it's like a doctor dealing with whether it's a thermometer whether it's a blood pressure equipment or any other analytics that go on for diagnostics. So it's important. So this is just a repetition of what we had done last time. I just wanted to let you know that this particular apparatus, this particular setup will only be able to give you the total mass concentrations which is good in and of itself. Just to caution again larger particles will tend to dominate the mass. So if you do this measurement somewhere in a village in let's say a desert area then the mass that you may get may be very large. Why? Because the particles are large course particles coming from dust coming from sand in the desert. So they tend to dominate. Whereas if you do a similar measurement at let's say a traffic junction in an urban area then the particles there's no dust, there is dust but assuming that the roads are clean, et cetera. Let's say that the contribution is coming only from vehicle emissions. Now vehicle emissions are small. It's almost like two orders of magnitude smaller than sand particles. So the contribution that they would have would be then 10 to the power of three times which means if it's 100 times lesser then you're talking about a million times less mass. One of the issues also comes up is whether the weighing machine that you have. Last time we had talked about doing this flow 10 liters per minute for 10 minutes. 10 liters per minute for 10 minutes. 10 liters per minute for 10 minutes which means total in the denominator would be 100 liters. But sometimes if the concentration is high then you'll get sufficient mass to be able to do this calculation. But if the particulate matter is small and the contribution to mass is less then that 100 liter may not be adequate for you to be able to get a detectable amount of mass. So therefore you may have to either run at a high Q value or for a longer time. Okay, again I spend a lot of time with students emphasizing the choice of Q and T. If you take high Q then you are taking the, you're getting a snapshot of what's happening immediately because the time is short. So in short time if you take high flow rate then you'll be able to get closer to the real time, closer to now what's happening. Whereas if you kept Q low, if you kept Q low and you extended T instead of 10 minutes you extended it to let's say three hours then what you will get on this filter is an average concentration over a period of those three hours. So there is enough, it looks very simplistic over here but there's enough science in this very small delta M by QT expression. One more time, delta M. There are many instruments that are available that are at a 10 microgram level of resolution but then sometimes for the work we do we require something which has a resolution of one microgram. Now the cost difference is huge between the 10 microgram instrument and the one microgram instrument. So I think a 10 microgram probably you get it for about one and a half, two lakhs. Whereas if you're looking at a one micro meter you easily shoot up to eight to 10 lakhs. So that's the kind of cost difference. Therefore the decision of whether you're going to use a larger Q or a larger T, that whole aspect comes in but one of these has to be larger for the mass to be collected. Okay so one last thing probably I keep thinking whatever I'm saying last is the last thing but then one more thought emerges and I think it's important to share this also. I use this entire lecture also to tell the students what does resolution mean, resolution of mass? And the example I give them is I say if you go to a vegetable store and you want to buy let's say potatoes, okay? The cost of potatoes is some X rupees per kg. Based on that what is the minimum resolution that the weighing balance should have? So assuming that nowadays the minimum exchange and currency is about one rupee, they would have to work backwards to see given a cost of potatoes, what should be the resolution of the weighing scale that they have? These days they have these single pan weighing balances. So they just put a basket on it with all the vegetables and then they just weigh that and then they even have a calculator over there that they figure out. But the important thing is what present? Are you going to buy it to the nearest gram? Are you going to buy it to the nearest 10 grams? Are you going to buy it to the nearest 100 grams? So that is going to determine is it the first place of decimal of a kg, second place of a decimal of a kg or third place of a decimal kg which is going to decide the choice of your scale. Then I go ahead and change the game a little bit and I say what if you went to a gold shop? If you go to a gold shop, the price per gram of gold is nowhere close to what we buy in a vegetable grocery shop. So certainly the order of cost of the material that you're buying shoots up by three orders of magnitude in which case then you have to do a calculation again that if you go to a jeweler shop, they don't deal with one rupee anymore. They probably deal with 10 rupees sometimes 50 rupees depending on if you're buying something worth thousands of rupees, they'd even round it off to the nearest 100 rupee. But if you're buying something which is about 700, 800 rupees then they'll probably round it off to 10 rupees. So I use this as an exercise to deal with the whole aspect of resolution. I'm not saying that people, students have not dealt with this in somewhere or the other like for example in physics or in chemistry or some other place they would have definitely dealt or even lengths and millimeters and micrometers and use of vernier calipers and micrometer screw gauges. They've dealt with it in some way or form. But a lot of times when I was studying, it remained an exercise to be carried out in a laboratory setting. I never really expanded it or extended it to my day-to-day usage for grocery shopping or sometimes when I go with my wife for jewelry shopping, I didn't go jewelry shopping till I got married. So after I got married, I started going jewelry shopping. So jewelry shopping, the whole idea of jewelry shopping, I excite the students even to consider going to a jewelry shop before getting married so they could actually go and talk to the jeweler and find out why did they choose a particular balance and if they go to the nearest one milligram or go to the 10 micrograms and what made that choice and how much does a weighing balance cost. And then I asked them to go to the grocery store and also check with the grocery stores as to how much does their single balance cost. So I do want students to begin to engage themselves in these measures because it's got to be like a, it's got to be like a sense that they have, like they would have a sense of how cold is water and what else, how cold is water and how much does one kilogram weigh. So if I pick up this bottle, I have a sense that it is about 200 milliliters. So it's about 200 grams. So I have a sense of how much it weighs. The first time I got into a shock was when I tried to pick up a bottle of mercury, the same volume of mercury, if you try and lift it, it'll be difficult to lift it. You'll be able to lift it, but it'll be difficult to lift. So 200 grams multiplied by a factor of let's say 14 for 13.6 specific gravity, that kind of shoots up to now kilograms, it may not be easy to pick up. That's one thing I say. Then the other thing I usually say is if they say this is a certain flow rate, let's say five liters per minute or a certain wind velocity, let's say two meters per second. It comes to, let's say the fan in the room. I actually asked them to know whether they know what that speed is. If I turn on the fan, will they be able to tell me what is the speed at this point? A lot of times people don't, it's difficult for students to be able to do that. So then I say, okay, now imagine if you were on a two wheeler on a scooter or if you were in a car and the wind was blowing in your face or if you were in a car and you put your hand out just for a little bit to see what the speed of the wind is, you get a sense of what is it that, what's that spin? So you see the speedometer, you say it's moving at 40 kilometers per hour, you stick your hand out, you have a sense of it. On a scooter especially, your hair begins to flow and you begin to get a pinch in your face, especially if it's raining. So these are some of the things. It looks quite simple, but I spend a lot of time on this. And as I said previously, I thought this was the last thing I was going to say, but here's another thing to say. Another thing to say is the vacuum pump. A lot of times people, students have not dealt with a vacuum pump, so it's important for them to get to know if they were to buy a vacuum pump, what are the kinds of characteristics that they would have to look for a vacuum pump. So the entire design of vacuum pump and which pump they would choose is also becomes then a part of the exercise that they do in the class. So that's as much I want to say about some of these components. Again, flow meters need to be calibrated. You have to have a balance, which is sensitive, which minimum resolution is to the demand of what a delta mask can be. And of course you have a regular stopwatch. Okay, so that's that. And move on to the next slide. So we really therefore are going to be looking at sizing of particles. We're going to be looking at sizing of particles. And I'd said not all particles are created equal and we even talked about ants and elephants. So ants is not what we're dealing with right now. We're dealing with elephants, baby elephants, dinosaurs. So we're dealing with these. We're not dealing with these. These I'll describe later that all gases are created equal. Ideal gas law applies to all the gases, but that's not the same when it comes to elephants. Particular matter don't follow. In fact, the known laws of physics, for very small molecule, for molecular size, and the known for particle size, which are in the Newtonian regime, larger particles. Newtonian was known. So somewhere in between, the small particles and the large particles is the physics that only recently because of development of instrumentation. So that's why we are focusing so much on it because a lot of this stuff you will not even find in textbooks. The normal textbooks that you would have for environmental studies will not cover most of these things. So we'll keep adding these as resources as we go along. Okay, so we got ants and elephants and we're dealing with elephants and we want to be able to size these baby elephants, elephants, dinosaurs, et cetera. So this is the graph that I shared with you last time. We said why we are interested in different sizes is because if you were to look at ambient measurements, there's course mode, there's accumulation mode and there's nucleation mode. And we said particles less than 10 micrometers are respirable, anything less than 2.5 as more of a contribution coming from anthropogenic sources. So therefore, it's important for us, it's of interest to us to be able to get size distributions. So this is my association of sizing. You take flour in a kitchen, you take wheat flour or you take rice flour in a kitchen and you basically do the searing. So the first time, I need to share something with you. When I was a kid, I used to think that maida, flour, maida, aata and suji or rava. I don't know, I'm sorry, I don't know the English words for these three, but rava, aata and maida. I used to think that they come from three different grains. So when you have a naan, it is made out of maida. You have roti that's made out of aata and then when you have some halwa, shira, that is made out of suji or rava and I used to think they come from three different grains. And one time my mother wanted to make baturas at home and there was no maida at home. So she took some of the regular aata, regular wheat flour and she put it in one of her chunnis, the head gear that my mother used to wear. She took a clean one. So she took that and she put this over there and she did chik-chik-chik-chik-chik a few times and what came out of it was maida and I at that point in time, not that I didn't know it already, but for that time onwards it was confirmed for me that my mother was a magic woman that she would actually make aata out of, maida out of aata, okay? So that is my association with sizing, sizing of particles. That is my association with sizing. So if you look at this, civil engineers, chemical engineers, mechanical engineers, a lot of people in the area, in the business of materials, when they're looking at sand, everybody's seen when any construction is going on that there'll always be a little sieve which is put at a, you know, with a stand like this and they will take the sand and then with a spade they will put it over there and some of the large, smaller particles will go through but some of the larger pebbles will stay out. Seving is done, it's always done. So this is typically, you know, sieves that are used for analysis of soil for example. They also have these shakers. So this is a typical sieve shaker that you stack these up and you put the soil on top and then you basically close the whole thing and you shake it up for a certain amount of time. And what that shaking will do is that it'll actually help that if a particle, the smallest size, the smallest dimension of that particular particle when it comes in a certain orientation, it'll basically go through the hole. So the smallest size will go through the hole in the sieve and so you do that for a certain amount of time and then ultimately what is left on each of these sieve trays is what couldn't go through and then therefore you have a size distribution. This is a very clear understanding that everybody has. So why am I spending so much time on it? Why I'm spending so much time on it is that this intuition that we have about sizing does not work for particles which are respirable. These are not, this particular application is not. See, can you just imagine now to pick up particles which would be all less than 10 micrometers. I mean how would you pick up particles less than 10 micrometers input? Oh, okay, suppose you say you took talcum powder. You took talcum powder and you put it on a sieve. So first of all you'll have to deal with having whole size which is 10 micrometer. Whole size which is 10 micrometer which you cannot see with your naked eye. You just barely be able to see with your naked eye. You'll be able to see it if you take the sieve and you put it against the light then you'll be able to see a hole through it. Otherwise you may not be able to see it. That's for 10 micrometer and we're talking about particles which are of the order of about 0.01 micrometer and less. So just any chance that you'll be able to get a sieve of that size is gonna be difficult. So you cannot deal with it as a sieve. Second thing is when you put the stack, when you put the stack over here, you're dealing with the particles settling by gravity. You're banking on gravity to separate these particles. But when it comes down to 10 micrometer particle, one micrometer particle, 0.1 micrometer particle, those particles are so small that if you shake them up they'll probably get entrained in the air and not settled down because they're small enough. They're large enough as elephants and large enough as dinosaurs, but they're small enough that all the ants around them will tend to keep them suspended. So they have a very low, very small settling rate so the chances of you to be able to actually separate them based on sieving is just next to impossible. So therefore we're in trouble. We're in trouble because we actually have been thinking of separation of size based on our understanding, our learnings have been that of sieving and suddenly I'm now left with, I don't know how to separate out these particles. I don't know how to size these particles. So I don't want to be alone in this dilemma, so I'd like to share that dilemma with you and have you do a little bit of work on it. So here's a class assignment now that I give to the class. So let me go ahead and flash the class exercise. There we go. The thought exercise, the thought exercise is how would you size and count? Size and count aerosol particles in the nanometer size range. These are particles which begin to be less than one micrometer. So particles which are less than one micrometer. How would you size them and count them? So I'm gonna connect with, so this is... Is Kaylee, we are back again. Kaylee, we are back again. All right, I like the way you guys respond. Very good. Okay, so somebody please take the microphone and tell me what ideas that might be there to size and count particles in the nanometer size range? Sir, it will be measured in micrograms per meter cube. But how would you do that? Particulate matter. High volume sampler for PM10. Excellent. And if it is 2.5, it is RSPM, Respirable Suspended Particulate Micro Sampler. Excellent. All right, very good. Very good. Excellent, excellent. Very good. Who else? Okay, GRI, ET, Kokatpalli, Hyderabad. How are you? Hyderabad. Good morning, sir. All right, excellent. So one of you please tell me, how would you deal with? How would you deal with sizing and counting particles in the nanometer size range? Hello, sir. Good morning, sir. Good morning. Sir, by using any instrument like particle size, we can determine, sir. Like microorganisms, we are using colony counters for counting the number of microorganisms and all. So by using particle sizes or spectrophotometers, we can determine these aerosols, sir. Excellent, excellent. All right, so very good. Thank you, thank you very much. Thank you very much. Next please. Okay, actually you can see very good. Hello everybody. Hi, very nice, okay. So one of you please share. Hello. Yeah, please. Sir, shall we use some fluorescence particle size measurement analyzer or some Temman also transmission electron microscope? Yeah, very good. So I think the thinking is all in the right direction. All right, so I'm gonna move on now. Just give you an example. I'd gone to meet with Professor Peter McMurray one time, okay, at the University of Minnesota. And one of the things I learned from him was that when we get an instrument in that lab, Professor Peter McMurray, he's one of the founding fathers of aerosol research at the University of Minnesota Department of Mechanical Engineering, Peter McMurray. So when Professor McMurray said, you know, when they get a new instrument in the lab, by the way, a lot of instruments get developed in that lab, okay, but if they get a new instrument from outside, the first thing that they do is they take the cover, they take out the cover, the top cover, they take the top cover and they throw it away and they take the manual and they have a ritual in all and all the students and all the other faculty get together and they have a ritual that they burn up the manual, okay? So I was a little, I looked at him like, you know, a little strangely, he says, no, we do that because we don't want the manufacturer of the instrument to tell us what we can or what we cannot do with this instrument. So I was quite excited by that idea to be able to open up the instrument and tinker around with it. And then some years later, I came back to India and the relationship that we have with instrument is very different, okay? We are very scared to open up an instrument. We don't want to touch it or spoil it. And it's, I think it's fair because a lot of money has gone into buying an instrument and you don't want to mess with it. But I'd, you know, I usually like to evoke this interest among students and I, anytime an instrument goes bad, I don't throw it. So there's no write-off of instruments as far as I'm concerned. Any instrument that has gone bad becomes now a resource for me for education. So I actually will use it in the lab, ask them to open it, ask them to remove it, ask them to break it if required and do all kinds of crazy things so that actually I have an appreciation of what is instrumentation, okay? So the relationship shifting between it being something that is something to be scared of, you know, I push in the sample and I push a button and I get some results out and then whatever those results are now, like are, you know, the universal truth. No, that's not the case, okay? You gotta have a healthy relationship with the instrument. I mean, you gotta know when it's working, you gotta know when it's not working. And the only way that you only will be able to do that if one understands the inside out of that instrument. So I'm gonna share with you some of the principles of these instruments that are used so that instead of it just being a name, it actually becomes something that you can design yourself, okay? So let me just go ahead and show you the next few slides. So what we're gonna be dealing with is inertial impactors, okay? Inertial impactors is what's at the heart of that principle of principle of inertial impactor is what you have is elephants that are suspended in ants, okay? So if the order of magnitude is three, okay, then the mass is going to be 10 to the power of nine, cubic relationship, okay? So you've got ants and you've got these elephants which are 10 to the power of nine times heavier than the ants, okay? So we understand inertia quite well. So if you were to take a gas stream and pass it through a jet like this, which has particles over here, and then you insert some kind of an impaction plate or some kind of an obstruction, this q over here, the flow over here, splits up to go in either direction because ants get a sense that there is obstruction so they still have to keep moving because they're at a certain velocity. So the ants are coming in, they go through this nozzle, suddenly there's an obstacle here so they start taking a turn, okay? All the ants start taking a turn, all the ants start taking a turn. However, some of the elephants, which are being pushed by the ants, okay? They try and take the same turn but they're too heavy to be able to take that turn because of their inertia, okay? So they therefore impact out. So a large particle moving at a high velocity through a nozzle when it experiences an obstacle it cannot take that bend that the fluid is taking and therefore will impact out, okay? So this is basically the impaction plate or the collection plate, this here is a nozzle, this is the gas flow rate, this is an elephant which basically couldn't make that turn along with all the ants. That's the principle, it's called inertial impaction, okay? I got this image from this URL so you're welcome to go there. Now instead of having one impaction stage like this, in an instrument, in a cascade impactor, you have a cascade of these stages and why you have cascade of these stages is because you want to change the size of the elephant that you're going to capture, okay? So let's take a look at this for example. So this is the first impaction plate, this is the second impaction plate, this is the third impaction plate. There can be as many as 10 impaction plates in an instrument, okay? Up to 10 stages. So when you look at this now, you look at particles that are all sizes. You have this large elephant, you have this small elephant and you got this baby elephant. So you got three sizes of elephants and of course ants you cannot see, okay? The gas stream that is coming through is ants, ants, ants, ants, ants, ants, right? Millions and millions of ants, billions and millions of ants and in there somewhere are also suspended some of these small baby elephants, medium-sized elephants, large elephants, okay? These are all coming along with the flow. These ants take a turn over here, ants take a turn over here. The small baby elephant and the medium-sized elephant are also small enough that in that small time that is available to them and start pushing them, so they also take that turn. However, the largest elephant, too bad tragedy, it couldn't take the turn, okay? So it impacted out on the surface. Oh, okay, so what do we do? We say that if on this surface I put some kind of a filter paper or forget filter paper, let me just take a piece of aluminum foil, okay? Metal foil, I put a piece of aluminum here which has been pre-wade, okay? It has been wade before and I run this instrument and this instrument is flowing at a rate of, let's say 30 liters per minute, I'm sampling 30 liters per minute and I run it for let's say 30 minutes. So I would have sampled 900 liters, okay? 900 liters and any elephant, any elephant that is bigger than this size, either this size or bigger than this size will impact out on this plate, will impact on this metal foil. So then after running it for 30 minutes, you take this foil and you wait again. The difference in the mass will give you the size of the particles or size of the elephants of a certain size, bigger than that certain size, okay? So that's the first stage, that's what you've done. Now, you go to the next stage but this time what you do is, you decrease the size of this nozzle. Notice this nozzle here, this opening over here is smaller than this opening, okay? The flow that is going is the same. So 30 liters per minute going through this nozzle. The same 30 liters per minute is going through this nozzle but because the nozzle's size is smaller, the velocity is much higher compared to this stage. So if the velocity is higher, the small elephant and the medium elephant which could not impact over here now will experience enough inertia that the medium size elephant will impact out but the smallest size elephant still escapes it. So now what you have is, you have large elephants which have deposited here, medium size that have deposited here. Now you take it to the next stage in which the diameter of the nozzle is even smaller. So now you'll collect the smallest size over here and then at the end of the whole cascade impactor, the whole set of 10 stage impaction stages, collection stages, you are left with a filter paper which removes the rest. And of course there's a vacuum pump over here which is sucking at the rate of 30 liters per minute. So then what happens is you take this filter paper or this metal foil, you take this metal foil, you take this metal foil. You have collected particles, you have collected elephants of different sizes. The largest one over here, the smallest one over here. So if you take the mass of particles collected on each of these stages and you plot them on the x-axis, you have the size of the particle and on the y-axis you have mass collected, you actually get a histogram. That histogram is a size distribution. It's really very simple. All you need to pay attention to is that the nozzle size over here, the nozzle size keeps decreasing as a result of which the particle which could not impact over here as it gets to higher and higher velocities, it has more and more inertia and therefore cannot take that 90 degree turn and therefore impacts out. So that's the principle. It's called a cascade impactor. Each of these stages is called an impaction stage. And these are quite popular when you want to get size distributions. Now I'll just give you some, show you some examples, okay? Oh, by the way, you know just in a previous lecture I had shown you the measurements that we had done at Parel and at Worley, you know, by Nitin Goyal. Those were actually taken by this instrument. So you actually did the size distributions of the atmospheric aerosols at these two locations at different times of the day, okay? Okay, the following slides are a PhD work of another student of mine. His name is Selva Kumar. So this is something which we did. Remember I said to you in the beginning of the lecture that I'm a stickler for calibration, okay? I'm a stickler for, you know, making sure that if I'm saying that it is five liters per minute, that it is five liters per minute. When I'm saying that it is three micrometers, that means it is three micrometers, okay? So you can check it under the microscope. You can take it to an optical micro, three micrometers is usually measurable under an optical microscope. But if it is smaller than one micrometer, then you might need to take it to a scanning electron microscope where you can do the sizing, okay? But you have to do some of those things to be able to make sure. But, you know, if you didn't want to go outside, and you just wanted to use whatever you have in the lab, for example, if you had a cascade impactor, and you want to know whether the cascade impactor is working properly or not, okay? So what we did was something interesting. What we said was, and this is not something new. It's done quite routinely. So what we did is we actually said, let's take a salt solution. Instead of clean water, we take salt solution. We take a little bit of water, and you mix a known amount of salt in it. And we then mix it up, make it into a solution, and then spray it. So spray meaning, like for example, you know, anytime you use begon spray, right? You go, it basically has a spray coming out. Or if you go for a haircut, they invariably will use some kind of a spray to wet your hair, okay? So the spray, you can actually see the droplets, okay? Those are the droplets. Now instead of water, if you put the salt solution, these water droplets, when they dried up, would turn into salt particles, okay? The salt solution, when it dries up, will become salt particles, okay? So in one case, we took five grams per liter of the salt. In another case, we took 40 grams per liter, which means eight times, okay? Five grams per liter, and then we took 40 grams per liter. That means we took a mass, which is eight times more. So then we expected that the particle size would double by, would double, would increase in size by double, factor of two. Why? Because the volume of the mass is dp cubed. So if I have the size multiplied by eight, the mass concentration multiplied by eight, instead of five grams per liter, if I took 40 grams per liter, I've increased the mass or increased the volume by eight times. So which means that the size of the diameter of the particle should double, okay? So that's a trick that we used, and we actually worked out pretty well, that for, let's say for five grams per liter, we got a size of about 0.6 micrometer, whereas for 40, we got a size of about 1.01. Not quite double, but all the same. And then we did another shot, where we multiplied it by a factor of eight again. But not 320, we took 300, because it reaches a point of saturation. The solubility limit comes in, so we kept it at 300, and it went from one to 1.9, almost doubling. So those are some of the things, which in a few doing research, it's important to check that. But I also see that as a way of evoking a sense of rigor, a sense of being particular about measurements among students, okay? So this is one of the exercises that we had done. I also talked a little bit about the chemistry of the particles. And so the particles that we collected at each of these stages, we actually did some chemistry on it, okay? So this is, we did some edacs analysis. And over here, what you see is different stages of that cascade impactor, okay? The biggest size were on stage zero, and the smallest size were on stage 10. The biggest size was 18 micrometers and above, and the smallest size was 56 nanometers, okay? So there's a whole range of particles here. And what you see over here is the chemistry. It's a 3D plot. Here you have the size. So the largest size is furthest away from you, and the smallest size is closest to you. This here is the chemistry. It is actually different elements that were detected using edacs. And it turns out that the largest particles, this is by the way from a wood gasifier. So we were interested in deposition of particles coming from a gasifier for many reasons. We won't get into the reasons, but our interest was to characterize these particles. So when you looked at the larger particles, they tend to be more of SiO2, ash particles, okay? So the chemistry was showing that large particles. By the time you got to the smaller particles, there was hardly any SiO2. So most of the SiO2 was therefore in the larger particles. But as you got to other elements, and you got to carbon, for example, some of the larger particles had no carbon at all, whereas the smaller particles were almost 100% carbon, okay? So this is something we talked about yesterday that depending on the source, depending on the source of particles, if it is coming from ash, then the chemistry would be very different. If it is coming from combustion, and it is carbonaceous, the chemistry would be very different. So this is another way that you could use a cascade impactor. You take the sample from each of those stages, which you know are different sizes, and you take and do the chemistry on this to be able to see the chemical composition. Okay, this is what particles look like. So there are many, many images available for particles from different sources or different sizes. A lot of times people use morphology of the particles to be able to identify them. Okay, particles are seldom spherical. Most of the particles in the atmosphere are not spherical. It's very rare that you'd have spherical particles unless they're droplets. If they're liquid droplets, then they tend to be spherical. But otherwise any crustal material, any suit particles, they tend to be irregular shapes, and they sometimes get to be like agglomerates and chains. So when we're talking about diameter of a particle, it's a difficult question to say, what would you call the diameter? So invariably when we're talking about diameter, we actually have a definition called the aerodynamic diameter. Okay, and what aerodynamic diameter means is that if I had a spherical particle of unit density, spherical particle of unit density, and I have this arbitrary particle with unknown density, some diameter, I don't know, but if these two are settling at the same rate, then the aerodynamic diameter of this particle, unknown size, unknown density, the aerodynamic diameter of this would be the diameter of this particular spherical particle of unit density. Okay, so that's aerodynamic diameter. Again, I'm not gonna spend too much time on it. I usually give this as a class assignment and also as homework, okay? Okay, the next few slides are from the work of Dr. Nitin Goyal. He did his PhD some years ago with me. Okay, so what I showed you was a 10 state cascade impactor, okay? At that point in time when he had bought it, it cost some eight lakhs. Some part of it came from some project money, part of it came from the departmental funds, okay? I don't know what the present cost is, but at that point we bought it for eight lakhs. Now, eight lakhs is quite expensive. I mean, if you wanted to buy an instrument and use it routinely for students, it's difficult, okay? It's very difficult to kind of have that kind of money be available. And one point in time at the most two or three students work and if you have a class of 25, 30 people or more, then it's really, you know, difficult to be able to distribute that same instrument to everybody. So what we said was, we said, okay, can we do some cheaper alternative? We want to make something indigenously. So as a part of the work that Nitin did, he actually developed the single stage impactor. So it only measures PM 2.5. So any particles that are greater than PM 2.5, they get impacted out. And anything which is smaller than PM 2.5 gets collected on a filter paper. So our interest was in PM 2.5. The same instrument you could also use for PM 1. We recently tested it for PM 1. All you had to do was, instead of operating at six liters per minute, you operate it at a different flow rate at a slightly higher flow rate. So particles larger than one micrometer instead of larger than 2.5 micrometer can impact out. Okay, so that's a play that you could do. You could even have students, you know, design one of these, fabricate one of these. This cost some 6,000 rupees, okay? It was made in a local workshop close to IIT campus, a place called Bikroli, which is close by, okay? So this is just what it looks like. It's made out of aluminum, simple connectors over here. Once you assemble it, these are the nuts and bolts you can have. These are the impaction stages, okay? These are the impaction stages. This is where the particles would deposit out. And then at the end, you would have a filter paper on all particles, for all particles that are smaller than 2.5 would get collected and that's what you could do chemical analysis or you could do the weighing, okay? All right. Okay, now, when we designed this, we called it the SSI, the single stage impactor. 6,000 rupees compared to 8 lakhs, okay? We just wanted to make sure that we were doing the right thing. I mean, does this, you know, indigenous instrument that we have made, does it actually compare well with the commercially used, well documented, well established 10 stage impactor, which is called Moody, the micro orifice uniform deposition impactor, okay? So we compared and they compared pretty well, okay? So our design was okay and we could then actually use a single stage impactor for class studies, lab studies and also for other research work. Okay, so that was inertial impaction. We can go all the way greater than anything, greater than 56 nanometers can be impacted out. Anything smaller than 56 nanometers, it now gets small enough that you can't differentiate between ants and elephants in terms of inertia, okay? So anything larger than 56 nanometers has, is available, is commercially being used. I think I believe there is another cascade impactor which has come which goes even smaller than 56 nanometers, but I haven't actually had a chance to check it out, okay? So inertial impactors, everybody, very simple, very routinely used. So two of you, you know, mentioned some of the instruments when you said that you use a PM 2.5 sampler or when you use a mini wall sampler, not a high volume sampler. In a high volume sampler, you don't use this principle of impaction. What you use is cyclone. So I'll talk about the cyclone in the next lecture, but what you do is, again, there, also you use the inertia of the particle to separate it out from the gas stream and all particles which are greater than 10 micrometers would get removed and you get particles, you get gas which is now removed of 10 micrometer particles. So everything that is left over there and then these PM 10, which is what is used in high volume samplers, okay? But in anything which is other than high volume sampler, mini wall samplers, you know, other samplers that are using, which are not optical in nature, which are actually mass based, by and large they're using this principle, okay? So that's the inside story of what's going on. And I just want you to know, I'll send you a paper, by the way, our entire intent in developing this particular device was that it is in the public domain, okay? We actually have all the drawings that are available. We tell you how to test it. We tell you how to get it fabricated. All those details are available. And so I'll even put it up as a challenge to you. Go ahead and for the next lab class that you have, you just give it to students, you probably need to give them a little bit of money, not more than total budget of about 10,000, which therefore they can go ahead and fabricate it and test it out. You will need a vacuum pump with this and of course the filter paper or the cascade impactor, the foil, you will need a weighing balance. So those are things that you will need. But all the other details are available in a paper in Current Science, I'll send you the link for it, in Current Science by Nitin Goyal and co-workers, so you can go look for it. I think it's somewhere 2008 or so, I'm not too sure, but somewhere about seven, eight years ago. Okay, so that's that. And so this part over here, which is again hidden, is actually now optical particle counting, okay? Optical, you use optical. So the principle is same as why is a cloud visible? Okay, it's visible because it scatters light. Why can you say between one glass of water, which is clear and one glass of water, which is turbid, so turbidity? Why would a certain sample of water or river water or even drinking water sometimes, why is it turbid? Why is that turbidity? Is because it has some suspended particles in it, which are scattering light. So if it is scattering light, that means it is an elephant. Now, sorry, I don't mean to say it in a way that ants don't scatter light. Ants are also scattering light. Ants are scattering light in the form of a blue sky. Okay, molecules, gas molecules are scattering the light, therefore you see a blue sky. But when you see white smoke, sorry, or white cloud or even white smoke, or you see black smoke, that is more because the elephants are now scattering light or in the case of black smoke, they're actually absorbing light. They're black, they absorb all light, so therefore you can't see it. You see it because it's black. You see it because you don't see it. You see it because you see light around it, but there's no light coming from that black spot, okay? So just to emphasize that in one case, you're scattering light. In the other case, light is being actually absorbed, okay? So the difference between black particle and a white scattering droplet is that white scattering droplet scatters all the light. It doesn't absorb any light, whereas a black particle absorbs light. So that's the basic principle. And what you would do is then you would actually say there are two parts to it, like scattering is no absorption, extinction is when there's absorption. The limit to measurement using an optical method, not microscope, even with the optical microscope, this is the limitation. The limitation is the same. You've got wavelength of light, which is in the 0.4 to 0.7 micrometer range. So you cannot distinguish anything smaller than that. The size of the probe is going to determine the characterization that you can do with it. So typically the size that we deal with is limited to about 0.1 micrometers when it comes down to optical sizes. You can use a single particle detection, which means one particle has to pass through the laser beam at one time, or you can take a cloud of particles. So for example, if you're looking at a turbidity or if you're looking at cloud, scattering from cloud, then that is a collection of particles. Turbidity is a collection of particles. So you can do single particle or you can do a cloud of particles. The principle is simple. You basically have a laser source of a certain wavelength, which is going through an optical cavity or optical space. And you have these particles of different shapes, different sizes, different refractive indices that are coming through. As soon as a particle passes through the laser beam, it will scatter light. And a good example of this is, I'm sure you've seen some of these laser shows, okay? They have these laser shows, rock shows, laser shows, sometimes on TV and some of these India's Got Talent or Nuchbelier or all of these different programs. They'll have a lot of laser beams going across and they also have smoke coming across. So they actually have to have the smoke because if they didn't have the smoke, the gas molecules will not scatter that light, okay? So the smoke is essential to be able to have a laser show. So that's basically the same principle over here. You have a laser beam coming through. Instead of a cloud of particles, you actually have a single particle going through it one at a time. So as this particle goes through, it scatters light. So as soon as a particle goes through, it scatters light. There is a detector here which actually sees, oops, there's a particle there, oops, there's a particle there, oops, there's a particle there. So every time a particle passes by, you actually get a signal. You get a signal, you get a signal. So every beep is a signal. That means the particle is gone. Now, the size of the particle is going to decide what is the size of the beep, okay? What is the signal? What is the size of the scattered light is going to decide, is going to be decided by the size of the particle. So in some sense, there's a graph over here which shows that if this is the particle size, then then some linear relationship between the size of the particle and the scattered signal. This is the eye scatter. This is the eye incident over here. This is the eye scatter. So this is the intensity of the light, okay? So this is some kind of a linear relationship over a limited size range. So this is the linear part which you can use. So if you know the amount of light scattered, you can go from here and then interpret the size of the particle, okay? Again, unlike the previous time where we dealt with an aerodynamic diameter, we are now dealing with optical diameter. The optical diameter is not the same as aerodynamic diameter. If this instrument says one micrometer and if that instrument says one micrometer, if you put them both under the optical microscope, they will not measure same. Very rarely will they measure it to be the same size, okay? So different instruments will give you different nominal diameters. So in one case, it is aerosol optical diameter, sorry, aerodynamic diameter. In this case, it is optical. And in the next case, it will be electrical mobility particle diameter. We'll get to that. So this is what it looks like, size. So you have to make sure that these particles are coming one at a time. If they don't come one at a time, if they come two at a time, then the amount of light scattered will be higher and the instrument will not interpret it as two particles but as one large particle. So you have to make sure there is no coincidence of these particles. It's called a coincidence error. You have to keep these particles sufficiently far apart from each other. And how you keep them far apart from each other is that even if you take, for example, smoke, which has very high concentration of particles, you take smoke sample, you have to dilute it with clean filtered air so that these particles can be separated about far enough so that when they pass through the laser beam, they pass one at a time, okay? This is typically the cross section. Just this one was just a simple schematic which I had drawn. This is a schematic from a commercial particle counter. This is what it looks like. Same thing. You have some way to generate a laser beam. Particles are coming through a jet over here. They pass through the space. There's some filter, which is a monitor, which is, sorry, there is a sensor which is monitoring the power which is coming from the laser so that it compares this against what gets scattered, et cetera. So there are internal checks. They have to do quality assurance and make sure that the instrument works under various conditions even when the intensity of the incident beam itself is decreasing, it basically normalizes for the scattered signal here, et cetera, okay? All right. So that's as far as optical particles is concerned. Let's move on to the next slide. This part over here is now electrical mobility. What is electrical mobility? Again, I think everybody understands from class 11, 12 from the electrostatics that we had studied that if a particle has a charge and you put it in an electric field, that it actually moves, okay? There's electrical mobility of a charged particle. So we use that principle for these particles which are otherwise quite small. They cannot be sieved. They cannot be, you know, even the mass may be sometimes too small to be collected on a filter paper over a short time. So you want to use some way to be able to keep them in suspended still in the gas phase, use these electrical properties to be able to size them in real time, okay? So what you do is you basically charge these particles, atmospheric particles, you charge them and you put them in an electric field and that basically will then size them. So you can, I'll show you the, I'll tell you what I mean by sizing, okay? This is by the way the URL for the instrument which is being sold by TSI. You're welcome to go see it. So this is basically now fairly complicated but okay, I'd like to just focus on this part, okay? You charge these particles. These particles are coming. The atmospheric particles are coming which are already charged and then they enter this particular window, okay? They enter this window. They have a certain charge distribution to them. They're all sizes. Small size, medium size, large size, all size of particles are entering. Each one of them, depending on the size, would have picked on a certain charge. So the size mass charge distribution is established. It is well known. And now you have this particle which is introduced here and this is a central tube and this is an outer shell and in this annular space, there's an electrical field. So particle of a certain size with a certain charge will follow a certain trajectory. If the charge is higher, it'll probably move faster. If the charge is smaller, it'll probably go for a longer trajectory, okay? So this trajectory now is going to be determined by the size and charge on that size, okay? So for one particular electrical field, strength, you will get one trajectory or one particle size to go through this window. So you take this particular window, the particles coming from this window, you take that size over here and you put it through what's called a condensation particle counter. I'll talk about that. So what you really done is, you have taken all the sizes in here. Of those sizes, you have taken particle which follows one particular trajectory, particles which has followed one particular trajectory, which means it is one particular size and then you pass it through a counter where it gets counted. So what this piece of instrument has done is done the sizing. It has picked up one particular size and now in this particular instrument over here, it is counting the number of particles of that particular size, okay? So you did one size. Now you want to pick up particles which are of a larger size. So what you do is you decrease the electrical field a little bit. So the earlier particle which was impacting here, now because the electrical field is lesser, we'll follow this new trajectory for the larger particle and now this particle will go through this particular same window and again go to CPC. So what you do is you keep scanning the electrical field, okay? So you keep going up and down on the electrical field. You keep scanning it up and down. The computer algorithm keeps track of when the electrical field was a particular value and at that point in time what is the particular size that would have hit this particular special window which then got counted, okay? So very neat. It's really neat. In real time, in 20 seconds, you'll actually get the entire size distribution of particles. Okay? This is state of the art. I told you that the Moody sampler cost about 8 lakhs. The OPC particle counter depending on again which grade you get, it varies anywhere from 10 lakhs to 32 lakhs. This instrument over here, the combination of these two instruments over here is about 45 lakhs, okay? So that's the amount that you would have to pay if you wanted real time measurements of particulate matter still suspended in gas phase. You didn't collect them on any surface. They're still in gas phase, okay? In gas phase, you actually got the size distribution. It's very critical especially when you're dealing with synthesis of nanopowders and you want to see whether a particular process change has influenced the size. You don't want to collect it on a paper and do some electron microscopy. You want to do it in real time, okay? So this is an advantageous instrument over there. Okay? This over here is now the next part. So this particular instrument had two parts to it which is the sciser and this is the counter. So what do you do? How do you count these particles? They're in the nanometer size range. This instrument can go up to three nanometers. So now you've got three nanometer particle but how are you going to count it, okay? So then you use a trick. You pass these particles which are coming up to three nanometer size. You pass them through a chamber which is saturated with alcohol. Nowadays they also have it with water but the routine, the one that I have used used to have alcohol, okay? This is alcohol vapor. So you have this section, a tube, a flow in which particles of three nanometer size or five nanometer size or 10 nanometer size depending on which size is coming through at that time is going through this space and this cavity is completely saturated with alcohol vapor, okay? Now temperatures are lower now. So this particle which was three nanometers or five nanometers or 10 nanometers actually now begins to have condensation begin to take place on it and that condensation makes this particle which is very small in size grow to a certain size which is now detectable optically. So the same principle that we had used for optical particle counting can now be used but the optical particle counted counters were limited to 100 nanometers but if you have particles smaller than 100 nanometers then optically you can't detect it. So here's the trick you use. You actually use alcohol vapor condensation on each of these individual particles. You make them grow in size and then you detect them optically, okay? So that's the nature of the instrument and that's how it works. So I actually have half a minute left. I'm gonna just quickly flip through these. I have one more lecture with you, okay? I have one more lecture with you so I will review everything that we have done include complete some of this work over here. I've got about eight slides which I will take over in the next time, okay? But let me just flip through. This is some more details. I've already talked about this. This is some of the other instruments that are used. This just again gives you the cost. I'll upload this on Moodle just now after this lecture, okay? You're more than welcome to see it. Again, you know, when we are done with elephants and dinosaurs, what do we left with ants? We haven't dealt with gas analysis, okay? So for gas analysis, you basically, these are the gases criteria pollutants. You have primary gases and secondary gases and then you left with, I said this in the beginning of the lecture, not all particles are created equal. However, all gases are created equal but the chemistry is different so you can use the chemistry to be able to use different detection methods. These are the different methods by the U.S. Environmental Protection Agency, oxides of sulphur, oxides of nitrogen, carbon monoxide and ozone. These are the different methods which are used to detect. I'm giving you these two papers to read by June 12th. Okay, before June 12th, before the course ends, I'd like for you to read these two classic papers on, so maybe spend half an hour on each. If you started to read each one of them rigorously, it'll probably take you six, seven hours, okay? But just spend half an hour on each to get a sense of what these papers are before June 12th and then you can spend some more time on it. Okay, ladies and gentlemen, sorry I'm, you know, two minutes over time. I appreciate your time and your patience and your attention. Have a lovely cup of tea, coffee, whatever you're having and I'll see you back in about 28 minutes. Thank you, bye-bye.