4.3. Aerodynamic Particle Sizer

Now this is a very specific company. That is why it is called as an RSPM. People design this based on the impactor, here it is a cyclone in this particular instrument. It doesn’t have to be a cyclone, can be anything and it uses a large filter paper which is a sheet that uses a glass microfiber filter of 8 by 10 inches which is like an A4 size sheet, large filter paper. And you can sample 8 hours and 1000 liters per minutes, large volume you can imagine how much of air is going through that so the amount of particulate it collects is significant. So at the end of it you weigh the filter paper before and after and you get total mass divided by total volume. So you have total volume. So, 1000 liters multiplied by 8 hours into 60 minutes per hour you get total volume in meter cube in liters, total volume is available here. And then whatever is a weight that you collect on the filter paper, you divide by this number you will get concentrations typically, the concentrations of PM in air are reported as micrograms per meter cubed, all standards are recorded in this unit microgram not milligrams per liter micrograms per meter cubed, very small amount.

High volume samplers, like the one referred to as RSPM, are specifically designed to measure particulate matter (PM) in the air. These devices use large filter papers to collect air samples over significant periods, often eight hours or more. After sampling, the filter is weighed before and after to determine the mass of particulate matter collected. The total volume of air sampled is calculated based on the flow rate (1000 liters per minute) multiplied by the duration of sampling (8 hours). The final concentration of PM is expressed in micrograms per cubic meter, reflecting the small quantities often present in ambient air.

Imagine a large sponge soaking up water from a bucket over a period of time. The sponge represents the filter paper that collects air particles like the water. Once the sponge is full, you weigh it to see how much water it absorbed. Similarly, in air sampling, we collect air over time, weigh the filter to see how much particulate matter is there, and express it in very small units since particles are often in minuscule quantities.

PM 2.5 is all 24-hour sampling intervals measurement, so, you cannot do anything. So, I cannot get any information on a lower timescale it is all 1-day average, 24-hour average only. But this is ok because it again depends on the objectives. The way we use this data is we are looking at long timescales, we are saying what happens this year, next year in 10 years, what will happen if this industry stops polluting, this industry starts polluting and so on.

Measurements of PM 2.5 are typically taken over a full 24-hour period, producing averages that provide insights into air quality trends rather than short-term fluctuations. This long timescale is essential for policymakers, who need to assess air quality improvements or deterioration over years. While this approach might seem limited, it serves the goal of understanding larger trends rather than minute-to-minute variations.

Think about measuring how much water a garden receives over the entire month instead of checking every day. By averaging the total water received each day, you can understand if the garden is receiving enough water over time, which helps in planning future irrigation. Similarly, 24-hour PM 2.5 measurements give insights into long-term air quality rather than just daily ups and downs.

What we are looking at in the previous 2 slides is PM 10, PM 2.5 one that is it done, but what if I want a distribution, I want distribution of particle sizes in different sizes how much is there in each size, then I can decide whether. So, this is what is called as cascade impactor, cascade impactor is just an impactor but you have multiple impactors that one place after the other. You can see this schematic diagram here that it is going in the first one, there is the first impact that goes through this one.

Cascade impactors are advanced tools used to measure the distribution of particle sizes within air samples. Unlike simpler samplers that merely measure total PM concentrations, the cascade impactor captures particles at different sizes as they pass through sequential stages or 'impactors.' Each stage filters out particles of a specific size, allowing researchers to analyze and categorize the size distribution of PM in the air continuously.

Imagine a set of sieves that separate different-sized grains of sand from a pile. Each sieve catches grains above a certain size, allowing smaller grains to pass. Similarly, cascade impactors work like these sieves, capturing various sized particles in the air at different stages, which helps us understand what types of pollutants are present.

This is a problem so when you are a regulatory agency you really like to know real time information; you would like to know what is going on real time so that you can focus your energies on fixing where the problem is which is a biggest source of pollution. And, like in investigations is really like, you know, proper investigation or you are looking for clues and you are kind of connecting with what is the cause of these things.

Regulatory bodies prioritize access to real-time data on particulate matter levels to respond effectively to pollution sources. Real-time monitoring helps identify pollution spikes, enabling quicker intervention and policy action. It functions like an ongoing investigation into air quality where continuous data collection supports actionable insights, directing efforts to areas needing immediate attention.

Think of a doctor monitoring a patient's heart rate in real time using a monitor. If the heart rate spikes, the doctor can respond immediately, adjusting treatment as needed. Similarly, by tracking air quality in real time, agencies can react quickly to pollution bursts and take steps to control them.

There are other methods of doing real time analysis. I am going to ask a question at the end of this discussion and you have to be prepared for that. There are optical methods in which there is a laser and it is based on scattering principle there is a particle light hits it, it scatters and is based on the scattering law, the amount of scattering and angular scattering depends on the size of the particle.

Advanced optical methods utilize lasers to measure particulate matter in real time. When light from the laser hits particles, it scatters. The scattering pattern—how much and in what direction—provides information about the size and concentration of the particles. This technology allows for continuous monitoring and immediate data processing.

Consider how rain droplets scatter sunlight, creating a rainbow. The size and angle of the droplets affect how light scatters. In particle measurement, when a laser hits airborne particles, the way they scatter light tells us about their size and density—similar to observing light effects in nature.

This is called as an aerodynamic particle sizer. And as the name suggests this exactly does what the aerodynamic diameter definition is. It takes the particle and it finds out what is the momentum of the particle, how fast it is moving at a certain size. And then based on that it calculates, what is the size of the particle? So, there are 2 lasers it calculates how much time it takes for a particle go from one place to another place, and it is a very sophisticated equipment called as APS, this is not an optical method. This is aerodynamic method.

The Aerodynamic Particle Sizer (APS) detects particles based on their aerodynamic properties. By measuring how long it takes for a particle to travel between two points using lasers, the APS calculates the particle's size based on its momentum and velocity. This sophisticated technique does not rely on traditional optical methods, making it a powerful tool in air quality measurement.

Imagine a runner going through a race. The time it takes them to complete a lap can give you insights into their speed and, in turn, their fitness. The APS uses a similar principle: it tracks particles across a set distance to determine their speed (momentum) and uses that information to figure out the size of the particles.

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