Is Delhi the PRIMARY source for Chennai Smog? Physics says NO

The plume picture has sparked a debate saying Delhi's bad air has traveled to Chennai resulting in Chennai smog.

Well, well, well, pictures without physics based explanation are extremely dangerous and some weather bloggers are proving their point simply using the images.

Here, I try to explain the physics (in my capacity from what I know about fluid dynamics and thermodynamics). Before that, let us lay downs some facts.

Fact: Chennai is reeling under a smog. No doubt about it. Some people are diverting the debate to smog vs fog. 

Realise that smog is nothing but smoke+fog.
So, fog is indeed a part and parcel of smog. The smoke particles get attached to the misty fog. Even if the fog goes away (due to solar convection), the smoke can hang around like a haze or drop down to ground levels (resulting in irritation and pollution to your face - when you wipe down, you will see brown or black dust). But it is a cycle, where fog will again form due to dew point concept. So let's not separate it out.

Myth: Is the primary source Delhi? 

Delhi may be a source, but not the primary. It is likely to contribute only to 10-15% of the Chennai smog. The primary source is local or sub-regional pollution.

 Let us start with the "famous" plume picture:

Before jumping into the physics, let us talk about some inconsistencies with the debate.

Inconsistency #1. Let's consider that the plume source is ONLY from our capital Delhi. 

How is it possible to have an intermediate "relatively low level" pollution zone and then sudden "high level" zone near Chennai?

How can air get diluted and then suddenly get concentrated 

(if we are only talking about the source from Delhi)?

Inconsistency #2: Chennai people are agreeing that it is due to weak winds. If it is indeed weak winds, the how is such a large momentum supplied to the plume (+the smoke) to travel some 2000 odd kms? 

Momentum is directly related to wind velocity and if wind is weak, how come particles stay suspended in them for such a long long time (almost 2-3 days travel from Delhi). Shouldn't particles get suspended out by then or fall some place else before reaching Chennai?

This is completely bizarre to me.

Inconsistency #3: In this 2000 km journey by the plume, it is assumed that there is absolutely no interaction with the topography (where topography help in producing lee wave phenomenon or other local wave phenomenon), which has the tendency to completely dilute the pollutants (by mixing with relatively cleaner air). Dilution means, if I start with 1000 ug/cu.m of polluted air, then due to physical laws of mixing, I end up with 100 ug/cu.m of air.

So going by this Delhi should be a MEGA MEGA gas chamber. 

Now, on to Physics:

1. From simple analytical plume theory, it is clear that pollutants get diluted (to 2-3 orders of magnitude) within 200-300 kms of the source. It has an exponential behaviour. Hence, Haryana air can reach Delhi in a deadly (but again somewhat diluted) fashion, but it is highly highly unlikely that Delhi's deadly air can reach Chennai (Fick's dilution law doesn't permit this).

2. If you look at the temperature profile, you see a clear inversion layer. 

Inversion layer is one where temperature suddenly starts to increase with height resulting in a highly stable system, which will trap all the pollutants. 

In the image, I have marked the inversion layer, which forms close to the ground (near the planetary boundary layer) during winter transition.

The winter transition is felt at lower latitudes when Northerly flow of winds happen, bringing relatively cold air down South. This aids in formation of the inversion layer, which traps the local pollutants. Such a wind flow (unfortunately) is routinely misunderstood or assumed as bad air coming from North. It's not the bad air, but most importantly cold air.

Like I already said, in the inversion layer, pollutants+mist (fog) can get trapped. Once sun rises, then fog will get evaporated, but smoke will stay on, causing irritation.

3. Given the dilution, topography effect, how can pollutants stay in air for such a long time, when they have short residence times (meaning a strong eddy can totally push them out of the boundary layer to upper atmosphere). So, in this 2000 km journey, there was not even a single eddy that was present. This is impossible, given that "atmosphere is a turbulent machine".

Final conclusion:

All of the above physics based reasoning, point to the fact that the primary source of Chennai Smog is local or sub-regional effect. Delhi may just be secondary supplier (at the most, even that is doubtful to me).

It is important to take the model outputs with a pinch of salt. Not all the times, model outputs may be correct, but physics is always right. So it is important to join the model outputs with physics before jumping to conclusions. 

Some weather bloggers simply put out this image and public can get convinced. But sometimes, what you always see may not be true and there may be some hidden truth to it.

Question lingering: Chennai sees smog during Bhogi or other festival. During October, November, Chennai sees acceptable air quality.

Well,  North east monsoon rains have a big role to play. 

1. But this time around weather dynamics are very different. The winds are completely unfavourable for Chennai, thanks to the two cyclones that formed in the Arabian Sea.

2. Winter-transition started pretty quickly in India. The onset of winter is in the north, but its  effect is felt throughout India (thanks to the sun hemispherical angle and the radiation effect). As already stated, winter time inversion layer is pretty shallow, which coincides with the Northerly wind pattern.

3. Chennai or for that matter any city in India is not new to pollution and smog. This time around it has coincided with multiple events, which are not in favour.

Hence, Delhi is NOT to be blamed for Chennai's pollution woes. 

It is time the city took care of its own problem rather than blaming others. Also, it is prudent NOT to put all the faith or blindly follow the local weather blogger's opinion. Do your own research or sometimes trust IMD.

Disclaimer: The content given here are all personal views and the author will not be held responsible or held liable for any oversight or misinterpretation.