Rapid growth of pollutant particles in cold urban air

Delhi, one of the most polluted cities in the world, every year around winter a cold toxic smog envelops the city. During the lockdown its air quality had improved, but still as winter came around and the city started to open up, so did the pollution. There are serious health implications because of these kinds of smog. This smog caused because of Particulate Matter (PM) in the atmosphere, it is the leading cause of lung disease and is thought to even contribute to neurological diseases like Alzheimer’s. A better understanding of the particulate matter that is causing smog is very much required to mitigate air-pollution.

The particles in the atmosphere are broadly classified into two categories: primary particles, which form directly from combustion, or other human activities and secondary particles, which form in the atmosphere from other gaseous pollutants like sulphuric acid, ammonia, nitric acid and other volatile organic compounds. The secondary particles condense and stick to the surface of other existing particles. The Winter smog is a mixture of both these kinds of particles. For secondary particles to survive they need to grow rapidly, but how they grow so rapidly in urban areas is still a puzzle. Observations have suggested that in urban air vapour and particles are in equilibrium, so the growth rate is low. But, researchers have found in a laboratory experiment that in the presence of common nitrogen emissions from vehicles the particles grow a lot faster.

These nitrogen compounds are thought to have concentrations near their equilibrium values in atmosphere, so they were thought to not play much of a role in particle growth. In the Upper Atmosphere, typically in cold temperatures, nitric acid and ammonia react to form ammonium nitrate. Ammonium nitrate condenses onto other particles and this significantly adds to the total no of particles.

Any sort of combustion in cities form high atmospheric concentrations of gaseous pollutants, these pollutants readily condense into tiny clusters, around 10nm in diameter. But, it’s a lot harder for these tiny clusters to combine and form larger ones, they just evaporate, because of an effect identified by Kelvin in 1871: Smaller clusters have greater curvatures so they tend to have higher surface vapour pressures in turn implying that they evaporate more readily if the particle is smaller.

As clusters grow, they reach a threshold size where they get caught by a larger particle and stick to its surface. As a result, the number of particles does not increase. The rate at which they condense onto a larger particle (an aerosol) is called the condensation sink rate. The ability of small, new clusters to grow into larger ones depends on the ratio of the condensation sink rate and the growth rate.

Fig. 1
Figure 1a: Particle nucleation and growth (particle growth rate, ddp/dt) at −10 °C from a mixture of 0.44 pptv sulfuric acid and 1,915 pptv ammonia at 60% relative humidity Figure 1b: Particle formation and growth under identical conditions to those in a, but with the addition of 24 pptv of nitric acid vapour formed via NO2 oxidation. After the particles cross 5nm in diameter they grow rapidly.
Figure 1c: Observed growth rates after activation versus the product of measured nitric acid and ammonia levels at +5 °C and −10 °C. 
(Courtesy of Wang, M. et al. Rapid growth of new atmospheric particles by nitric acid and ammonia condensation, Nature)

Figure 1 shows the values observed by researchers working at the CLOUD(Cosmic’s Leaving OUtdoor droplets (does that short form make any sense?)) chamber at CERN, a chamber which essentially simulates the atmosphere by varying the temperatures and gas concentrations. It also contains a proton synchrotron which serves as an artificial source of cosmic rays, also simulating the atmospheric conditions. Now coming back to the figure, Figure 1a is particle diameter growth under normal conditions i.e no activation with any nitrogen compounds whereas, Figure 1b shows the diameter growth when activated with Nitric acid vapour, when particle reaches. a diameter of 5nm it grows very rapidly. Figure 1c, shows the growth rate at -10°C and 5°C. It is clearly evident that we need lesser concentration of nitrogen compounds at -10°C than at 5°C to get the same growth rate, it implies that in colder temperatures particles grow more easily.

Figure 2: The CLOUD chamber at CERN
(Image: Maximillien Brice/ CERN)

Majority of ammonia formed in urban areas is due to the catalytic converters in vehicles and that in rural areas is associated with cow dung. The most common pathway for formation of ammonium nitrate keeps it in chemical equilibrium with ammonia and nitric acid. So, it is unlikely that this mechanism will increase the density of particles, unless the atmosphere is super-saturated with nitric acid and ammonia with respect to ammonium nitrate i.e, nitric acid and ammonia are in a lot higher concentration than the product (ammonium nitrate), they quickly react to restore equilibrium in turn increasing. the number of particles. Even a small amount of supersaturation is enough to increase the growth rate by a lot.

Because ammonium nitrate is semi volatile, it was thought to not play an important role in particle growth, where low vapour pressures are required. So sulphuric acid was thought to be the main constituent for particle growth, but this isn’t true as demonstrated earlier by the researchers at CLOUD, it’s the saturation ratio not the vapour pressure that governs the particle growth rate. Furthermore, nitric acid and ammonia vapours are 1000 times more abundant in the atmosphere than sulphuric acid making the rate of growth a lot higher.

In experiments done at CLOUD, the researchers found that at any temperature lower than 5°C , ammonium nitrate and sulphuric reacted to form small clusters of molecules. Once these clusters of ammonium sulphate reached a certain threshold size, as discussed previously, ammonium nitrate condensed onto this cluster resulting in rapid particle growth above 100nm/hr – as long as the temperature remained higher than -15°C. If the temperature was colder than -15°C, which occurs in humid airflows above certain clouds, then the nitric acid reacted with ammonia forming ammonium nitrate which grew further by additional condensation.

The “threshold size” at which rapid growth happens depends on the concentrations of nitric acid and ammonia. Once that threshold is reached rapid growth happens until the equilibrium is restored.

Why is this observation so important?

In the relatively clean and “cold” upper atmosphere, where nitric acid is abundant due to electrical storms and ammonia, through convection from liquid water in clouds makes this observation all the more important.

More these particles grow, more is the pollution and more are the health risks.

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