Physicists Have Figured Out What Helps Smog Particles Form Very Quickly
Physicists from CERN found that aerosol droplets can form very quickly due to concentrated nitric acid vapors and ammonia compounds in the air. This explains why smog forms in megacities even in winter, and also suggests that a similar process can take place in the upper atmosphere, thus affecting global warming, according to the results of the experiments published in the scientific journal Nature.
"This explains how new aerosol particles can form in the highly polluted air of megacities. In addition, we now know how aerosols can occur in the upper atmosphere, where they can strongly affect the climate," said the study's leader, Professor Neil Donahue of Carnegie Mellon University.
Aerosols are very small drops of liquids. Due to their size, they can float in the air for a very long time. It is these particles that make up the fog, and they play a crucial role in the formation of clouds and urban smog.
Back in 2009, specialists from the European organization for nuclear research (CERN) created a special CLOUD installation that can reproduce the formation of these particles and study how the physical processes that are associated with their growth and accumulation in the atmosphere affect human health and the climate of the planet.
In particular, back in 2011, scientists discovered that sulfuric acid and ammonia, which were previously considered one of the main "progenitors" of aerosols, participate in the formation of only a small proportion of them in the atmosphere. Two years later, CERN physicists, including Russian researchers from the RAS Institute of Physics, found that other nitrogen compounds, including amines and nitric acid, are involved in the formation and growth of aerosols.
In a new series of similar experiments, Donahue and his CLOUD colleagues became interested in how another nitrogen compound, ammonium nitrate, affects aerosol formation. The molecules of this substance themselves can almost not evaporate into the atmosphere, but they can be formed on the surface of existing aerosol drops of ammonia and nitric acid molecules.
Based on these considerations, scientists tracked how aerosol particles are formed at different temperatures. To do this, they launched the CLOUD installation pairs of both substances. It turned out that thanks to ammonium nitrate and its initial components, aerosol particles grow much faster at low temperatures – by tens or even hundreds of times.
Scientists note that such conditions, as well as the necessary concentrations of ammonia and nitric acid vapors, may well develop in winter in densely populated megacities. This explains how smog can occur in conditions where scientists previously thought its formation was limited by the low growth rate of aerosol particles.
Scientists suggest that the same thing can happen in the upper atmosphere of the Earth. The temperatures that prevail there are low enough for the particles to form directly from ammonium nitrate. As a result, aerosols can form there as quickly as possible. Their drops, according to physicists, can play an important role in cooling the planet and in other climatic processes, the nature of which has yet to be studied.