Smog-forming particles may grow quickly in supersaturated urban air

Particles form and accrete in different atmospheric environments. Rapid growth is essential to their survival and contributes to episodes of urban smog. But how those particles grow so quickly in urban environments remains a puzzle. Now Neil Donahue of Carnegie Mellon University and colleagues have shown in a laboratory experiment that common vehicle pollutants may drive the particle-formation process. They found that gaseous nitric acid and ammonia, at temperatures and vapor pressures typical in winter months at many locations, condense into ammonium nitrate particles that grow large enough to survive in a saturated atmosphere.

Ammonium nitrate tends to exist at chemical equilibrium with its nitric acid and ammonia constituents, so it’s unlikely to directly contribute to particle growth unless the atmosphere becomes supersaturated with those gases. To investigate the conditions that may drive supersaturation and new particle formation, Donahue and his colleagues studied mixtures of gaseous nitric acid, ammonia, and sulfuric acid in CERN’s CLOUD chamber (pictured above). The researchers subjected the gases to a range of atmospheric temperatures, pressures, and electrical charges. They used mass spectrometry to identify the resulting secondary compounds and then measured the size distribution. At temperatures below 5 °C, ammonia and nitric acid condensed onto preexisting particles; below –15 °C, the gases nucleated directly to form particles of ammonium nitrate. The particles grew through condensation at rates above 100 nm/hr—fast enough to account for the rapid accumulation of new, large particles that are observed in urban environments.

The researchers propose that variations in temperature and emission sources in cold urban settings provide the conditions necessary for localized supersaturation of nitric acid and ammonia. By imposing regulations to control nitrogen emissions, urban planners could help to improve air quality by reducing the concentration of newly formed aerosols and the growth of existing ones. (M. Wang et al., Nature 581, 184, 2020 .)