Aerosol particles can form and grow in Earth’s upper troposphere in an unexpected way. The new mechanism may represent a major source of cloud and ice seed particles in areas of the upper troposphere where ammonia is efficiently transported vertically. Aerosol particles are known to generally cool the climate by reflecting sunlight back into space and by making clouds more reflective. How new aerosol particles form in the atmosphere remains relatively poorly known?
Using mixtures of sulfuric acid, nitric acid and ammonia vapors in the chamber at atmospheric concentrations. The CLOUD team found that these three compounds form new particles synergistically at rates much faster than those for any combination of two of the compounds. The CLOUD researchers found that the three vapors together form new particles 10 to 1000 times faster than a sulfuric acid–ammonia mixture. Once the three-component particles form, they can grow rapidly from the condensation of nitric acid and ammonia alone to sizes where they seed clouds.
The CLOUD measurements show that these particles are highly efficient at seeding ice crystals. When a supercooled cloud droplet freezes, the resulting ice particle will grow at the expense of any unfrozen droplets nearby. The CLOUD researchers went on to feed their measurements into global aerosol models that include vertical transport of ammonia by deep convective clouds.