Ammonia Catalyzed Formation of Sulfuric Acid in Troposphere: The Curious Case of a Base Promoting Acid Rain.

Electronic structure calculations have been performed to investigate the role of ammonia in catalyzing the formation of sulfuric acid through hydrolysis of SO3 in Earth's atmosphere. The uncatalyzed process involves a high activation barrier and, until date, is mainly known to occur in Earth's atmosphere only when catalyzed by water and acids. Here we show that hydrolysis of SO3 can be very efficiently catalyzed by ammonia, the most abundant basic component in Earth's atmosphere. It was found, based on magnitude of relative potential energies as well as rate coefficients, that ammonia is the best among all the catalysts studied until now (water and acids) and could be a considerable factor in formation of sulfuric acid in troposphere. The calculated rate coefficient (at 298 K) of ammonia catalyzed reaction has been found to be ∼105-107 times greater than that for water catalyzed ones. It was found, based on relative rates of ammonia and water catalyzed processes, that in troposphere ammonia, together with water, could be the key factor in determining the rate of formation of sulfuric acid. In fact, ammonia could surpass water in catalyzing sulfuric acid formation via hydrolysis of SO3 at various altitudes in troposphere depending upon their relative concentrations.