Ammonium nitrate particles formed in upper troposphere from ground ammonia sources during Asian monsoons

The rise of ammonia emissions in Asia is predicted to increase radiative cooling and air pollution by forming ammonium nitrate particles in the lower troposphere. There is, however, a severe lack of knowledge about ammonia and ammoniated aerosol particles in the upper troposphere and their possible effects on the formation of clouds. Here we employed satellite observations and high-altitude aircraft measurements, combined with atmospheric trajectory simulations and cloud-chamber experiments, to demonstrate the presence of ammonium nitrate particles and also track the source of ammonia that forms into the particles. We found that during the Asian monsoon period, solid ammonium nitrate particles are surprisingly ubiquitous in the upper troposphere from the Eastern Mediterranean to the Western Pacific—even as early as in 1997. We show that this ammonium nitrate aerosol layer is fed by convection that transports large amounts of ammonia from surface sources into the upper troposphere. Impurities of ammonium sulfate allow the crystallization of ammonium nitrate even in the conditions, such as a high relative humidity, that prevail in the upper troposphere. Solid ammonium nitrate particles in the upper troposphere play a hitherto neglected role in ice cloud formation and aerosol indirect radiative forcing.