Upper Tropospheric Particle Formation from Anthropogenic Pollution

The major remaining uncertainty in climate projections stems from the aerosol-cloud-climate interactions in the atmosphere. Therefore, an accurate representation of aerosol particles and clouds is the foundation of any effort to forecast long-term climate change. Aerosol particles in the upper free troposphere are especially important because they are thought to provide at least one-third of global cloud condensation nuclei (CCN). However, the precursor vapors and associated mechanisms that drive particle formation at altitude are poorly understood.

This talk will present recent results that fill the knowledge gap on how anthropogenic pollution drives particle formation in the cold upper troposphere, with experiments performed in the CERN CLOUD chamber. We show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high levels of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region. Once particles have formed, the co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid particle growth to CCN sizes with only trace sulfate. Furthermore, our measurements show that these CCN are also highly efficient ice nucleating particles (INP), comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid multi-acid HNO₃–H₂SO₄–NH₃ nucleation in the upper troposphere and producing INP that spread across the mid-latitude Northern Hemisphere.