Stratospheric Air Intrusions Promote Global-Scale New Particle Formation (Invited)

New particle formation (NPF) in the free troposphere is an important source of global CCN, thereby affecting the cloud properties and the Earth’s radiative balance. The prevailing view is that in the remote free troposphere, new particles are formed predominantly in cloud outflow regions because convective clouds loft precursors (e.g., dimethyl sulfide) from the boundary layer and remove most of the preexisting particles that would otherwise compete with NPF as sinks for condensable vapors. However, several studies suggest that the frequently observed nucleation mode particles in the upper troposphere (UT) are unlikely to be fully explained by NPF associated with convective clouds. In this study, we present observational evidence of NPF in the UT induced by stratospheric air intrusion (SAI), i.e., dynamic descent of stratospheric air into the troposphere, using airborne measurements during the NASA’s North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) and Atmospheric Tomography Mission (ATom). During the SAI events, the mixing of ozone-rich stratospheric air with more moist free tropospheric background leads to increased hydroxyl radical (OH) concentrations. Such mixing is most frequent near the tropopause, where the sulfur dioxide (SO₂) mixing ratios are high. The combination of increased SO₂ and OH levels results in enhanced sulfuric acid concentrations, thereby promoting NPF. Such NPF occurs frequently and over large geographic regions, likely representing an important particle source in the midlatitude free troposphere.