Upper Tropospheric Particle Formation by Stratospheric Air Intrusion

Jiaoshi Zhang, Xianda Gong, Ewan Crosbie, Glenn Diskin, Karl Froyd, Sam Hall, Agnieszka Kupc, Richard Moore, Jeff Peischl, Andrew Rollins, Joshua P. Schwarz, Michael Shook, Chelsea Thompson, Kirk Ullmann, Christina Williamson, Armin Wisthaler, Lu Xu, Luke Ziemba, Charles Brock, JIAN WANG, Washington University in St. Louis

     Abstract Number: 512
     Working Group: Remote and Regional Atmospheric Aerosol

Abstract
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. Over the remote marine atmosphere, NPF has been frequently observed in the outflow region of convective clouds that loft precursors (e.g., dimethyl sulfide) from the boundary layer and remove the pre-existing 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.