Stratospheric air intrusions promote global-scale new particle formation

The prevailing view is that in the free troposphere, new particles are formed predominantly in convective cloud outflows. We present another mechanism using global observations.

We find that during stratospheric air intrusion events, the mixing of descending ozone-rich stratospheric air with more moist free tropospheric background results in elevated hydroxyl radical (OH) concentrations. Such mixing is most prevalent near the tropopause where the sulfur dioxide (SO 2 ) mixing ratios are high. The combination of elevated SO 2 and OH levels leads to enhanced sulfuric acid concentrations, promoting particle formation.