An unrecognized mode of small particles in the lower stratosphere.

Analysis of recent in situ data reveals a persistent mode of organic-rich aerosol particles in the stratosphere below 19 kilometers at nitrous oxide (N2O) > 270 parts per billion by volume, with a number geometric mean diameter of ~0.03 to 0.11 μm (0.08 to 0.2 μm in surface and 0.11 to 0.3 μm in volume). This mode, composed mostly of organic-rich particles transported from the troposphere, is poorly sensed by satellites and most balloon-borne optical measurements but dominates the surface area for heterogeneous reactions and the sink for condensable vapors. These small particles grow in size and decrease in concentration as they mix with older stratospheric air. A global chemistry-climate model fails to replicate the characteristics of these particles, suggesting that model improvements are necessary for accurate assessment of proposed geoengineering efforts.