Seminar

Abstract

Aerosol particles in the stratosphere have long been measured by satellite and balloon-borne sensors. These sensors, however, have limited sensitivity for particles smaller than 0.15 μm. While some evidence for particles smaller than 0.1 μm in the stratosphere has been published, these observations are mostly limited to altitudes below 12 km and ozone levels lower than 500 ppbv–at the very bottom of the stratosphere. As a result, the size distribution of smaller particles in the stratosphere, although contributing significantly to the aerosol surface area and sink for condensable species, which is very significant in the lower stratosphere, remains poorly quantified.

In 2023, we conducted in situ measurements of the size and composition of aerosol particles in the stratosphere, ranging from 3 nm to 2.4 μm at altitudes of up to 19 km, and ozone levels of 3000 ppbv. We found the widespread presence of a mode of aerosol particles in the lowest part of the stratosphere (N2O > 270 ppbv) with surface-area-weighted mean diameters from ~0.09 to 0.22 µm. This mode, poorly sensed by satellites and current balloon instruments, dominates the surface area available for heterogeneous chemistry and the sink for condensable vapors. It is composed mostly of particles transported from the troposphere and is rich in organics. As this air of recent tropospheric origin mixes with older stratospheric air containing larger particles consisting mostly of sulfuric acid and trace meteoric and spacecraft metals, this mode of tropospheric particles increases in size and diminishes in concentration due to coagulation and mixing. The presence of this small tropospheric aerosol mode may be especially important for proposed stratospheric aerosol injection (geoengineering) efforts in the lower stratosphere.

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Ming Lyu received her PhD in Atmospheric-Analytical chemistry from the University of Alberta, Canada, for work on wildfire brown carbons from boreal peatland. She joined CIRES/NOAA Chemical Sciences Lab in 2022 as a research scientist, where her work focuses on the processes and microphysical properties of aerosols in the upper troposphere and lower stratosphere.