A large source of cloud condensation nuclei from new particle formation in the tropics
Cloud condensation nuclei (CCN) can affect cloud properties and therefore Earth’s radiative balance1–3. New particle formation (NPF) from condensable vapours in the free troposphere has been suggested to contribute to CCN, especially in remote, pristine atmospheric regions4, but direct evidence is sparse, and the magnitude of this contribution is uncertain5–7 . Here we use insitu aircraft measurements of vertical profiles of aerosol size distributions to present a globalscale survey of NPF occurrence. We observe intense NPF at high altitudes in tropical convective regions over both Pacific and Atlantic oceans. Together with the results of chemical-transport models, our findings indicate that NPF persists at all longitudes as a global-scale band in the tropical upper troposphere, covering about 40 per cent of Earth’s surface. Furthermore, we find that this NPF in the tropical upper troposphere is a globally important source of CCN in the lower troposphere, where CCN can affect cloud properties. Our findings suggest that the production of CCN as new particles descend towards the surface is not adequately captured in global models, which tend to underestimate both the magnitude of tropical upper tropospheric NPF and the subsequent growth of new particles to CCN sizes.
New particles form in the atmosphere when condensing gases form stable clusters with diameters of more than 1.5 nm or so. Growth by condensation and coagulation may enable particles to reach diameters of more than around 60 nm, at which point they can act as CCN. Atmospheric observations are required to guide the incorporation of NPF mechanisms into models. Large numbers of small particles havepreviously been observed at high altitude in the tropics10–12, because deep convective clouds loft condensable vapours and remove most larger particles that would otherwise compete with NPF as sinks for these vapours13 (Fig. 1). Newly formed particles grow to CCN sizes in subsiding air outside of the convective clouds. Global-scale measurements are needed to understand the scale and impact of NPF in the upper troposphere. However, satellites cannot detect particles with diameters of less than 100 nm, and previous in situ observations have been of regional scale11–13. To address this, as part of the NASA Atmospheric Tomography Mission (ATom)15 we conducted in situ, global-scale measurements of particle size distributions over the Pacific and Atlantic oceans during multiple seasons, with near pole-to-pole coverage and systematic profiling between.
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