New particle formation (NPF) was predicted to contribute to a major fraction of free tropospheric particle number and cloud condensation nuclei (CCN) concentrations by global models. At high altitudes, pre-existing particle concentrations are low, leading to limited condensational sinks for nucleation precursor gases, and temperatures are cooler compared to lower altitudes, whereas radiation is higher. These factors would all be in favor of nucleation to occur with an enhanced frequency at high altitudes. In the present work, long term data from six altitude stations (and four continents) at various altitudes (from 1465 to 5240 m a.s.l) were used to derive statistically relevant NPF features (frequency, formation rates, and growth rates) and seasonal variability. The combined information together with literature data showed that the frequencies of NPF events at the two Southern hemisphere (SH) stations are some of the highest reported thus far (64% and 67%, respectively). There are indications that NPF would be favored at a preferential altitude close to the interface of the free troposphere (FT) with the planetary boundary layer (PBL) and/or at the vicinity with clouds, which otherwise inhibit the occurrence of NPF. Particle formation rates are found to be lower at high altitudes than at low altitude sites, but a higher fraction of particles are formed via the charged pathway (mainly related to positive ions) compared to boundary layer (BL) sites. Low condensational sinks (CS) are not necessarily needed at high altitudes to promote the occurrence of NPF. For stations at altitudes higher than 1000 m a.s.l., higher CSs favor NPF and are thought to be associated with precursor gases needed to initiate nucleation and early growth.
Bibliographical noteFunding Information:
Data analyzed for the present work were acquired within the frame of the project ACTRIS-2 (Aerosols, Clouds, and Trace gases Research InfraStructure) under the European Union-Research Infrastructure Action in the frame of the H2020 program for "Integrating and opening existing national and regional research infrastructures of European interest" under Grant Agreement NO654109. Measurements performed at PUY, MDO and CHC received support from CNRS-INSU and Ministry for Research and Education under ACTRIS-FR and long-term monitoring aerosol program SNO-CLAP. Measurements at CHC were made possible due to the support of UMSA through the Institute for Physics Research and the support from IRD (Institut de Recherche pour le Développement) under Jeune Equipe program CHARME awarded to LFA and by Labex OSUG@2020 (Investissements d'avenir-ANR10 LABX56). Measurements at PYR were carried out in the framework of the UNEP-ABC (Atmospheric Brown Clouds) and Ev-K2-CNR SHARE (Stations at High Altitude for Research on the Environment) projects. The contribution of CNRS through the PICS bilateral program between CNR and CNRS and through the LEFE-INSU program is gratefully acknowledged.
© 2019 by the authors.
- High altitude
- Mountain research stations
- New particle formation