Abstract
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.
Highlights
New particle formation (NPF) is a key process driving the aerosol number in the atmosphere; it is the source of over half of the atmosphere’s cloud condensation nuclei, influencing cloud properties and Earth’s energy balance
We examined the influence of the very specific conditions encountered at high altitudes on NPF, such as cloud presence at the sites, higher ion concentrations, and lower condensational sinks (CS), in comparison to boundary layer (BL) sites
No seasonal (JFJ and CHC), negative ion formation rates were higher than the positive ion formation rates, which variation of nucleation rates measured over a year long period or more at Northern hemisphere high was not the case for the Puy de Dôme Station (PUY) station located at the lowest altitude or for the average of the BL sites
Summary
New particle formation (NPF) is a key process driving the aerosol number in the atmosphere; it is the source of over half of the atmosphere’s cloud condensation nuclei, influencing cloud properties and Earth’s energy balance. According to Merikanto et al (2009) [8], 45% of global low-level cloud CCN at 0.2% supersaturation could derive from nucleation, with a significant fraction created in the free troposphere (FT) (35%) This modeling study revealed that nucleation is expected to be a major contributor to the total particle number concentration in the upper troposphere [8]. (2016) [12] combined model and chamber experiments at low temperatures representative of high altitudes, suggesting a large source of new particles via nucleation in the upper free troposphere as well. We gathered information from experimental NPF studies performed at high altitude (above 1000 m a.s.l.) ground-based stations in order to provide an overall picture of the frequency of occurrence of the process as well as particle formation and growth rates with their seasonal variability. We examined the influence of the very specific conditions encountered at high altitudes on NPF, such as cloud presence at the sites, higher ion concentrations, and lower CS, in comparison to BL sites
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