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This study investigates the processes and conditions favouring the formationof nanoparticles (diameter<10 nm) which are frequently observed on highmountains reaching the low free troposphere. This was done through ananalysis of a data set collected at Izaña Global Atmospheric WatchObservatory (Canary Islands; 2367 m above sea level). This highmountain supersite is located well above the stratocumulus layercharacteristic of the subtropical oceanic tropospheres. At night, when thecatabic flow regime is well established, free troposphere aerosols weremeasured. The development of orographic buoyant upward flows during daylightresulted in an increase of water vapour, SO2 and NOyconcentrations. These ascending airflows perturbed the free troposphere andresulted in high concentrations of 3–10 nm particles (N3–10) due to newparticle formation. An analysis of the 5-min average time series allowedthe identification of two main types of N3–10 event. In Type I events alinear relationship between N3–10 and SO2 was observed (r2coefficients 0.70–0.95 and a mean slope of 11 cm−3 ppt−1for 5-min averaged data; SO2 concentrations from tens to hundreds ofppt). These particles seem to be formed during upward transport (probablywithin or after the outflows of clouds typically located below Izaña).During Type II events, no correlation between SO2 and N3–10 wasobserved and 3–10 nm particles were formed in-situ at noon and during theafternoon due to the condensation of vapours linked to photochemistry. Newparticle formation was observed almost every day owing to the favourableconditions associated with the entry of boundary layer air in the low freetroposphere, even if SO2 concentrations are rather low at Izaña(tens to hundreds of ppt). The low surface area of pre-existing particles,low temperature and high radiation intensity clearly favoured the formationof nanoparticles. The low surface area of pre-existing particles in theupward flows is furthered by in-cloud particles scavenging in thestratocumulus layer typically located below Izaña. The highertemperature and the presence of coarse Saharan dust particles decrease theefficiency of the new particle formation mechanisms in summer. Thus, the"N3–10 versus SO2" slope (for r2>0.7 cases) was higher inautumn and winter (~15 cm−3 ppt−1 as average) thanin summer (2–8 cm−3 ppt−1). These field observationssuggest that elevated mounts that reaches the free troposphere may act assource regions for new particles

Atmospheric nanoparticle observations in the low free troposphere during upward orographic flows at Izaña Mountain Observatory