Climatology of new particle formation at Izana mountain GAW observatory in the subtropical North Atlantic

A climatology of new particle formation (NPF) events at high altitude in the subtropical North Atlantic is presented. A 4-year data set (June 2008-June 2012), which includes number size distributions (10-600 nm), re-active gases (SO2, NOx, and O-3), several components of solar radiation and meteorological parameters, measured at Izana Global Atmosphere Watch (GAW) observatory (2373m above sea level; Tenerife, Canary Islands) was analysed. NPF is associated with the transport of gaseous precursors from the boundary layer by orographic buoyant upward flows that perturb the low free troposphere during daytime. On average, 30% of the days contained an NPF event. Mean values of the formation and growth rates during the study period were 0.46 cm(-3) s(-1) and 0.42 nm h(-1), correspondingly. There is a clearly marked NPF season (May-August), when these events account for 50-60% of the days per month. Monthly mean values of the formation and growth rates exhibit higher values in this season, 0.49-0.92 cm(-3) s(-1) and 0.48-0.58 nm h(-1), respectively. During NPF events, SO2, UV radiation and upslope winds showed higher values than during non-events. The overall data set indicates that SO2 plays a key role as precursor, although other species seem to contribute during some periods. Condensation of sulfuric acid vapour accounts for most of the measured particle growth during most of the year (similar to 70 %), except for some periods. In May, the highest mean growth rates (similar to 0.6 nm h(-1)) and the lowest contribution of sulfuric acid (similar to 13 %) were measured, suggesting a significant involvement of other condensing vapours. The SO2 availability seems also to be the most influencing parameter in the year-to-year variability in the frequency of NPF events. The condensation sink showed similar features to other mountain sites, showing high values during NPF events. Summertime observations, when Izana is within the Saharan Air Layer, suggest that dust particles may play a significant role acting as coagulation sink of freshly formed nucleation particles. The contribution of dust particles to the condensation sink of sulfuric acid vapours seems to be modest (similar to 8% as average). Finally, we identified a set of NPF events in which two nucleation modes, which may evolve at different rates, occur simultaneously and for which further investigations are necessary.