High occurrence of new particle formation events at the Maido high-altitude observatory (2150 m), Reunion (Indian Ocean)

This study aims to report and characterise the frequent new particle formation (NPF) events observed at the Maido observatory, Reunion, a Southern Hemisphere site located at 2150m (a.s.l.) and surrounded by the Indian Ocean. From May 2014 to December 2015, continuous aerosol measurements were made using both a differential mobility particle sizer (DMPS) and an air ion spectrometer (AIS) to characterise the NPF events down to the lowest particle-size scale. Carbon monoxide (CO) and black carbon (BC) concentrations were monitored, as well as meteorological parameters, in order to identify the conditions that were favourable to the occurrence of nucleation in this specific environment. We point out that the annual NPF frequency average (65 %) is one of the highest reported so far. Monthly averages show a bimodal variation in the NPF frequency, with a maximum observed during transition periods (autumn and spring). A high yearly median particle growth rate (GR) of 15.16 nm h(-1) is also measured showing a bimodal seasonal variation with maxima observed in July and November. Yearly medians of 2 and 12 nm particle formation rates (J(2) and J(12)) are 0.858 and 0.508 cm(3) s(-1), respectively, with a seasonal variation showing a maximum during winter, that correspond to low temperature and RH typical of the dry season, but also to high BC concentrations. We show that the condensation sink exceeds a threshold value (1 : 04 x 10(3) s(-1)) with a similar seasonal variation than the one of the NPF event frequency, suggesting that the occurrence of the NPF process might be determined by the availability of condensable vapours, which are likely to be trans-ported together with pre-existing particles from lower altitudes.