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. New particle formation (NPF) is a key atmospheric process which may beresponsible for a major fraction of the total aerosol number burden at theglobal scale, including in particular cloud condensation nuclei (CCN). NPFhas been observed in various environments around the world, but somespecific conditions, such as those encountered in volcanic plumes, remainpoorly documented in the literature. Yet, understanding such naturalprocesses is essential to better define pre-industrial conditions and theirvariability in climate model simulations. Here we report observations of NPFperformed at the high-altitude observatory of Maido (2165 m a.s.l., LaReunion Island) between 1 January and 31 December 2015.During this time period, three effusive eruptions of the Piton de la Fournaise,located ∼39  km away from the station, were observed anddocumented, resulting in 29 d of measurement in volcanic plume conditionsto be compared with 250 “non-plume days”. This dataset is, to ourknowledge, the largest ever reported for the investigation of NPF intropospheric volcanic plume conditions, and it allowed for the first time astatistical approach to characterize the process and also assessment of itsrelevance with respect to non-plume conditions. NPF was observed on 90 %of the plume days vs. 71 % of the non-plume days during the 4 months whenthe eruptions occurred. The events were on average detected earlier on plumedays, most likely benefiting from larger amounts of precursors available atthe site prior to nucleation hours. The overall effect of the plumeconditions on the particle growth rate was limited. However, with theexception of September, particle formation rates were significantly higheron plume days. The signature of the volcanic plume on the aerosol spectra upto dp=600  nm was further investigated based on the analysis andfitting of the particle size distributions recorded under in-plume and off-plumeconditions. The spectra recorded prior to nucleation hours, in the absence offreshly formed particles, featured a significant contribution of particleslikely formed via heterogeneous processes at the vent of the volcano (andassimilated to volcanic primary particles) to the concentrations of the twoaccumulation modes on plume days. Later on in the morning, theconcentrations of the nucleation and Aitken modes showed importantvariations on plume days compared to event days outside of plume conditions.The spectra recorded on event days, under in-plume and off-plume conditions, werefurther used to provide an average size distribution of the particles ofvolcanic origin, which clearly highlighted the dominant contribution ofsecondary over primary particles (93 %) to the total concentrationmeasured on NPF event days within a volcanic plume. In a next step, particularattention was paid to the concentration of particles with dp>50  nm ( N50 ), used as a proxy for potential CCN population.The contribution of secondary particles to the increase in N50 was themost frequent in plume conditions, and the magnitude of the increase wasalso more important on plume days compared to non-plume days. Finally, in orderto further evaluate the effect of volcanic plume conditions on theoccurrence of NPF, we analysed the variations of the condensation sink (CS)and [ H2SO4 ], previously reported to play a key role in theprocess. Over the investigated months, higher CS (calculated prior tonucleation hours) were observed in plume conditions and coincided with high SO2 mixing ratios. Those most likely compensated for the strengthenedloss rate of the vapours and favoured the occurrence of NPF, suggesting atthe same time a key role of H2SO4 in the process. This lasthypothesis was further supported by the correlation between the formationrate of 2 nm particles ( J2 ) and [ H2SO4 ], and by the fairapproximation of J2 that was obtained by means of a recentparameterization of the binary nucleation of H2SO4 – H2O .This last result demonstrates that in the absence of direct measurements of[ H2SO4 ] and sub-3 nm particle concentrations, estimates of J2 could be fairly estimated from the knowledge of SO2 mixing ratiosonly. Finally, the use of the parameterization for ion-induced binarynucleation also highlighted the likely significant contribution of ion-induced nucleation for [ H2SO4 ] below ∼ 8 × 10 8  cm −3 .

New particle formation in the volcanic eruption plume of the Piton de la Fournaise: specific features from a long-term dataset