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

Abstract. New particle formation (NPF) is a key atmospheric process which may be responsible for a major fraction of the total aerosol number burden at the global scale, including in particular cloud condensation nuclei (CCN). NPF has been observed in various environments around the world, but some specific conditions, such as those encountered in volcanic plumes, remain poorly documented in the literature. Yet, understanding such natural processes is essential to better define preindustrial conditions in climate model simulations, as those form the baseline to calculate the radiative forcing caused by anthropogenic emissions. Here we report observations of NPF performed at the high-altitude observatory of Maïdo (2165 m a.s.l., La Réunion Island) between 1st January and 31st December 2015. During this time period, 3 effusive eruptions of the Piton de la Fournaise, located ~ 39 km away from the station, were observed and documented, resulting in 36 days of measurement in volcanic plume conditions to be compared with 250 non-plume days. This dataset is, to our knowledge, the largest ever reported for the investigation of NPF in tropospheric volcanic plume conditions, and allowed for the first time a statistical approach to characterize the process and also assess its relevance with respect to non-plume conditions. NPF was observed on 86 % of the plume days vs 71 % of the non-plume days during the 4 months when the eruptions occurred. The events were on average detected earlier on plume days, most likely benefiting from larger amounts of precursors available at the site prior to nucleation hours compared to non-plume days, during which condensable species were in contrast transported from lower altitude by the mean of convective processes. Surprisingly, the overall effect of the plume conditions on the particle growth rate was limited. However, with the exception of September, particle formation rates were significantly higher on plume days. The signature of the volcanic plume on the aerosol spectra up to dp = 600 nm was further investigated based on the analysis and fitting of the particle size distributions recorded in the different conditions. The spectra recorded prior to nucleation hours, in absence of freshly formed particles, featured a significant contribution of particles likely formed via heterogeneous processes at the vent of the volcano (and assimilated to volcanic primary particles) to the concentrations of the 2 accumulation modes on plume days. Later on in the morning, the concentrations of the nucleation and Aitken modes showed important variations on plume days compared to event days outside of plume conditions. The spectra recorded on event days, in and off-plume conditions, were further used to provide an average size distribution of the particles of volcanic origin, which clearly highlighted the dominant contribution of secondary over primary particles (96 %) to the total concentration measured on NPF event days within volcanic plume. In a next step, particular attention 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 of N50 was the most frequent in plume conditions, and the magnitude of the increase was also more important on plume days compared to non-plume days. Last, in order to further evaluate the effect of volcanic plume conditions on the occurrence of NPF, we analysed the variations of the condensation sink (CS) and [H2SO4], previously reported to play a key role in the process. Over the investigated months, higher CS (calculated prior to nucleation hours) were observed in plume conditions, and coincided with high SO2 mixing ratios. Those most likely compensated for the strengthened loss rate of the vapour and favoured the occurrence of NPF, suggesting at the same time a key role of H2SO4 in the process. This last hypothesis was further supported by the correlation between the formation rate of 2 nm particles (J2) and [H2SO4], and by the fair approximation of J2 that was obtained by the mean of a recent parameterisation of the binary nucleation of H2SO4 – H2O. This last result was of high interest as it also demonstrated that in absence of direct measurement of [H2SO4] and sub-2nm particles concentration, estimates of J2 could be obtained from the knowledge of SO2 mixing ratios only.