New particle formation in the volcanic eruption plume of the Piton de la Fournaise: specific features from a long-term dataset
Authors: Rose Clémence , Foucart Brice , Picard David , Colomb Aurélie , Metzger Jean-Marc , Tulet Pierre , Sellegri Karine 
Published: October 2019

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 pre-industrial conditions and their
variability in climate model simulations. Here we report observations of NPF
performed at the high-altitude observatory of Maïdo (2165 m a.s.l., La
Réunion 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 and
documented, resulting in 29 d 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 it allowed for the first time a
statistical approach to characterize the process and also assessment of its
relevance 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 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. 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 under in-plume and off-plume
conditions. The spectra recorded prior to nucleation hours, in the 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 two
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, under in-plume 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 (93 %) to the total concentration
measured on NPF event days within a 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 in 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. Finally, 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 vapours 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 means of a recent
parameterization 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 ratios
only. Finally, the use of the parameterization for ion-induced binary
nucleation also highlighted the likely significant contribution of ion-induced nucleation for [H2SO4] below ∼8×108 cm−3.