New particle formation at urban and high-altitude remote sites in the south-eastern Iberian Peninsula
Authors: Casquero-Vera Juan Andrés , Lyamani Hassan , Dada Lubna , Hakala Simo , Paasonen Pauli , Román Roberto , Fraile Roberto , Petäjä Tuukka , Olmo-Reyes Francisco José , Alados-Arboledas Lucas 
Published: November 2020

Abstract. A substantial fraction of the atmospheric aerosols originates from
secondary new particle formation (NPF), where atmospheric vapours are
transformed into particles that subsequently grow to larger sizes, affecting
human health and the climate. In this study, we investigate aerosol size
distributions at two stations located close to each other
(∼ 20 km) but at different altitudes: urban (UGR; 680 m a.s.l., metres above sea level) and high-altitude remote (SNS; 2500 m a.s.l.) sites, both in the area
of Granada, Spain, and part of AGORA observatory (Andalusian Global
ObseRvatory of the Atmosphere). The analysis shows a significant
contribution of nucleation mode aerosol particles to the total aerosol
number concentration at both sites, with a contribution of 47 % and 48 %
at SNS and UGR, respectively. Due to the important contribution of NPF
events to the total aerosol number concentrations and their high occurrence
frequency (> 70 %) during the study period, a detailed analysis
of NPF events is done in order to get insight into the possible mechanisms
and processes involved in NPF events at these contrastive sites. At SNS, NPF
is found to be associated with the transport of gaseous precursors from
lower altitudes by orographic buoyant upward flows. NPF events at the SNS site
are always observed from the smallest measured sizes of the aerosol size
distribution (4 nm), implying that NPF takes place in or in the vicinity of
the high-altitude SNS station rather than being transported from lower altitudes.
Although NPF events at the mountain site seem to be connected with those
occurring at the urban site, growth rates (GRs) at SNS are higher than those
at the UGR site (GR7−25 of 6.9 and 4.5 nm h−1 and GR4−7 of 4.1
and 3.6 nm h−1 at SNS and UGR, respectively). This fact could have
special importance for the production of cloud condensation nuclei (CCN) and
therefore for cloud formations which may affect regional/global climate,
since larger GRs at mountain sites could translate to a larger survival
probability of NPF particles reaching CCN sizes, due to the shorter time period needed
for the growth. The analysis of sulfuric acid (H2SO4) shows that
the contribution of H2SO4 is able to explain a minimal fraction
contribution to the observed GRs at both sites (< 1 % and
< 10 % for the 7–25 and 4–7 nm size ranges, respectively), indicating
that other condensing vapours are responsible for the majority of particle
growth, as well as the differing growth rates between the two sites. Results
also show that the condensation sink (CS) does not play a relevant role in
NPF processes at both sites and points to the availability of volatile
organic compounds (VOCs) as one of the main factors controlling the NPF
events at both sites. Finally, a closer analysis of the NPF events that were
observed at the SNS site during a Saharan dust episode that occurred during the
field campaign was carried out, evidencing the role of TiO2 and
F2O3 together with VOCs in promoting new particle formation during
this dust intrusion event. Although further investigation is needed to
improve our understanding in this topic, this result suggests that climate
effects of mineral dust and NPF are not disconnected from each other as it
was commonly thought. Therefore, since mineral dust contributes to a major
fraction of the global aerosol mass load, dust–NPF interaction should be
taken into account in global aerosol-climate modelling for better climate
change prediction.