Ground-based observation of clusters and nucleation-mode particles in the Amazon
Authors: Wimmer Daniela , Buenrostro Mazon Stephany , Manninen Hanna Elina , Kangasluoma Juha , Franchin Alessandro , Nieminen Tuomo , Backman John , Wang Jian , Kuang Chongai , Krejci Radovan , Brito Joel , Goncalves Morais Fernando , Martin Scot Turnbull , Artaxo Paulo , Kulmala Markku , Kerminen Veli-Matti , Petäjä Tuukka 
Published: September 2018

Abstract. We investigated atmospheric new particle formation (NPF) in the
Amazon rainforest using direct measurement methods. To our knowledge this is
the first direct observation of NPF events in the Amazon region. However,
previous observations elsewhere in Brazil showed the occurrence of nucleation-mode particles. Our measurements covered
two field sites and both the wet and dry
season. We measured the variability of air ion concentrations (0.8–12 nm)
with an ion spectrometer between September 2011 and January 2014 at a
rainforest site (T0t). Between February and October 2014, the same
measurements were performed at a grassland pasture site (T3) as part of the
GoAmazon 2014/5 experiment, with two intensive operating periods (IOP1 and
IOP2 during the wet and the dry season, respectively). The GoAmazon 2014/5
experiment was designed to study the influence of anthropogenic emissions on
the changing climate in the Amazon region. The experiment included basic
aerosol and trace gas measurements at the ground, remote sensing
instrumentation, and two aircraft-based measurements. The results presented in this work are from measurements performed at ground
level at both sites. The site inside the rainforest (T0t) is located 60 km
NNW of Manaus and influenced by pollution about once per week. The pasture
(T3) site is located 70 km downwind from Manaus and influenced by the Manaus
pollution plume typically once per day or every second day, especially in
the afternoon. No NPF events were observed inside
the rainforest (site T0t) at ground level during the measurement period.
However, rain-induced ion and particle bursts (hereafter, “rain events”)
occurred frequently (643 of 1031 days) at both sites during the wet and dry
season, being most frequent during the wet season. During the rain events,
the ion concentrations in three size ranges (0.8–2, 2–4, and 4–12 nm)
increased up to about 104–105 cm−3. This effect was most
pronounced in the intermediate and large size ranges, for which the background
ion concentrations were about 10–15 cm−3 compared with 700 cm−3 for the cluster ion background. We observed eight NPF events at the pasture site during the wet season. We
calculated the growth rates and formation rates of neutral particles and
ions for the size ranges 2–3 and 3–7 nm using the ion spectrometer data.
The observed median growth rates were 0.8 and 1.6 nm h−1
for 2–3 nm sized ions and particles, respectively, with larger growth rates
(13.3 and 7.9 nm h−1) in the 3–7 nm size range. The
measured nucleation rates were of the order of 0.2 cm−3 s−1 for
particles and 4–9×10-3 cm−3 s−1 for ions. There was no
clear difference in the sulfuric acid concentrations between the NPF event
days and nonevent days (∼9×105 cm−3).
The two major differences between the NPF days and nonevent days were a
factor of 1.8 lower condensation sink on NPF event days (1.8×10-3 s−1) compared to nonevents (3.2×10-3 s−1) and different air
mass origins. To our knowledge, this is the first time that results from ground-based
sub-3 nm aerosol particle measurements have been obtained from the Amazon
rainforest.