Biogenic cloud nuclei in the central Amazon during the transition from wet to dry season

The Amazon basin is a vast continental area in
which atmospheric composition is relatively unaffected by
anthropogenic aerosol particles. Understanding the proper-
ties of the natural biogenic aerosol particles over the Ama-
zon rainforest is key to understanding their influence on re-
gional and global climate. While there have been a number
of studies during the wet season, and of biomass burning par-
ticles in the dry season, there has been relatively little work
on the transition period – the start of the dry season in the
absence of biomass burning. As part of the Brazil–UK Net-
work for Investigation of Amazonian Atmospheric Composi-
tion and Impacts on Climate (BUNIAACIC) project, aerosol
measurements, focussing on unpolluted biogenic air masses,
were conducted at a remote rainforest site in the central Ama-
zon during the transition from wet to dry season in July 2013.
This period marks the start of the dry season but before sig-
nificant biomass burning occurs in the region.
Median particle number concentrations were 266 cm−3,
with size distributions dominated by an accumulation mode
of 130–150 nm. During periods of low particle counts, a
smaller Aitken mode could also be seen around 80 nm. While
the concentrations were similar in magnitude to those seen
during the wet season, the size distributions suggest an en-
hancement in the accumulation mode compared to the wet
season, but not yet to the extent seen later in the dry sea-
son, when significant biomass burning takes place. Submi-
cron nonrefractory aerosol composition, as measured by an
aerosol chemical speciation monitor (ACSM), was domi-
nated by organic material (around 81 %). Aerosol hygro-

scopicity was probed using measurements from a hygro-
scopicity tandem differential mobility analyser (HTDMA),
and a quasi-monodisperse cloud condensation nuclei counter
(CCNc). The hygroscopicity parameter, κ , was found to be
low, ranging from 0.12 for Aitken-mode particles to 0.18 for
accumulation-mode particles. This was consistent with pre-
vious studies in the region, but lower than similar measure-
ments conducted in Borneo, where κ ranged 0.17–0.37.
A wide issue bioaerosol sensor (WIBS-3M) was deployed
at ground level to probe the coarse mode, detecting pri-
mary biological aerosol by fluorescence (fluorescent biolog-
ical aerosol particles, or FBAPs). The mean FBAP number
concentration was 400 ± 242 L−1; however, this ranged from
around 200 L−1 during the day to as much as 1200 L−1 at
night. FBAPs dominated the coarse-mode particles, compris-
ing between 55 and 75 % of particles during the day to more
than 90 % at night. Non-FBAPs did not show a strong diur-
nal pattern. Comparison with previous FBAP measurements
above canopy at the same location suggests there is a strong
vertical gradient in FBAP concentrations through the canopy.
Cluster analysis of the data suggests that FBAPs were dom-
inated (around 70 %) by fungal spores. Further, long-term
measurements will be required in order to fully examine the
seasonal variability and distribution of primary biological
aerosol particles through the canopy.
This is the first time that such a suite of measurements has
been deployed at this site to investigate the chemical compo-
sition and properties of the biogenic contributions to Ama-
zonian aerosol during the transition period from the wet to the dry season, and thus provides a unique comparison to the
aerosol properties observed during the wet season in previ-
ous similar campaigns. This was also the first deployment of
a WIBS in the Amazon rainforest to study coarse-mode parti-
cles, particularly primary biological aerosol particles, which
are likely to play an important role as ice nuclei in the region.