Introduction
The Amazon basin consists of the world's largest rainforest, covering an area
of 5.5 million km2. The Amazon rainforest is one of the few
continental regions where atmospheric processes are minimally influenced by
anthropogenic emissions, particularly during the wet season, and ambient
conditions can represent, to some extent, those of the pristine
pre-industrial era . Concentrations and properties of
aerosol particles are largely governed by biogenic emissions of both primary
biological aerosol particles (PBAP) and biogenic volatile organic compounds
(BVOCs), which contribute to secondary organic aerosol (SOA). On a regional
scale, in the wet season, the hydrological cycle is strongly influenced by
these biogenic aerosol emissions, which provide most of the cloud
condensation nuclei and thereby influence the radiation balance and cloud
lifetime . In the dry season, by contrast, widespread
biomass burning can result in a substantially increased aerosol optical depth
over large areas of Amazonia, as well as modified cloud properties and
suppressed precipitation .
Previous studies in the pristine Amazon rainforest showed that fine particles
(which account for most of the cloud condensation nuclei) consist mostly of
secondary organic material derived from oxidized biogenic gases
. A lack of evidence for new
particle formation during ground-based measurements
implies that nucleation processes occur
at higher altitudes, and new particles are entrained into the boundary layer
from aloft . Larger supermicron particles are
dominated by primary biological aerosol particles (PBAP) released from
rainforest biota , which can play a
significant role as ice nuclei . These PBAP consist of
wind-driven particles, such as pollen, bacteria, and plant debris, as well as
actively ejected material, such as fungal and plant spores. Nonbiological
particles observed in the Amazon in the supermicron size range largely
consist of advected Saharan dust and sea salt from the Atlantic
.
The low aerosol number concentrations in the pristine Amazon rainforest
(typically a few hundred cm-3) mean that CCN activation in convective
clouds is often aerosol limited . It is clear that there is
a strong coupling between the rainforest biosphere and the hydrological cycle
in the Amazon basin, with biogenic aerosol particles providing the nuclei for
clouds, which, in turn, sustain the rainforest through precipitation
.
Improving our knowledge of these processes is necessary for understanding the
influence the Amazon rainforest has on regional and global climate and
atmospheric composition, and how changing land use and climate in Amazonia
will impact this . To this end, the Brazil–UK Network
for Investigation of Amazonian Atmospheric Composition and Impacts on Climate
(BUNIAACIC) was established to define and nurture a framework within which
future UK contributions to studies in these areas may be coordinated. As part
of the BUNIAACIC project, a short-term intensive measurement campaign was
undertaken at a pristine rainforest site in July 2013. The main focus of
this study was to look at natural (biogenic) aerosol at this site at the
beginning of the dry season (also referred to as the transition from wet to
dry season), and to compare to previous measurements made during the wet
season at the same location . Here, we present the results
of this study.
Methodology
Measurement site and sampling
The measurements were conducted at a remote site in the pristine Amazonian
rainforest between 4 and 28 July 2013, during the transition from the
wet to dry season. This is around the start of the dry season but before
significant biomass burning takes place. In July 2013, the total rainfall
measured was 153 mm, mostly concentrated at the start and end of the month
(during the measurement period itself, the rainfall was 77 mm). For the
purposes of comparison, the AMAZE-08 campaign, which was conducted at the
same site, had 370 mm of rainfall over the course of 5 weeks during the wet
season . In this study, the quartile ranges in temperature
were 24–29 ∘C during the daytime, and 23–25 ∘C at night;
relative humidity (RH) was 72–92 % by day and
85–96 % at night. By contrast, the conditions during AMAZE-08 were cooler
and more humid, with temperature ranging 23–27 ∘C during the
day and 22–24 ∘C at night; RH ranging 88–99 % by day
and 96–100 % at night .
Sampling was done at the TT34 tower (2∘35′40′′ S,
60∘12′33′′ W; elevation 110 m), in the Reserva Biológica do Cuieiras, approximately
60 km NNW of the city of Manaus in Brazil (see Fig. ). The site
is representative of near-pristine conditions, and no biomass burning takes
place within the reservation; however, the site can be affected by regional
transport of pollutants including emissions from Manaus and biomass burning
. Locally, accommodation for researchers and a
60 kW diesel generator were situated 0.33 and 0.72 km, respectively, in a
WNW direction from the tower. Intensive measurement campaigns have taken
place at this site in the past e.g., and long-term
measurements have been conducted since 2008 .
During this experiment, local time was 4 h behind UTC.
Location of the sampling site, shown by the red markers. The yellow
rectangle represents the bounding box around Manaus used to flag air masses
influenced by pollution from the city.
A laminar sample flow of about 17 L min-1 was drawn through a 3/4 in.
OD stainless steel line from a height of 39 m (about 10 m above canopy
height) down to a ground-level air-conditioned container, in which the
instruments were housed. Before entering the container, the sample was passed
through an automatic regenerating adsorption aerosol dryer .
This kept the RH in the sample flow between 20 and 40 %. For the range
of flows rates during this campaign the transmission range has previously
been calculated from 4 nm to 7 µm .
Instruments drawing off this dried sample flow included a hygroscopicity
tandem differential mobility analyser (HTDMA; University of Manchester), and
a cloud condensation nuclei counter (CCNc; CCN-100, Droplet Measurement
Technologies). Upstream from these instruments, the sample flow
(2 L min-1) was further dried to an RH of between 15 and 25 % with
a Nafion dryer operating with a counterflow of dry compressed air. The flow
then passed through an electrical ionizer (model 1090, MSP Corporation),
providing a charge-neutralized aerosol sample to the instruments. These same
instruments were deployed in Borneo during the OP3 project
. Further details of the HTDMA and CCNc are given below.
Core instruments running at the site, on the same inlet, included a multi-angle
absorption photometer (MAAP; model 5012, Thermo-Scientific), a
condensation particle counter (CPC; model 3772, TSI), and an aerosol chemical
speciation monitor (ACSM; Aerodyne Research Inc.). The ACSM was used to
measure mass concentrations of particulate ammonium, nitrate, sulfate,
chloride, and organic species in the submicron size range. Mass calibration
was obtained by sampling monodisperse ammonium nitrate and ammonium
sulfate. The instrument collection efficiency was calculated to be 1 during
BUNIAACIC, through the comparison of the mass concentration of species
measured by the ACSM and MAAP (black carbon equivalent; BCe) with
the integrated mass of the SMPS. Further instrumental details and data
post-processing is given by . A weather station (Davis, USA) at
the top of the tower provided meteorological data (wind speed and direction,
temperature, RH, etc.).
As well as the instruments in the container, a wide issue bioaerosol sensor
(WIBS; model 3M, University of Hertfordshire) was operated in a weatherproof
box on the ground, a short distance from the base of the tower, with a short
(1 m) 1/4 in. OD stainless steel inlet (more details are provided below). Other
core instruments running at the site, but not used in this study, are
detailed by .
HTDMA measurements
In the HTDMA , a dry aerosol sample is
mobility size-selected with the first DMA and then humidified to a set RH.
The second DMA is then used to measure the size distribution of the
humidified aerosol, to give the distribution of growth factor (defined as the
ratio of humidified to dry aerosol diameter: D/D0) as a function of RH
and dry diameter (GFRH,D0). Quality assurance and inversion of the
data was performed using the TDMAinv toolkit of .
During normal operation, the first DMA cycled through five mobility sizes
(45, 69, 102, 154, and 269 nm; calibrated values), and the
monodisperse flow, after the first DMA, was humidified to a target RH of
90 %. The RH measured in DMA2 remained fairly stable (±2 %) for most of
the measurement period, and the variation was accounted for by correcting the
data to the target RH within the inversion toolkit .
In addition to this normal mode of operation, humidograms were run on
21 and 23 July. In this mode, cycling through three dry sizes (45, 102,
and 269 nm), the RH in the second DMA was gradually varied between 45 and
95 % in order to determine how the GF of ambient aerosol varies with RH.
In both DMAs, a ratio of 10 : 1 was maintained between the sheath and sample
flows, and these were calibrated using an airflow calibrator (Gilibrator-2,
Sensidyne). The first DMA was size calibrated at the start of measurements
using NIST-traceable polystyrene latex spheres (PSL; Fisher Scientific),
sizes 100, 150, 200, and 300 nm, nebulized with an aerosol generator (model
ATM 226; TOPAS). Dry scans (in which the sample is not humidified between the
DMAs) were run on an approximately weekly basis in order to monitor the size
offset between the two DMAs and to define the width of the DMA transfer
functions . The HTDMA was further verified by sampling
nebulized ammonium sulfate, monitoring the growth factors for a range of
RH (68–92 %) at a given size (140 nm), and comparing to modelled
values ADDEM; . More details of the calibration
procedures for this instrument are given by .
CCNc measurements
The CCNc operated downstream of a DMA (model 3081,
TSI), the voltage of which was controlled with a classifier (TSI, model
3080),
stepping discretely through a mobility size range of 16–325 nm. This
quasi-monodisperse aerosol sample flow was then split isokinetically between
the CCNc and a CPC (TSI, model 3010). The flow into the CPC was further
diluted with filtered air by a factor of 2 in order to match the flow into
the CCNc. Inside the CCNc, the aerosol flowed through a wetted column with a
temperature gradient, providing supersaturated conditions in which a
proportion of the particles activated and were detected by an optical
particle counter (OPC) at the bottom of the column. Throughout the
deployment, the CCNc cycled through five calibrated supersaturation set points:
0.15, 0.26, 0.47, 0.80, and 1.13 %. The ratio of activated particles to
total particles (measured by the CPC) can be determined as a function of dry
particle diameter and supersaturation (the activated fraction, AF). By
fitting a sigmoid curve function to this activation spectrum, the dry
diameter at which 50 % of particles activate (D50) was derived. The
hygroscopicity parameter, κ , was then derived from
D50 and supersaturation using the κ-Köhler model. In
addition, the total number of CCN (NCCN) was calculated by
integrating the number size distribution above D50.
The DMA was calibrated using PSLs of the same sizes as with the HTDMA. The
CCNc was calibrated by flowing nebulized ammonium sulfate into the system
and determining the supersaturation at which 50 % of the particles of a given
dry size activate. This critical supersaturation is then compared to modelled
values ADDEM; to determine the slope and offset.
Bio-aerosol measurements
Fluorescent biological aerosol particles (FBAPs) in the optical size range of
0.5≤Dp≤20 µm were detected using the WIBS-3M
, which operates on the principle of
ultraviolet-light-induced fluorescence of molecules common to most biological
material, specifically tryptophan and the co-enzyme NADH. Two sequential
pulses of UV light are provided by filtered Xenon lamps at 280 and 370 nm
to excite tryptophan and NADH, respectively. Fluorescence is then detected in
the ranges of 310–400 and 400–600 nm following the tryptophan excitation,
and 400–600 nm following the NADH excitation (i.e. three fluorescence channels;
FL1, FL2, and FL3, respectively). In addition, the WIBS-3M provides a
dimensionless particle asymmetry factor (Af) as a proxy for particle
morphology, as detailed by . Particles smaller than 0.8 µm
were rejected from analysis due to low counting efficiency.
The baseline fluorescence of the instrument is measured during so-called
forced trigger (FT) sampling periods, where the instrument triggers the flash
lamps and records the resultant fluorescence in the absence of aerosol in the
sample volume. The mean fluorescence in a FT period is treated as the
baseline fluorescence of the optical chamber during the sample period. For a
particle to be considered fluorescent (FBAP) it must exhibit a fluorescence
greater than a threshold value, defined as the baseline fluorescence plus 3
standard deviations, in any channel. During data processing, the threshold
value for each channel is subtracted from the single-particle fluorescence
data and the value is clipped at zero with all values greater than zero being
considered significantly fluorescent compared to the instrument baseline. All
reported fluorescence measurements are relative to the applied threshold and
not the absolute detector intensities. This is consistent with previous
studies using this instrument ,
and a detailed description of this data processing method is
provided by . The thresholds remained consistent over 58
FT periods throughout the measurements at 112.4±3.9 for channel FL1,
284.6±7.8 for FL2, and 164.6±5.7 for FL3. The ambient threshold
determination method was not used here due to the
majority of particles being fluorescent in nature.
Size calibration of the WIBS-3M consisted of using PSLs with a physical
diameter of 1.0 µm. Blue fluorescent latex spheres
(1.0 µm diameter; Thermo Scientific) were also used to ensure that the excitation
and fluorescence channels were operating correctly. The WIBS-3M inlet was
operated at a total flow rate of 2.3 L min-1 (±5 %). Of this, 90 % was
directed through a HEPA filter and used as a sheath flow, constraining the
remaining 0.23 L min-1 for the scattering chamber sample flow from which particle
concentrations were derived.
Particles with fluorescent magnitudes above the threshold are termed FBAPs, as
they represent a lower limit of PBAP, some of which may not be detected by
this method if their fluorescence goes undetected, or they simply do not
fluoresce . Nonbiological fluorescent material
can also be detected by the WIBS should its excitation and emission profile
match that of the instrument. Generally, the identified interferents are
smaller than the detection limit of the instrument. Polycyclic aromatic
hydrocarbons (PAHs), such as naphthalene, and soot-containing PAHs have been
shown to fluoresce in FL1 ; however, they would
not be expected to be seen in significant concentrations outside of the
pollution events at such a remote site. Mineral dusts contain a small subset
of fluorescent aerosol within their population (≈ 10 %), and given
their ubiquitous nature may present a significant source of interferents to
the UV-LIF method . Their observed fluorescent intensity,
however, is considerably weaker than is observed for biofluorophores
, and if they were present in any significant
concentrations they would likely form their own cluster in the cluster
analysis discussed in Sect. . It
should also be noted that the technique does not distinguish between
biological particles and fluorescent material attached to nonbiological
particles (e.g. dust).
This instrument has previously been deployed in Borneo, and further details
of its operation are given by . In this experiment, the
instrument was positioned in a small clearing, a few metres away from the
rainforest understorey. It should be noted that the WIBS-3M measurements only
ran until 10 July, and thus did not overlap with the other principle
measurements (HTDMA, CCNc, ACSM) which began on 10 July.
Meteorological conditions were fairly consistent over the whole measurement
period, and so all measurements discussed here are considered representative
of the same general period (i.e. the transition from wet to dry season).
Removal of pollution episodes
In order to focus on the natural (biogenic) aerosol,
for comparison with the wet season, it was necessary to exclude periods
affected by pollution. While the site is described as pristine, it can
nevertheless be affected by local emissions and regional transport of
pollutants: biomass burning emissions from outside the reserve, the urban
plume from Manaus, and pollution from the nearby diesel generator.
For each day of the campaign 7-day back trajectories were calculated using
the HYSPLIT model at 30 min intervals and six altitudes
above TT34 (0, 250, 500, 1000, 2000, and 4000 m a.s.l). The horizontal and
vertical wind fields employed here were from the NCEP/NOAA 1∘×1∘ Global Data Assimilation System (GDAS) reanalysis product. These
back trajectories were used to identify air masses arriving at TT34 which had
either passed over Manaus or passed nearby active fire zones. The results
were largely the same between 0 and 2000 m, with very little influence from
the upper-level flow at 4000 m.
A bounding box was drawn between -3.16 and -2.88∘ latitude and
between -60.12 and -59.81∘ longitude to define the Manaus influence zone
(see Fig. ), and any back trajectory passing over this box at
any altitude up to 2000 m was flagged. Air masses potentially impacted by
biomass burning were identified by coupling the back trajectory measurements
to satellite-detected fires as measured by the MODIS instrument. This
operates on the Aqua and Terra satellites, which have local overpass times in
the morning and afternoon, respectively. The fire detection data (specific
product: MCD14ML) were produced by the University of Maryland and acquired
from the online Fire Information for Resource Management System (FIRMS;
https://earthdata.nasa.gov/data/near-real-time-data/firms/about). At each
location along the back trajectories, the surrounding 1∘ box was
interrogated for any fire counts at the nearest Terra/Aqua overpass. If any
were present, this trajectory was flagged as potentially influenced by biomass
burning. This technique is subject to uncertainties associated with
trajectory errors e.g. and the detection of fires in
cloudy scenes, or false detection of fires , and
therefore can only be considered qualitative. Finally, data were investigated
for possible contamination from the generator if the local wind direction was
in the range of 270–340∘; however, there were no instances of
generator contamination during the measurement periods of this study.
In the event of any flag, the black carbon data (from the MAAP) were checked
along with the particle counts (where available), and aerosol data were
excluded if the pollution flag coincided with a significant increase in these
concentrations. No other increases in black carbon concentrations were seen
outside the flagged periods. Approximately 28 % of the HTDMA and CCNc data
were removed in this way, with 5 % of the data being flagged as possibly
impacted by biomass burning and most of the rest removed due to the Manaus urban
plume. The ACSM, which was not necessarily operating at the same times as the
HTDMA and CCNc, had approximately 9 % of its data removed due to pollution
flags (almost entirely due to the urban plume from Manaus).
Particle number size distribution averaged over the entire
measurement campaign. Box-and-whisker plots show the median, interquartile
ranges, and 5th and 95th percentiles, and lines and markers show mean
dN/dlogDp. Also shown are κ derived from the
D50 from the CCNc, and growth factor from the HTDMA, both as a function
of particle diameter. Error bars represent ±1 standard deviation. Note
that the HTDMA and CCNc/particle size data have been averaged over slightly
different measurement periods, as shown in Figs.
and .
Results and discussion
Size distributions
The particle number size distribution recorded over the measurement period of
this study can be seen in Fig. . This shows a broad
accumulation-mode peak at 130–150 nm with a median number concentration of
266 cm-3 (calculated from the integral of the size
distribution curve). Despite observing aerosol number concentrations
comparable to previous observations during the wet season, the shape of the
distribution resembles those measured in the dry season, although the
concentrations during the latter are considerably higher at 2200 cm-3
. Figure
shows the size distribution again combined with the coarse-mode FBAP
measurements from the WIBS. In terms of number concentration, the submicron
modes dominate the coarse mode by a factor of 103. The WIBS
measurements are discussed in further detail below.
Combined log–log plot of total particle number size distribution (as
measured with the SMPS) with FBAP number size distribution (from the WIBS).
It should be noted that the SMPS measured the mobility diameters of particles
in a dried sample, while the WIBS measured optical diameters at ambient
humidity.
The time series of total particle counts (integrated from size
distributions; top panel) and particle number size distribution (bottom
panel). Shaded areas represent pollution episodes removed from the data. Any
other gaps are due to instrument downtime.
The size distribution, however, was quite variable over the period of the
measurements, as can be seen in the time series in
Fig. , and varied with total particle number
concentrations. Median size distributions observed when the total number
concentration was above or below 200 cm-3 are shown in
Fig. . During periods of low particle counts, an
Aitken mode is also seen, with a mode around 80 nm, while the size
distribution during episodes of higher concentrations is dominated by the
accumulation mode, possibly masking the smaller mode. Such a size
distribution profile, dominated by accumulation-mode aerosols, has also been
reported during the dry season in western Amazonia, in the deforestation arc,
during biomass burning events , albeit with substantially
higher concentrations.
Median and interquartile ranges of particle number size
distributions observed during high (> 200 cm-3) and low
(< 200 cm-3) total particle number concentrations.
Submicron nonrefractory aerosol composition from the ACSM
measurements along with equivalent black carbon from the MAAP measurements:
concentration (top panel) and mass fraction (bottom panel). The pie chart
shows the average proportions over the measurements. Shaded areas represent
pollution episodes removed from the data. Any other gaps are due to
instrument downtime.
The time series of normalized RH-corrected (to 90 %) growth
factor distributions derived from HTDMA measurements, for all five dry
diameters. Shaded areas represent pollution episodes removed from the data.
Any other gaps are due to instrument downtime and humidograms.
Composition
Submicron nonrefractory aerosol composition, as measured by the ACSM during
the period of this study, is illustrated in Fig. . The mean
mass loadings for organic material, sulfate, and nitrate were 2.13±0.75,
0.11±0.04, and 0.08±0.03 µg m-3, respectively (±1 standard deviation).
Organic material dominated the submicron aerosol, comprising around 81 % of
the total mass (86 % of nonrefractory material), on average. Such a high
fraction of organics compares well with previous observations in the Amazon
basin . BCe
concentrations are also shown, with a mean mass loading of 0.25±0.01 µg m-3. This is consistent with previous wet season
measurements in the Amazon .
The mass fractions of nonrefractory aerosol and BCe are
shown in the bottom panel of Fig. . Due to the noise in the
ammonium signal (see Fig. ), resulting from concentrations
below the limit of detection of 0.3 µg m-3 for the ACSM, it
was necessary to estimate the ammonium from the nitrate and sulfate mass
loadings for the purpose of mass fraction calculations. The time series of
mass fractions show that, while the mass loadings vary considerably,
particularly in organics, the composition is relatively consistent as a
proportion of the aerosol mass. Organic mass fractions remain steady around
81 % of the total mass, until 22 and 23 July, when a slight increase
in BCe is seen.
Levoglucosan, a major constituent of biomass burning aerosol, fragments in
AMS and ACSM instruments at a mass-to-charge ratio (m/z) of 60
, and so the fraction f60 is frequently used as a
marker for biomass burning . The mean f60
from the ACSM data in this study, after removal of pollution episodes, was
0.19± 0.07 %. This is well below 0.3 %, which is considered to
be the upper limit for background air masses not affected by biomass burning
. It should be noted that previous studies in the Amazon
have observed that a large fraction of the biomass-burning related organic
aerosols do not present a significant f60 signal, due to long-range
transport . It can be said, however, that the relatively low
f60 observed here suggests that, on average, these measurements were not
strongly impacted by local biomass burning emissions.
Previous studies have successfully identified FBAP markers on ambient aerosol
in the Amazon using an aerosol mass spectrometer , a
method which relies strongly on the high-resolution capabilities of the
instrument used at the time. Given the unity mass resolution of the ACSM,
similar methodology has not been applied here.
The time series of total, FBAP, and non-FBAP number concentrations, as
measured by the WIBS-3M. Shaded areas represent pollution episodes removed
from the data. Any other gaps are due to instrument downtime.
Aerosol water uptake
The HTDMA ran from 13 to 28 July. Figure
shows the time series of RH-corrected GF distributions for all dry sizes,
as derived from the HTDMA data using the TDMAinv toolkit. These largely
exhibit a single mode at each size, roughly in the range of 1.2–1.4. Some
variability can be seen, for example on 21 and 23 July, but for the
most part, peak growth factors remained relatively stable over the
measurement period. This is consistent with the stable mass fractions seen in
the composition data from the ACSM (see Fig. , bottom panel).
The variability and slight decrease in GF at some sizes on 22 and
23 July may also be attributed to the slight increase in the mass fraction
of BCe (Fig. ). High peak growth factors (> 1.6)
can briefly be seen on the night of 15 July, shortly before a
pollution event; however, without composition data available on that day
(Fig. ), it is difficult to speculate as to the nature of this.
Mean peak growth factors and derived κ from HTDMA
measurements for each dry diameter, along with ± standard deviation.
D0 (nm)
GF
κ
45
1.19±0.08
0.09±0.10
69
1.20±0.08
0.09±0.09
102
1.28±0.08
0.12±0.10
154
1.32±0.07
0.15±0.09
249
1.36±0.10
0.17±0.09
Mean derived parameters from CCNc measurements for each set
supersaturation (SS), along with ± standard deviation.
SS (%)
D50 (nm)
κ
NCCN (cm-3)
0.15
152±9.5
0.18±0.03
87±35
0.26
105±5.5
0.18±0.03
161±60
0.47
78±4.2
0.13±0.02
212±74
0.80
56±3.0
0.12±0.02
248±82
1.13
45±3.4
0.12±0.03
268±86
Smaller, more hygroscopic (GF >1.5) modes can be seen at the lower dry
diameters, while the larger particles also show a hydrophobic mode in the
growth factor distribution. The contribution of the hydrophobic mode to the
larger particles is small (<10 % in number) and may be due to some unknown
local anthropogenic influence that was not accounted for. The averages of the
growth factor at the peak of the growth factor distribution (i.e. the
dominant mode) are shown in Table and Fig. .
They show an increase with dry diameter, reflecting the difference between
Aitken- and accumulation-mode aerosol: organic mass fractions are highest in
the Aitken mode, while elevated sulfate mass fractions have been previously
seen in the accumulation mode . It should be
noted, however, that the elevated sulfate events observed by
were likely linked to long-range transport of biomass
burning aerosol from Africa, which, due to a combination the African burning
season and large-scale circulation, tends to impact the Amazon forest more
often during the wet season .
The campaign averages of the CCNc-derived parameters, D50, κ, and
NCCN are given for each set supersaturation in
Table . The κ values are also plotted against D50 in
Fig. . Consistent with the growth factor data, and with
previous measurements at this site , they show more
hygroscopic particles at larger diameters (κ≈0.12 below
100 nm, and κ≈0.18 around the accumulation mode).
Reconciliation between subsaturated and supersaturated particle water uptake for
these measurements has already been investigated by .
They showed that there was agreement within the variability of the data, with
a slightly underestimated hygroscopicity from the HTDMA data compared to the
CCNc at lower supersaturations (larger dry diameters). The analysis of
considered the full data set without separating out the
pollution events; however, performing the same analysis on the clean data
did not result in any significant difference.
FBAP measurements
Measurements of biological particles in the Amazon are
important as they are considered to have a strong influence on clouds as ice
nuclei . The WIBS-3M operated uninterrupted from the
morning of 3 July to 10 July. The mean particle number
concentration measured by the WIBS-3M during this period was 464±250 L-1
(1 standard deviation), while the mean FBAP number
concentration was 400±242 L-1 (i.e. accounting for 86 % of
the particles in the size range of the instrument). The time series of number
concentrations for the duration of this period is shown in Fig. .
This shows coarse-mode particles were dominated by FBAP
number concentrations, which exhibited a strong diurnal cycle with
concentrations varying from around 200 L-1 during the daytime up to
as much as 1200 L-1 at night. The diurnal variation (Fig. )
shows that FBAP number concentrations plateaued from
around 21:00 through the night, began to drop from 05:00, reached a minimum
by 11:00 and started increasing again from 15:00. The FBAP fraction was
highest (more than 90 %) at night, and remained high until around 08:00 –
even after FBAP number concentrations began decreasing. This dropped to
between 55 and 75 % during the day, helped in part by an apparent increase
in non-FBAP concentrations, before steadily increasing in line with the FBAP
concentrations through the late afternoon/early evening.
Diurnal variations in total, FBAP, and non-FBAP number
concentrations, as measured by the WIBS-3M, as well as the fraction of FBAPs,
and the combined number concentrations of clusters Cl1 and Cl2. Shown are the
means (lines and markers), medians and interquartile ranges (boxes), and 5th
and 95th percentiles (whiskers). Also shown at the bottom are the mean
diurnal variations in temperature and RH, measured on the tower above the
canopy, for the same period.
There are a number of factors driving the diurnal cycle in coarse-mode
particles, as discussed by . Previous studies at this and
a nearby site, utilizing electron and light microscopy, have identified the
FBAPs as predominantly fungal spores . Similar
diurnal cycles have been seen in airborne fungal spore densities at other
tropical rainforest locations . The observed
nighttime peak in FBAP number concentrations in Fig.
is consistent with nocturnal sporulation driven by increasing RH (see bottom
panel; note that RH is measured above the canopy). The dependence of fungal
spore release on meteorological conditions, however, varies greatly according
to species, and any relationship is nontrivial . FBAP
number concentrations begin dropping several hours before any decrease in RH,
and the FBAP fraction also remains high (Fig. ). This
suggests that the morning decrease in FBAPs is not necessarily due to a
cessation of emission processes, but may also be the result of a breakup of
the nocturnal boundary layer around sunrise
. and
suggest that the nighttime increase in coarse-mode particles is due, at
least in part, to the shallow nocturnal boundary layer. The slight increase
in non-FBAP concentrations during the day may be a result of enhanced
particle exchange through the canopy, facilitated by sporadic turbulent
events, as described by , bringing non-FBAPs that had
originated elsewhere into the space below canopy.
Particle number size distributions measured with the WIBS-3:
(a) mean size distributions for total, FBAPs, and non-FBAPs; and
(b) diurnal variation of size distribution for FBAPs and non-FBAPs
(note that the colour scales are not the same).
Figure shows the number size distributions reported by the
WIBS-3M during the measurement period. Again, FBAPs clearly dominate the
particle number concentrations for Dp>1 µm; however,
non-FBAP concentrations are higher for particles smaller than
1 µm measured by the WIBS-3M (i.e. down to the instruments
50 % detection diameter of 0.8 µm). The FBAP number size
distribution shows a peak at around 1.8 µm, while the non-FBAP
distribution is characterized by a flatter, broader peak between 0.8 and
1.3 µm. Nonfluorescent particles at this site have previously
been identified as mineral dust, nonfluorescent biological aerosol, and
inorganic salts . Caution must be applied when
interpreting the submicron fluorescent aerosol fraction due to the reduced
fluorescent counting efficiency for particles Dp<0.8 µm , which may lead to an underestimation
of the fluorescent aerosol fraction at small sizes.
Solutions to the Ward linkage cluster analysis, showing mean (±1
standard deviation) intensity in each fluorescence channel (FL1–3), optical
particle diameter (Dp), and asymmetry factor (Af). The
intensities are referenced to the FT + 3 standard deviation threshold
representing an intensity of zero, as discussed is Sect. .
Fluorescent intensities and asymmetry factors are in arbitrary units.
Cl1
Cl2
Cl3
FL1 (280 nm)
1400±302
478±386
386±533
FL2 (280 nm)
120±96
33±47
351±212
FL3 (370 nm)
94±106
47±73
721±379
Dp(µm)
2.5±1.3
1.9±1.0
2.3±1.1
Af
30.9±15.0
30.2±15.7
29.0±15.1
A Ward linkage cluster analysis using the z score normalization was applied
to the data, where the optimum number of retained distinct clusters was
determined using the Calinski–Harabasz criterion. Prior to analysis, all
nonfluorescent and saturated particles, and particles smaller than
0.8 µm in diameter were excluded, resulting in approximately
15 % of the single-particle data being rejected. The asymmetry factor and
size inputs were converted to log space prior to normalization and
clustering. Complete information on this technique is given by
, who used the same instrument model in the same way as
in this study. This analysis revealed three distinct fluorescent classes of
particles (Cl1–3). The statistical parameters of each cluster are shown in
Table . It can be seen that Cl1 and Cl2 display similar
characteristics; specifically, they mainly fluoresce in FL1 with weak
fluorescence in the other channels, although the intensities are greater for
Cl1, suggesting they are distinct subclasses. The two clusters correlate
strongly (r2=0.86) with each other; hence, both have been combined in
Fig. . They show similar fluorescent signatures to the
clusters attributed to fungal spores by
based on comparison with other sampling techniques and the diurnal emission
pattern. In this study, they show higher concentration overnight
(Fig. ), and a strong correlation to RH
(Fig. ). Together, clusters Cl1 and Cl2 contribute
approximately 70 % to the total fluorescent particle concentration,
regardless of time of day, suggesting that FBAPs were dominated by fungal
spores during this study. A third cluster, Cl3, shows very low concentrations
(around 20 L-1), with no strong diurnal trend; however, there are
insufficient data to speculate upon the nature of this cluster (such as
response to rainfall). The asymmetry factor for each cluster was around 30,
suggesting that the particles are aspherical in nature. A similar value of
asymmetry factor was observed by in the ambient fungal
cluster, further suggesting that clusters Cl1 and Cl2 are representative of
fungal spores.
PBAP classification via the comparison of single-particle fluorescent
signatures to laboratory samples is an ongoing area of research
e.g.. Such direct comparison for this purpose is not
possible here due to differences in the instruments used (i.e. different
excitation/detection wavebands and optical chamber design). Even comparing
results between the same model of instrument with identical detector/filter
configurations has been difficult due to the current
lack of a robust fluorescence calibration method.
Diurnal means of total particle number concentrations in clusters 1
and 2, plotted against RH.
Comparison with previous studies
Submicron aerosol
Aerosol water uptake studies have previously been conducted at the TT34 site
by using size-selected CCNc measurements, and at Balbina
(110 km NE of TT34) by using a HTDMA, both during the wet
season. HTDMA and CCNc measurements were also made at Balbina during the
transition from wet to dry season by . In addition, HTDMA
measurements from pasture land in southwest Amazonia at the end of the dry season/beginning
of wet season are presented by and
. This study represents the first measurements with HTDMA
and monodisperse CCN instruments at TT34 during the transition from wet to
dry seasons. Concurrent CCNc and HTDMA measurements have also been conducted
in Borneo, southeast Asia, by , providing a useful comparison with
a different tropical rainforest region.
The HTDMA growth factor measurements of showed a similar
pattern to this study: a dominant mode of less hygroscopic particles
(GF ≈ 1.16–1.32), accompanied at times by a hydrophobic mode
(GF < 1.06; particularly at the larger particle sizes), and a more hygroscopic mode
(GF ≈ 1.38–1.54). The growth factors of the less hygroscopic
particles are compared in Fig. , along with the other studies
(note that define “less hygroscopic” as
GF ≈ 1.17–1.5). All the measurements showed a similar increase in growth factor with
dry diameter. The growth factor values from this study were slightly higher
than those of and , but the difference is
within the variability of the measurements, and probably within the
variability that has been seen between different HTDMA instruments
. The moderately hygroscopic particles
(GF = 1.26) observed by exhibited growth factors in the same
range as the other studies in Amazonia; however, in this case, the hydrophobic
mode (GF ≈ 1.05–1.13) was dominant for all but the larger particles
(> 135 nm). Furthermore, strong diurnal cycles (daytime increases in the
fraction of moderately hygroscopic particles) were observed
, which were not seen during the current study.
In contrast to the current study, the measurements of and
were conducted in a region that has undergone heavy land
use change and is strongly influenced by anthropogenic sources
, which may contribute to the observed diurnal pattern.
Mean growth factor for the dominant, less hygroscopic mode plotted
against dry diameter, comparing this to previous studies in Amazonia and
Borneo. The data from and represent
less hygroscopic and moderately hygroscopic particles (respectively) during the
wet season. The definitions differ slightly between the studies in terms of
GF range, but the modes represented here broadly fit into the less
hygroscopic classification of . Error bars represent
±1 standard deviation.
In contrast to the studies from Amazonia, aerosol growth factors measured in
Borneo were somewhat higher, in the range of 1.3–1.7 (Fig. ).
This can be explained by the fact that, while the site in the
Amazon benefited from a fetch of hundreds of kilometres of undisturbed
rainforest, the site in Borneo was heavily influenced by marine air masses
. As discussed by , the sulfate
loadings in Borneo were substantially higher than in Amazonia, even in air
masses from across the island, which, with sulfate being more hygroscopic
than organic aerosol, is a possible explanation for the higher growth
factors.
Humidogram (dependency of growth factor on RH), taken between 14:00
and 20:30 UTC on 21 July. The fainter points at higher RH were taken between
13:30 and 14:30 UTC on 23 July. The humidogram data from ,
and the humidogram fit from are also shown, for
comparison. The black line shows the modelled humidogram for ammonium
sulfate for reference.
The results of the humidogram are shown in Fig. , and compared
to the humidogram data from Borneo and the humidogram fit
for the wet season data of . Growth factors in Borneo were
higher across the RH range than in Amazonia. As with previous measurements,
no deliquescence behaviour was seen in this study.
Values of κ derived from the HTDMA and CCNc measurements during these
studies are compared in Fig. , as a function of dry
diameter. Here, the κ from HTDMA measurements is derived using the
average growth factor, rather than the peak of the less hygroscopic mode, for
direct comparison with the CCNc-derived values. The various measurements in
Amazonia showed very similar κ, largely agreeing within the
variability of the individual measurements. It can be said that water uptake
measurements in Amazonia are consistent, and, as noted by ,
show κ to be around half that typically seen in other continental
regions .
The HTDMA-derived κ from the Borneo experiment shows more hygroscopic
aerosol than in Amazonia, as discussed above; however, the CCNc-derived values
are more in line with those in Amazonia. This discrepancy has been noted
previously and possible reasons for it are discussed by and
. These were mainly related to differences in the
instrument setups and how they treat the aerosol. It should be noted that the
discrepancy in the data from the Borneo experiment was the largest amongst a
number of data sets studied by , but the reason for this
is not clearly understood.
Comparing κ as a function of diameter for this and previous
studies in Amazonia and Borneo. Filled circles represent HTDMA-derived
values, while empty circles are CCNc-derived values. Error bars represent
±1 standard deviation, where these data are available. The values for
and were calculated by
.
In general, the particle concentrations and hygroscopic properties observed
during this study were similar to those seen during wet season measurements
in the Amazon rainforest. The main difference seen was that size
distributions in this study were more strongly dominated by the accumulation
mode: similar to those seen in the dry season , but in
clean conditions with significantly lower number concentrations. Under these
conditions, cloud droplet formation in convective clouds in this region is
likely to be aerosol limited . Previous modelling studies
have suggested this is the case during the wet season , in
contrast to the dry season during periods of intense biomass burning when
droplet number is largely controlled by the updraft velocity
.
In terms of composition, submicron nonrefractory aerosol concentration
during this experiment showed significantly higher concentration
(≈ 2.5 µg m-3) than observed at the remote sites in
central Amazonia in previous years during the wet season, ranging from
0.4 µg m-3 to 0.6 µg m-3
. Conversely, the concentration is significantly
lower than reported during the dry season (8.9 µg m-3)
, due to this transitional period having no extensive
biomass burning activities, albeit with already reduced wet deposition due to
reduced precipitation. Interestingly, despite the marked changes in ambient
concentration, very little difference is observed in terms of relative
contributions considering this and previous studies, being strongly dominated
by organics (≈ 80 %), followed by sulfate and minor contribution of
nitrate and ammonium .
It is not clear how meteorological conditions influence the differences
between this study and those in the wet season. The warmer, dryer conditions
of this study might result in more evaporation of SOA, whereas more
precipitation in the wet season would lead to more washout, and therefore
lower loadings. From the results, it would seem that the latter effect
dominates, resulting in the higher organic loadings and accumulation-mode
aerosol seen in this study, but there may be other, more complex factors.
Long-term measurements would be needed to fully investigate the influence of
meteorology on particles in the Amazon.
Coarse-mode aerosol
conducted measurements of FBAPs at the TT34 tower using an
ultraviolet aerodynamic particle sizer (UV-APS) during the AMAZE-08 campaign.
In contrast to this study, the AMAZE-08 measurements were taken during the
wet season (February–March) from the top of the tower (i.e. above
canopy). It is also worth comparing with the measurements of
, who used the same WIBS-3 instrument to sample the aerosol
above and below canopy in the rainforest of northeast Borneo.
The median number concentration of FBAP observed below the canopy in the
current study was 363 L-1, while the UV-APS measurements at the top
of the tower by were around a fifth of this at 73 L-1
(also median). In an intercomparison between the two different
measurement techniques, found that, while there was
agreement in total number concentrations, the counts in the fluorescence
channels of the WIBS (particularly FL1) were substantially higher than the
UV-APS fluorescence counts, which would at least partly explain the difference
here. The wetter, more humid conditions during the wet season measurement
period of would be expected to favour emission
. On the other hand, the higher rainfall would
also result in enhanced wet deposition during the wet season, especially
above canopy. At other locations, saw concentrations in
Borneo often in excess of 1500 L-1 below canopy and around 200 L-1
above, using the same instrument at each site, while
observed substantially higher concentrations of fungal
spores in the understorey than in the canopy during measurements in
Queensland, Australia. Strong vertical gradients in biological particles are
therefore regularly seen in rainforest environments, and would be an
additional factor in the differences observed between the measurements at
TT34. In a remote tropical rainforest in China, estimated
fungal spore concentrations to be around 50 L-1 based on chemical
analysis of filters, and found higher concentrations associated with rainfall
events. A global modelling study by found simulated
surface annual mean concentrations of fungal spores to be around 100 L-1
over tropical forests (including central Amazonia), which is
consistent with this and other measurements at this site.
The fraction of FBAPs in this study was, on average, 85 % of total coarse-mode
particles (and as much as 90 %), whereas it was 24 % in the AMAZE-08 campaign
(41 % in unpolluted conditions). The higher fraction at ground level would be
expected, being closer to the source, whereas above canopy there is a
stronger influence from nonfluorescent particles from external sources.
found fungal spores accounted for 35 % of coarse-mode
particles, also in central Amazonia, but their filter samples were taken at a
pasture site adjacent to the rainforest. In Borneo, as in the Amazon, there
was a higher fraction below the canopy (55 %) than above (28 %), however not
as high as the 86 % observed in this study. Reasons for this difference are
unclear, but may include a stronger influence in Borneo of nonfluorescent
particles from external sources, such as the nearby coast. More consistent
with the current study were the scanning electron microscopy (SEM)
measurements reported by and , which
attributed 80 % of coarse-mode particles to primary biological aerosol during
the AMAZE-08 campaign, and also identified particles likely to be fungal
spores.
One difference between the measurements of this study and others is the
position of the mode in the FBAP number size distribution.
report the peak at 2.5 µm, while observe the
peak around 2.3 µm. By contrast, the peak in this study was
1.8 µm. The difference between the two measurements at TT34 is likely
due to the different measurement techniques, with the UV-APS found to be less
sensitive to smaller fluorescent particles .
Diurnal variations between this study and that of were
similar; however, reported an additional increase in FBAP
number concentrations in the afternoon in Borneo. This increase coincided
with a peak in RH, and it is believed that this is linked
. In this study, the RH increased more gradually through
the afternoon and evening (see Fig. , bottom panel),
which may explain the lack of afternoon peak in FBAPs compared to the Borneo
results. also do not observe a mid-afternoon peak in
FBAPs.
Conclusions
Measurements of aerosol concentrations and water uptake
properties were conducted at a remote site in pristine Amazonian rainforest
in July 2013, during the transition from the wet to dry season. Back
trajectories and wind sectors were examined in conjunction with black carbon
concentrations in order to exclude any pollution episodes and ensure the
aerosols measured were representative of background aerosol over the
rainforest.
With any pollution episodes removed from the data, particle concentrations
were low, with a median of 266 cm-3. The particle size distributions
were largely dominated by an accumulation mode around 130–150 nm, with a
smaller Aitken mode apparent during periods of lower particle counts. Based
on previous measurements contrasting wet and dry seasons ,
the results here may reflect the transition between the two seasons, with
periods consistent with each at different times (but without considering any
influence from biomass burning).
Aerosol chemical composition, as measured with an ACSM, was dominated by
organic material, comprising around 81 % of the total mass of nonrefractory
aerosol and BCe. The mass fraction of organics was relatively
consistent over the measurement period.
Aerosol water uptake and hygroscopicity was measured using an HTDMA and a
CCNc. Good agreement was found between the measurements of both instruments.
Particle growth factors from the HTDMA varied little over most of the
measurement period and were typically between 1.2 and 1.4 (low hygroscopicity
mode). Aerosol hygroscopicity was found to be low (κ=0.12) for
Aitken-mode particles, and increased slightly to κ=0.18 for
accumulation-mode particles. This is consistent with previous measurements
at or near this site, and with the observation that Aitken-mode particle
composition is dominated by organic material, while accumulation-mode
particles exhibited higher sulfate mass fractions .
Particles in the size range of 0.5≤Dp≤20 µm were
measured using the WIBS-3M, which distinguishes fluorescent (representing a
subset of primary biological aerosols, or FBAPs) and nonfluorescent particles.
FBAPs dominated the coarse-mode aerosol, accounting for as much as 90 %.
Concentrations of FBAPs followed a strong diurnal cycle, with maximum
concentrations during the night. This is likely driven by a combination of
the dependence of emission processes on meteorological conditions and the
diurnal cycle of the boundary layer.
The results from this study were also compared to measurements conducted in
Borneo in 2008 , contrasting the
vast green ocean of the Amazon rainforest to the island rainforest
geography of southeast Asia. In the submicron range, aerosol hygroscopicity was
greater in Borneo, possibly due to the stronger marine influence of that
region . Coarse-mode particles at both locations were
dominated by FBAPs (probably mostly fungal spores). Below canopy, the Amazon
exhibited a higher fraction of FBAPs than Borneo, though higher FBAP
concentrations were seen at the latter.