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 properties of the natural biogenic aerosol particles over the Amazon rainforest is key to understanding their influence on regional and glob...
| Main Authors: | , , , , , , , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-08-01
|
| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/9727/2016/acp-16-9727-2016.pdf |
| Summary: | The Amazon basin is a vast continental area in which
atmospheric composition is relatively unaffected by anthropogenic aerosol
particles. Understanding the properties of the natural biogenic aerosol
particles over the Amazon rainforest is key to understanding their influence
on regional and global climate. While there have been a number of studies
during the wet season, and of biomass burning particles 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
Network for Investigation of Amazonian Atmospheric Composition and Impacts on
Climate (BUNIAACIC) project, aerosol measurements, focussing on unpolluted
biogenic air masses, were conducted at a remote rainforest site in the
central Amazon during the transition from wet to dry season in July 2013.
This period marks the start of the dry season but before significant biomass
burning occurs in the region.
<br><br>
Median particle number concentrations were 266 cm<sup>−3</sup>, 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 enhancement in
the accumulation mode compared to the wet season, but not yet to the extent
seen later in the dry season, when significant biomass burning takes place.
Submicron nonrefractory aerosol composition, as measured by an aerosol
chemical speciation monitor (ACSM), was dominated by organic material (around
81 %). Aerosol hygroscopicity was probed using measurements from a
hygroscopicity tandem differential mobility analyser (HTDMA), and a
quasi-monodisperse cloud condensation nuclei counter (CCNc). The
hygroscopicity parameter, <i>κ</i>, was found to be low, ranging from 0.12
for Aitken-mode particles to 0.18 for accumulation-mode particles. This was
consistent with previous studies in the region, but lower than similar
measurements conducted in Borneo, where <i>κ</i> ranged 0.17–0.37.
<br><br>
A wide issue bioaerosol sensor (WIBS-3M) was deployed at ground level to
probe the coarse mode, detecting primary biological aerosol by fluorescence
(fluorescent biological aerosol particles, or FBAPs). The mean FBAP number
concentration was 400 ± 242 L<sup>−1</sup>; however, this ranged from
around 200 L<sup>−1</sup> during the day to as much as 1200 L<sup>−1</sup> at
night. FBAPs dominated the coarse-mode particles, comprising between 55 and
75 % of particles during the day to more than 90 % at night. Non-FBAPs did not
show a strong diurnal 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 dominated (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.
<br><br>
This is the first time that such a suite of measurements has been deployed at
this site to investigate the chemical composition and properties of the
biogenic contributions to Amazonian 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 previous similar campaigns.
This was also the first deployment of a WIBS in the Amazon rainforest to
study coarse-mode particles, particularly primary biological aerosol
particles, which are likely to play an important role as ice nuclei in the
region. |
|---|---|
| ISSN: | 1680-7316 1680-7324 |