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

James D. Whitehead, Eoghan Darbyshire, Joel Brito, Henrique M.J. Barbosa, I.A. Crawford, Rafael Stern, Martin W. Gallagher, Paul Kaye, James D. Allan, H. Coe, Paulo Artaxo, G. B. McFiggans

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    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.
    Original languageEnglish
    Pages (from-to)9727-9743
    Number of pages16
    JournalAtmospheric Chemistry and Physics
    Issue number15
    Publication statusPublished - 3 Aug 2016


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