Biogenic cloud nuclei in the Amazon
Authors: Whitehead J. D. , Darbyshire E. , Brito J. , Barbosa H. M. J. , Crawford I. , Stern R. , Gallagher M. W. , Kaye P. H. , Allan J. D. , Coe H. , Artaxo P. , McFiggans G. 
Published: January 2016

Abstract. 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 above the canopy at a remote rainforest site in the Amazon, during the transition from wet to dry seasons, in July, 2013. This period marks the start of the dry season, but before signiﬁcant 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 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 non-refractory aerosol composition, as measured by an Aerosol Chemical Speciation Monitor (ACSM), was dominated by organic material (86 %). 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, 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 previous studies in the region, but lower than similar measurements conducted in Borneo, where κ ranged 0.17–0.37, possibly due to a stronger marine inﬂuence at that location, bringing higher sulphate loadings than are typically seen in the Amazon. 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 FBAP). The mean FBAP number concentration was 404 ± 237 l−1, however this was subject to a strong diurnal cycle, and ranged from around 200 l−1 during the day to as much as 1200 l−1 at night. FBAP dominated the coarse mode particles, comprising more than 90 % of particles detected by the WIBS-3 during the night. This proportion was also subject to a diurnal cycle, dropping to between 55 % and 75 % during the day, since non-FBAP 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. Application of Ward linkage cluster analysis using the z-score normalisation to the data suggests that FBAP were dominated (around 70 %) by fungal spores. Further, long-term measurements will be required in order to fully examine the seasonal variability, and distribution through the canopy of primary biological aerosol particles. 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 dry seasons, and thus provides a unique contrast 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 is likely to play an important role as ice nuclei in the region.