Measurements and comparison of primary biological aerosol above and below a tropical forest canopy using a dual channel fluorescence spectrometer 1Centre for Atmospheric Science, University of Manchester, M13 9PL, UK
2Science and Technology Research Institute, University of Hertfordshire, AL10 9AB, UK
Received: 25 Aug 2009 – Published in Atmos. Chem. Phys. Discuss.: 11 Sep 2009Abstract. Aerosol particle size distributions were measured below and above a tropical
rainforest canopy in Borneo, Malaysia, in June/July 2008 using the WIBS-3: a
single particle dual channel fluorescence spectrometer. Material in the size
range 0.8–20 μm was characterized according to optical equivalent
diameter (DP), morphology and fluorescence at 310–400 nm and
400–600 nm following excitation at 280 nm and 370 nm respectively.
Particles fluorescent after both excitations are likely to be fluorescent
primary biological aerosol particles (FBAP).
Revised: 11 Apr 2010 – Accepted: 19 Apr 2010 – Published: 17 May 2010
Measured FBAP number concentration (NFBAP) at both sites exhibited
clear diurnal cycles. The largest variability was observed in the
understorey, where NFBAP reached a minimum of 50–100 L−1 in
late morning. In mid afternoon it exhibited strong transient fluctuations as
large as 4000 L−1 that were followed by sustained concentrations of
1000–2500 L−1 that reduced steadily between midnight and sunrise.
Above the canopy FBAP number ranged from 50–100 L−1 during the daytime
to 200–400 L−1 at night but did not exhibit the transient enhancements
seen in the understorey. The strong FBAP fluctuations were attributed to the
release of fungal spores below the canopy and appeared to be linked to
elevated relative humidity.
The mean FBAP number fraction in the size range 0.8 μm<DP<20 μm
was 55% in the understorey and 28% above canopy. A
size mode at 2 μm<DP<4 μm appears at both
sites and is primarily FBAP, which dominated the coarse (DP≥2.5 μm)
number concentration at both sites, accounting for
75% in the understorey and 57% above the canopy. In contrast, the
concentration of non-fluorescent particles (NNON) at both sites was
typically 200–500 L−1, the majority of which occupied a size mode at
0.8<DP<1.5 μm. Enhanced understorey NNON was
observed daily in mid-afternoon and also at midday on three occasions: the
former coincided with the FBAP enhancements and measured approximately 10%
of their magnitude; the latter occurred independently of the NFBAP
diurnal cycle and comprised particles smaller than 2 μm. Particle
diameter of 3–5 μm is consistent with smaller fungal spores,
though absolute identification of biological species is not possible with the
UV-LIF technique. Based on the measured FBAP and non-fluorescent particle
abundances and their observed recovery times following rain showers, FBAP
originated beneath the canopy while the non-fluorescent material was
transported from further away. It is concluded that these separate sources
contributed the majority of the aerosol measured by the WIBS-3 at both sites.
Citation: Gabey, A. M., Gallagher, M. W., Whitehead, J., Dorsey, J. R., Kaye, P. H., and Stanley, W. R.: Measurements and comparison of primary biological aerosol above and below a tropical forest canopy using a dual channel fluorescence spectrometer, Atmos. Chem. Phys., 10, 4453-4466, doi:10.5194/acp-10-4453-2010, 2010.