Atmospheric aerosol measurements were conducted near Danum Valley, in theMalaysian state of Sabah, North-East Borneo, as part of the OP3 and ACESprojects, in April and June/July 2008. Here, aerosol fluxes and diurnalvariability in and above the rainforest canopy were examined in order to gainan understanding of their behaviour in the surface layer of the South-EastAsian rainforest. Aerosol fluxes were calculated by eddy covariance frommeasurements above the rainforest canopy on the Global Atmosphere Watch (GAW)tower. Upward fluxes were seen on most mornings between 09:00 and 11:00 localtime and this could be attributed to venting of the nocturnal boundary layeras it broke up in the morning. Measurements were also conductedwithin the canopy and trunk space at a nearby site. Profiles in aerosol number concentrationswere investigated using GRIMM Optical Particle Counters (OPCs) at variouslevels within the rainforest canopy and trunk space, as well as a single OPC on a verticallymoving platform. These showed an overnight increase in larger particles (1–20 μm) at all levels, but much more prominently near the top ofthe canopy, which could be attributed to fog formation. At ground level,number concentrations in this size range correlated with enhancements inbiological aerosol concentrations, measured using a Wide Issue BioaerosolSpectrometer (WIBS) located near the forest floor, suggesting that coarseparticle number concentrations were dominated by biological aerosols. Acomparison of particle number concentrations (in the size range0.5–1.0 μm) between above canopy and the trunk space showed correlations,despite turbulence data suggesting persistent decoupling between the twomeasurement sites. These correlations often relied on a shift of the particletime-series against each other, implying a time delay in observations betweenthe sites, which varied according to time of day. This lag time was shortestduring the middle of the day by a significant margin. This was not observedfor aerosols larger than 1.0 μm. Further evidence of daytimecoupling between above canopy and the trunk space in terms of aerosol measurements isimplied by comparison of measurements from an Aerosol Mass Spectrometer (AMS)at the GAW tower and simultaneous bag sampling at the in-canopy site,subsequently analysed with the AMS. Transport of particles through the canopyseems to occur through large-scale, sporadic turbulent events, suggestingthat the coupling between the canopy space and the air above is due to theseventilation events.
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