Construction of a Biofluorescence Optical Particle Counter

Russell Greaney, Oliver Ryan, S. Gerard Jennings, Colin D. O’Dowd,

O’Dowd et al. [1] found that submicrometer marine spray aerosol contained a significant fraction of organic matter is associated with the seasonality of plankton biological activity as determined from satellite ocean-colour products.  When plankton activity is high, the chemical composition of the aerosol is dominated by organic matter that possesses characteristics similar to organic matter found at the ocean surface. Laser induced fluorescence will probe the organic content of marine aerosols through excitation of fluorescence of chlorophyll-a contained in the phytoplankton.  In-vivo chlorophyll-a has a main broad absorbance peak at ~440 nm and two main fluorescence peaks at ~670 nm and ~720 nm.  The aerosol is probed with 405 nm 50 mW laser that has a wavelength to allow for maximum absorption and induce a maximum yield of fluorescence. The instrument will stimulate and collect a total fluorescence signal from each probed aerosol particle in order to determine the ratio of elastic scattered light at laser wavelength to fluoresced light.  Simultaneously it will allow particle sizing through collection of the elastically scattered laser light, and a flux measurement by counting the particles that pass. The laser choice should, theoretically enable scattering measurements to infer diameters down to 50 nm (but instrumental limitations may prevent this), while still stimulating chlorophyll-a at high efficiency. The energy output of 50 mW is sufficiently high to reliably induce the fluorescence, based on existing experimental data available in the literature, e.g. [2-4]. The fluorescence quantum yield is very low for chlorophyll-a in phytoplankton, ranging from 2% to 7% [3] or 0.6 – 3% [4].  The fluorescence yield depends on, for example, type of phytoplankton, light exposure history, other absorbing pigments in the phytoplankton, competing energy processes, available nutrients and surrounding medium [5, 6]. The fluorescence signature also has a very short lifetime on the order of nanoseconds making the fluorescence output almost simultaneous to the elastic scattered light signal.  This necessitates the use of highly sensitive photomultiplier tubes (PMTs) to maximise data signal-to-noise.

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