Primary biological aerosol (PBA) particles include bacteria, fungal spores, pollen and animal and plant debris. They are ubiquitous in the atmosphere and form a transmission vector for animal and plant disease. Several studies also suggest PBA are important in cloud ice processes despite being present at relatively low concentrations. Biodiversity changes caused by human activity may therefore have climate impacts through cloud-aerosol interactions, and improved methods are required to characterise PBA size distributions, sources and sinks on a global scale.Natural and anthropogenic sources contribute PBA while several processes remove it from the atmosphere. It therefore exhibits strong temporal and geographical variations in concentration. Published measurements involve several collection and offline analysis techniques, which are difficult to compare directly and generally feature low time resolution. This means PBA are poorly characterised globally and specific meteorological influences are difficult to identify. Light-induced fluorescence (LIF) offers a rapid, consumable-free method of measuring PBA size distributions, however false-positive results can contribute outdoors and LIF cannot resolve specific biological species.This thesis describes the use of a Wide Issue Bioaerosol Spectrometer version 3 (WIBS-3): a novel LIF instrument that records particle diameter, shape and fluorescence following excitations at two wavelengths. Its response to samples of PBA, non-PBA and calibration particles was assessed in the laboratory, and three outdoor studies are discussed at length: Below and above a tropical rainforest canopy (Malaysia); in an urban location (Manchester); and at a high-altitude background site (Puy de Dôme, France). The likely sources of fluorescent aerosol are identified at each site: Fungal spore release was triggered by rainfall in Borneo; increased fluorescent concentrations were linked to traffic activity in Manchester and a strong diurnal cycle in France was attributed to boundary layer depth. The limitations and benefits of the different WIBS-3 measurement channels are also discussed, along with comparisons of fluorescent particle concentrations with published and in-situ PBA measurements.False-positives are found to play a strong role outdoors away from PBA sources such as tropical forest, and a ubiquitous background of fluorescent non-PBA was detected using one excitation wavelength. A shorter excitation wavelength appears more capable of discriminating between PBA and false-positives. Basic PBA emission rates are calculated using data from these studies and tropical rainforests are concluded to be the largest source of PBA mass globally.