Intercomparison of Multiple UV-LIF Spectrometers Using the Aerosol Challenge Simulator

Abstract

Measurements of primary biological aerosol particles (PBAPs) have been conducted worldwide using ultraviolet light-induced fluorescence (UV-LIF) spectrometers. However, how these instruments detect and respond to known biological and non-biological particles, and how they compare, remains uncertain due to limited laboratory intercomparisons. Using the Defence Science and Technology Laboratory, Aerosol Challenge Simulator (ACS), controlled concentrations of biological and non-biological aerosol particles, singly or as mixtures, were produced for testing and intercomparison of multiple versions of the Wideband Integrated Bioaerosol Spectrometer (WIBS) and Multiparameter Bioaerosol Spectrometer (MBS). Although the results suggest some challenges in discriminating biological particle types across different versions of the same UV-LIF instrument, a difference in fluorescence intensity between the non-biological and biological samples could be identified for most instruments. While lower concentrations of fluorescent particles were detected by the MBS, the MBS demonstrates the potential to discriminate between pollen and other biological particles. This study presents the first published technical summary and use of the ACS for instrument intercomparisons. Within this work a clear overview of the data pre-processing is also presented, and documentation of instrument version/model numbers is suggested to assess potential instrument variations between different versions of the same instrument. Further laboratory studies sampling different particle types are suggested before use in quantifying impact on ambient classification.

Detection of Airborne Biological Particles in Indoor Air Using a Real-Time Advanced Morphological Parameter UV-LIF Spectrometer and Gradient Boosting Ensemble Decision Tree Classifiers

“…The fungal samples used in this study are fungal material extracts intended for allergenic testing which have undergone chemical processing with acetone and are not naturally occurring whole spores. It is not clear how these may differ from naturally emitted spores, however, the fluorescent spectra of the processed samples are broadly consistent with those from other studies which examined live cultured fungal samples [23,38,40]. SEM images of the aerosolized fungal samples were made and these are presented in Forde et al, [23], where the samples were observed to be fibrous in nature and often amalgamated when rod-shaped or filament morphologies are expected.…”

“…A historic limitation of UV-LIF methods is that older spectrometers do not offer enough spectral resolution or morphological detail to unambiguously classify particles (e.g., Wideband Integrated Bioaerosol Spectrometer, WIBS and the UV-APS) due to the conflation of PBAP classes. More sophisticated UV-LIF spectrometers are now becoming available which offer much greater spectral resolution and particle shape information which should significantly improve PBAP classification capability [21][22][23][24][25]. While real time UV-LIF spectrometers may not offer the specificity of offline methods, their capacity for high time resolution detection makes them ideally suited for the investigation of rapid and dynamic changes in the indoor environment and as such provide critical complementary information on real-time dispersion.…”

“…Additionally we require that for a particle to be classified as fluorescent it must exhibit fluorescence in a minimum of 2 channels to filter out spurious measurements and noise caused by the grating as suggested by Könemann et al [24]. We choose to use 9σ thresholding in our analysis as this has the effect of removing ubiquitous weakly fluorescent non-biological interferents (e.g., dust and soot) from the population to be analysed while having only a very minor impact on PBAP which tends to be much more fluorescent [23,38]. In the next step, we normalise each individual particle's fluorescent spectra by the sum of the fluorescent intensity over all channels.…”

“…Several PBAP of interest were sampled using the Aerosol Challenge Simulator (ACS) at the Defence Science and Technology Laboratory (Dstl) at Porton Down, Wiltshire, United Kingdom. A full description of the site and experimental procedure is provided in Forde et al [23]. A brief description is now provided; known concentrations of test challenge particles are generated and introduced into a sampling manifold system via separate challenge aerosol and background sample mixing chambers as required.…”

“…The exhaust flow and aerosol stream was then passed to a double ultra-low particulate air filter section. Dry powders (all fungal material and pollen samples) were aerosolised using a modified TSI small-scale powder disperser (SSPD, model 3433) [23]. Liquid bacterial samples were dispersed into the ACS using a medical nebuliser from diluted starting stocks containing approximately 1 × 10 8 CFU/mL in suspension.…”