Spectral Intensity Bioaerosol Sensor (SIBS): an instrument for spectrally resolved fluorescence detection of single particles in real time

Abstract

Abstract. Primary biological aerosol particles (PBAPs) in the atmosphere are highly relevant for the Earth system, climate, and public health. The analysis of PBAPs, however, remains challenging due to their high diversity and large spatiotemporal variability. For real-time PBAP analysis, light-induced fluorescence (LIF) instruments have been developed and widely used in laboratory and ambient studies. The interpretation of fluorescence data from these instruments, however, is often limited by a lack of spectroscopic information. This study introduces an instrument – the Spectral Intensity Bioaerosol Sensor (SIBS; Droplet Measurement Technologies (DMT), Longmont, CO, USA) – that resolves fluorescence spectra for single particles and thus promises to expand the scope of fluorescent PBAP quantification and classification. The SIBS shares key design components with the latest versions of the Wideband Integrated Bioaerosol Sensor (WIBS) and the findings presented here are also relevant for the widely deployed WIBS-4A and WIBS-NEO as well as other LIF instruments. The key features of the SIBS and the findings of this study can be summarized as follows. Particle sizing yields reproducible linear responses for particles in the range of 300 nm to 20 µm. The lower sizing limit is significantly smaller than for earlier commercial LIF instruments (e.g., WIBS-4A and the Ultraviolet Aerodynamic Particle Sizer; UV-APS), expanding the analytical scope into the accumulation-mode size range. Fluorescence spectra are recorded for two excitation wavelengths (λex=285 and 370 nm) and a wide range of emission wavelengths (λmean=302–721 nm) with a resolution of 16 detection channels, which is higher than for most other commercially available LIF bioaerosol sensors. Fluorescence spectra obtained for 16 reference compounds confirm that the SIBS provides sufficient spectral resolution to distinguish major modes of molecular fluorescence. For example, the SIBS resolves the spectral difference between bacteriochlorophyll and chlorophyll a and b. A spectral correction of the instrument-specific detector response is essential to use the full fluorescence emission range. Asymmetry factor (AF) data were assessed and were found to provide only limited analytical information. In test measurements with ambient air, the SIBS worked reliably and yielded characteristically different spectra for single particles in the coarse mode with an overall fluorescent particle fraction of ∼4 % (3σ threshold), which is consistent with earlier studies in comparable environments.

Real-time sensing of bioaerosols: Review and current perspectives

“…In all cases, particle sizing should periodically be calibrated and monitored, e.g., using NIST-traceable PSL particles. Various instruments provide proxies for particle shape, but the quality of these measurements varies between instrument types and their application should involve careful calibration with known particle shapes (Gabey et al 2010;Healy et al 2012;K€ onemann et al 2019;Savage et al 2017).…”

“…Several noncommercial WIBS versions were produced (i.e., revisions 3 and 4; UH) with slightly different optical and electronic configurations, and DMT has manufactured two commercial models (4A and NEO). Detailed technical descriptions can be found elsewhere (Foot et al 2008;Gabey et al 2010;Kaye et al 2007, Kaye et al 2005K€ onemann et al 2019;Perring et al 2015). All WIBS versions are generally similar: Elastic scatter from a 635-nm laser is used to determine particle size and asymmetry, two xenon flashlamps are filtered to produce narrow excitation wavebands centered at 280 and 370 nm, and two wideband photomultiplier detection channels ($310-400 nm and $420-650 nm) provide three channels of fluorescence detection.…”

“…The Spectral Intensity Bioaerosol Spectrometer (SIBS, DMT) is a commercial LIF instrument built on the optical block of the WIBS, modified to disperse emission spectra into 16 channels (300-720 nm) following excitation pulses from filtered xenon flashlamps centered at 285 and 370 nm (K€ onemann et al 2019). The instrument has been described in detail, characterized in the laboratory (K€ onemann et al 2019), and recently applied to ambient aerosol (K€ onemann et al 2018a;Nasir et al 2018). The SIBS provides significantly increased spectral resolution relative to WIBS units and may be able to discriminate between aerosol types with higher certainty once appropriate data analysis techniques (e.g., clustering or machine learning) are applied.…”

“…LIF bioaerosol data are generally analyzed in one of a few ways: (i) counting particles above a given threshold as fluorescent, and, often, interpreting this as a lower limit proxy for PBAP (e.g., Gabey et al 2010;Huffman, Treutlein, and P€ oschl 2010), (ii) assigning types based on response in different fluorescent channels (Perring et al 2015;Wright et al 2014), or (iii) using classification algorithms utilizing resolved emission spectra from multichannel instruments (K€ onemann et al 2019;Ruske et al 2017). The uncertainties associated with each strategy can dramatically affect interpretation of data, as discussed below.…”

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

“…The PLAIR Rapid-E, as based upon the principles described by Kiselev et al (2011Kiselev et al ( , 2013 [16,17], features fluorescence detection over 32-wavelength bins following single channel excitation, in addition to a larger particle detection range of 1-100 µm. Other examples include the Spectral Intensity Bioaerosol Sensor (SIBS), which features dual wavelength excitation (285 nm and 370 nm) and 16-channel fluorescence detection (302-721 nm) [18][19][20].…”