Systematic characterization and fluorescence threshold strategies for the wideband integrated bioaerosol sensor (WIBS) using size-resolved biological and interfering particles

Abstract

Abstract. Atmospheric particles of biological origin, also referred to as bioaerosols or primary biological aerosol particles (PBAP), are important to various human health and environmental systems. There has been a recent steep increase in the frequency of published studies utilizing commercial instrumentation based on ultraviolet laser/light-induced fluorescence (UV-LIF), such as the WIBS (wideband integrated bioaerosol sensor) or UV-APS (ultraviolet aerodynamic particle sizer), for bioaerosol detection both outdoors and in the built environment. Significant work over several decades supported the development of the general technologies, but efforts to systematically characterize the operation of new commercial sensors have remained lacking. Specifically, there have been gaps in the understanding of how different classes of biological and non-biological particles can influence the detection ability of LIF instrumentation. Here we present a systematic characterization of the WIBS-4A instrument using 69 types of aerosol materials, including a representative list of pollen, fungal spores, and bacteria as well as the most important groups of non-biological materials reported to exhibit interfering fluorescent properties. Broad separation can be seen between the biological and non-biological particles directly using the five WIBS output parameters and by taking advantage of the particle classification analysis introduced by Perring et al. (2015). We highlight the importance that particle size plays on observed fluorescence properties and thus in the Perring-style particle classification. We also discuss several particle analysis strategies, including the commonly used fluorescence threshold defined as the mean instrument background (forced trigger; FT) plus 3 standard deviations (σ ) of the measurement. Changing the particle fluorescence threshold was shown to have a significant impact on fluorescence fraction and particle type classification. We conclude that raising the fluorescence threshold from FT + 3σ to FT + 9σ does little to reduce the relative fraction of biological material considered fluorescent but can significantly reduce the interference from mineral dust and other non-biological aerosols. We discuss examples of highly fluorescent interfering particles, such as brown carbon, diesel soot, and cotton fibers, and how these may impact WIBS analysis and data interpretation in various indoor and outdoor environments. The performance of the particle asymmetry factor (AF) reported by the instrument was assessed across particle types as a function of particle size, and comments on the reliability of this parameter are given. A comprehensive online supplement is provided, which includes size distributions broken down by fluorescent particle type for all 69 aerosol materials and comparing threshold strategies. Lastly, the study was designed to propose analysis strategies that may be useful to the broader community of UV-LIF instrumentation users in order to promote deeper discussions about how best to continue i...

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“…A number of instrumental factors, including gain settings of detectors used for fluorescence detection, can significantly influence observed fluorescence intensities, making direct comparisons, even within similar instrumentation, challenging at best. In this context it is worth noting that the data reported here were recorded with the same WIBS unit used by Savage et al (2017). Moreover, Savage et al (2017) measured three PSL samples (2.0 µm green, 2.0 µm red, and 2.1 µm bluesame type and manufacturer, but different batches as those used in present work) and the obtained results agree with the results in Table 3.…”

“…In this context it is worth noting that the data reported here were recorded with the same WIBS unit used by Savage et al (2017). Moreover, Savage et al (2017) measured three PSL samples (2.0 µm green, 2.0 µm red, and 2.1 µm bluesame type and manufacturer, but different batches as those used in present work) and the obtained results agree with the results in Table 3. This suggests that the WIBS-derived PSL fluorescence intensities in Table 3 can be compared to the fluorescence intensities of the extent library of fluorescent materials and standard organisms in Savage et al (2017).…”

“…Thus, within a distribution of particles of a single composition, the fluorescence intensity is expected also to be Gaussian in nature. This assumption breaks down when observed fluorescence intensity saturates the detector but can still be instructive for comparison of particles, as was discussed by Savage et al (2017).…”

“…Emission detected in the 310 to 400 nm band from the 370 nm excitation is not detected because of PMT saturation. A detailed technical description of the WIBS series can be found elsewhere (e.g., Foot et al, 2008;Kaye et al, 2000Kaye et al, , 2005Savage et al, 2017;Stanley et al, 2011). The voltage settings used for all data presented here are PMT1 (AF) 400 V, PMT2 (particle sizing and FL1 emission) 450 mV, and PMT3 (FL2, FL3 emission) 732 mV.…”

“…Many LIF instruments deployed for the rapid detection of bioaerosol particles have become commonly used within the bioaerosol community, and a growing number of instruments are commercially available (e.g., Huffman and Santarpia, 2017). The WIBS-4A, in particular, has been used for the purposes of both laboratory validations and longer-term ambient measurements (e.g., Healy et al, 2012;Hernandez et al, 2016;Huffman et al, 2013;O'Connor et al, 2013;Perring et al, 2015;Robinson et al, 2013;Savage et al, 2017;Toprak and Schnaiter, 2013). The WIBS-4A provides information about particle size, a light scattering asymmetry factor (AF, broadly related to particle shape), and fluorescence properties for individual particles in real time.…”