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

摘要

Atmospheric particles of biological origin, also referred to as bioaerosolsor primary biological aerosol particles (PBAP), are important to varioushuman health and environmental systems. There has been a recent steepincrease in the frequency of published studies utilizing commercialinstrumentation 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 bothoutdoors and in the built environment. Significant work over several decadessupported the development of the general technologies, but efforts tosystematically characterize the operation of new commercial sensors haveremained lacking. Specifically, there have been gaps in the understanding ofhow different classes of biological and non-biological particles caninfluence the detection ability of LIF instrumentation. Here we present asystematic characterization of the WIBS-4A instrument using 69 types ofaerosol materials, including a representative list of pollen, fungal spores,and bacteria as well as the most important groups of non-biologicalmaterials reported to exhibit interfering fluorescent properties. Broadseparation can be seen between the biological and non-biological particlesdirectly using the five WIBS output parameters and by taking advantage ofthe particle classification analysis introduced by Perring et al. (2015).We highlight the importance that particle size plays on observedfluorescence properties and thus in the Perring-style particleclassification. We also discuss several particle analysis strategies,including the commonly used fluorescence threshold defined as the meaninstrument background (forced trigger; FT) plus 3 standard deviations() of the measurement. Changing the particle fluorescence thresholdwas shown to have a significant impact on fluorescence fraction and particletype classification. We conclude that raising the fluorescence thresholdfrom FT + 3 to FT + 9 does little to reduce therelative fraction of biological material considered fluorescent but cansignificantly reduce the interference from mineral dust and othernon-biological aerosols. We discuss examples of highly fluorescentinterfering particles, such as brown carbon, diesel soot, and cotton fibers,and how these may impact WIBS analysis and data interpretation in variousindoor and outdoor environments. The performance of the particle asymmetryfactor (AF) reported by the instrument was assessed across particle types asa function of particle size, and comments on the reliability of thisparameter are given. A comprehensive online supplement is provided, whichincludes size distributions broken down by fluorescent particle type for all69 aerosol materials and comparing threshold strategies. Lastly, thestudy was designed to propose analysis strategies that may be useful to thebroader community of UV-LIF instrumentation users in order to promote deeperdiscussions about how best to continue improving UV-LIF instrumentation andresults.