Recent fluorescence-based measurements of biological particles with the WIBS-4A

Biological particles (bacteria, fungi/fungal spores, viruses, algae and fragments of biological material) may play a significant role in modifying cloud properties by acting as ice nuclei at relatively warm temperatures (e.g., -2 C), and thus have an indirect effect on climate forcing. Little is known, however, regarding the abundance and distribution of biological particles and their importance to cloud microphysics in different environments. On-line, continuous measurement systems offer the potential to measure biological systems at high time resolution and sensitivity, providing greater insight into their distribution in the atmosphere and dispersal mechanisms. The WIBS-4A (Wideband Integrated Bioaerosol Sensor) detects fluorescent biological material in real-time associated with individual particles. It measures five properties: a) optical size via light scattering, b) fluorescent emissions in the wavelength range 310-400 following excitation by 280 nm light, c) fluorescent emissions in the wavelength range 420-650 following excitation by 280 nm light, d) fluorescent emissions in the wavelength range 420-650 following excitation by 360 nm light, and e) particle asymmetry factor based on intensities of forward scattered light onto a 4-element detector. Together, these properties aid the classification of sampled particles that contain biofluorophores such as tryptophan or NADPH, which are found in biological particles. Here we present results from a series of laboratory, ground-based and aircraft-based measurements of biological particles using the WIBS-4A. The studies include airborne measurements of fluorescent biological particles over the United States, ground-based measurements at an eastern US coastal site and a continental high alpine site, and laboratory measurements of a variety of biological and non-biological particles. Our analysis focused on both the characterization of the instrument response as well as demonstration of its suitability for performing ambient measurements and potential artifacts. We also present recommendations for field operation of the instrument, sample system design considerations, and data analysis approaches.