Real-time Measurements of Fluorescent Biological Aerosol Particles in the Atmosphere and in Clouds

Some types of biological particles are known to efficiently nucleate ice at relatively warm temperatures in the atmosphere, with the potential to influence cloud microphysical properties and climate. However, the prevalence (or lack thereof) of these particle types in different parts of the atmosphere and in clouds is a matter of debate. Current techniques are mostly limited to near-ground sampling or to limited aircraft sampling with complex instrumentation. Many types of biological particles fluoresce when exposed to ultraviolet light, and the Wideband Integrated Bioaerosol Sensor (WIBS-4A) takes advantage of this characteristic to perform real-time measurements of fluorescent biological aerosol particles (FBAP).

The multi-year IDEAS program, sponsored by the National Science Foundation, allows for instrument testing and training of students in airborne research. During the IDEAS test campaign conducted in fall of 2013, the WIBS-4A was flown on the National Center for Atmospheric Research G-V aircraft to measure concentrations of fluorescent biological particles from different potential sources and at various altitudes over the U. S. western plains states. Measurements were also made using the WIBS behind a counterflow virtual impactor (CVI) to sample cloud particles of different types. Aerosol particle types and immersion freezing ice nuclei temperature spectra were also investigated in clear and cloudy air using offline analyses of particles collected on filters.

Most of the analysis focused on particles fluorescing in two WIBS channels representative of biological particles: fluorescent channel 1 at 310-400 nm (after a 280 nm excitation), and fluorescent channel 3 at 420-650 nm (after a 370 nm excitation). Clear-air profiles of particles showed a strong dependence of biological particles on altitude (or temperature). Concentrations of FBAP larger than 800 nm ranged from about 10-70 per liter in the boundary layer but concentrations were usually 1-2 orders of magnitude lower in the free troposphere. Clear-air FBAP concentrations at mixed-phase cloud temperatures colder than 260K were often <1 per liter. The fraction of all large particles that fluoresced in the two WIBS channels was between 0.00 and 0.10, with an overall mean of 0.02 for this region and season.

A previous ground-based study near Manitou Springs, Colorado, suggested that the population of ice nucleating particles at the forested site, as measured by a continuous flow diffusion chamber (CFDC), was dominated by biological particles. During IDEAS, focused vertical profiles were made over the same site with the WIBS instrument. About 25 per liter (5-10% of large particles sampled) appeared to be biological above the forest canopy, with a decline at higher altitudes. Low-altitude measurements from the airborne filter samples gave similar ice nuclei spectra as the ground-based CFDC measurements, with about 0.1 per liter of all large particles active as ice nuclei at 253K and about 1 per liter initiating ice formation at 248K.

Preliminary measurements of FBAP within ice crystals in an orographic cloud at 252K indicated about 0-2 per liter fluoresce in both WIBS channels, although as many as 10 per liter fluoresced in WIBS channel 3 only. For a deep frontal cloud, between 0-2 per liter of residuals from droplets or ice crystals fluoresced in both channels. Time-averaging of the data reduced counting uncertainty in measured number concentration to about 1 per liter.

Further refinements will be made to FBAP concentrations using computations of inlet and sampling line aspiration and transmission efficiencies. At the meeting, these measurements will be presented and implications for effects of biological particles on cloud formation will be discussed.