Using Tryptophan as an Indicator of pH in Bioaerosol Particle Fluorescence for Temporal Resolution of Aerosol Chemistry

HUNTER RICHARDS, Zhenyu Ma, Allen E. Haddrell, Herek L. Clack, University of Michigan

     Abstract Number: 513
     Working Group: Aerosol Chemistry

Abstract
Introduction
While the role of pH in respiratory virus inactivation has recently been discussed [1], measuring the change in pH of aerosols carried in the rapidly changing dynamics of exhaled breath is required to better connect this relationship to transmission of human respiratory diseases. To better understand the potential effect that an increase or decrease in pH has on the inactivation of certain respiratory viruses, it is first necessary to characterize the change in pH of a bioaerosol over time. Common fluorophores found in bioaerosol particles such as amino acids like tryptophan [2] can be exploited to interrogate pH change in exhaled bioaerosols that are relevant to the transmission of infectious disease. In our previous experiments, it was determined that a higher pH correlates with stronger 310-400 nm fluorescence, in response to 280 nm excitation, for aerosols containing tryptophan. From these results, using tryptophan as a pH-dependent fluorophore opens the possibility to monitor in situ the change in pH of bioaerosols.

In this study, a wideband integrated bioaerosol sensor (WIBS, Droplet Measurement Technologies) is used to analyze the fluorescence intensity of aerosolized solutions within a closed chamber containing a CO₂ sensor, using tryptophan fluorescence as an indicator of pH. An exploration of the effect of relevant environmental parameters, such as relative humidity and CO₂ concentrations, on aerosol pH provides insights into the impact on viral inactivation, climate implications, and air pollution control methods.

Experimental Procedure
A solution of 5% mass fraction of solute (MFS) sodium chloride (NaCl) and sodium bicarbonate (NaHCO3) with tryptophan was aerosolized using a handheld nebulizer (Mesh Nebulizer Model YM-3R9) into a closed chamber. A portable CO₂ monitor (SAF Aranet4) recorded temporal changes in CO₂ concentrations in the chamber while a Wideband Integrated Bioaerosol Sensor (WIBS) instrument sampled aerosols from the chamber to measure aerosol fluorescence intensity over time.

The WIBS instrument sampled from the chamber at a flow rate of 0.3 liters per minute for 10-20 second intervals at designated points in time over a 90 minute period. This period of time was sufficient to identify the rapidly-changing aerosol chemistry during the first 10 minutes post-aerosolization, with stable conditions forming for each solution after roughly 50 minutes in our prior experiments to establish optimal experimental procedures. The WIBS instrument measures bioaerosol fluorescence intensity of the aerosols sampled from the chamber at predetermined time intervals by using a Xenon flash lamp to excite tryptophan at its known excitation wavelength of 280 nm, which produces fluorescent emission at 310-400 nm wavelength.

The chamber conditions were adjusted for relative humidity and CO₂ concentration to determine the effect of these parameters on aerosol pH.

Results
The recorded fluorescence intensity spectra of sampled aerosols generated from bulk solutions of known pH were plotted in terms of relative abundance. At a higher bulk solution pH, aerosols generated from a solution mixture of NaCl and NaHCO3 with added tryptophan exhibit a higher fluorescence intensity. With the introduction to the chamber of the aerosolized NaCl + NaHCO₃ solutions, the CO₂ concentration in the chamber steadily rose over the duration of the chamber experiment before reaching a relatively stable condition after 40 minutes. Pairing the temporal evolution of CO₂ concentration in the chamber with the time-resolved fluorescence intensity of aerosols sampled by the WIBS instrument, fluorescence intensity decreased as the CO₂ concentration increased correlating with an increase in aerosol pH.

Summary and Conclusions
As CO₂ concentration increases over time, a reduction in fluorescence intensity was observed for the solution containing tryptophan as a fluorophore. This indicates an increase in bioaerosol pH as CO₂ concentrations increase. Using WIBS to monitor fluorescence intensity of bioaerosols mimicking exhaled breaths paired with CO₂ concentration monitoring can help better understand the relationship between pH and virus inactivation.

References
[1] Luo, Beiping, et al. “Expiratory Aerosol pH: The Overlooked Driver of Airborne Virus Inactivation.” Environmental Science & Technology (2023), 486-497.
[2] Pan, Yong-Le. “Detection and characterization of biological and other organic-carbon aerosol particles in atmosphere using fluorescence.” Journal of Quantitative Spectroscopy and Radiative Transfer, Volume 150 (2015), pages 12-35