Measurements of Surrogate Respiratory Sessile Droplet pH and Implications for Exhaled Respiratory Aerosol and Airborne Disease Transmission

Respiratory aerosol pH has been proposed as a key factor driving the infectivity loss of SARS-CoV-2 viruses and influenza A virus in exhaled aerosols, thus affecting the airborne transmission of respiratory diseases. Sodium bicarbonate acts as a principal buffer in biological systems, regulating blood pH and the CO₂ balance between gas and liquid phases. Upon exhalation, changes in gas-phase conditions alter aerosol composition and pH. Despite Raman spectroscopy being used to quantify atmospherically relevant aerosol pH, the kinetics of CO₂ partitioning and pH variability in respiratory droplets remain poorly understood. In this paper, a method to investigate the HCO₃^–/CO₃^2– equilibrium in a surrogate respiratory fluid system within sessile droplets is proposed to elucidate the pH evolution of an exhaled respiratory aerosol. The enzymatic catalysis of CO₂ hydration and H₂CO₃ dehydration is explored. Experimental results were used to benchmark the ResAM model, which simulates respiratory aerosol droplet thermodynamics and pH evolution. Simulated pH evolution profiles of picoliter droplets show size independence. Simulations for both sessile droplets and respiratory aerosols show that carbonic anhydrase significantly increases the rate of pH increase, and gas-phase CO₂ levels are important for determining the final droplet pH. Consequences for understanding the aerobiological pathways for virus transmission are considered.