Enhancing the Measurement Accuracy of Integrated Photoacoustic Nephelometers to Better Characterize Infrared Optical Properties of Stratospheric Aerosol Injection Candidates
PRABHAV UPADHYAY, Taveen Kapoor, Benjamin Sumlin, Rajan K. Chakrabarty, Washington University in St. Louis
Abstract Number: 259
Working Group: Instrumentation and Methods
Abstract
Stratospheric Aerosol Injection (SAI) is a geoengineering technique aimed at counteracting rising global temperatures by introducing sunlight-reflecting aerosols into the stratosphere to reduce global albedo. Ideally, SAI materials should not absorb shortwave solar and longwave terrestrial radiation (200-4000 nm) that can potentially heat the stratosphere. To constrain the types of suitable candidate SAI materials, the optical properties must be fully understood through laboratory measurements of their scattering and absorption behaviors. One instrument capable of this task is the integrated photoacoustic-nephelometer (IPN), which simultaneously measures these parameters. Improving the sensitivity and reducing the noise of near-IR IPN measurements is vital for providing accurate data to radiative transfer models that can predict the impact that SAI may have.
Noise in IPN measurements comprises gas flow noise, ambient acoustic noise from sampling inlets and instrument walls, laser power noise, and sensor and electronic noise. Our IPNs currently use a single-pass, single-resonator configuration with three sensors (two photodiodes, one microphone) that measure the extinction and scattering of light and sound waves due to the photoacoustic effect from light absorption in the photoacoustic resonator. The primary source of measurement uncertainty is fluctuating laser power, which is influenced by ambient conditions and operational parameters that affect the laser's operating temperature and consequently its power output. The instrument has difficulty distinguishing real signals from laser fluctuations near the detection limit. This effect is present even in laboratory conditions and is exacerbated by highly dynamic ambient conditions during field measurements.
To mitigate these uncertainties in the single-resonator design, we have integrated an additional sensor to monitor the unattenuated laser power and correct the changing background signals due to laser power fluctuations in real-time. Additionally, we are investigating enhancements in sensitivity through computational acoustic and fluid dynamics simulations to analyze the effects of resonator geometry on acoustic amplification and flow noise, and the positioning of sampling inlets and outlets on acoustic noise. This study aims to refine the design of IPNs to improve measurements of aerosol scattering and absorption coefficients in the near-IR spectrum.