Optical Properties of Light Absorbing Aerosol Particles at Elevated Relative Humidity
GWEN RACHEL LAWSON, Simon Xi Chen, Justin Langridge, Kate Szpek, Michael Cotterell, University of Bristol
Abstract Number: 191
Working Group: Aerosol Physics
Abstract
Interactions of aerosols with light affect the overall radiative forcing of the Earth’s climate. Despite this importance, they remain one of the largest uncertainties in climate models, limiting future improvements as illustrated by the IPCC report (2021). The absorption of light by atmospheric aerosols and the role of volatile components are not well understood. Photoacoustic spectroscopy (PAS) has been shown to provide accurate absorption measurements for sub-micron, dry aerosol particles devoid of any volatile components (Langridge et al., 2013). PAS uses a periodically modulated laser to irradiate an aerosol sample. Light absorbing aerosols are heated by the laser and transfer their heat to the surrounding bath gas, which undergoes periods of adiabatic expansion and contraction, generating acoustic waves. The amplitude of the acoustic wave is related to the absorption coefficient through calibration. PAS relies on efficient heat flux from the particles to the bath gas to generate this acoustic wave. When there is water or other volatile compounds present, the measured absorption is low biased for sub-micron particles since the additional heat flux pathway via evaporation has a lower conversion efficiency than the sensible heat flux. Extending the current PAS technique to facilitate measurements of absorbing aerosols at high relative humidity represents a key improvement to allow measurements of aerosol absorption under atmospherically relevant conditions. We use cavity ring down spectroscopy (CRDS) to measure the extinction coefficients of hydrated aerosol particles, at a range of elevated relative humidities, to characterize the hygroscopic growth. We then explore the potential to use the phase shift, the difference in phase between the waveforms of the periodically modulated laser and the generated sound wave, to correct the measured absorption for the presence of water at elevated relative humidity. This would allow PAS to measure aerosol absorption outside of the current limitation of dry conditions.