AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Measurements of Absorption Enhancement at High Relative Humidity Using Short-Pulse Photoacoustic Spectroscopy
JAMES RADNEY, Christopher Zangmeister, National Institute of Standards and Technology
Abstract Number: 458 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract The absorption strength of aerosols is enhanced at high relative humidity compared to dry conditions. This enhancement can arise from multiple mechanisms ranging from the development of a core-shell morphology or the particle growth through water uptake. While this enhancement is well documented, its direct measurement using photoacoustic spectroscopy has remained elusive; to date, photoacoustic measurements of humidified aerosols have observed minimal enhancement relative to expectations. In many instances, signal depression is actually observed. Photoacoustic signals are generated from the heating and concomitant cooling of aerosols by a high intensity source. This heating and cooling generates a pressure wave that can be detected by a microphone. At high humidity, some of the absorbed energy volatizes particle bound water instead of being transferred completely into thermal energy for the acoustic wave. While volatizing water does generate some signal (as an increase in pressure), it represents only a fraction of the thermal energy that should have been released. Here, we will demonstrate a photoacoustic technique using a short-pulse supercontinuum laser with a high repetition rate (78 MHz, 650 ps pulse width) that allows for the measurement of absorption enhancement at high relative humidity (> 75%). We will also show that for ethylene soot, the absorption enhancement can be greater than 50% even when a change in particle mass (as measured using an aerosol particle mass analyzer) is not detectable and that significant changes in the absorption Angstrom exponent are also observed.