Abstract View
Accurate Aerosol Absorption Measurements at Reduced Sample Pressures
MICHAEL COTTERELL, Kate Szpek, David Tiddeman, Jim Haywood, Justin Langridge, University of Bristol
Abstract Number: 221
Working Group: Instrumentation and Methods
Abstract
Aerosol optical properties relate directly to their microphysical parameters and are used to interrogate differences in particle composition and mixing state. Recent years have seen optical spectroscopy on aerosols providing new insights into aerosol chemistry, reaction rates, water uptake in response to increasing humidity, and morphological evolution as aerosols age. Aerosol light absorption is poorly constrained in climate models, arising partly from a lack of accurate and direct observations of aerosol absorption. Photoacoustic spectroscopy (PAS) is the technique of choice for contact-free aerosol absorption measurements, and recent years have seen PAS instruments deployed in the field from aircraft measurement platforms for spatially resolved atmospheric aerosol measurements. The accuracy of these absorption measurements depends chiefly on the fidelity of instrument calibration and its dependence on sample pressure; measurements from aircraft platforms sample at varying pressures typically over the range 400 – 1000 hPa.
Strategies using O3-laden gas samples at varying pressures are often employed to calibrate PAS instruments operating at visible wavelengths. We show that such calibration approaches enable high accuracy aerosol absorption measurements at ambient (~1000 hPa) pressures but provide for increasingly inaccurate calibrations as the pressure is progressively reduced below 1000 hPa. Instead, we show that the PAS instrument can be calibrated at ambient pressure and then a miniature speaker used to quantity the pressure response function for the microphone transducer. In this way, we show that we accurately measure aerosol absorption at reduced pressure for sub-micrometre diameter aerosols consisting of dyed polystyrene latex spheres or nigrosin dye. These results will be of utmost interest to those measuring aerosol absorption using PAS from airborne platforms or those calibrating PAS instruments for ground based or laboratory measurements.