Quartz-Enhanced Photoacoustic Spectroscopy for Field Calibration of Atmospheric Aerosol Absorption Properties

Accurate measurements of absorption by atmospheric aerosols are needed for the development and verification of models to predict climate change. However, instruments deployed in field conditions, especially for long-term or unattended operation, can have greatly increased measurement uncertainties compared to their laboratory counterparts. Field-capable instrumentation with improved accuracy and sensitivity is needed. Accuracy may be improved by direct absorption measurements using photoacoustic absorbance spectroscopy PAS), but conventional PAS sensors are relatively large and suffer from susceptibility to background noise. Smaller, lighter direct absorbance instrumentation with high accuracy is needed to enable widespread measurements of high fidelity in field studies. A variant of PAS is proposed for improved field calibrations that uses a quartz tuning fork QTF) as the sensing element. The QTF enables a very high level of immunity to background noise and offers a smaller sample volume than conventional PAS. The method would measure absorption of black carbon and brown carbon aerosols with high sensitivity using a multi-wavelength laser source to differentiate between them. The proposed feasibility study will determine how an aerosol affects a QTF, predict the detectability limits of particle concentration, and test these limits experimentally. Once feasibility has been established, a preliminary design for a Phase II prototype instrument will be produced. MetroLaser plans to commercialize the technology being developed in this effort in the form of a highly sensitive instrument that can be used in field studies from both stationary and airborne platforms. Its inherently high accuracy will make it suitable for calibrating other instrumentation. State and federal government agencies concerned with environmental studies, as well as universities, should benefit from the proposed instrument due to its ease of use and low measurement uncertainty.