A photophonic instrument concept to measure atmospheric aerosol absorption

Abstract

A laboratory model of an instrument concept to measure the absorption of solar radiation by atmospheric aerosols was designed, built and tested. The concept was based on the photophonic phenomenon discovered by Bell and an acoustic resonator developed by Helmholtz.

The design consisted of two chambers: an aerosol chamber and a reference chamber combined into a double Helmholtz resonator configuration. The radiation from the visible light source was amplitude modulated by a mechanical chopper. The modulated light beam was passed through the chambers and pressure variations resulted from energy absorbed by the aerosol in the chamber. The pressure signal was sensed by microphones, then the electrical signal amplified and processed by a differential amplifier.

The testing showed the instrument had sufficient sensitivity and low enough system noise to measure an absorption coefficient of about 10⁻⁶/meter. Methods of signal improvement and noise reduction were discussed and tested.

The results showed the instrument could measure absorption coefficients within the range expected by the earth's atmospheric aerosols.

The instrument design was not optimized for maximum signal or minimum noise, but the justifiable conclusion was reached that the concept showed the promise of leading to a useful instrument in the measurement of atmospheric aerosol absorption and an improvement over the present instruments.