Optical hygrometer using differential absorption of hydrogen Lyman-alpha radiation

Area of Science:

• Atmospheric Science
• Optical Instrumentation
• Spectroscopy

Background:

• Accurate measurement of atmospheric water vapor is crucial for understanding weather and climate.
• Existing hygrometers face challenges with accuracy, response time, and interference from other atmospheric components.

Purpose of the Study:

• To design and build an optical hygrometer for atmospheric field studies.
• To utilize the differential absorption of the Lyman-alpha line for precise water vapor quantification.
• To overcome limitations of previous instruments, including light source instability and O(2) absorption.

Main Methods:

• Differential absorption measurement along two paths: one with sample air, one with dry reference air.
• Utilizing the H Lyman-alpha line for water vapor absorption detection.
• Employing a dynamic range adjustment through sample dilution with reference air.

Main Results:

• The optical hygrometer achieves high accuracy (3%) at typical tropospheric water vapor concentrations.
• Detection of water vapor concentrations as low as ~3.1 x 10(14) molecules/cm³ (0.01 g/m³) is possible.
• The instrument demonstrates a fast response time of less than 1 second.
• The absorption cross-section of water vapor at the Lyman-alpha wavelength was determined to be (1.64 +/- 0.045) x 10⁻¹⁷ cm².

Conclusions:

• The developed optical hygrometer is a robust and accurate tool for atmospheric field studies.
• The differential absorption technique effectively mitigates interference and light source variations.
• The instrument's performance characteristics make it suitable for real-time atmospheric water vapor monitoring.