System and method for measuring aerosol or trace species
Inventors
Murphy, Daniel M. • Gordon, Timothy D.
Assignees
United States, Secretary Of Commerce • Government of the United States of America
Publication Number
US-9709491-B1
Publication Date
2017-07-18
Expiration Date
2036-03-28
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Abstract
Embodiments of the present invention relates to a system for measuring trace species in a sample gas. The present invention uses an open-path configuration including an optical cell with mirrors at each end, a long slotted rod with holes in the end for an optical bean to pass through, and a slotted tube surrounding the slotted rod such that air can pass through the rod in a transverse direction when the slots are aligned. Embodiments of the present invention further includes a flow configuration for purging sample gas from the optical cell, and cleaning mirrors, with calibrated or zero air through inlets in front of each mirror.
Core Innovation
Embodiments of the present invention relate to a system for measuring trace species in a sample gas using an open-path cavity that allows aerosol to flow freely through an optical cavity without any intervening inlet or tubing. The open-path aerosol extinction system is capable of isolating the optical cavity from wind and rain while still providing an open path for aerosols, enabling the characterization of extinction of highly humidified and/or coarse aerosol.
The system incorporates an optical cell with mirrors at each end, a long slotted rod with apertures for an optical beam to pass through, and a slotted tube surrounding the rod. The tube can be rotated concentrically around the rod to align or misalign the slots, enabling air to pass through during measurement or sealing the cavity for zeroing and calibration. This configuration includes a flow design to purge sample gas and clean the mirrors with calibrated or zero air introduced through inlets in front of each mirror.
Claims Coverage
The claims cover a system and an optical cell for measuring trace species in sample gas with inventive features related to the optical cavity configuration, flow control, and detection system.
Open-path optical cavity with slotted rod and rotatable slotted tube
The system includes a rod positioned between two curved mirrors forming an optical cavity, where the rod has a linear slot capable of receiving sample gas and apertures for the laser beam to pass through. A tube concentric with and surrounding the rod has opposing linear slots that can be rotated to align or misalign with the rod slot, allowing air to pass through or sealing the cavity from ambient air.
Mirror chambers with calibrated gas inlets
The optical cell includes first and second mirror chambers housing the curved mirrors, each having inlet ports to receive and direct calibrated gas in front of the mirrors, enabling purging and preventing contamination during zeroing and calibration.
Housing with airflow and turbulence control
A housing accommodates the optical cell components with a vertically oriented slot, at least one fan to draw ambient air through the slot, and a flow straightener to reduce turbulence in the optical cavity.
Fiber collimator and photodetector arrangement
The system includes a fiber collimator behind the second curved mirror to collect the laser beam exiting the cavity, focus it into a fiber optic cable, and transmit to a photodetector for signal measurement.
Processor for decay rate calculation and synchronization
A processor receives signals from the photodetector, calculates the decay rate of the optical cavity, determines the level of trace species, and synchronizes the measurement of transmitted light with laser source operation.
The independent claims detail a system for measuring trace species utilizing an open-path optical cavity with a novel slotted rod and rotatable tube mechanism, coupled with mirror chambers having calibrated gas inlets, a specialized housing for airflow management, and a detection system including fiber collimation and processor-based signal analysis.
Stated Advantages
The open-path aerosol extinction system avoids confounding effects from concentration gradients between the light source and distant target.
The system analyzes a more localized and well-defined sample compared to previous designs.
Background signals can be easily minimized by zeroing the instrument, improving measurement accuracy.
Documented Applications
Measuring scattering and absorption properties of aerosols, particularly in high humidity and coarse particle environments such as in or near clouds where existing instruments perform poorly.
Characterizing gases that are too reactive or sticky to be measured with closed-path instruments, allowing open-path measurement and effective zeroing.
Monitoring pollution conditions related to visibility and aerosols near solar arrays and other environments requiring pollution quantification.
Measuring particle flux using eddy covariance methods with high time resolution, overcoming past technical limitations.
Deployment on research aircraft for vertical aerosol extinction profiling to better constrain ground- and satellite-derived aerosol optical depth retrievals and reduce radiative forcing uncertainty, especially in twilight zones around clouds and dust-dominated regions.
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