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Final Report: A Sensitive and Affordable Compact Ammonia MonitorEPA Contract Number: EPD08015
Title: A Sensitive and Affordable Compact Ammonia Monitor
Investigators: Shorter, Joanne H
Small Business: Aerodyne Research Inc.
EPA Contact: Manager, SBIR Program
Project Period: March 1, 2008 through August 31, 2008
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2008) RFA Text | Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Air Pollution
Emission of ammonia to the atmosphere is a major environmental concern and a potential health hazard. However, the accurate measurement of ammonia sources requires the development of a high sensitivity, fast response, autonomous, continuous ammonia instrument capable of monitoring present emissions, quantifying the effectiveness of control measures, and evaluating the effects of ammonia emissions on local and regional soils, groundwater, and atmospheric environments. The goal of our SBIR program is to develop a low cost and an extremely compact ammonia monitor based on a novel astigmatic multipass cell. The multipass cell will allow the design of an instrument that is reduced to its optical essentials; little more than a laser, an absorption volume, and a detector. With the elimination of components and complexity, we expect to achieve a lower cost instrument while maintaining high sensitivity of 0.3 parts per billion (ppbv) in a 1 second measurement.
The goal of this program was to investigate the feasibility of developing a lower coast, yet highly sensitive, rapid response instrument for autonomous real-time monitoring of ammonia emissions. The primary research task of the program was to develop a novel multipass absorption cell that will allow the design of a simpler, lower cost instrument. Through the elimination of components and complexity, a lower cost instrument is possible while maintaining sensitivity.
We studied the feasibility of developing a lightweight laser spectrometer based on a novel approach to coupling light from a quantum cascade laser (QCL) into a low-pressure multipass cell. Unlike other Aerodyne Research, Inc. (ARI) astigmatic Herriot cells, we looked at a novel in-line construction. In this arrangement, the laser is at one end of the cell and the light exits at the opposite end of the cell. The optical train outside the cell can be made very compact and simplified, allowing the instrument to be very robust and resistant to misalignment.
In the Phase I program we have investigated the design of the in-line cell and examined the advantages and disadvantages of such a cell. In addition, a variation of this design where the cell is “flooded” with the light was studied. We also have looked at another design of a multipass cell that also could yield significant cost savings by allowing the use of lower cost mirrors and other components.
The flood-fill in-line cell is a variation on the original in-line cell concept wherein we illuminate the cell with a wide-angle beam. The illuminating beam spread angle would be on the order of 0.1 radians, as opposed to a conventional narrow-beam with a spread angle of 0.01 radians. Thus, the magnification of the laser beam would be reduced by a factor of 10, and simpler coupling optics could be used. The output beam would be expected to contain a mixture of path lengths, but that effect may be reduced by appropriate masking of the input beam. The mixture of path lengths also could offer an advantage, because light that arrives at the detector after a small number of passes would exhibit little absorption, and could be used for pulse-normalization.
We have demonstrated that the flood-fill in-line cell configuration can be made to have an output beam with most of the light exiting with the designed pass number, by masking the input beam. The disadvantage of this approach is that the optical transmission of the cell is rather low. Another significant finding from the ray-trace studies of flood-fill illumination is that the output spots are large and oddly shaped. Further examination of output spots for a large number of cases has resulted in a comprehensive model of aberrations in astigmatic cells.
Our studies of the last novel design of a multipass cell indicate that it is a promising route to a low-cost instrument. It could yield significant cost savings by allowing the use of lower cost mirrors and other components. We will file for patent protection of our new cell configuration if further studies continue to be promising.
In the Phase I program we demonstrated the feasibility of developing a lower cost, sensitive quantum cascade laser ammonia monitor through the development of a novel multipass absorption cell. We have identified two possible cell designs for the instrument: an in-line astigmatic multipass cell design based on the ARI AMACs, and a second novel cell design using low-cost components. The incorporation of either new cell along with lower cost collection optics and simpler cooling approaches for the laser and detector will lead to a lower cost, more commercial air monitoring instrument for ammonia and other important atmospheric trace gases.