TECHNOLOGY AREA(S): Battlespace, Sensors, Weapons

ACQUISITION PROGRAM: Air Ocean Tactical Applications - Surface Atmospheric Sensing Capabilities (Non-ACAT)

OBJECTIVE: Develop, demonstrate, and transition an open-celled cavity ring-down instrument to measure ambient light extinction at multiple wavelengths, including the capability to measure in the near infrared used by directed energy systems, such as 1.06 or 1.61 microns.

DESCRIPTION: Direct measurements of ambient light extinction are difficult to obtain. Sampling the atmosphere into a closed celled system results in particle/droplet loss and disrupts temperature and humidity field for those that remain. Long path instruments by nature measure an integrated signal along a path rather than at a point, and can be impacted be refraction effects. Cavity Ring-Down Spectrometers (CRS) essentially package a long path signal between two ultra-high fidelity mirrors spaced closely together. A beam is propagated across a cavity. When terminated, the time the remaining energy decays (or rings down) is recorded. This intensity decay rate can be directly related to ambient extinction to accuracies demonstrated to better than 5% [Ref 10]. There are a number of research-grade CRS systems used by the scientific community to measure both aerosol particles and gasses, and have been demonstrated on both ground and airborne deployments. Current examples applications from the peer reviewed literature, include characterizing air pollution and haze in mega cities [Ref 7]. When coupled with a comparable light scatting instrument, CRS�s can help measure atmospheric aerosol absorption to high fidelity [Ref 1] index of refraction [Ref 3]. Examination of carefully selected absorption lines can provide real time air chemistry measurements [Refs 6, 8].

For directed energy purposes (such as supporting test range activities and later operations), engineers must account for atmospheric extinction and absorption in the ambient environment, including ambient aerosol particles to fogs and mists. CRS systems have demonstrated the ability to monitor extinction to high fidelity in closed cavities. However, application of this technology to monitor ambient conditions will require a more open-celled system than is traditionally used by the community. Closed celled systems suffer from particle losses of larger particles in airflow plumbing, and particle working by the instruments optical block lead to an evaporation of water on the particles [Ref 11]. A second challenge is that while CRS systems are commonly applied in visible wavelengths for climate and air quality applications, for directed energy applications the CRS must be modified to perform in near infrared at wavelengths utilized by these systems, such as 1.06 microns. Finally, to help characterize the optical environment two or preferably three wavelengths should be measurable, including at least one in the visible portion of the spectrum. From three wavelengths, the relative contribution of fine mode particles such as pollution and smoke can be differentiated from larger particles such as dust, sea spray, and fog [Refs 5, 9].

Nominal Performance Targets: (Guidelines, not requirements. Proposal should address projected capabilities of specific technical approach)
� Ambient light extinction range of 0.005 to 10 per km
� At least two wavelengths, in the red and near infrared, with three preferable (with the addition of green)
� Response time on the order of seconds
� Easily deployable to the field with reasonable weight (<40 lbs.), power (<500 W), and minimal operator interface
� Proofed for typical ambient environments, including inclement weather, marine environments and heavy dust
� Full data stored on-board

PHASE I: Design a specific sensor engineering concept. Conduct an ambient environment demonstration as a proof-of-concept. Prepare a Phase II plan.

PHASE II: Further develop the concept into an instrument for deployment on a test range and/or at sea during a directed energy field test, including further developing the user interface and an instrument housing that is rugged enough to be used shipboard in a maritime environment. Provide a final technical report and deliver the prototype instrument for further use and evaluation.

PHASE III DUAL USE APPLICATIONS: Support the Navy in transitioning the instrument to deployment to the fleet. There is an ever-increasing use of near IR systems for civilian applications and research, including sensors for transportation and air quality.

REFERENCES:

1. Al Fischer, A., and Smith, G. D., �A Portable, Four Wavelength Single-Cell Photoacoustic Spectrometer for Ambient Aerosol Absorption.��� Aerosol Science and Technology, 52:4, 383-406,2018.� DOI: 10.1080/02786826.2017.1413231

2. Baynard, T., Lovejoy, E.d R., Pettersson, A., Brown, St. S., Lack, D., Osthoff, H., Massoli, P., Ciciora, S., Dube, W.P., and Ravishankara, A.R. �Design and Application of a Pulsed Cavity Ring-Down Aerosol Extinction Spectrometer for Field Measurements.� Aerosol Science and Technology, 41:4,447-462, (2007) DOI: 10.1080/02786820701222801

3. Dinar, E., Riziq, A. A., Spindler, C., Erlick, C., Kiss, G., Rudich, Y., �The Complex Refractive Index of Atmopsheric and Model Humic-like Substances (HULIS) retrieved by a Cavity Ring Down Aerosol Spectrometer (CRD-AS), Faraday Discussions, 137, 279-295, 2008, DOI: 10.1039/b703111d.

4. Gordon, T.D., Wagner, N.L., Richardson, M.S., Law, D.C., Wolfe, D., Eloranta, E.W., Brock, C.A., Erdesz, F., and Murphy, D.M., �Design of a Novel Open-Path Aerosol Extinction Cavity Ringdown Spectrometer.� Aerosol Science and Technology, 49:9, 717-726, 2015. DOI: 10.1080/02786826.2015.1066753.

5. Kaku, K. C., Reid, J. S., O'Neill, N. T., Quinn, P. K., Coffman, D. J.. and Eck T. F., (2014), Verification and application of the extended spectral deconvolution algorithm (SDA+) methodology to estimate aerosol fine and coarse mode extinction coefficients in the marine boundary layer, Atmospheric Measurement Technology, 7, 3399-3412, 2014, DOI:10.5194/amt-7-3399-2014.

6. Laj, P., et al., �Measuring Atmopsheric Composition Change.� Atmospheric Environment, 43:33, 5351-5414, 2009, DOI 10.1016/j.atmosenv.2009.08.020.

7. Li, R., Hu, Y., Li, L., Fu, H., and Chen, J., �Real-time aerosol optical properties, morphology and mixing states under clear, haze and fog episodes in the summer of urban Beijing,� Atmos. Chem. Phys., 17, 5079-5093, 2017, https://doi.org/10.5194/acp-17-5079-2017.

8. Li, Z. Y, Hu, R. Z., Xie, P. H., Chen, H., Wu, S. Y., Wang, F. Y., Wang, Y. H., Ling, L. Y., Liu, J. G., and Liu, W. Q., �Development of a Portable Cavity Ring Down Spectroscopy Instrument for Simultaneous, In situ Measurement of NO3 and N2)5, Optics Express, 26.10, A433-A449, DOI 10.1364/OE.26.00A433.

9. O'Neill, N.T., Eck, T.F., Smirnov, A., Holben, B.N., and Thulasiraman,S., �Spectral Discrimination of Coarse and Fine Mode Optical Depth, J. Geophysical Research, 108:D17, 4559, 2003.doi:10.1029/2002JD002975,

10. Petersson, A., Lovejoy, E. R., Brock, C. A., Brown, S. S., Ravishankara, A. R., �Measurements of Aerosol Optical Extinction at 532 nm with Pulsed Cavity Ringdown Spectroscopy, J. Aerosol Science, 35:8, 995-1011, 2004, DOI: 10.1016/j.jaerosci.2004.02.008.

11. Reid, J. S., Brooks, B., Crahan, K. K., Hegg, D. A., Eck, T. F., O'Neill, N., de Leeuw, G., Reid, E. A., and Anderson K. D., �Reconciliation of Coarse Mode Sea-salt Aerosol Particle Size Measurements and Parameterizations at a Subtropical Ocean Receptor Site,� J. Geophysical. Research, 111, D02202, 2006. DOI:10.1029/2005JD006200.

KEYWORDS: Meteorology; Aerosols; Atmospheric Spectroscopy; Electro-optical Propagation; Directed Energy; Electromagnetic Maneuver Warfare