Aerosol Spectral Absorption Measurement for Near UV through Near Infrared Wavelengths

RT&L FOCUS AREA(S): Directed energy; General Warfighting Requirements TECHNOLOGY AREA(S): Battlespace Environments; Sensors OBJECTIVE: Develop an instrument or measurement technology that can measure light absorption by ambient aerosol particles (~0.05 to 20+ mm) including pollution, smoke, irregular dust, complex obscurants, etc. at user-defined wavelengths over the wavelength range of 340 nm to 2.2 um and can show good fidelity for both fine and coarse mode particles, preferably based on in situ rather than filter-based methods. What is required is the bulk absorption coefficient, and developers are free to pursue integrating single particle measurements or bulk volumetric methods. DESCRIPTION: Light scatter and absorption by aerosol particles can impact numerous Navy systems, from retrieval errors or biases in satellite-based radiance measurements used for sea surface temperature and aerosol data assimilation to atmospheric attenuation and beam quality degradation related to absorption and thermal blooming for directed energy systems. Spectral absorption is also often used to estimate the chemical composition of aerosol types (e.g., dust, black carbon, brown carbon). However, each of these applications examines a relatively small wavelength range over the total spectrum that the Navy utilizes. This STTR topic is for the development of instrumentation or technologies that can measure or derive aerosol absorption over wider wavelength ranges from 340 nm to 2.2 um at multiple wavelengths, with a minimum of three and preferably more wavelengths as requested by the buyer at the time of construction. If the entire range cannot be met in a single instrument design, preference will be given to the 500 nm to 2.2 um range, followed by the 340 nm to 670 nm range. All proposed methodologies will be considered but in situ technologies suitable for rapid response field site applications or aircraft use are preferred. Technologies can be direct measurements of absorption or the difference between extinction and scattering. PHASE I: Develop a concept for an instrument or measurement technology that can measure light absorption by ambient aerosol particles at user-defined wavelengths over the wavelength range of 340 nm to 2.2 um and can show good fidelity for both fine and coarse mode particles, preferably based on in situ rather than filter-based methods, but proposals of all manner of technology solutions will be considered. Target uncertainties are +/- 20% in absorption coefficient or 0.5 Mm-1, whichever is greater. By the end of Phase I awardee is expected to demonstrate efficacy of the proposed technology in controlled laboratory conditions. Develop a Phase II plan. PHASE II: Demonstrate the working prototype instrument at Navy field sites or in conjunction with earth science field campaigns of opportunity including comparison with a commercially available mid-visible band instrument. Several stages of refinement are expected based on field test findings. We expect the instrument to be man portable (e.g., individual components <50 lbs), with reasonable site installation requirements (e.g., <600 W). Housing design for open celled instruments must be able to withstand reasonable marine weather conditions (rain, sea salt corrosion, winds to 60 knots). Closed cell or filter based methods must include a plan for sampling to ensure ambient conditions are being represented. PHASE III DUAL USE APPLICATIONS: The need for this technology is in association with Navy test range and operational system support for optics applications sensitive to aerosol absorption. However, we expect the instrument to be useful for Earth system science research (such as remote sensing and climate research), air quality composition monitoring, and combustion systems engineering. Instrument refinement for all of these applications would be expected in Phase III. REFERENCES: Backman et al., On Aethalometer measurement uncertainties and an instrument correction factor for the Arctic, Atmos. Meas. Tech., 10,2017, pp 5039-5062, https://doi.org/10.5194/amt-10-5039-2017 Hoffer et al. Brown carbon absorption in the red and near-infrared spectral region. Atmos. Meas. Tech., 10, 29 November 2016 (revised 15 May 2017), pp. 2353-2359, https://amt.copernicus.org/articles/10/2353/2017/amt-10-2353-2017.pdf Pandey, A. et al. Aerosol light absorption from optical measurements of PTFE membrane filter samples: sensitivity analysis of optical depth measures. Atmos. Meas. Tech., 12, 10 July 2018 (revised 10 January 2019), pp. 1365-1373. https://amt.copernicus.org/articles/12/1365/2019/amt-12-1365-2019.pdf Ogren, J. A. Comment on Calibration and Intercomparison of Filter-Based Measurements of Visible Light Absorption by Aerosols. Aerosol Sci. Tech., 44, 2010, pp. 589-591. https://www.tandfonline.com/doi/pdf/10.1080/02786826.2010.482111 Schnaiter et al. Measurement of wavelength-resolved light absorption by aerosols using a UV-vis extinction cell. Aerosol Sci. and Tech., 39, 2010, pp. 249-260. https://www.tandfonline.com/doi/pdf/10.1080/027868290925958 Smith et al. Measuring black carbon spectral extinction in the visible and infrared. J. Geophys. Res. Atmos., 120, 2015, pp. 9670-9683. https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015JD023564