Non Technical Summary
AThe global reactive nitrogen cycle includes both anthropogenic and biogenic sources and is largely responsible for the formation of nitrate aerosols, including ammonium sulfate, ammonium bisulfate, and ammonium nitrate. These aerosols are the main constituents of PM2.5 pollution, with approximately 50% of the PM2.5 in the Eastern United States composed of nitrate aerosols. PM2.5 pollution has been implicated in a wide array of adverse health effects, including premature mortality, cancer, and other lung-related diseases. Moreover, PM2.5-related illnesses have significant economic impact, with one study suggesting that particulate matter control would result in a $4B annual savings in Shanghai alone. These aerosols result from the reaction of atmospheric ammonia (NH3) with nitrogen oxides (NOx). The former is primarily due to agricultural sources which have resulted in global NH3 emissions increasing from 23 Tg/year to 60 Tg/year over the past 70 yearsfrom nitrogen-based fertilizers (33%) and livestock production (66%).Recent research has shown that "... commonly used averaging strategies for the predictions of long-term average fluxes from long-term average measurements of NH3 concentrations (e.g. from denuder or passive sampler records) ... are biased." This study suggested that slow, monthly ammonia averages miss high-frequency flux data, leading to substantially incorrect NH3 dry deposition fluxeswith relative errors approaching 100 %. Similarly, relaxed eddy accumulation techniqueshave underreported ammonia fluxes by 20 - 70 %. Thus, there is a pressing need for new technologies that can make rapid, highly-accurate measurements of ammonia at agriculturally-relevant sites.In this Small Business Innovative Research program, Nikira Labs Inc. proposes to develop a novel, open-path, cavity-enhanced spectroscopy system for the accurate, rapid quantification of ammonia (NH3) flux.The resulting system can be used to measure ammonia flux from agricultural activities and gauge emission mitigation strategies.

Classification

Knowledge Area (KA)	Subject of Investigation (SOI)	Field of Science (FOS)	Percent
133	0410	2000	100%

Knowledge Area
133 - Pollution Prevention and Mitigation;

Subject Of Investigation
0410 - Air;

Field Of Science
2000 - Chemistry;

Goals / Objectives
Objective 1Fabricate an open-path, cavity-enhanced spectroscopy system for measurements of ammonia eddy flux.Objective 2Laboratory test and deploy the Phase I analyzer to determine its analytical performance.Objective 3Design a Phase II flux instrument that is suitable for long-term field deployment.

Project Methods
In this SBIR effort, Nikira Labs Inc. will fabricatean open-path, cavity-enhanced spectroscopy system that uses near-infrared, telecommunications-grade lasers to quantify ammonia flux in ambient air.In conventional cavity-enhanced absorption spectrometry (CEAS), a gas sample is introduced into a high-finesse optical cavity comprised of two highly-reflective mirrors (R > 99.98 % typical). A narrow linewidth laser (~ 3 MHz) is coupled into the cavity through the front mirror and the transmitted intensity is detected through the rear mirror. In cavity ringdown spectroscopy (CRDS), the first variant of CEAS, the laser is mode and frequency matched to the cavity and then rapidly shutoff to quantify a ringdown time that is a direct measurement of optical loss (mirror loss and gas absorption) at a specific laser wavelength. Alternatively, the intensity of laser light transmitting through the cavity can be directly detected and provide a similar measurement of optical loss via Integrated Cavity Output Spectroscopy (ICOS). This latter CEAS variant was later superseded by Off-Axis ICOSwhich was far more robust and suitable for commercial deployment. Regardless of the specific CEAS method, they all benefit from a high-finesse optical cavity that provides from a very long effective optical pathlength, Leff = L/(1-R), that, for highly reflective mirrors (R ~ 99.99%) approaches 10,000 meters! This enormous effective pathlength allows for very small absorptions to give rise to detectable changes in ringdown time or intensity. All commercial CEAS instruments to date pull samples through a fine particulate filter and into he optical cavity. As noted above, this sampling methodology is unsuitable for ammonia flux, since the stickiness and solubility of ammonia make it difficult to retain a 10 Hz sampling rate.In this SBIR, Nikira Labs Inc. will utilize a new CEAS variant that exploits an open-path cavity to measure ammonia without passing the air sample through any wetted materials. The high-reflectivity mirrors will be protected by purge flows from filtered micropumps to prevent soiling. The laser will be coupled into the cavity via a large-beam collimator (akin to Off-Axis ICOS) and scanned over an ammonia absorption feature near 1530 nm at a rate of 300 - 1000 Hz. The transmitted intensity will be detected by a high-gain, low-noise InGaAs detector. Each scan will be accompanied by a ringdown measurement to provide the optical pathlength accompanying the data. The ammonia absorption features (and other adjacent water absorptions) will be fit to pressure-broadened Voigt profiles, and Beer's Law will be used to calculate the ammonia concentration at a requested data rate ranging from 20 - 0.1 Hz for eddy flux or long-term monitoring observations respectively. The embedded computer with data acquisition board (DAQ) is used to tune the laser, collect the detector signal, and analyze the spectra. A data logger will be used to log the ammonia concentration, wind speed/direction from the sonic anemometer, temperature, pressure, and other collocated analyzers. Finally, the ammonia flux will be determined using conventional eddy flux calculations.If periodic zeroing is required, the inlet of the micropumps can be equipped with an ammonia scrubber and the cavity can be closed via a rotating sleeve26 to remove ammonia, particulates, and other absorbing gases (e.g. water vapor) from the measurement zone. This feature will also provide a confirmation of system operation and can be used to adjust the exposed pathlength and change the instrument dynamic range if necessary (e.g. in cases of high ammonia and/or particulates, the sleeve can be rotated to provide a shorter effective pathlength for the open-path).[i] O'Keefe, A. and Deacon, D.A., 1988. Cavity ring?down optical spectrometer for absorption measurements using pulsed laser sources.Review of Scientific Instruments,59(12), pp.2544-2551.[ii] O'Keefe, A., Scherer, J.J. and Paul, J.B., 1999. CW integrated cavity output spectroscopy.Chemical Physics Letters,307(5-6), pp.343-349.[iii] Baer, D.S., Paul, J.B., Gupta, M. and O'Keefe, A., 2002. Sensitive absorption measurements in the near-infrared region using off-axis integrated-cavity-output spectroscopy.Applied Physics B,75(2-3), pp.261-265.[iv] 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., 2015. Design of a novel open-path aerosol extinction cavity ringdown spectrometer.Aerosol Science and Technology,49(9), pp.717-726.[v] Aubinet, M., Vesala, T. and Papale, D. eds., 2012.Eddy covariance: a practical guide to measurement and data analysis. Springer Science & Business Media.[vi] Eugster, W. and Merbold, L., 2015. Eddy covariance for quantifying trace gas fluxes from soils.Soil,1(1), pp.187-205.