An open-path QCL-based instrument with sub-ppbv sensitivity for NH3 eddy covariance measurement
Authors: Wang Yin , Wang Kai , Kang Peng , Lu Yin , Zhen Xiaojie , Liu Gang , Zheng Xunhua 
Published: March 2020

&lt;p&gt;Ammonia (NH&lt;sub&gt;3&lt;/sub&gt;) emissions from farmlands and livestock are attracting more and more attention. There is an urgent need for ground-based instruments that can acquire the spatial and temporal variability in NH&lt;sub&gt;3&lt;/sub&gt; concentrations and emissions, particularly in field environments where power and shelter are not readily available. However, accurate measurements of atmospheric NH&lt;sub&gt;3&lt;/sub&gt; is of great challenges due to its reactive nature. Conventional NH&lt;sub&gt;3&lt;/sub&gt; instruments are subject to drawbacks, such as slow response time, limited precision, intensive maintenance, or high power consumption due to the use of the closed-path tube, optics, and vacuum pump.&lt;/p&gt;&lt;p&gt;We have developed an open-path instrument for fast (10 Hz) and sub-ppbv sensitivity measurements of atmospheric NH&lt;sub&gt;3&lt;/sub&gt; concentration. The instrument is based on second-harmonic (2f) wavelength modulated laser absorption spectroscopy technique (WM-LAS), which employs a distributed-feedback semiconductor quantum cascade laser (DFB-QCL) and a HgCdTe (MCT) photodetector. An open-path Herriott cell configuration with a 0.5 m physical path and 46 m optical path-length is used for selective and sensitive detection of the mid-infrared absorption transition of NH&lt;sub&gt;3&lt;/sub&gt; at 9.06 &amp;#956;m [1]. There is no delay due to sample adsorption. The instrument has a precision (1&amp;#963; noise level) of 0.53 ppbv and 0.15 ppbv at a sampling frequency of 10 Hz and 1 Hz, respectively. The entire NH&lt;sub&gt;3&lt;/sub&gt; instrument has a weight of ~7 kg and dimensions of 84 cm (length) and 20 cm (diameter). It can be powered by rechargeable lithium batteries, with a total power consumption of as low as 50 W. The instrument has strong environmental adaptability and is suitable for field deployment in various environments. It can be used in ground-based or vehicle-based measurements of atmospheric NH&lt;sub&gt;3&lt;/sub&gt; concentration.&lt;/p&gt;&lt;p&gt;With the good performance in terms of response time and precision, this instrument is an ideal tool for NH&lt;sub&gt;3&lt;/sub&gt; flux measurements based on the eddy covariance (EC) technique [2]. An EC flux system was built based on the open-path ammonia instrument, which also included a CSAT3 sonic anemometer (Campbell Scientific&amp;#174;) and LI-7500 (LICOR&amp;#174;) for water vapor (H&lt;sub&gt;2&lt;/sub&gt;O) and carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;) measurements. The system was installed at a rice paddy field with a typical Chinese-style rice-duck symbiosis system in Jiangsu province, China. Experiments showed that the lower detection limit of the EC system for NH&lt;sub&gt;3&lt;/sub&gt; flux was around 17ng m&lt;sup&gt;-2&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt;.&lt;/p&gt;&lt;p&gt;References:&lt;/p&gt;&lt;p&gt;[1] Miller, D. J., Sun, K., Tao, L., and Zondlo, M. A.: Open-path, quantum cascade-laser-based sensor for high-resolution atmospheric ammonia measurements, Atmos. Meas. Tech., 7, 81&amp;#8211;93,2014.&lt;br&gt;[2] McDermitt, D., Burba, G., Xu, L., Anderson, T., Komissarov, A., Riensche, B., Schedlbauer, J., Starr, G., Zona, D., Oechel, W., Oberbauer, S., and Hastings, S.: A new low-power, open-path instrument for measuring methane flux by eddy covariance, Appl.Phys. B, 102, 391&amp;#8211;405, 2011.&lt;/p&gt;