An open-path ammonia analyzer for eddy covariance flux measurement

• An open-path QCL-based NH 3 analyzer was tested for EC flux measurements. • The analyzer showed good performance in terms of accuracy, sensitivity and stability. • Flux detection limit was estimated at 7.1 ± 1.1 μg N m −2 h −1 under the given test conditions. • The analyzer is suitable for quantifying the biosphere-atmosphere exchanges of NH 3 . Although biosphere-atmosphere exchanges of ammonia (NH 3 ) are of great environmental importance regarding, e.g., eutrophication and aerosol-related human health problems, their magnitudes are not well constrained due to the challenges in in-situ measurements of NH 3 fluxes. In this paper we introduce a portable, low-power and quantum cascade laser-based open-path analyzer for eddy covariance (EC) measurement of NH 3 fluxes particularly at sites without grid power. This analyzer minimizes the sampling artifacts due to surface adsorption of NH 3 by using an open-path optical Herriott cell configuration, while also increases its portability and convenience in field deployment by assembling most instrumental components in a highly integrated and compact enclosure. Its performance was tested through laboratory experiments and one-week EC measurements at a subtropical rice paddy. Its uncertainty in measuring NH 3 concentration is ± 15%, being comparable to those of the currently available high-sensitivity NH 3 analyzers. The flux random error averaged at 26 ± 28% during the entire experimental period. The instrumental noise of the analyzer with a sampling frequency of 10 Hz is estimated to be 0.286 and 0.302 nmol mol ‒1 (ppbv) under the laboratory and field conditions, respectively. Under the given test conditions, the detection limit of the EC system is 7.1 ± 1.1 μg N m −2 h −1 for half-hourly NH 3 flux measurement. Nearly 91% of the half-hourly fluxes observed during the experimental period were larger than the mean detection limit. Such instrumental sensitivity is adequate for quantifying NH 3 emissions from most source ecosystems, and also possible to monitor deposition fluxes at ecosystems away from strong sources with low NH 3 concentrations. Availability of this open-path NH 3 analyzer provides an access to a better understanding of the biosphere-atmosphere NH 3 exchanges and to a more reliable estimate of the NH 3 inventory at ecosystem or larger scales.