An open-path ammonia analyzer for eddy covariance flux measurement

Abstract

Although biosphere-atmosphere exchanges of ammonia (NH3) 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 NH3 fluxes. In this paper we introduce a portable, low-power and quantum cascade laser-based open-path analyzer for eddy covariance (EC) measurement of NH3 fluxes particularly at sites without grid power. This analyzer minimizes the sampling artifacts due to surface adsorption of NH3 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 NH3 concentration is ± 15%, being comparable to those of the currently available high-sensitivity NH3 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 NH3 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 NH3 emissions from most source ecosystems, and also possible to monitor deposition fluxes at ecosystems away from strong sources with low NH3 concentrations. Availability of this open-path NH3 analyzer provides an access to a better understanding of the biosphere-atmosphere NH3 exchanges and to a more reliable estimate of the NH3 inventory at ecosystem or larger scales.