TRANC – a novel fast-response converter to measure total reactive atmospheric nitrogen

Abstract

Abstract. The input and loss of plant available nitrogen (reactive nitrogen: N r ) from/to the atmosphere can be an important factor for the productivity of ecosystems and thus for its carbon and greenhouse gas exchange. We present a novel converter for reactive nitrogen (TRANC: Total Reactive Atmospheric Nitrogen Converter), which offers the opportunity to quantify the sum of all airborne reactive nitrogen compounds ( N r ) in high time resolution. The basic concept of the TRANC is the full conversion of all N r to nitrogen monoxide (NO) within two reaction steps. Initially, reduced N r compounds are being oxidised, and oxidised N r compounds are thermally converted to lower oxidation states. Particulate N r is being sublimated and oxidised or reduced afterwards. In a second step, remaining higher nitrogen oxides or those generated in the first step are catalytically converted to NO with carbon monoxide used as reduction gas. The converter is combined with a fast response chemiluminescence detector (CLD) for NO analysis and its performance was tested for the most relevant gaseous and particulate N r species under both laboratory and field conditions. Recovery rates during laboratory tests for NH 3 and NO 2 were found to be 95 and 99 %, respectively, and 97 % when the two gases were combined. In-field longterm stability over an 11-month period was approved by a value of 91 % for NO 2 . Effective conversion was also found for ammonium and nitrate containing particles. The recovery rate of total ambient N r was tested against the sum of individual measurements of NH 3 , HNO 3 , HONO, NH + 4 , NO − 3 , and NO x using a combination of different well-established devices. The results show that the TRANC-CLD system precisely captures fluctuations in N r concentrations and also matches the sum of all individual N r compounds measured by the different single techniques. The TRANC features a specific design with very short distance between the sample air inlet and the place where the thermal and catalytic conversions to NO occur. This assures a short residence time of the sample air inside the instrument, and minimises wall sorption problems of water soluble compounds. The fast response time (e-folding times of 0.30 to 0.35 s were found during concentration step changes) and high accuracy in capturing the dominant N r species enables the converter to be used in an eddy covariance setup. Although a source attribution of specific N r compounds is not possible, the TRANC is a new reliable tool for permanent measurements of the net N r flux between ecosystem and atmosphere at a relatively low maintenance and reasonable cost level allowing for diurnal, seasonal and annual investigations.

Measuring the biosphere-atmosphere exchange of total reactive nitrogen by eddy covariance

“…The calibration gas was fed into the system (by switching a solenoid valve) at the TRANC inlet on the field. The sensitivity of the NO analyser showed a minor drift over time of < 20 % in three months and the conversion efficiency for NO 2 was always close to 100 % (see also Marx et al, 2012).…”

“…The TRANC was operated in two slightly different versions in 2006 and 2007. In the preliminary version of 2006 the first c. 15 cm of the steel inlet tube were not actively heated and stayed close to ambient temperature, whereas in the final version (2007) the 1 / 4 inlet tube was actively heated from the very first millimetre, leading to inner wall temperatures always > 100 • C (see Marx et al, 2012). The operating temperatures of the alloy (870 • C) and gold tubes (300 • C) in the converter were not affected by this difference.…”

“…Among these are a dynamic chamber system for NO and NO 2 (Sect. (Thomas et al, 2009;Wolff et al, 2010) was operated at the site (see also Marx et al, 2012). The general availability of concentration and flux measurements during the two study periods is summarised in Table 1.…”

“…1). A detailed description of the converter and tests for its conversion efficiency are given by the companion paper by Marx et al (2012). The TRANC was operated in two slightly different versions in 2006 and 2007.…”

“…The total reactive nitrogen converter (TRANC) custom-built according to Marx et al (2012) was used for fast response detection of N r . The basic principle of the converter is to thermally convert all reduced and oxygenated N r compounds to nitric oxide (NO), which is then analysed by a commercially available NO detector.…”