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

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 NH₃ and NO₂ 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 NO₂. 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 NH₃, HNO₃, HONO, NH₄⁺, NO₃⁻, 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.