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

[摘要] The input and loss of plant available nitrogen (reactive nitrogen: N_r)from/to the atmosphere can be an important factor for the productivity ofecosystems and thus for its carbon and greenhouse gas exchange. We present anovel converter for reactive nitrogen (TRANC: TotalReactive Atmospheric Nitrogen Converter), which offers the opportunity to quantify the sum of allairborne reactive nitrogen compounds (∑N_r) in high timeresolution. The basic concept of the TRANC is the full conversion of allN_r to nitrogen monoxide (NO) within two reaction steps. Initially,reduced N_r compounds are being oxidised, and oxidised N_r compoundsare thermally converted to lower oxidation states. Particulate N_r isbeing sublimated and oxidised or reduced afterwards. In a second step,remaining higher nitrogen oxides or those generated in the first step arecatalytically 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 relevantgaseous and particulate N_r species under both laboratory and fieldconditions. Recovery rates during laboratory tests for NH₃ and NO₂were found to be 95 and 99%, respectively, and 97% when the twogases were combined. In-field longterm stability over an 11-month period wasapproved by a value of 91% for NO₂. Effective conversion was alsofound for ammonium and nitrate containing particles. The recovery rate oftotal ambient N_r was tested against the sum of individual measurementsof NH₃, HNO₃, HONO, NH₄⁺, NO₃⁻, and NO_xusing a combination of different well-established devices. The results showthat the TRANC-CLD system precisely captures fluctuations in ∑N_rconcentrations and also matches the sum of all individual N_r compoundsmeasured by the different single techniques. The TRANC features a specificdesign with very short distance between the sample air inlet and the placewhere the thermal and catalytic conversions to NO occur. This assures ashort residence time of the sample air inside the instrument, and minimiseswall sorption problems of water soluble compounds. The fast response time(e-folding times of 0.30 to 0.35 s were found during concentration stepchanges) and high accuracy in capturing the dominant N_r species enablesthe converter to be used in an eddy covariance setup. Although a sourceattribution of specific N_r compounds is not possible, the TRANC is anew reliable tool for permanent measurements of the net ∑N_r fluxbetween ecosystem and atmosphere at a relatively low maintenance andreasonable cost level allowing for diurnal, seasonal and annualinvestigations.

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