Intercomparison Campaign of NO3 and N2O5 Detection Techniques at the Atmosphere Simulation Chamber SAPHIR

The nitrate radical and its equilibrium partner N2O5 are of central importance for the chemistry and the loss of nitrogen oxides in the nighttime atmosphere. Since the first atmospheric detection of NO3 by Differential Optical Absorption Spectroscopy (DOAS) in the early eighties no other technique was available for about twenty years. Within the last decade new complementary techniques have been developed which allow sub-ppt in-situ measurements of NO3 with high time resolution on mobile platforms. A striking advantage of most of these instruments is their capability to measure concurrently the concentration of N2O5 via thermal conversion to NO3 in an additional heated inlet. Some instruments have already been deployed to field campaigns, however, there has been no systematic comparison between them. In order to strengthen the community's confidence in the ability to measure atmospheric NO3 and N2O5 accurately an intercomparison campaign was conducted in June 2007 at Forschungszentrum Jülich. Research groups from England, Ireland, Japan, the United States, and Germany gathered at the atmosphere simulation chamber SAPHIR in Jülich. SAPHIR is ideally suited for instrument validation, i.e., it allows the controlled production and destruction of NO3 and N2O5 and it provides a well mixed gas volume of 270 m3 for multiple instruments to sample from. All participating instruments applied optical spectroscopy for NO3 detection: four techniques based on Cavity Ring-Down Spectroscopy, three using Cavity Enhanced Absorption Spectroscopy, two Laser- Induced Fluorescence instruments, and one folded long-path DOAS took part. All instruments have measured simultaneously during ten chamber experiments covering different chemical scenarios. The experiments were selected to test the specific instrumental performance as a function of varying trace gas concentrations (O3, NO2, reactive hydrocarbons), humidity, and aerosols. In this contribution we will present an overview and selected results of the campaign discussing accuracy, precision, and time response of the different instruments.

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