Intercomparison of NO3 radical detection instruments in the atmosphere simulation chamber SAPHIR

[摘要] The detection of atmospheric NO₃radicals is stillchallenging owing to its low mixing ratios (≈ 1 to300 pptv) in the troposphere. While long-path differentialoptical absorption spectroscopy (DOAS) has been a well-establishedNO₃detection approach for over 25 yr, newly sensitivetechniques have been developed in the past decade. This publicationoutlines the results of the first comprehensive intercomparison ofseven instruments developed for the spectroscopic detection oftropospheric NO₃.Four instruments were based on cavityring-down spectroscopy (CRDS), two utilised open-path cavity-enhancedabsorption spectroscopy (CEAS), and one applied "classical"long-path DOAS.The intercomparison campaign "NO3Comp" was held atthe atmosphere simulation chamber SAPHIR in Jülich (Germany) in June2007. Twelve experiments were performed in the well-mixed chamber forvariable concentrations of NO₃, N₂O₅, NO₂,hydrocarbons, and water vapour, in the absence and in the presence ofinorganic or organic aerosol.The overall precision of the cavityinstruments varied between 0.5 and 5 pptv for integrationtimes of 1 s to 5 min; that of the DOAS instrument was9 pptv for an acquisition time of 1 min.TheNO₃data of all instruments correlated excellently with theNOAA-CRDS instrument, which was selected as the common referencebecause of its superb sensitivity, high time resolution, and mostcomprehensive data coverage.The median of the coefficient ofdetermination (r²) over all experiments of the campaign(60 correlations) is r² = 0.981(quartile 1 (Q1): 0.949; quartile 3 (Q3): 0.994; min/max: 0.540/0.999).Thelinear regression analysis of the campaign data set yielded very smallintercepts (median: 1.1 pptv; Q1/Q3: −1.1/2.6 pptv;min/max: −14.1/28.0 pptv), and the slopes of theregression lines were close to unity (median: 1.01; Q1/Q3: 0.92/1.10;min/max: 0.72/1.36).The deviation of individual regression slopes fromunity was always within the combined accuracies of each instrumentpair.The very good correspondence between the NO₃measurements by all instruments for aerosol-free experiments indicatesthat the losses of NO₃in the inlet of the instruments weredetermined reliably by the participants for the correspondingconditions.In the presence of inorganic or organic aerosol, however,differences in the measured NO₃mixing ratios were detectableamong the instruments.In individual experiments the discrepanciesincreased with time, pointing to additional NO₃ radicallosses by aerosol deposited onto the filters or on the inlet walls of theinstruments. Instruments using DOAS analyses showed no significanteffect of aerosol on the detection of NO₃. No hint of a crossinterference of NO₂ was found. The effect of non-Lambert–Beerbehaviour of water vapour absorption lines on the accuracy of theNO₃ detection by broadband techniques was small and wellcontrolled.The NO3Comp campaign demonstrated the high quality,reliability and robustness of performance of current state-of-the-artinstrumentation for NO₃ detection.

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