Comparison of N₂O₅ mixing ratios during NO3Comp 2007 in SAPHIR

Abstract. N₂O₅ detection in the atmosphere has been accomplished using techniques which have been developed during the last decade. Most techniques use a heated inlet to thermally decompose N₂O₅ to NO₃, which can be detected by either cavity based absorption at 662 nm or by laser-induced fluorescence. In summer 2007, a large set of instruments, which were capable of measuring NO₃ mixing ratios, were simultaneously deployed in the atmosphere simulation chamber SAPHIR in Jülich, Germany. Some of these instruments measured N₂O₅ mixing ratios either simultaneously or alternatively. Experiments focused on the investigation of potential interferences from, e.g., water vapour or aerosol and on the investigation of the oxidation of biogenic volatile organic compounds by NO₃. The comparison of N₂O₅ mixing ratios shows an excellent agreement between measurements of instruments applying different techniques (3 cavity ring-down (CRDS) instruments, 2 laser-induced fluorescence (LIF) instruments). Datasets are highly correlated as indicated by the square of the linear correlation coefficients, R², which values were larger than 0.96 for the entire datasets. N₂O₅ mixing ratios well agree within the combined accuracy of measurements. Slopes of the linear regression range between 0.87 and 1.26 and intercepts are negligible. The most critical aspect of N₂O₅ measurements by cavity ring-down instruments is the determination of the inlet and filter transmission efficiency. Measurements here show that the N₂O₅ inlet transmission efficiency can decrease in the presence of high aerosol loads, and that frequent filter/inlet changing is necessary to quantitatively sample N₂O₅ in some environments. The analysis of data also demonstrates that a general correction for degrading filter transmission is not applicable for all conditions encountered during this campaign. Besides the effect of a gradual degradation of the inlet transmission efficiency aerosol exposure, no other interference for N₂O₅ measurements is found.