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NO3-COMP

Overview and objectives of this Quality Assurance activity:

In the atmosphere NO3 radicals are primarily formed by the oxidation of NO2 with O3. NO3 is quickly photolyzed during the day, therefore significant concentrations are normally only detected at night. N2O5 is the product of the reversible reaction of NO3 with NO2. It is relatively unstable at elevated temperatures. Therefore NO2, NO3, and N2O5 establish a strongly temperature dependent equilibrium. NO3 radicals readily react with a variety of hydrocarbons and aldehydes controlling their removal from the atmosphere during the night. N2O5 was found to hydrolyze quickly on wet surfaces to form HNO3 in the condensed phase constituting the most important atmospheric loss reaction of oxidized nitrogen compounds. Like OH radicals during the day, NO3 radicals are the most important oxidizing agent during the night and thus are responsible for the self-cleaning of the nocturnal atmosphere.

So far the only technique feasible for field detection of NO3 radicals was Differential Optical Absorption Spectroscopy (DOAS). Some few measurements using matrix isolation combined with electron spin resonance were published but this technique was only rarely used due to its low time resolution.
In the last years, several new very sensitive and fast in situ techniques have been developed for NO3 detection. An advantage of these novel instruments is their common capability to detect N2O5 indirectly via thermal decomposition to NO3 in a heated inlet.
The new techniques utilize Cavity Ring Down Spectroscopy (CRDS), Cavity Enhanced Absorption Spectroscopy (CEAS), Laser Induced Fluorescence (LIF), and Chemical Ionization Mass Spectrometry (CIMS) to detect NO3. Except DOAS all instruments need in-field calibration since the NO3 radicals have to be transferred into the detection instruments involving losses in the inlet. The calibration, however, has proven to be difficult and constitutes a potential source of error. In contrast DOAS is a lossless in-situ technique which is inherently calibrated by the use of well documented absorption cross section of NO3.

Instrument intercomparison campaign

For summer of the year 2007 a comprehensive instrument intercomparison for NO3 radicals and N2O5 detection methods is being planning at the Atmosphere Simulation Chamber SAPHIR. The SAPHIR chamber is large enough to host a number of instruments simultaneously. The chamber experiments will be conducted under well controlled, atmosphere-like chemical conditions in SAPHIR. Critical instrumental parameters like the dependence of NO3 signal on water vapor and on NO2 will be investigated in a series of chamber studies.


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