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Abstract:
At nighttime, when concentrations of the OH-radical are low, the nitrate radical, NO3, over takes the role of major initiator of the oxidation of many organic trace gases, especially those containing one or more double bonds. In contrast to daytime, where the lifetime of NO3 is very short due to its photolysis and reaction with NO, NO3 can reach mixing ratios of several tens of ppt at night. NO3 can also react with NO2 to form N2O5. As N2O5 is thermally stable, the three trace-gases usually exist in equilibrium:
NO3 + NO2 + M ⇌ N2O5 + M
Measurements of NO3 and N2O5 are central to our understanding of the fate of NOx at night. Loss of NO3 to gas-phase reactions (forming e.g. organic nitrates) has a different impact on NOx than formation of N2O5 which may hydrolyse on aerosol to form particulate nitrate.
During the ACROSS campaign in Rambouillet Forest (France), a recently built two-channel cavity-ring-down spectrometer was deployed for the first time to record mixing ratios of NO3 and N2O5 at night over a period of several weeks. NO3 was detected directly at 662nm in one channel while N2O5 was first converted to NO3 in a thermal dissociation inlet before being detected in the same way.
In this work, we describe the new instrument in detail and compare obtained data with those measured by an established cavity-ring-down instrument. We show that, at a sampling height of about 6m, NO3 and N2O5 mixing ratios were low and frequently below the detection limit of both instruments; the likely reasons for this are discussed.
