First ground-deployment of a new small-footprintcavity-ring-down spectrometer for NO3 and N2O5 in a temperate forest during the ACROSS campaign

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 NO_x at night. Loss of NO₃ to gas-phase reactions (forming e.g. organic nitrates) has a different impact on NO_x 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.