Field studies of reactive nitrogen chemistry (nur Englisch)

The photochemically driven, OH-initiated oxidation processes during the day are supplemented or, for some classes of volatile organic compounds (VOCs) such as terpenes or CH₃SCH₃, even surpassed by night-time reactions with the NO₃ radical. NO₃ can initiate and propagate nocturnal radical chemistry, linking HO₂ and NOx chemistry and significantly impact on the oxidation rates of several classes of atmospheric traces gases. In air masses with sufficient NO₂, N₂O₅ mixing ratios often exceed those of NO₃ and the heterogeneous hydrolysis of N₂O₅ on tropospheric aerosol can substantially modify the amount of reactive nitrogen available for daytime photochemical O₃ production. We measure pptv levels NO₃ and N₂O₅ in the field using cavity-ring-down spectroscopy (see Methods, Field 1).

For a recent publication see Crowley et al., Atmos. Chem. Phys. Discuss, 10, 2795-2812, 2010.

Through its photolysis, NO₂ is the source of O₃ formation in the troposphere. Its oxidation by reaction with the OH radical or reaction with organic peroxy radicals converts NO₂ into the longer-lived reservoir species HNO₃ and PANs. A short lifetime with respect to deposition means that HNO₃ formation represents a sink of NOx in the lower troposphere, whereas PAN is sufficiently long lived that it may be transported from polluted regions to cleaner ones, where, via thermal decomposition, it re-releases NO₂. The PANs and O₃ have similar formation routes and speciated PAN measurements can be used as traces of photochemical O₃ generation in different environments. We measure HNO₃ and speciated PANs using chemical ionisation mass spectrometry (Methods, Field 2) whereas NO₂ is measured by cavity-ring-down spectroscopy (see Methods, Field 3). An extension to the NO₂ measurement using thermal dissociation also enables measurements of PANs (but not speciated) and alkyl-nitrates.