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The atmospheric reactivity of the NO3 radical

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Liebmann,  Jonathan Michael
Atmospheric Chemistry, Max Planck Institute for Chemistry, Max Planck Society;

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Citation

Liebmann, J. M. (2018). The atmospheric reactivity of the NO3 radical. PhD Thesis, Universität, Mainz.


Cite as: https://hdl.handle.net/21.11116/0000-0003-1269-3
Abstract
The nitrate radical (NO3), formed from oxidation of NO2 by O3, controls the oxidizing capacity of the Earth's atmosphere during nighttime. Reaction of NO3 with (biogenic) volatile organic compounds (VOCs) and subsequent deposition into the particle phase represents an important loss process for nitrogen oxides (NO + NO2 = NOx) cleansing the Earth's atmosphere. The rate of loss of NOx due to reaction of the NO3 radical with VOCs can be accessed via the NO3-reactivity thus the inverse lifetime. This work focuses on i) the development of the first instrument for measuring NO3-reactivity ii) the deployment of the instrument in field campaigns and interpretation of the data with special regard to boundary layer dynamics and VOCs iii) the assessment of the fractional contribution of the NO3-reactivity as NOx loss process during day and night. The instrument developed uses cavity ring down spectroscopy (CRDS) for the detection of synthetically generated NO3 radicals after transmissions through a flow-tube reactor. The change in the NO3 mixing ratio upon modulation of the bath gas between zero air and ambient air is used to derive the NO3-reactivity. The instrument can measure NO3 reactivities from 0.005 s−1 up to 45 s−1 using an automated dilution procedure. The uncertainty of the measurements is strongly dependent on the ambient NO and NO2 mixing ratios and is 16% at the center of its dynamic range (0.01-0.4 s−1). During field campaigns in the boreal forest and at mountain tops, the variability in the reactivity was found to be driven mainly by the dynamics of the boundary layer. In the nocturnal boundary layer, very high reactivities were measured whereas in the residual layer very low reactivities partly below the detection limit of the instrument were measured. A strong vertical gradient in NO3-reactivity was found in the nocturnal boundary layer which disappeared due to efficient mixing during daytime. NO3 mixing ratios, calculated using the NO3-reactivity, were in good agreement with the entire measured NO3 mixing ratios. From organic trace gas measurements it was found that the reactivity was mainly determined by reaction with monoterpenes though their concentrations were insufficient to account for the measured NO3-reactivity. We suggest that sesquiterpenes, which were not covered by the trace gas analyzes, play an important role here. In forested regions, the high values of NO3-reactivity imply that even during the day at least 20% of the formed NO3 radicals react with organic trace substances and thus represent a NOx loss that has been neglected in the literature so far. A comparison of the NO3-reactivity with the OH-reactivity found only a minor correlation as already suggested by the different rate constants.