Simulation of the diurnal variations of the oxygen isotope anomaly (Delta O-17) of reactive atmospheric species
Morin, S. ; Sander, R. ; Savarino, J.
The isotope anomaly (?17O) of secondary atmospheric species such as nitrate (NO3?) or hydrogen peroxide (H2O2) has potential to provide useful constrains on their formation pathways. Indeed, the ?17O of their precursors (NOx, HOx etc.) differs and depends on their interactions with ozone, which is the main source of non-zero ?17O in the atmosphere. Interpreting variations of ?17O in secondary species requires an in-depth understanding of the ?17O of their precursors taking into account non-linear chemical regimes operating under various environmental settings.
This article reviews and illustrates a series of basic concepts relevant to the propagation of the ?17O of ozone to other reactive or secondary atmospheric species within a photochemical box model. We present results from numerical simulations carried out using the atmospheric chemistry box model CAABA/MECCA to explicitly compute the diurnal variations of the isotope anomaly of short-lived species such as NOx and HOx. Using a simplified but realistic tropospheric gas-phase chemistry mechanism, ?17O was propagated from ozone to other species (NO, NO2, OH, HO2, RO2, NO3, N2O5, HONO, HNO3, HNO4, H2O2) according to the mass-balance equations, through the implementation of various sets of hypotheses pertaining to the transfer of ?17O during chemical reactions. <br>The model results confirm that diurnal variations in ?17O of NOx predicted by the photochemical steady-state relationship during the day match those from the explicit treatment, but not at night. Indeed, the ?17O of NOx is "frozen" at night as soon as the photolytical lifetime of NOx drops below ca. 10 min. We introduce and quantify the diurnally-integrated isotopic signature (DIIS) of sources of atmospheric nitrate and H2O2, which is of particular relevance to larger-scale simulations of ?17O where high computational costs cannot be afforded.
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