Simulation of the diurnal variations of the oxygen isotope anomaly (Delta O-17) of reactive atmospheric species

The isotope anomaly (Delta O-17) of secondary atmospheric species such as nitrate (NO3-) or hydrogen peroxide (H2O2) has potential to provide useful constrains on their formation pathways. Indeed, the Delta O-17 of their precursors (NOx, HOx etc.) differs and depends on their interactions with ozone, which is the main source of non-zero Delta O-17 in the atmosphere. Interpreting variations of Delta O-17 in secondary species requires an in-depth understanding of the Delta O-17 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 Delta O-17 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, Delta O-17 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 Delta O-17 during chemical reactions. The model results confirm that diurnal variations in Delta O-17 of NOx predicted by the photochemical steady-state relationship during the day match those from the explicit treatment, but not at night. Indeed, the Delta O-17 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 Delta O-17 where high computational costs cannot be afforded.