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

The isotope anomaly (Δ ¹⁷O) of secondary atmospheric species such as nitrate (NO ₃⁻) or hydrogen peroxide (H ₂O ₂) has potential to provide useful constrains on their formation pathways. Indeed, the Δ ¹⁷O of their precursors (NO _x, HO _x etc.) differs and depends on their interactions with ozone, which is the main source of non-zero Δ ¹⁷O in the atmosphere. Interpreting variations of Δ ¹⁷O in secondary species requires an in-depth understanding of the Δ 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 Δ 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 NO _x and HO _x. Using a simplified but realistic tropospheric gas-phase chemistry mechanism, Δ ¹⁷O was propagated from ozone to other species (NO, NO ₂, OH, HO ₂, RO ₂, NO ₃, N ₂O ₅, HONO, HNO ₃, HNO ₄, H ₂O ₂) according to the mass-balance equations, through the implementation of various sets of hypotheses pertaining to the transfer of Δ ¹⁷O during chemical reactions.

The model results confirm that diurnal variations in Δ ¹⁷O of NO _x predicted by the photochemical steady-state relationship during the day match those from the explicit treatment, but not at night. Indeed, the Δ ¹⁷O of NO _x is "frozen" at night as soon as the photolytical lifetime of NO _x drops below ca. 10 min. We introduce and quantify the diurnally-integrated isotopic signature (DIIS) of sources of atmospheric nitrate and H ₂O ₂, which is of particular relevance to larger-scale simulations of Δ ¹⁷O where high computational costs cannot be afforded.