Abstract
Abstract. The isotope anomaly (Δ17O) of secondary atmospheric species such as nitrate (NO3−) or hydrogen peroxide (H2O2) has potential to provide useful constrains on their formation pathways. Indeed, the Δ17O of their precursors (NOx, HOx etc.) differs and depends on their interactions with ozone, which is the main source of non-zero Δ17O in the atmosphere. Interpreting variations of Δ17O in secondary species requires an in-depth understanding of the Δ17O 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 Δ17O 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, Δ17O 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 Δ17O during chemical reactions. The model results confirm that diurnal variations in Δ17O of NOx predicted by the photochemical steady-state relationship during the day match those from the explicit treatment, but not at night. Indeed, the Δ17O 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 Δ17O where high computational costs cannot be afforded.
Highlights
Unraveling chemical mechanisms at play in the atmosphere requires finding creative ways to test the predictions of models which describe them
We introduce and quantify the diurnally-integrated isotopic signature (DIIS) of sources of atmospheric nitrate and H+O2 reaction leads to increasing the 17OHO2+HO2 (H2O2), which is of particular relevance to larger-scale simulations of 17O where high computational costs cannot be afforded
Of particular interest is the development of measurements of the isotope anomaly ( 17O) of oxygen-bearing species (Thiemens, 2006). 17O is defined as δ17O−0.52×δ18O, with δx O=Rx /RVx SMOW−1 (x =17 or 18) where Rx refers to the xO/16O elemental ratio in the species of interest and in Vienna Standard Mean Ocean Water (VSMOW), taken as a reference
Summary
Unraveling chemical mechanisms at play in the atmosphere requires finding creative ways to test the predictions of models which describe them. Most studies to date have relied on concentration measurements to validate model results. Of particular interest is the development of measurements of the isotope anomaly ( 17O) of oxygen-bearing species (Thiemens, 2006). Ozone (O3) possesses a distinctive isotope anomaly inherited from nonmass dependent fractionation NMDF during its formation in the atmosphere (Marcus, 2008). In contrast to conventional isotopic ratios which are strongly affected by isotopic fractionation, 17O is fairly insensitive to mass-dependent fractionation. The vast majority of chemical reactions induce mass-dependent fractionation, which in general do not strongly modify 17O. It can be reasonably assumed that 17O is transferred as is during
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