NOx cycle and the tropospheric ozone isotope anomaly: an experimental investigation
Item
Title (Dublin Core)
NOx cycle and the tropospheric ozone isotope anomaly: an experimental investigation
Description (Dublin Core)
The oxygen isotope composition of nitrogen oxides (NO<sub>x</sub>) in the atmosphere is a useful tool for understanding the oxidation of NO<sub>x</sub> into nitric acid / nitrate in the atmosphere. A set of experiments was conducted to examine change in isotopic composition of NO<sub>x</sub> due to NO<sub>x</sub>–O<sub>2</sub>–O<sub>3</sub> photochemical cycling. At low NO<sub>x</sub> / O<sub>2</sub> mixing ratios, NO<sub>x</sub> became progressively and nearly equally enriched in <sup>17</sup>O and <sup>18</sup>O over time until it reached a steady state with Δ<sup>17</sup>O values of 39.3 ± 1.9‰ and δ<sup>18</sup>O values of 84.2 ± 4‰, relative to the isotopic composition of the initial O<sub>2</sub> gas. As the mixing ratios were increased, the isotopic enrichments were suppressed by isotopic exchange between O atoms, O<sub>2</sub>, and NO<sub>x</sub>. A kinetic model was developed to simulate the observed data and it showed that the isotope effects occurring during O<sub>3</sub> formation play a dominant role in controlling NO<sub>x</sub> isotopes and, in addition, secondary kinetic isotope effects or isotope exchange reactions are also important during NO<sub>x</sub> cycling. The data and model were consistent with previous studies which showed that the NO + O<sub>3</sub> reactions occur mainly via the transfer of the terminal atoms of O<sub>3</sub>. The model predicts that under tropospheric concentrations of NO<sub>x</sub> and O<sub>3</sub>, the timescale of NO<sub>x</sub>–O<sub>3</sub> isotopic equilibrium ranges from hours (for ppbv NO<sub>x</sub> / O<sub>2</sub> mixing ratios) to days (for pptv mixing ratios) and yields steady state Δ<sup>17</sup>O and δ<sup>18</sup>O values of 45‰ and 117‰ respectively (relative to Vienna Standard Mean Ocean Water (VSMOW)) in both cases. Under atmospheric conditions when O<sub>3</sub> has high concentrations, the equilibrium between NO<sub>x</sub> and O<sub>3</sub> should occur rapidly (h) but this equilibrium cannot be reached during polar winters and/or nights if the NO<sub>x</sub> conversion to HNO<sub>3</sub> is faster. The experimentally derived rate coefficients can be used to model the major NO<sub>x</sub>–O<sub>3</sub> isotopologue reactions at various pressures and in isotope modeling of tropospheric nitrate.
Creator (Dublin Core)
Michalski, G.
Bhattacharya, S. K.
Girsch, G.
Date (Dublin Core)
2018-01-15
Type (Dublin Core)
Text
Format (Dublin Core)
application/pdf
Identifier (Dublin Core)
10.5194/acp-14-4935-2014
https://acp.copernicus.org/articles/14/4935/2014/
Source (Dublin Core)
eISSN: 1680-7324
Language (Dublin Core)
eng
