Constraining Historical NOx Concentrations Using Variations in Atmospheric 18O18O: A Status Report
Abstract
Historical concentrations of nitrogen oxides, ozone, and other short-lived tropospheric oxidants are not well known. The best observational constraints come from starch-paper measurements (i.e., via the Schönbein method) made in Europe during the late 1800s; they suggest that tropospheric ozone increased several-fold during the 20th century, but uncertainties in those measurements and other paleo-oxidant indicators have limited the accuracy of observations available for validating long-term chemistry-climate models. Isotopic bond ordering in molecular oxygen could be exploited as a new constraint on past oxidant concentrations: Rare isotopes in O2 are re-arranged through O(3P) + O2 isotope exchange reactions in the atmosphere, resulting in relative proportions of 18O18O that depend on temperature, O2 residence time, and O(3P) concentration. Increased tropospheric O(3P) concentrations, arising from increasing emissions and photolysis of nitrogen oxides, likely altered the relative proportions of 18O18O near the surface. In this sense, the isotopic bond ordering in tropospheric O2 (i.e., Δ36) records information about the concentrations of O(3P) and its principal tropospheric source, NO2. I will present recent progress towards this application, including improved constraints on the global budget from measurements of stratospheric air, biological isotopic fractionation, and an experimental photochemical calibration of Δ36. Preliminary measurements of air from the Antarctic firn indicate a decrease in Δ36 of 0.1‰ over the past century. If the tropospheric residence time of O2 has remained unchanged, the data suggest that global surface O(3P) concentrations have increased ~50% during this time period. This result may be consistent with Schönbein-method measurements if large increases in NOx emissions occurred mainly over continental regions, with limited increases over the oceans. Extending this record further into the past could test predictions about NOx emissions from biomass burning, soil, and lightning in different climate regimes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.A43L..04Y
- Keywords:
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- 0365 ATMOSPHERIC COMPOSITION AND STRUCTURE Troposphere: composition and chemistry;
- 0365 ATMOSPHERIC COMPOSITION AND STRUCTURE Troposphere: composition and chemistry;
- 0330 ATMOSPHERIC COMPOSITION AND STRUCTURE Geochemical cycles;
- 1041 GEOCHEMISTRY Stable isotope geochemistry