Evolution of tropospheric O3 from preindustrial to present inferred from 18O18O in polar firn and ice
Abstract
Tropospheric O3 is a greenhouse gas (with radiative forcing of 0.4 ± 0.2 W m-2), a key atmospheric oxidant, and a criteria air pollutant in urban areas. While its concentrations in the present-day atmosphere are well-characterized, the Fifth Assessment Report of the Intergovernmental Panel on Climate Change reports low confidence in our knowledge of its preindustrial concentrations. Ultimately, this low confidence stems from O3 not being preserved in paleoatmospheric records, sparse and uncertain measurements in the early 20th century, and uncertainties in its atmospheric budget. Recently, the clumped-isotope composition of tropospheric O2 has been proposed as a proxy for preindustrial tropospheric O3 concentrations because it is expected to respond to changes in gross rates of O + O2 photochemical cycling. Increasing tropospheric O3 leads to faster rates of O + O2 photochemical cycling, and relative proportions of 18O18O in O2 (i.e., its Δ36 value) closer to equilibrium at tropospheric temperatures. We report new measurements of 18O18O in polar firn and three different ice cores that show a 0.03 ± 0.02‰ decrease in Δ36 value (95% confidence) that likely begins after 1950 C.E. This 20th-century decrease in Δ36 value implies an increase in tropospheric O3 burden of 25% since preindustrial times. Transient model simulations from 1850 - 2015 C.E. using the Goddard Institute for Space Studies E2 model slightly overestimate the decrease in Δ36 value, although the timing of the change is consistent with the clumped-isotope data. Together, these results imply that the tropospheric O3 burden increased modestly during the 20th century, ruling out more extreme scenarios for the anthropogenic radiative forcing due to tropospheric O3.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.C52A..03Y
- Keywords:
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- 3337 Global climate models;
- ATMOSPHERIC PROCESSESDE: 0724 Ice cores;
- CRYOSPHEREDE: 1621 Cryospheric change;
- GLOBAL CHANGEDE: 4901 Abrupt/rapid climate change;
- PALEOCEANOGRAPHY