24 Jul 2020

24 Jul 2020

Review status: a revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

A 3D-model inversion of methyl chloroform to constrain the atmospheric oxidative capacity

Stijn Naus1, Stephen A. Montzka2, Prabir K. Patra3,4, and Maarten C. Krol1,5 Stijn Naus et al.
  • 1Meteorology and Air Quality, Wageningen University and Research, the Netherlands
  • 2NOAA Global Monitoring Laboratory, Boulder, CO, USA
  • 3Research Institute for Global Change, JAMSTEC, Yokohama, Japan
  • 4Center for Environmental Remote Sensing, Chiba University, Chiba, Japan
  • 5Institute for Marine and Atmospheric Research, Utrecht University, the Netherlands

Abstract. Variations in the atmospheric oxidative capacity, largely determined by variations in the hydroxyl radical (OH), form a key uncertainty in many greenhouse and other pollutant budgets, such as that of methane (CH4). Methyl chloroform (MCF) is an often-adopted tracer to indirectly put observational constraints on variations in OH. We investigated the budget of MCF in a 4DVAR inversion using the atmospheric transport model TM5, for the period 1998–2018, with the objective to derive information on interannual variations in OH and in its spatial distribution.

We derived interannual variations in the global oxidation of MCF that bring simulated mole fractions of MCF within 1–2 % of the assimilated observations from the NOAA-GMD surface network at most sites. Additionally, the posterior simulations better reproduce aircraft observations used for independent validation. The derived OH variations showed robustness with respect to the prior MCF emissions and the prior OH distribution. The interannual variations were typically small (< 3 %/year), with no significant longterm trend in OH.

The inverse system found strong adjustments of the latitudinal distribution of OH, with systematic increases in tropical OH and decreases in extra-tropical OH (both up to 30 %). These spatial adjustments were driven by intrahemispheric biases in simulated MCF mole fractions, which have not been identified in previous studies. Given the unexpectedly large amplitude of these adjustments and a residual bias in intrahemispheric gradients, we suggest a reversal in the extratropical ocean sink of MCF in response to declining atmospheric MCF abundance (as hypothesized in Wennberg et al., 2004). This reversal provides a more realistic explanation for the biases, possibly complimentary to adjustments in the OH distribution.

While we identified significant added value in the use of a 3D transport model over simpler box models, we also found a trade-off in computational expense and convergence problems. However, although the signals are small compared to assuming interannually repeating OH, the derived variations better match the global MCF observations and are relevant for studying the budget of e.g. CH4.

Stijn Naus et al.

Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement

Stijn Naus et al.


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Short summary
Following up on previous box model studies, we employ a 3D transport model to estimate variations in the hydroxyl radical (OH) from observations of methyl chloroform (MCF). We derive small interannual OH variations that are consistent with variations in the El Nino Southern Oscillation. We also find evidence for release of MCF from the oceans in atmospheric gradients of MCF. Both findings highlight the added value of a 3D transport model, since box model studies did not identify these effects.