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

Naus, S., S. Montzka, P. K. Patra, and M. C. Krol (2021), A 3D-model inversion of methyl chloroform to constrain the atmospheric oxidative capacity, Atmos. Chem. Phys., doi:10.5194/acp-2020-624.

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 atmosphericMCF 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
20 budget of e.g. CH4.

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Research Program: 
Tropospheric Composition Program (TCP)