Inter-model comparison of global hydroxyl radical (OH) distributions and their impact on atmospheric methane over the 2000-2016 period

Yuanhong Zhao 1 Marielle Saunois 1 Philippe Bousquet 1 Xin Lin 1, 2 Michaela Hegglin 3 Josep G. Canadell 4 Robert B. Jackson 5, 6, 7 Didier Hauglustaine 1 Sophie Szopa 1 Ann R. Stavert 8 Nathan Luke Abraham 9, 10 Alex T. Archibald 9, 10 Slimane Bekki 11 Makoto Deushi 12 Patrick Jöckel 13 Béatrice Josse 14 Douglas Kinnison 15 Ole Kirner 16 Virginie Marécal 14 Fiona M. O'Connor 17 David Plummer 18 Laura Revell 19, 20 Eugene Rozanov 21, 19 Andrea Stenke 19 Sarah Strode 22, 23 Simone Tilmes 24 Edward J. Dlugokencky 25 Bo Zheng 1
Abstract : The modeling study presented here aims to estimate how uncertainties in global hydroxyl radical (OH) distributions, variability, and trends may contribute to resolve discrepancies between simulated and observed methane (CH4) changes since 2000. A multi-model ensemble of 14 OH fields were analysed and were aggregated into 64 scenarios to force the offline atmospheric chemistry transport model LMDz with a standard CH4 emission scenario over the period 2000–2016. The multi-model simulated global volume-weighted tropospheric mean OH concentration ([OH]) averaged over 2000–2010 ranges between 8.7 × 105 and 12.8 × 105 molec cm−3. The inter-model differences in tropospheric OH burden and vertical distributions are mainly determined by the differences in the nitrogen oxide (NO) distributions, while the spatial discrepancies between OH fields are mostly due to differences in natural emissions and VOC chemistry. From 2000 to 2010, most simulated OH fields show an increase of 0.1–0.3 × 105 molec cm−3 in the tropospheric mean [OH], with year-to-year variations much smaller than during the historical period 1960–2000. Once ingested into the LMDz model, these OH changes translated into a 5 to 15 ppbv reduction in CH4 mixing ratio in 2010, which represent 7 %–20 % of the model simulated CH4 increase due to surface emissions. Between 2010 and 2016, the ensemble of simulations showed that OH changes could lead to a CH4 mixing ratio uncertainty of > ±30 ppbv. Over the full 2000–2016 time period, using a common state-of-the-art but non-optimized emission scenario, the impact of [OH] changes tested here can explain up to 54 % of the gap between model simulations and observations. This result emphasizes the importance of better representing OH abundance and variations in CH4 forward simulations and emission optimizations performed by atmospheric inversions.
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Yuanhong Zhao, Marielle Saunois, Philippe Bousquet, Xin Lin, Michaela Hegglin, et al.. Inter-model comparison of global hydroxyl radical (OH) distributions and their impact on atmospheric methane over the 2000-2016 period. Atmospheric Chemistry and Physics Discussions, European Geosciences Union, 2019, pp.(Under Review). ⟨10.5194/acp-2019-281⟩. ⟨insu-02088245⟩

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