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

摘要

The modeling study presented here aims to estimate how uncertainties inglobal hydroxyl radical (OH) distributions, variability, and trends maycontribute to resolving discrepancies between simulated and observed methane(CH4) changes since 2000. A multi-model ensemble of 14 OH fields wasanalyzed and aggregated into 64 scenarios to force the offlineatmospheric chemistry transport model LMDz (Laboratoire de Meteorologie Dynamique) with a standard CH4 emissionscenario over the period 2000–2016. The multi-model simulated globalvolume-weighted tropospheric mean OH concentration ([OH]) averaged over2000–2010 ranges between 8.7×105 and 12.8×105 molec cm−3. The inter-model differences in tropospheric OH burden andvertical distributions are mainly determined by the differences in thenitrogen oxide (NO) distributions, while the spatial discrepancies betweenOH fields are mostly due to differences in natural emissions and volatile organic compound (VOC)chemistry. From 2000 to 2010, most simulated OH fields show an increase of0.1–0.3×105 molec cm−3 in the tropospheric mean [OH],with year-to-year variations much smaller than during the historical period1960–2000. Once ingested into the LMDz model, these OH changes translatedinto a 5 to 15 ppbv reduction in the CH4 mixing ratio in 2010, whichrepresents 7 %–20 % of the model-simulated CH4 increase due tosurface emissions. Between 2010 and 2016, the ensemble of simulations showedthat OH changes could lead to a CH4 mixing ratio uncertainty of>±30 ppbv. Over the full 2000–2016 time period, using acommon state-of-the-art but nonoptimized emission scenario, the impact of[OH] changes tested here can explain up to 54 % of the gap between modelsimulations and observations. This result emphasizes the importance ofbetter representing OH abundance and variations in CH4 forwardsimulations and emission optimizations performed by atmospheric inversions.