Quantifying the causes of differences in tropospheric OH within global models

Global mean tropospheric OH differs by as much as 80% among various global models, for reasons that are not well understood. We use neural networks (NNs), trained using archived output from eight chemical transport models (CTMs) that participated in the Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols and Transport Model Intercomparison Project (POLMIP), to quantify the factors responsible for differences in tropospheric OH and resulting CH4 lifetime (tau(CH4)) between these models.

Annual average tau(CH4), for loss by OH only, ranges from 8.0 to 11.6 years for the eight POLMIP CTMs. The factors driving these differences were quantified by inputting 3-D chemical fields from one CTM into the trained NN of another CTM. Across all CTMs, the largest mean differences in tCH4 (Delta tau(CH4)) result from variations in chemical mechanisms (Delta tau(CH4) = 0.46 years), the photolysis frequency (J) of O-3 -> O(D-1) (0.31 years), local O-3 (0.30 years), and CO (0.23 years).