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Quantifying the causes of differences in tropospheric OH within global models
Author(s) -
Nicely Julie M.,
Salawitch Ross J.,
Canty Timothy,
Anderson Daniel C.,
Arnold Steve R.,
Chipperfield Martyn P.,
Emmons Louisa K.,
Flemming Johannes,
Huijnen Vincent,
Kinnison Douglas E.,
Lamarque JeanFrançois,
Mao Jingqiu,
Monks Sarah A.,
Steenrod Stephen D.,
Tilmes Simone,
Turquety Solene
Publication year - 2017
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2016jd026239
Subject(s) - troposphere , atmospheric sciences , isoprene , chemical transport model , environmental science , hydroxyl radical , greenhouse gas , photodissociation , ozone , atmospheric chemistry , seasonality , climatology , meteorology , tropospheric ozone , radical , chemistry , photochemistry , physics , geology , mathematics , organic chemistry , oceanography , copolymer , polymer , statistics
The hydroxyl radical (OH) is the primary daytime oxidant in the troposphere and provides the main loss mechanism for many pollutants and greenhouse gases, including methane (CH 4 ). 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 CH 4 lifetime ( τ CH4 ) between these models. Annual average τ 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 τ CH4 (Δ τ CH4 ) result from variations in chemical mechanisms (Δ τ CH4 = 0.46 years), the photolysis frequency ( J ) of O 3 → O( 1 D) (0.31 years), local O 3 (0.30 years), and CO (0.23 years). The Δ τ CH4 due to CTM differences in NO x (NO + NO 2 ) is relatively low (0.17 years), although large regional variation in OH between the CTMs is attributed to NO x . Differences in isoprene and J (NO 2 ) have negligible overall effect on globally averaged tropospheric OH, although the extent of OH variations due to each factor depends on the model being examined. This study demonstrates that NNs can serve as a useful tool for quantifying why tropospheric OH varies between global models, provided that essential chemical fields are archived.