Resolving the Model-Observation Discrepancy in the Mesospheric and Stratospheric HOx Chemistry

Content Provider	Semantic Scholar
Author	Li, King-Fai Zhang, Qiong Wei Wang, Shuhui Sander, Stanley P. Yung, Yuk L.
Copyright Year	2017
Abstract	We examine the middle atmospheric odd-hydrogen (HO_x) chemistry by comparing the Aura Microwave Limb Sounder (MLS) OH and HO_2 measurements with a photochemical model simulation. The model underestimates mesospheric OH and HO_2 concentrations if the standard chemical kinetic rates are used, whether the model H_2O and O_3 are constrained with observations or not. To resolve the discrepancies, we adjust the kinetic rate coefficients of three key reactions (O + OH → O_2 + H, OH + HO_2 → H_2O + O_2, and H + O_2 + M → HO_2 + M) and the O2photo absorption cross section at Lyman-α (121.57 nm) using the Bayesian optimal estimation. A much better model-observation agreement can be achieved if the kinetic rate coefficients for H + O_2 + M → HO_2 + M is increased by 134–310%, and the O_2 photo absorption cross section at Lyman-α is reduced by 33–54%, while the kinetic rate coefficients for O + OH → O_2 + H and OH + HO_2 → H_2O + O_2 remain consistent with the current laboratory values. The kinetic rate coefficient for H + O_2 + M → HO_2 + M requires a very large adjustment beyond the uncertainty limits recommended in the NASA Data Evaluation, suggesting the need for future laboratory measurements. An alternative explanation is that the radiative association reaction, H + O_2 → HO_2 + hν, plays a significant role, which has never been measured. Our results demonstrate that high quality satellite observations can be used to constrain photochemical parameters and help improve our understanding of atmospheric chemistry.
Starting Page	607
Ending Page	624
Page Count	18
File Format	PDF HTM / HTML
DOI	10.1002/2017EA000283
Alternate Webpage(s)	http://authors.library.caltech.edu/81117/2/ess2157-sup-0001-2017EA000283-S01.pdf
Alternate Webpage(s)	https://doi.org/10.1002/2017EA000283
Volume Number	4
Language	English
Access Restriction	Open
Content Type	Text
Resource Type	Article