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. The hydroxyl radical (OH) determines the capability of atmosphericself-cleansing and is one of the significant oxidants in atmosphericphotochemistry reactions. Global OH has been monitored bysatellites with the traditional limb mode in the past decades. Thisobserved mode can achieve the acquisition of high-resolution vertical OH data but cannotobtain enough horizontal OH data for inverting high-precision OHconcentrations because OH has a high reactivity that makes OHconcentrations extremely low and distributions complicated. The doublespatial heterodyne spectrometer (DSHS) is designed to obtain higher-resolution and more detailed OH data. This sensor can measure OHby the three-dimensional limb mode to obtain comprehensive OH datain the atmosphere. Here we propose a new tomographic retrieval algorithm based on the simulated observation data because the DSHS willwork officially on the orbit in the future. We build up an accurateforward model. The main part of it is the SCIATRAN radiative transfermodel which is modified according to the radiation transmissiontheory. The error in results obtained by the forward model is ±44.30 % in the lower atmosphere such as at a 21 km height anddecreases gradually until the limit of observation altitude. We alsoconstruct the tomographic retrieval algorithm of which the core is alookup table method. A tomographic-observation database is built upthrough the atmospheric model, the spatial information (the positionof the target area and satellite position), the date parameters, theobservation geometries, OH concentrations, and simulated observationdata. The OH concentrations can be found from it directly. If thereare no corresponding query conditions in the tomographic-observationdatabase, the cubic spline interpolation is used to obtain the OHconcentrations. The inversion results are given, and the errors in themincrease as the altitudes rise until about a 41 km height thenstart to decrease. The errors in the inversion results reach themaximum of about ±25.03 % at the 41 km height and decrease to±8.09 % at the limited observation height. They are alsosmall in the lower atmosphere at ±12.96 % at 21 km. In summary, the tomographic retrieval algorithm can obtain more accurate OH concentrations even in the loweratmosphere where the OH data are not high quality and avoids the setting ofinitial guess values for solving the iteration problems. Our researchnot only provides support for the scientific theory of the construction of theDSHS but also gives a new retrieval algorithm idea for otherradicals.

Tomographic retrieval algorithm of OH concentration profiles using double spatial heterodyne spectrometers