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An algorithm setup for the operational Aerosol Layer Height productfor TROPOMI on the Sentinel-5 Precursor mission is described anddiscussed, applied to GOME-2A data, and evaluated with lidarmeasurements. The algorithm makes a spectral fit of reflectance at theO<sub>2</sub> A band in the near-infrared and the fit window runs from758 to 770 nm. The aerosol profile is parameterised bya scattering layer with constant aerosol volume extinction coefficientand aerosol single scattering albedo and with a fixed pressurethickness. The algorithm's target parameter is the height of thislayer. In this paper, we apply the algorithm to observations fromGOME-2A in a number of systematic and extensive case studies, and wecompare retrieved aerosol layer heights with lidarmeasurements. Aerosol scenes cover various aerosol types, bothelevated and boundary layer aerosols, and land and sea surfaces. Theaerosol optical thicknesses for these scenes are relativelymoderate. Retrieval experiments with GOME-2A spectra are used toinvestigate various sensitivities, in which particular attention isgiven to the role of the surface albedo.  From retrieval simulations with the single-layer model, we learn thatthe surface albedo should be a fit parameter when retrieving aerosollayer height from the O<sub>2</sub> A band. Current uncertainties insurface albedo climatologies cause biases and non-convergences whenthe surface albedo is fixed in the retrieval. Biases disappear andconvergence improves when the surface albedo is fitted, whileprecision of retrieved aerosol layer pressure is still largely withinrequirement levels. Moreover, we show that fitting the surface albedohelps to ameliorate biases in retrieved aerosol layer height when theassumed aerosol model is inaccurate. Subsequent retrievals withGOME-2A spectra confirm that convergence is better when the surfacealbedo is retrieved simultaneously with aerosol parameters. However,retrieved aerosol layer pressures are systematically low (i.e., layerhigh in the atmosphere) to the extent that retrieved values no longer realistically represent actual extinction profiles. Whenthe surface albedo is fixed in retrievals with GOME-2A spectra,convergence deteriorates as expected, but retrieved aerosol layerpressures become much higher (i.e., layer lower in atmosphere). Thecomparison with lidar measurements indicates that retrieved aerosollayer heights are indeed representative of the underlying profile inthat case. Finally, subsequent retrieval simulations with two-layeraerosol profiles show that a model error in the assumed profile (twolayers in the simulation but only one in the retrieval) is partlyabsorbed by the surface albedo when this parameter is fitted. This isexpected in view of the correlations between errors in fit parametersand the effect is relatively small for elevated layers (less than100 hPa). If one of the scattering layers is near thesurface (boundary layer aerosols), the effect becomes surprisinglylarge, in such a way that the retrieved height of the single layer is above thetwo-layer profile.  Furthermore, we find that the retrieval solution, once retrievalconverges, hardly depends on the starting values for thefit. Sensitivity experiments with GOME-2A spectra also show thataerosol layer height is indeed relatively robust against inaccuraciesin the assumed aerosol model, even when the surface albedo is notfitted. We show spectral fit residuals, which can be used for furtherinvestigations. Fit residuals may be partly explained by spectroscopicuncertainties, which is suggested by an experiment showing theimprovement of convergence when the absorption cross section is scaledin agreement with Butz et al. (2013) and Crisp et al. (2012), anda temperature offset to the a priori ECMWF temperature profile isfitted. Retrieved temperature offsets are always negative and quitelarge (ranging between −4 and −8 K), which is not expectedif temperature offsets absorb remaining inaccuracies in meteorologicaldata. Other sensitivity experiments investigate fitting of stray lightand fluorescence emissions. We find negative radiance offsets andnegative fluorescence emissions, also for non-vegetated areas, butfrom the results it is not clear whether fitting these parametersimproves the retrieval.  Based on the present results, the operational baseline for the AerosolLayer Height product currently will not fit the surface albedo. Theproduct will be particularly suited for elevated, optically thickaerosol layers. In addition to its scientific value in climateresearch, anticipated applications of the product for TROPOMI areproviding aerosol height information for aviation safety and improvinginterpretation of the Absorbing Aerosol Index

atmospheric measurement techniques

Evaluation of the operational Aerosol Layer Height retrieval algorithm for Sentinel-5 Precursor: application to O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; A band observations from GOME-2A