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. Retrievals of vertical profiles of key atmospheric gases provide a criticallong-term record from ground-based Fourier transform infrared (FTIR) solarabsorption measurements. However, the characterization of the retrievedvertical profile structure can be difficult to validate, especially for gaseswith large vertical gradients and spatial–temporal variability such as watervapor. In this work, we evaluate the accuracy of the most common water vaporisotope ( H 2 16 O , hereafter WV) FTIR retrievals in the lower and uppertroposphere–lower stratosphere. Coincident high-quality vertically resolvedWV profile measurements obtained from 2010 to 2016 with balloon-borne NOAAfrost point hygrometers (FPHs) are used as reference to evaluate theperformance of the retrieved profiles at two sites: Boulder (BLD), Colorado, and atthe mountaintop observatory of Mauna Loa (MLO), Hawaii. For a meaningfulcomparison, the spatial–temporal variability has been investigated. Wepresent results of comparisons among FTIR retrievals with unsmoothed andsmoothed FPH profiles to assess WV vertical gradients. Additionally, weevaluate the quantitative impact of different a priori profiles in theretrieval of WV. An orthogonal linear regression analysis shows the bestcorrelation among tropospheric layers using ERA-Interim (ERA-I) a prioriprofiles and biases are lower for unsmoothed comparisons. In Boulder, wefound a negative bias of 0.02±1.9  % ( r=0.95 ) for the 1.5–3  km layer. A larger negative bias of 11.1±3.5  % ( r=0.97 ) was found inthe lower free troposphere layer of 3–5  km attributed to rapid verticalchange of WV, which is not always captured by the retrievals. The biasimproves in the 5–7.5  km layer ( 1.0±5.3  %, r=0.94 ). The biasremains at about 13 % for layers above 7.5  km but below 13.5  km . At MLOthe spatial mismatch is significantly larger due to the launch of the sondebeing farther from the FTIR location. Nevertheless, we estimate a negativebias of 5.9±4.6  % ( r=0.93 ) for the 3.5–5.5  km layer and 9.9±3.7  % ( r=0.93 ) for the 5.5–7.5  km layer, and we measure positive biases of 6.2±3.6  % ( r=0.95 ) for the 7.5–10  km layer and 12.6 % andgreater values above 10  km . The agreement for the first layer issignificantly better at BLD because the air masses are similar for both FTIRand FPH. Furthermore, for the first time we study the influence of differentWV a priori profiles in the retrieval of selected gas profiles. Using NDACCstandard retrievals we present results for hydrogen cyanide (HCN), carbonmonoxide (CO), and ethane ( C2H6 ) by taking NOAA FPH profiles as theground truth and evaluating the impact of other WV profiles. We show that theeffect is minor for C2H6 (bias  % for all WV sources) among allvertical layers. However, for HCN we found significant biases between 6 %for layers close to the surface and 2 % for the upper troposphere dependingon theWV profile source. The best results (reduced bias and precision and r  valuescloser to unity) are always found for pre-retrieved WV. Therefore, werecommend first retrieving WV to use in subsequent retrieval of gases.

Tropospheric water vapor profiles obtained with FTIR: comparison with balloon-borne frost point hygrometers and influence on trace gas retrievals