Tropospheric water vapor profiles obtained with FTIR: comparison with balloon-borne frost point hygrometers and influence on trace gas retrievals
Authors: Ortega Ivan , Buchholz Rebecca R. , Hall Emrys G. , Hurst Dale F. , Jordan Allen F. , Hannigan James W. 
Published: February 2019

Abstract. Retrievals of vertical profiles of key atmospheric gases provide a critical
long-term record from ground-based Fourier transform infrared (FTIR) solar
absorption measurements. However, the characterization of the retrieved
vertical profile structure can be difficult to validate, especially for gases
with large vertical gradients and spatial–temporal variability such as water
vapor. In this work, we evaluate the accuracy of the most common water vapor
isotope (H216O, hereafter WV) FTIR retrievals in the lower and upper
troposphere–lower stratosphere. Coincident high-quality vertically resolved
WV profile measurements obtained from 2010 to 2016 with balloon-borne NOAA
frost point hygrometers (FPHs) are used as reference to evaluate the
performance of the retrieved profiles at two sites: Boulder (BLD), Colorado, and at
the mountaintop observatory of Mauna Loa (MLO), Hawaii. For a meaningful
comparison, the spatial–temporal variability has been investigated. We
present results of comparisons among FTIR retrievals with unsmoothed and
smoothed FPH profiles to assess WV vertical gradients. Additionally, we
evaluate the quantitative impact of different a priori profiles in the
retrieval of WV. An orthogonal linear regression analysis shows the best
correlation among tropospheric layers using ERA-Interim (ERA-I) a priori
profiles and biases are lower for unsmoothed comparisons. In Boulder, we
found 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 in
the lower free troposphere layer of 3–5 km attributed to rapid vertical
change of WV, which is not always captured by the retrievals. The bias
improves in the 5–7.5 km layer (1.0±5.3 %, r=0.94). The bias
remains at about 13 % for layers above 7.5 km but below 13.5 km. At MLO
the spatial mismatch is significantly larger due to the launch of the sonde
being farther from the FTIR location. Nevertheless, we estimate a negative
bias 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 % and
greater values above 10 km. The agreement for the first layer is
significantly better at BLD because the air masses are similar for both FTIR
and FPH. Furthermore, for the first time we study the influence of different
WV a priori profiles in the retrieval of selected gas profiles. Using NDACC
standard retrievals we present results for hydrogen cyanide (HCN), carbon
monoxide (CO), and ethane (C2H6) by taking NOAA FPH profiles as the
ground truth and evaluating the impact of other WV profiles. We show that the
effect is minor for C2H6 (bias <0.5 % for all WV sources) among all
vertical layers. However, for HCN we found significant biases between 6 %
for layers close to the surface and 2 % for the upper troposphere depending
on the
WV profile source. The best results (reduced bias and precision and r values
closer to unity) are always found for pre-retrieved WV. Therefore, we
recommend first retrieving WV to use in subsequent retrieval of gases.