## 4 Conclusions  
For the accurate measurement of water self-broadening coefficients and their temperature dependence, we designed a new spectrometer, operating in a well-controlled environment (water bath) to achieve temperature stabilities better than 90 mK, and that is capable of measuring accurate and metrology-compatible self-broadening line data for H\({}_{2}\)O at 1.37 \(\upmu\)m. The self-broadening coefficient of the H\({}_{2}\)O line at 7299.43 cm\({}^{-1}\) and its temperature coefficient has been determined with GUM-compatible relative combined uncertainties of 2.8% and 1.6%, \(k=1\), respectively. With different gas cells in similar spectroscopic systems (without water immersion), the air broadening coefficient of the H\({}_{2}\)O line at 7299.43 cm\({}^{-1}\) and its temperature coefficient have been determined with relative combined uncertainties of 1.6% and 2.2%, respectively. Compared to HITRAN 2020 data, significant improvements in the uncertainties by factors up to 4.4 have been achieved. The traceability of the results to the SI has been addressed via input parameters as the measured gas pressures and temperatures (that are traceable to respective PTB standards). The results presented in this work (uncertainties \(<\) 3%) are seen to already be fit for purpose in atmospheric H\({}_{2}\)O concentration measurements (employing the H\({}_{2}\)O line at 7299.43 cm\({}^{-1}\)) as well as H\({}_{2}\)O measurements in other applications covering the temperature range from 293 K to 573 K (extrapolations/usage outside this temperature range, e.g., down to 200 K, might be considered where appropriate) where absorption data have to be fitted with an appropriate line profile, e.g., a Voigt profile.  
Experiment concept: V.E. Experiment realization and data collection: J.A.N. Data evaluation and discussion: J.A.N. and V.E. Paper conceptualization writing--review and editing: J.A.N., O.W. and V.E. All authors have read and agreed to the published version of the manuscript.  
Part of this work was supported via the EMRP ENV07 MeteoMet and ENV58 MeteoMet2 projects ([[https://www.meteomet.org/](https://www.meteomet.org/)]([https://www.meteomet.org/](https://www.meteomet.org/)), accessed on the 29 April 2021). European Metrology Research Programme (EMRP) is jointly funded by the EMRP participating countries within EURAMET and the European Union.  
Not applicable.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multirow{2}{*}{**Value**} & \multirow{2}{*}{\begin{tabular}{c} **Standard** \\ **Uncertainty** \\ \end{tabular} } & \multicolumn{2}{c}{**Relative**} & \multirow{2}{*}{\
\begin{tabular}{c} **Index** \\ **Standard** \\ **Uncertainty** \\ \end{tabular} } \\  & & & **(\(k=1\))** & \\ \hline Partial pressure & & & & \\ of H\({}_{2}\)O (\(p_{\text{H2O}}\)) & 28.6 hPa \({}^{*}\) & 0.34 hPa & 1.20\% & 39\% \\ \hline Lorentzian line & & & & \\ width (\(\Delta v_{L}\)) & 0.024 cm\({}^{-1}\) & 3.6 \(\times\) 10\({}^{-4}\) cm\({}^{-1}\) & 1.50\% & 61\% \\ \hline \multicolumn{4}{c}{**H\({}_{2}\)O**} \\
\begin{tabular}{c} **self-broadening** \\ **coefficient** \\ **(Y\({}_{\text{H2O}}\))** \\ \end{tabular} & **0.430 cm\({}^{-1}\)/atm** & & **2.00\% b** \\ \hline \hline \end{tabular}  
* 28.6 hPa = 0.02822 atm. b This corresponds to an expanded uncertainty of 4 % (\(k=2\)).  
\end{table}
Table 2: Uncertainty budget for the self-broadening coefficient \(\gamma_{\text{H2O}}\) in this work.