## 2 Materials and Methods
By fitting a model equation to these wavelength markers using the Levenberg-Marquardt algorithm [30], the wavelength can be determined at any point in time during the scan. The single scan measurement signals of the multipass cell and the reference signal of the Perspec chamber clearly show the dominant baseline slope due to the triangular amplitude modulation of the laser by the current tuning. The difference in the total transmission of the signals is due to the different absorption paths as well as broadband scattering and effects of optical components (mirrors, fibers and windows), but is taken into account in the evaluation for each absorption profile. Furthermore, the signals show differently pronounced water absorption in the range of 1370 nm, depending on the respective water concentration in the exhaust air of the acoustic levitator. To extract the H2O absorption line area from the raw signal, a polynomial baseline (including all disturbing effects mentioned before), as well as a Voigt line shape, was fitted to the measured spectra using a non-linear Levenberg-Marquardt algorithm [31; 32; 33]. In order to determine the Voigt line shape, we chose an empirical approximation proposed by [PERSON] et al. [31], which describes the Voigt profile as the weighted sum of a Lorentzian and a Gaussian function as follows:  
\[\varphi(\
u-\
u_{0})=c_{L}\frac{1}{\pi}\frac{\delta\
u_{V}}{\left(\
u-\
u_{0} \right)^{2}+\delta\
u_{V}{}^{2}}+c_{G}\frac{\sqrt{\pi\cdot}\delta\
u_{V}}{ \sqrt{\pi\cdot}\delta\
u_{V}}\exp\left(\frac{-\ln 2\cdot\left(\
u-\
u_{0} \right)^{2}}{\delta\
u_{V}{}^{2}}\right). \tag{3}\]  
In this model equation, the weighting factors \(c_{L}\) and \(c_{G}\) are third order polynomials and can be calculated with the dimensionless profile parameter \(d\) using Equations (4)-(6). \[c_{G} =0.32460-0.61825\cdot d+0.17681\cdot d^{2}+0.12109\cdot d^{3} \tag{4}\] \[c_{L} =0.68188+0.61293\cdot d-0.18384\cdot d^{2}-0.11568\cdot d^{3}\] (5) \[d=\frac{\delta\
u_{L}-\delta\
u_{D}}{\delta\
u_{L}+\delta\
u_{D}} \tag{6}\]  
The profile parameter \(d\) is computed from the half width at half maximum (HWHM) of the Doppler broadened, Gaussian line shape \(\delta\
u_{D}\), and the HWHM of the collision broadened, Lorentzian line shape \(\delta\
u_{L}\). \(\delta\
u_{V}\) denotes the HWHM of the resulting Voigt profile and was approximated using the method of [PERSON] and Longbothum [34]. From the derived line areas, the absorber number densities are calculated by applying the extended Lambert-Beer law (Equation (2)). The temperature-dependent line strength \(S(T)\) was taken from HITRAN16 [26]. In order to derive the mole fractions from the determined number densities via the ideal gas law, the gas temperature was measured and the gas pressure was assumed to be constant at 1 atm (atmospheric conditions, 1013.25 hPa). This evaluation procedure has already been used several times in the literature and is described in detail [15; 16; 20]. Figure 3 shows the extracted water absorption line of a single measured scan, and the fitted data for a high water concentration (left) and for a low water concentration (right), as they occur during a single droplet drying experiment, plotted as optical density (\(OD=-\ln(I/I_{0})\)) over a linear wavenumber scale, after the removal of amplitude modulation and correction for offset and transmission fluctuations. The two water absorption lines in Figure 3 show good agreement between measured and fitted data. The relative deviation between the measured absorption line profile and the fitted Voigt profile is plotted under the absorption profiles. Here, a larger deviation can be seen at lower concentrations due to stronger noise. In addition, small systematic residual deviations can be seen in both curves. These deviations have already been observed and discussed by [PERSON] et al. (2019) and are due to the Voigt profile used, which potentially does not optimally reproduce the absorption line.