## 2 Materials and Methods
### Data Acquisition System and Parameters
#### 2.1.2 Proton Transfer Reaction Time-of-Flight Mass Spectrometer  
In addition to validating the vehicle-mounted CWML, we also detected the VOCs in navigation monitoring experiments. PTR-TOF-MS is the in situ sensor, and the sampling port of PTR-TOF-MS is located at the front end of the car roof to avoid the mis-sampling of the exhaust gases from the vehicle. The PTR-TOF-MS instrument used in this study consists of an ion source, a drift tube, and a mass spectrometer. The basic principle of PTR-TOF-MS relies on a proton-transfer reaction. To ensure the proton-transfer reaction required for ionization, the proton affinity (PA) of the VOC to be measured must be higher than that of water, i.e., PA\({}_{\text{VOC}}\) > PA\({}_{\text{H2O}}\) (PA\({}_{\text{H2O}}\) = 691 kJ/mol) must be satisfied [32]. Most VOCs possess a proton affinity higher than water, which is suitable for their ionization by H\({}_{3}\)O\({}^{+}\). In addition, the major components of air [33] (such as oxygen, nitrogen, and carbon dioxide) have a proton affinity lower than water. Hence, they are not ionized, which does not affect target detection. In PTR-TOF-MS, the water vapor discharge from the ion source generates the active ion H\({}_{3}\)O\({}^{+}\). The sampling port introduces the ambient air into the drift tube, where the air undergoes a proton-transfer reaction with the reagent ion H\({}_{3}\)O\({}^{+}\)[34].  
\[\text{H}_{3}\text{O}^{+}+\text{R}\rightarrow\text{RH}^{+}+\text{H}_{2}\text{O}, \tag{1}\]  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Component** & **Qualification** & **Specification** \\ \hline \multirow{8}{*}{Transmitter} & Wavelength & 1.55 \(\mu\)m \\  & Pulse energy & 145 \(\mu\)J \\  & Pulse repetition & 10 kHz \\  & Pulse width & 400 ns \\  & AOM frequency & 80 MHz \\ \hline \multirow{8}{*}{Transceiver} & Laser mode & Pulse \\  & Scan mode & Conical \\  & Elevation angle & 60\({}^{\circ}\) \\  & Start angle & 0\({}^{\circ}\) \\  & Step angle & 90\({}^{\circ}\) \\ \hline \multirow{8}{*}{Data acquisition} & Sampling frequency & 1 GHz \\  & Sampling points & 400 \\ \cline{1-1}  & Range resolution & 60 m \\ \cline{1-1}  & Blind range & 60 m \\ \cline{1-1}  & Gate number & 128 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Coherent wind measurement Lidar system parameters.  
Figure 4: Raw power spectrum data.  
where R is the VOC molecule to be measured, and RH\({}^{+}\) is the production. The count rates cps (H\({}_{3}\)O\({}^{+}\)) and cps (RH\({}^{+}\)) of H\({}_{3}\)O\({}^{+}\) and RH\({}^{+}\) were measured by the mass detector, and the absolute concentrations of R were deduced as follows:  
\[[\text{R}]=\frac{[\text{RH}^{+}]}{[\text{H}_{3}\text{O}^{+}]_{0}}\times\frac{1 }{\text{k}t}=\frac{\text{cps}(\text{RH}^{+})}{\text{cps}(\text{H}_{3}\text{O}^ {+})}\times\frac{1}{\text{k}t}, \tag{2}\]  
where [R] represents the absolute concentration of the substance R to be measured, [H\({}_{3}\)O\({}^{+}\)]\({}_{0}\) is the initial concentration of H\({}_{3}\)O\({}^{+}\), k is the rate constant for the protonation reaction, and \(t\) is the average time for the ion to pass through the reaction zone.