## 2 Materials and Methods  
### TDLAS Spectrometer  
A schematic drawing of the experimental setup including the TDLAS spectrometer and the SPR is presented in Figure 1a. The TDLAS sensor comprised two distributed-feedback (DFB) diode lasers (Nanoglu, Gerbrunn, Germany), one operating around 770 nm for K(g) detection with DAS [16], and another that probed two H\({}_{2}\)O absorption peaks in the vicinity of 1398 nm using scanned calibration-free WMS (CF-WMS) to simultaneously retrieve path-averaged H\({}_{2}\)O concentration and H\({}_{2}\)O-weighted, path-averaged gas temperature [31]. Two-line thermometry is a viable alternative to thermocouples, specifically in high-temperature reacting flows [25; 32; 33]. The CF-WMS technique ensures high sensitivity, time resolution and baseline stability, which is beneficial in applications where large and unexpected changes in gas temperature and concentration may occur and a background signal without absorbers is difficult to obtain. The 770 nm laser was scanned with a 200 Hz triangular wave (10 averages) around the electronic D1 transition of atomic K at 769.9 nm. Due to the large line strength (\(\sim\)10\({}^{6}\) cm\({}^{-2}\)/atm) of this transitions from the ground state to the first excited state, very low detection limits in the can be achieved. However, a large product of K line strength, concentration and absorption path length, can lead to optically thick conditions, with very high absorbance around the absorption line centre and transmitted light levels below the photodetector threshold. A curve fitting algorithm previously presented by [PERSON] et al. [16] was used to analyse these saturated spectra, essentially by fitting the wings of the line shape. The 1398 nm laser was scanned with a triangular wave at 40 Hz (10 averages) to which a sinusoidal modulation at 8 kHz was added to perform CF-WMS. H\({}_{2}\)O concentration and temperature were simultaneously retrieved by least-squares fitting simulated to measured _1f_-normalized _2f_-WMS spectra, with temperature, concentration and nonlinear modulation amplitude as fitting parameters [31]. In all cases, the absorption path length was set to 20 mm, corresponding to the average flame size estimated visually. The two laser beams were coupled into a single optical fibre via a fibre combiner (Newport, F-CPL-B12355, Deere AvenueIrvine, CA, USA) to simplify light distribution to the SPR. When passing above the biomass particle in the reactor, the co-aligned, collimated laser beams had a diameter of 1.2 mm. After exiting the SPR, the two-wavelength light was separated by a dichroic beam splitter and focused on suitable photodetectors (Thorlabs, PDA20 CS and PDA36A, Newton, NJ, USA) for K(g) and H\({}_{2}\)O/temperature detection in a wavelength-multiplexed manner. Narrowband optical filters (Thorlabs, FB770-10 and FB1400-12) were installed in front of the detectors to ensure minimal interference due to stray light from the flame (during devolatilization, see Figure 1c) or the glowing biomass particle (during char conversion, see Figure 1d). Focusing the light on the photodetectors helped to minimize potential beam steering due to the flame. Synchronized waveform  
Figure 1: (**a**) Schematic drawing of the experimental setup including the two DFB lasers, the single particle reactor (SPR) and the optical detection system. FC—fibre combiner, BS—beam splitter, PD—photodetector, OF—optical filter. (**b**) Photograph of the SPR with TDLAS optics mounted. (**c**) Representative photograph of a biomass particle inside the SPR during devolatilization. (**d**) Representative photograph of a biomass particle inside the SPR during char conversion. generation (National Instruments, PXI-5402, Cary, NC, USA) and data acquisition (National Instruments, PXIe-6356) boards installed in a PC were used to scan and modulate the lasers and acquire the detector signals, respectively.