## 2 Materials and Methods
The ratio of radiative emission, detected by the sensor, is interpreted as follows:  
\[\frac{GS_{\lambda 1}}{GS_{\lambda 2}}=\frac{b_{1}\ \tau_{1}\ \Delta\lambda_{1}\ \lambda_{2}^{5+\kappa}\ exp\left(\frac{hc}{\lambda_{2}kT}\right)}{b_{2}\ \tau_{2}\ \Delta\lambda_{2}\ \lambda_{1}^{5+\kappa}\ exp\left(\frac{hc}{\lambda_{1}kT}\right)}\]  
where \(GS_{\lambda i}\) is the greyscale output for the filtered camera sensor after transformation to a radial profile, \(b_{i}\) is the wavelength-specific calibration constant, relating the filtered camera sensor response to blackbody spectral intensity, \(c\) is the speed of light, \(h\) is the [PERSON]'s constant, \(k\) is the [PERSON]'s constant, \(T\) is the temperature of the particle (assumed to be equal to the surrounding gas), \(\lambda_{i}\) is the rated wavelength of the bandpass filter, \(\tau_{i}\) is the transmissivity of the filter and \(\Delta\lambda_{i}\) is the full width at half maximum (FWHM) of the bandpass. \(b_{i}\) is calibrated using a blackbody furnace (Newport Oriel 67032, Irvine, CA, USA) while confirming the linear response of the camera sensor to intensity for a range of  
Figure 1: Schematic of the SpraySyn burner configuration (**a**) and an image of a typical flame used to pilot the synthesis spray flame (**b**). exposure times. The soot dispersion index, \(\alpha\), is assumed to be unity under typical flame conditions [16]. Thermocouple measurements of the pilot and spray flames are taken to complement the pyrometry measurements. A fine-wire type R thermocouple (125 micron wire, Omega P13R-005, Norwalk, CT, USA) is coated with a Y\({}_{2}\)O\({}_{3}\)/BeO ceramic, as outlined in previous studies [17; 18], to minimize catalytic surface reactions on the platinum wire [19]. Thermocouple readings are corrected for energy losses at the thermocouple bead, based on the known heat transfer behavior of fine-wire thermocouples in flames [20; 21]. Temperature readings are recorded at 100 Hz to capture the flame temperature before the measurement is disrupted by soot deposits on the thermocouple bead. The current conditions are chosen to minimize temperature measurement challenges, previously reported in heavier sooting flames [22; 23]. The two-flame series currently studied are outlined in Table 1. The pilot fuel is ethylene gas kept at the same flow rate for all flames. Pilot nitrogen is also kept at the same flow rate across all flames in a flow independent of the pilot oxygen. The first series of flames examines the effect of excess oxygen in the pilot flame by systematically increasing the pilot oxygen flow rate to decrease the pilot flame equivalence ratio from 0.5 to 0.4. The liquid particle feedstock is coal tar distillates (Koppers Inc. carbon black feedstock, Pittsburgh, PA, USA) diluted in toluene (65% toluene, 35% distillate by volume in the first series) to decrease viscosity and to minimize the disruption due to liquid coking at the atomization nozzle. The second flame series keeps the pilot flow rates constant while steadily increasing the percent volume of coal tar distillate in the liquid feed from pure toluene to 75% coal tar distillates by volume (at 3 mL/min liquid feed flow rate). Future optimization of the atomization process is required to minimize the liquid coking of undiluted coal tar distillates. Series 1 uses an atomizing N\({}_{2}\) flow rate of 9.9 SLPM, while series 2 uses 10.8 SLPM. The flame-naming system uses \"p\" to denote the pilot flame equivalence ratio, \"t\" to denote the liquid feed toluene percent volume and \"d\" to denote the liquid feed coal tar distillates percent volume. The p43t65d35 condition is shared among the two series with the difference being a higher flow atomizing N\({}_{2}\) flow rate designated by the L and H labels. The sheath nitrogen flow rate is kept at 120 SLPM for all flames.