3.2.1 Carbon emissions

The gaseous emission profiles of carbon monoxide ( $CO$ ) and carbon dioxide ( $C{O_2}$ ) provide a comprehensive understanding of the combustion process efficiency relative to the fuel’s carbon content ( $100 - H{\% }$ ). As listed in Table 1, the highly refined SPKs, namely ATJ and HEFA, exhibit carbon mass fractions of 84.6% and 84.9%, respectively. The hydrocarbon group type analysis confirms that these low-carbon fuels are almost entirely devoid of aromatic compounds, with ATJ comprising 99.8% isoparaffins and HEFA comprising 89.4% isoparaffins. Jet A, JP 8 and JP 5 are characterised by an intermediate carbon content, ranging from 85.67% to 86.46%. These established fuels are composed of n-paraffins, isoparaffins and substantial cycloparaffin components, with JP-5 exhibiting a peak cycloparaffin content of 47.37%. Moreover, the aromatic components of these fuels vary, accounting for 13.56%–20.36% of the total composition. At the upper extreme of carbon density lies SAK, a pure blending component with a carbon mass fraction of 89.8% composed entirely of 100% aromatic ring structures.

Figure 8.

Carbon monoxide emissions relative to baseline fuel Jet A at various heat release rates of tested fuels.

Figure 8. Long description

A bar graph comparing carbon monoxide emissions relative to Jet A at various heat release rates for different fuels. The horizontal axis represents the heat release rate in kilojoules per second (kJ/sec) with values at 41.5, 43.81, 46.11, 48.42, and 50.72. The vertical axis represents the carbon monoxide emissions relative to Jet A, ranging from 0.6 to 1.6. The graph includes multiple bars for each heat release rate, each representing a different fuel type. The fuels are color-coded as follows: JP8 (blue), JP5 (orange), ATJ (yellow), SAK (purple), J50A50 (green), J50H50 (light blue), ASA K4 (light green), ASA K8 (pink), ASA K17 (dark red), ASA K25 (dark blue), HSA K17 (brown), and HSA K25 (black). Notable trends include higher carbon monoxide emissions for SAK at heat release rates of 46.11 and 48.42 kJ/sec, and generally lower emissions for most other fuels across the heat release rates.

Figure 9.

Carbon dioxide emissions relative to baseline fuel Jet A at various heat release rates of tested fuels.

The $CO$ and $C{O_2}$ , as shown in Figs 8 and 9, respectively, are quantified relative to a Jet A baseline across HRRs of 41.5 to 50.72 kJ/sec. The $CO$ emission data, presented in Fig. 8, shows a limited chemical reaction rate during the intermediate stages of hydrocarbon oxidation. The isoparaffin dominant ATJ and its highly paraffinic low-level blend, ASAK4, consistently exhibit the highest oxidation efficiency, yielding the lowest relative $CO$ across the testing matrix. At the peak HRR condition of 50.72 kJ/sec, the ASAK4 blend registered a $CO$ fraction of 0.704 relative to Jet A, which can be attributed to the molecular simplicity of the aliphatic chains [Reference Sarathy, Javed, Karsenty, Heufer, Wang, Park, Elwardany, Farooq, Westbrook, Pitz, Oehlschlaeger, Dayma, Curran and Dagaut33]. The high hydrogen content of these fuels guarantees the rapid generation of an abundant pool of hydroxyl ( $O{H^ \bullet }$ ) radicals. Because the primary conversion of intermediate carbon monoxide to fully oxidised carbon dioxide is governed by the highly specific elementary reaction $CO + O{H^ \bullet } \to C{O_2} + {H^ \bullet }$ [Reference Lachkov and Chin34], the radical-rich environment provided by the low-carbon paraffinic fuels could ensure that this reaction proceeds to completion rapidly, effectively depleting $CO$ concentrations before the exhaust gases exit the combustor.

In contrast, SAK (100% aromatic) from Fig. 8 demonstrates that maximising the carbon mass fraction induces extreme kinetic bottlenecking. SAK produced $CO$ about 1.232 times that of Jet A at 50.72 kJ/sec, which increased sharply to 1.462 at an HRR of 46.11 kJ/sec. The elevated carbon fraction of 89.8% in SAK is locked within benzene rings. The activation energy required to shatter these aromatic structures is substantially higher than the energy required to cleave paraffinic bonds. Consequently, these carbon-dense molecules resist early thermal degradation and tend to form highly stable radical intermediates, thereby limiting reaction rates. Furthermore, the severe hydrogen deficiency of SAK (10.2%) heavily restricts the formation of the necessary $O{H^ \bullet }$ radical pool. The combination of delayed carbon breakdown and radical deficiency effectively freezes the oxidation chain at the $CO$ intermediate stage, leading to massive accumulation of carbon monoxide. This kinetic penalty is clearly observable as a linear scaling function within the ‘drop-in’ blending series; increasing the SAK fraction in the ATJ base from 4% (ASAK4) to 25% (ASAK25) linearly drives the relative $CO$ from 0.704 up to 0.880 at peak HRR, reflecting the systematic stoichiometric substitution of highly reactive isoparaffins with kinetically resistant aromatics.

The $C{O_2}$ emission trends detailed in Fig. 9 represent the terminal stage of the hydrocarbon oxidation sequence and are a function of both the total carbon content and the efficiency of the aforementioned $CO$ to $C{O_2}$ conversion. Assuming complete combustion, $C{O_2}$ production for a constant HRR is theoretically proportional to the fuel’s carbon mass fraction and inversely proportional to its gravimetric energy density. Among the evaluated fuels, the HEFA-based approved drop-in blend J50H50 consistently yielded the highest relative $C{O_2}$ emissions across the varying HRRs. It was the only blend to frequently exceed the Jet A baseline, reaching a peak relative value of approximately 1.02. Conversely, the low-carbon, hydrogen-rich SAFs, such as ATJ and the ASAK blends, generally operate below the Jet A baseline. Because these fuels possess superior net heats of combustion (e.g. 43.8 MJ/kg for ATJ), a lower total fuel mass flow rate and therefore less absolute elemental carbon is required to achieve the target 50.72 kJ/sec heat release, fundamentally lowering the absolute $C{O_2}$ output.

However, a direct comparison between Figs 8 and 9 exposes a profound inverse kinetic relationship driven by incomplete combustion in the carbon-dense fuels. For SAK at an HRR of 46.11 kJ/sec, the massive $CO$ spike to 1.462 corresponds to a notably suppressed $C{O_2}$ value of 0.902. Despite introducing the highest concentration of elemental carbon into the combustor, SAK fails to produce a proportionately high $C{O_2}$ signature because its carbon is kinetically trapped as carbon monoxide. The $C{O_2}$ production is artificially stunted not by a lack of initial carbon mass, but by the kinetic inability to oxidise the intermediate $CO$ species due to the aromatics inducing $O{H^ \bullet }$ radical deficiency. The variation in HRRs further exacerbates this inverse relationship. Across the entire fuel test matrix, a systematic reduction in HRR from 50.72 down to 41.5 kJ/sec corresponds to a uniform rise in relative $CO$ emissions and a general decline or flattening of relative $C{O_2}$ emissions. As the fuel mass flow is throttled back, flame temperatures within the combustor drop. The critical $CO + O{H^ \bullet } \to C{O_2} + {H^ \bullet }$ conversion is a highly exothermic process that is exponentially dependent on localised temperatures. At lower HRRs, thermal decay exponentially slows the rate of the final oxidation step, leading to an unavoidable accumulation of intermediate $CO$ before exhaust. Ultimately, the chemical kinetic limitations imposed by extreme carbon mass fractions and low operating temperatures manifest as poor combustion stability. Pure SAK experienced a complete flame blowout at the 43.81 and 41.5 kJ/sec conditions. The reduction in thermal energy, combined with the extreme activation energy required for aromatic ring dissociation and severe radical depletion, led to the final oxidation step quenching entirely, rendering the flame impossible to sustain. Paradoxically, combustion failures were also universally observed for the pure HEFA pathway and the low-aromatic HSAK4 and HSAK8 blends across all tested HRR conditions. While these highly isoparaffinic fuels exhibit optimal chemical kinetics for minimising $CO$ emissions, their physical properties likely result in suboptimal spray atomisation in this specific combustor. Coupled with a lack of dense aromatic structures required to anchor the flame front spatially, this resulted in complete combustion failure. This stark dichotomy demonstrates that while minimising the $100 - H{\% }$ carbon mass fraction perfectly optimises gaseous emission kinetics, the strategic reintroduction of carbon via aromatic compounds, as seen in the stable ASAK25 blend, is a strict thermophysical requirement to ensure flame stability across the full operating envelope of the gas turbine.

For HRR 50.72 kJ/sec, the order of CO and CO $_{2}$ formation in fuels is as follows:

CO: $\,ASAK4 \lt ATJ \lt J50H50 \lt ASAK8 \lt J50A50 \lt JP8 \lt ASAK17 \lt HSAK25 \lt JP5 \lt HSAK17 \lt ASAK25 \lt SAK$ .

CO $_{2}$ : $HSAK17 \lt J50A50 \lt SA{K^{\rm{*}}} \lt HSAK25 \lt JP8 \lt ASAK25 \lt ASAK4 \lt ASAK17 \lt ASAK8 \lt ATJ \lt J50H50 \lt JP5$ .

*The low CO $_{2}$ value for SAK is an artifact of severe incomplete combustion (kinetic trapping as CO), rather than true oxidation efficiency.

When evaluating empirical data solely by absolute emission mitigation, the pure isoparaffinic ATJ pathway and its ultra-low-aromatic derivative, ASAK4 (96% ATJ, 4% SAK), are unequivocally the highest-performing fuels in the evaluated fuel matrix. These low-carbon, hydrogen-rich blends consistently achieved the highest oxidation efficiency, yielding the lowest relative $CO$ across the testing matrix. For ASAK4, the highest HRR is 0.704 relative to JetA. Simultaneously, their higher net heat of combustion ensures that $C{O_2}$ remains systematically suppressed below the conventional Jet A baseline. Their maximised generation of the hydroxyl ( $O{H^ \bullet }$ ) radical provides rapid, complete carbon conversion without the intermediate incomplete oxidation of the carbon-dense aromatic fuels. However, when fuel performance is considered through the necessary lens of gas turbine operability, specifically the thermophysical requirements for stable flame anchoring and ASTM D7566 density compliance, the ASAK25 blend (75% ATJ, 25% SAK) appears as the optimal holistic candidate in terms of carbon emissions. While ASAK25 incurs a quantifiable, linear penalty in $CO$ production relative to pure ATJ due to its 25% aromatic fraction, it successfully restricts both $CO$ and $C{O_2}$ emissions to levels that are highly competitive with, or superior to, the conventional JP-5 and JP-8 benchmarks. Most critically, ASAK25 achieves this emission profile while demonstrating absolute combustion stability across the entire HRR range, successfully avoiding lean blowout, which was observed in both the highly paraffinic HEFA, HSAK4, HSAK8 and the SAK.

3.2.2 NOx emissions

The measured nitrogen-based emission characteristics across the evaluated HRR reveal a profound thermochemical trade-off inherent in highly refined sustainable aviation fuels. In terms of nitrogenous species, only $N{O_x}$ ( $NO$ and $N{O_2}$ ) emissions were observed, while no ${N_2}O$ emissions were detected across any of the tested fuel pathways. As indicated in the experimental methodology, the uncertainty bounds for NOx measurements are 3.80–4.70 PPM on a wet basis $N{O_x}$ emissions before correction for 16% oxygen, as shown in Fig. 10. Across all evaluated fuels, the data demonstrate a strict positive correlation between increasing HRR, combustor temperature and $N{O_x}$ formation. As shown in Fig. 10, as the combustor HRR increases from 41.5 to 50.72 kJ/sec, the baseline Jet A combustor temperature increases from 726.1 ${{\rm{\;}}^ \circ }$ C to 904.0 ${{\rm{\;}}^ \circ }$ C as shown in Fig. 6, yielding a corresponding increase in $N{O_x}$ from 13.90 to 16.73 PPM. This overarching formation trend is fundamentally governed by the extended Zeldovich mechanism, which dictates that the formation of thermal $N{O_x}$ is exponentially dependent on peak localised flame temperatures and residence time [Reference Miller and Bowman35–Reference Xing, An, Zhang and Kurose37]. As thermal energy within the primary zone scales with HRR, the high activation energy required to dissociate diatomic atmospheric nitrogen ( ${N_2}$ ) is more readily exceeded, accelerating the oxidation process.

Figure 10.

Dry NOx emissions corrected for 16% ${O_2}$ at various heat release rates of tested fuels.

Figure 10. Long description

A scatter plot representing the relationship between heat release rate and dry NOx emissions for various tested fuels. The horizontal axis represents the heat release rate in kilojoules per second (kJ/sec), ranging from 40 to 52. The vertical axis represents the dry NOx emissions in parts per million (PPM), ranging from 10 to 30. The plot includes data points for several fuels, each represented by different symbols and colors. The fuels include JetA, SAK, HSAK25, ASAK25, JP8, JP5, ATJ, J50A50, J50H50, HSAK17, ASAK4, ASAK8, and ASAK17. The data points show varying levels of NOx emissions at different heat release rates, with some clustering and patterns visible among the different fuels.

However, while the combustor temperatures remain remarkably similar across the fuel test matrix for a given HRR with Jet A and ATJ both registering approximately 904 ${{\rm{\;}}^ \circ }$ C and 905 ${{\rm{\;}}^ \circ }$ C at peak power, respectively, their specific $N{O_x}$ outputs diverge drastically. The pure synthetic pathway, ATJ, consistently produces the highest $N{O_x}$ emissions across the entire testing matrix, peaking at 23.44 PPM. This stark discrepancy between global temperature and pollutant production is driven by the fuel’s highly isoparaffinic chemical composition [Reference Xing, An, Zhang and Kurose37]. ATJ possesses the highest hydrogen content and a superior net heat of combustion. The rapid oxidation kinetics and high energy density of these simple branched alkanes create a localised, microscopic ‘hot spot’ within the primary combustion zone that may closely approach the fuel’s theoretical adiabatic flame temperature. It is within these intense, localised thermal regions that thermal $N{O_x}$ generation is profoundly accelerated, penalising the otherwise clean-burning synthetic fuel.

Conversely, the conventional jet fuels (Jet A, JP 8, JP 5) and the 100% aromatic SAK cluster at the lowest end of the emission spectrum, restricting $N{O_x}$ production to a range of approximately 14 to 16.7 PPM at the highest HRR. The empirical data from the fuel blends conclusively demonstrate that this suppression is chemically driven; with the introduction of aromatics, both the localised flame temperatures and the rapid oxidation kinetics of the base fuel are fundamentally altered. The substantial activation energy required to break the aromatic bonds acts as a chemical heat sink within the primary zone. By absorbing local thermal energy to drive endothermic aromatic ring dissociation, these cyclic structures effectively decelerate the overall oxidation rates and limit the peak localised temperatures. This thermal suppression limits the Zeldovich mechanism that requires intense heat to dissociate nitrogen.

Consequently, within the ASAK formulation series, $N{O_x}$ emissions scale inversely with the introduced aromatic fraction. The ultra-low aromatic ASAK4 blend closely mirrors the high emissions of the pure ATJ base (23.36 PPM at 50.72 kJ/sec). However, as the SAK is progressively added, the high-density ‘drop-in’ ASAK25 blend successfully suppresses the $N{O_x}$ signature down to 19.33 PPM at peak power. This mechanism underscores a fundamental compromise in modern aviation fuel design. While pure isoparaffinic SAFs eliminate intermediate carbon monoxide through rapid localised heat release, the strategic blending of aromatic molecules remains a critical requirement to temper localised flame temperatures, control oxidation rates and keep thermal $N{O_x}$ emissions within viable regulatory limits.

3.2.3 Total hydrocarbons (THC) emissions

Total unburned hydrocarbons (THC) measure macroscopic combustion inefficiency and represent fuel fragments escaping complete oxidation. For all fuels evaluated, data show an inverse, non-linear correlation between HRR and THC emissions, as shown in Fig. 11. As the HRR increases from 41.5 kJ/sec to 50.72 kJ/sec, the amounts of THC decrease. This change is due to physical and chemical improvements that occur at higher combustor temperatures. In general, the increased thermal energy speeds up heat transfer to the injected fuel, causing rapid fuel evaporation and reducing the droplet residence time. Higher temperatures also speed up hydrocarbon breakdown, so more vaporised fuel achieves complete oxidation within the combustor’s set residence time.

Figure 11.

Total unburned hydrocarbons relative to baseline fuel Jet A at various heat release rates of tested fuels.

When controlling for HRR, the THC profiles show a clear difference in combustion completeness, driven entirely by fuel chemistry. Highly refined SPKs have much better oxidation efficiency than conventional jet fuels. The pure ATJ and ASAK4 produce the lowest THC across the entire operating range, with respective relative THC fractions of 0.251 and 0.237 at the highest HRR. Hydrocarbon group analysis shows why: ATJ has 99.8% isoparaffins. When injected, these branched aliphatic structures have low C–C bond dissociation energies, so they break down quickly by thermal cracking and beta-scission. This makes the original hydrocarbon mass turn almost instantly into a reactive pool of small alkyl radicals. As a result, the fuel bypasses slow initial breakdown steps, leaving little unburned in the exhaust.

Conversely, adding aromatics significantly constrains chemical reaction rates and increases unburned hydrocarbon levels. Aromatic benzene rings require much higher activation energy for breaking the aromatic rings than straight-chain or branched paraffins. As 100% aromatic SAK blends into pure ATJ, combustion completeness drops in direct proportion. At peak power, increasing SAK moves the relative THC from 0.237 for ASAK4 to 0.384 for ASAK25. Since more thermal energy is needed to break these aromatic structures, their degraded derivatives are more likely to survive the primary zone. The extreme of this kinetic resistance is observed in the pure SAK pathway, where slow ring breakdown starves the flame of reactive intermediates, leading to blowout at low thermal conditions.

JP 8 and JP 5 emit much higher THC emissions than synthetic blends, showing an important link between how these fuels act physically and chemically. Although JP 8’s aromatic content is lower (13.56%) than that of high blend synthetics like ASAK25, it still produces the highest relative THC among stable fuels. This is due to the highly mixed composition of conventional fuels, which includes n-paraffins, complex cycloparaffins and heavier aromatics. By contrast, highly synthesised SPKs have a narrow boiling point range, while conventional fuels have a broad, multicomponent boiling range. The heavier components in conventional fuels vaporise slowly, allowing high-boiling-point droplets to linger and potentially escape the primary combustion zone or persist into the quenching zone, leading to higher baseline THC emissions. Also, J50A50 and J50H50 show consistently similar behaviour across the HRR below the baseline Jet fuel.

This indicates fuels rich in branched isoparaffins maximise primary zone efficiency. They bypass kinetic bottlenecks and almost eliminate unburned hydrocarbon fragments across all conditions. In contrast, the structural resilience of aromatics works as a kinetic sink. This penalty is directly proportional to the volumetric aromatic fraction. Thermal energy in the combustor governs combustion completeness, making low power conditions critical for controlling unburned hydrocarbon emissions. Ultra-low aromatic blends are offering the highest overall oxidation efficiency.

3.2.4 Particulate matter and soot emissions

The PM emissions for the evaluated fuel test matrix were compared at the peak HRR of 50.72 kJ/sec. As demonstrated in Fig. 6, the combustor temperature profiles for all fuels exhibit minimal variation at this specific HRR compared to other HRRs, establishing it as the ideal baseline for an isolated comparison of emission characteristics and thus minimising any interference from thermal fluctuations. PM emissions are rigorously quantified through normalised particle number-size distributions in Fig. 12, aggregated number and mass concentrations in Fig. 13 and the optically derived FSN in Fig. 14. As illustrated in Fig. 12, the particulate profiles for all tested fuels exhibit a log-normal distribution heavily dominated by ultrafine ‘nucleation mode’ particles, with particle diameters generally peaking between 10 nm and 20 nm. The conventional Jet A baseline produces the most aggressive nucleation profile, yielding the highest peak amplitude and an overwhelming total particle number concentration of 3,179,670/scm ${^3}$ in Fig. 13. In stark contrast, highly refined isoparaffinic synthetic fuels drastically suppress this nucleation spike. The pure ATJ pathway and the ultra-low aromatic ASAK4 blend restrict their total number concentrations to 975,187/scm ${^3}$ and 932,266/scm ${^3}$ , respectively. This reduction demonstrates the superior, clean-burning nature of highly branched aliphatic chains, which crack efficiently into light gaseous intermediates rather than condensing into solid particulate nuclei.

Figure 12.

Particulate matter normalised particle number–size distribution of various tested fuels at HRR 50.72 kJ/sec.

Figure 12. Long description

Panel A: A line graph showing the particle matter emissions for various tested fuels at a heat release rate of 50.72 kJ per second. The horizontal axis represents the particle diameter in nanometers, ranging from 10 to 600 nanometers. The vertical axis represents the normalized particle number size distribution in units of D N per D log D p, ranging from 0 to 7 times 10 to the power of 6. Multiple lines represent different fuels, each with distinct colors and line styles. The lines show various trends, with some peaking at different particle diameters and others showing more gradual changes.

Figure 13.

Particulate matter number–mass concentrations of various tested fuels relative to fuel aromatic content at HRR 50.72 kJ/sec.

Figure 13. Long description

Panel A: A bar graph comparing particulate matter number-mass concentrations of various tested fuels relative to fuel aromatic content. The horizontal axis represents different fuel types, including Jet A, JP 8, JP 5, ATJ, SAK, J50A50, J50H50, HSAK17, HSAK25, ASAK4, ASAK8, ASAK17, and ASAK25. The vertical axis on the left represents PM number concentration in units of number per cubic centimeter, ranging from 0 to 3.5 times 10 to the power of 6. The vertical axis on the right represents PM mass concentration in milligrams per cubic centimeter, ranging from 0 to 160. Each bar represents a different fuel type, with colors indicating the aromatic content percentage, ranging from blue (low aromatic content) to red (high aromatic content). The PM mass concentration is also plotted as a line graph with circular markers. Notable trends include higher PM number concentrations for Jet A and SAK fuels, and lower concentrations for JP 5 and ASAK4 fuels. The PM mass concentration line graph shows varying levels across different fuel types, with peaks at Jet A and SAK fuels.

Figure 14.

Filter smoke number for various tested fuels relative to their aromatic content at HRR 50.72 kJ/sec.

While particle number concentrations highlight primary nucleation phenomena, the overall PM mass concentration in Fig. 13 and FSN Fig. 14 serve as direct, corroborating metrics of total exhaust soot concentration. Both exhibit a profound, direct correlation with fuel aromaticity and cycloparaffinic content. Pure ATJ, consisting of 99.8% isoparaffins and a negligible 0.2% aromatics, produces the lowest soot signature of the suite, yielding a PM mass of just 12.07 mg/sm ${^3}$ and a baseline FSN near 0.2. Conversely, the introduction of aromatics results in a substantial increase in soot formation. Aromatic ring structures do not easily undergo beta-scission into smaller alkyl radicals [Reference Sarathy, Javed, Karsenty, Heufer, Wang, Park, Elwardany, Farooq, Westbrook, Pitz, Oehlschlaeger, Dayma, Curran and Dagaut33]; instead, they act as direct chemical precursors to PAHs [Reference Richter and Howard38]. Through hydrogen abstraction and sequential carbon addition, PAHs rapidly grow and condense into solid primary soot particles [Reference Calcote and Manos39]. This process fundamentally links the aromaticity of the fuel to the total soot mass concentration in the exhaust. Consequently, the 100% aromatic SAK produces an extreme PM mass of 154.92 mg/sm ${^3}$ and a corresponding FSN spike exceeding 2.6. This PAH-driven mechanism also explains the inherently elevated PM mass in complex conventional distillates like JP-5, which yields 98.41 mg/sm ${^3}$ due to its dense matrix of 20.36% aromatics and 47.37% cycloparaffins.

SAK data expose a critical physical divergence between particle number and particle mass, highlighting the mechanisms of soot agglomeration. Despite generating extreme mass and FSN measurements, SAK exhibits one of the lowest total particle number concentrations at just 749,841/scm ${^3}$ . Observing Fig. 12, the SAK size distribution is uniquely broad, flattened and shifted towards larger aerodynamic diameters compared to the sharp, narrow nucleation spike of Jet A. In highly aromatic, carbon-dense primary combustion zones, an overabundance of PAH precursors leads to massive initial soot inception [Reference Calcote and Manos39]. Because the spatial concentration of these primary particles is exceptionally high, their collision frequency increases exponentially. This causes the tiny nucleation particles to rapidly coagulate and agglomerate into much larger, heavier fractal aggregates before exiting the combustor. Consequently, the exhaust contains fewer individual particles, but those surviving aggregates are significantly denser, yielding the extreme mass and optical FSN readings.

Fuel blends confirm that PM mass and FSN scale predictably with aromatic addition while demonstrating the immediate mitigation benefits of SAF dilution. The J50A50 and J50H50 semi-synthetic formulations are a 50/50 volumetric blend of Jet A with ATJ and HEFA, respectively, effectively halving the complex aromatic matrix of the conventional baseline. This stoichiometric dilution proportionately disrupts early-stage soot inception, slashing the massive Jet A nucleation spike to 1,551,173/scm ${^3}$ for J50A50, significantly reducing the mass to 30.70 mg/sm ${^3}$ and dropping the FSN to approximately 0.45 and 0.3, respectively. Within the ‘drop-in’ formulation series, increasing the SAK fraction from 4% to 25% drives the PM mass from 17.98 mg/sm ${^3}$ (ASAK4) to 57.01 mg/sm ${^3}$ (ASAK25), with FSN scaling proportionally from roughly 0.25 to 0.85. A parallel PAH-driven trend is observed in the HEFA-based blends, where increasing aromaticity from 18.77% in HSAK17 to 27.33% in HSAK25 drives the total PM mass from 41.46 mg/sm ${^3}$ to 48.67 mg/sm ${^3}$ and pushes smoke numbers upward. Ultimately, while blends like ASAK25 and HSAK25 incur a measurable soot penalty relative to pure SAFs to satisfy hardware density requirements, they successfully bypass the extreme agglomeration regime of pure SAK and maintain ultrafine particle counts well below the conventional Jet A baseline.