<p>Mechanism of spray breakup in a swirl coaxial aerated injector (SCAI) that couples internal effervescent mixing with a swirl stream is reported. Shadowgraph experiments span liquid flow rate, injection pressure, gas-to-liquid ratio (GLR) and swirl momentum ratio (MR) to map how parametric variations influence the near-field atomization pathway. The images reveal a consistent sequence of mechanisms—intact core, interfacial waves, ligament formation and fine fragment clouds—whose location and intensity shift systematically with different operating points. Increasing liquid injection pressure tightens the core and advances breakup toward the lip; increasing GLR promotes early stripping, broad liquid-film shedding and a wider droplet-size spectrum; increasing liquid flow rate (at fixed aeration) lengthens the intact region and delays fragmentation; adding coaxial swirl shortens the intact length, widens the cone and redistributes fragments into an annular shell. To quantify these changes, we apply snapshot proper orthogonal decomposition (POD) to large image datasets captured at each operating point in order to isolate the coherent structures. Four spectral modes were sufficient for every test case. These findings provide a reduced-order description that links injector settings to breakup physics and serve as guidance for injector spray design.</p>