Figure 1: Stacked plots of accumulated PM₂.₅ mass and fractional contribution (top) and scatter plots of emission rates (bottom) for different combustion phases alongside averaged values for all tests under different secondary air configurations. Note: The char-burn phase for test repetitions 3 and 4 for the 30 SLPM secondary air configuration was not included due to data acquisition and control system failure. The phase-specific contributions for these tests were evaluated using two burn phases only. Figure 2: Fuel burn rate, residual oxygen, and peak local temperature during low and high PM₂.₅ emission events throughout the main burn phase for different secondary air flow configurations. Different color markers represent different secondary airflow configurations, and their size in high emissions panels (B and D) qualitatively indicates the magnitude of the PM₂.₅ emission rate. The line marks the tentative 80th/20th percentile temperature threshold.