The ship-plume sulfur chemistry was investigated for the ITCT 2K2 (Intercontinental Transport and Chemical Transformation 2002) ship-plume experiment, using the ship-plume photochemical/dynamic model developed in this study. In order to evaluate the performance of the model, the model-predicted mixing ratios of SO 2 and H 2 SO 4 were compared with those observed. From these comparisons, it was found that the model-predicted levels were in reasonable agreements with those observed (0.56≤R≤0.71), when the pH of sea-salt particles (pH ss ) was ≤~6.5. The ship-plume equivalent lifetimes of SO 2 τSO2eq were also estimated/investigated for this particular ship-plume case. The magnitudes of τSO2eq were found to be controlled by two main factors: (i) the mixing ratios of in-plume hydroxyl radicals (OH) and (ii) pH ss . The former is governed primarily by stability conditions of the marine boundary layer (MBL), when the ship NO x emission rate is fixed. The latter determines if the heterogeneous oxidation of dissolved SO 2 occurs via reaction with hydrogen peroxide (H 2 O 2 , when pH ss <6.5) or with ozone (O 3 , when pH ss >6.5). According to the multiple ship-plume photochemical/dynamic model simulations, the estimated τSO2eq over the entire ship plumes ranged from 10.32 to 14.32h under moderately stable (E) to stable (F) MBL conditions. These values were clearly shorter than the background SO 2 lifetime τSO2b of 15.18–23.20h. In contrast, τSO2eq was estimated to be 0.33h when the pH ss remained at ~8.0 (a rather unlikely case). In addition, the SO 2 loss budget was further analyzed to estimate the influences of the two main factors on the ship-plume sulfur chemistry. The changes in the loss budget with pH ss clearly showed a shift in the dominant SO 2 loss processes from heterogeneous SO 2 conversion (when pH ss >~6.5) to the gas-phase oxidation of SO 2 by OH (when pH ss <~6.5).