Ship-plume sulfur chemistry: ITCT 2K2 case study

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 SO2 and H2SO4 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 (pHss) was ≤~6.5. The ship-plume equivalent lifetimes of SO2 (τ(eq)(SO(2))) were also estimated/investigated for this particular ship-plume case. The magnitudes of τ(eq)(SO(2)) were found to be controlled by two main factors: (i) the mixing ratios of in-plume hydroxyl radicals (OH) and (ii) pHss. The former is governed primarily by stability conditions of the marine boundary layer (MBL), when the ship NOx emission rate is fixed. The latter determines if the heterogeneous oxidation of dissolved SO2 occurs via reaction with hydrogen peroxide (H2O2, when pHss<6.5) or with ozone (O3, when pHss>6.5). According to the multiple ship-plume photochemical/dynamic model simulations, the estimated τ(eq)(SO(2)) over the entire ship plumes ranged from 10.32 to 14.32 h under moderately stable (E) to stable (F) MBL conditions. These values were clearly shorter than the background SO2 lifetime (τ(b)(SO(2))) of 15.18-23.20 h. In contrast, τ(eq)(SO(2)) was estimated to be 0.33 h when the pHss remained at ~8.0 (a rather unlikely case). In addition, the SO2 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 pHss clearly showed a shift in the dominant SO2 loss processes from heterogeneous SO2 conversion (when pHss>~6.5) to the gas-phase oxidation of SO2 by OH (when pHss<~6.5).