Heterogeneous sulfate production in an urban fog

Heterogeneous production of sulfate in an urban fog has been investigated using data collected during the SCAQS program in the Los Angeles area, for the period of 10–11 December 1987. Fog was observed during the night of 10 December and the early morning hours of 11 December near the coast of southern California. Measurements at several sites (Hawthorne, downtown Los Angeles, etc.) indicated a significant increase in sulfate concentration during the afternoon of 11 December. Trajectory analysis suggests that these high sulfate concentrations were associated with the arrival at the receptor sites of air parcels that passed through the fog layer the previous night. To quantify the contribution of aqueous-phase processes to the above sulfate levels, a detailed trajectory model was employed to simulate the gas-phase processes during that episode. The model, using the available information about SO₂ and sulfate emissions, initial conditions and meteorology, successfully explained the sulfate levels in air parcels that did not pass through the fog layer, but underestimated by as much as 2.5 the sulfate levels of the trajectories through the fog. Sensitivity/uncertainty analysis indicated that the presence of sulfate beyond that attributable to gasphase chemistry (around 10μg⁻³) cannot be attributed to uncertainties in the model parameters (e.g. initial conditions, emissions, mixing heights, deposition velocities). The episode was then simulated using a full gas- and aqueous-phase chemistry model and the analysis indicated that heterogeneous sulfate formation in fog droplets under the conditions of the episode can indeed explain the observed sulfate.