Effect of marine boundary layer clouds on tropospheric chemistry as analyzed in a regional chemistry transport model

[1] A regional chemistry transport model is used to investigate whether clouds affect tropospheric species concentrations, particularly ozone. This study focuses on boundary layer clouds in a 20degrees by 20degrees region centered over Hawaii. We find that O-3 is depleted by 6% in the cloud-capped boundary layer during a 120-hour integration period. The O-3 loss rate is calculated to be 320 pptv d(-1) for the region, which was dominated by low NOx (< 10 pptv). Aqueous chemistry and cloud radiative effects contribute nearly equally to the O-3 depletion at the pH of 4.5 used in this simulation. When higher pH values are considered, more O-3 depletion occurs. For example, at pH of 5.5, O-3 is depleted by 17% during the 120-hour integration. At pH > 4.5 the superoxide ion is favored, and therefore reaction of superoxide and ozone in the cloud drops increases. We discuss the importance of including transport and physical processes (e. g., deposition) on the O-3 loss rate. We find that O-3-rich air is transported into the marine boundary layer via subsidence and diffusion, allowing for more O-3 to be depleted in an absolute sense compared to when transport and physical processes do not occur. However, the relative change in O-3 is smaller in the boundary layer when transport and physical processes are included because large eddies, which are represented by vertical diffusion in the model, maintain relatively high background O-3 levels in the boundary layer.