We measured the concentrations of aerosol methanesulfonate (MSA) and non-sea salt sulfate (NSS) in the remote Pacific marine boundary layer (MBL) at Christmas Island (157¿ W, 2¿ N) in July and August of 1994. The project-average MSA displayed a distinct diurnal variation, decreasing to 8.6 ppt at sunrise and increasing to 12.1 ppt by sunset. The average NSS diurnal variation ranged from 196 ppt at sunrise to 235 ppt at sunset. Large-particle dry deposition may account for 10--20% of the observed nighttime decrease, with entrainment of cleaner free tropospheric air responsible for the rest. The entrainment velocity inferred from the nighttime decrease averaged 0.5¿0.2 cm/s. A simple model suggests that NSS and MSA were produced at rates of about 74 and 6 ppt per day, respectively. Between 30 and 40% of the daily dimethylsulfide (DMS) flux forms NSS and 3% forms MSA. ¿ American Geophysical Union 1996