The effect of NO3 uptake at night into marine aerosol and cloud droplets on (1) sulfur oxidation, (2) NO3 ambient concentrations, and (3) NO3 atmospheric lifetime is examined using a simple chemical kinetics model. It is shown that the atmospheric lifetime toward multiphase removal of NO3 is <10 min in presence of cloud droplets. We also show that at low mixing ratios of H2O2 (<150 pmol/mol), NO3 can enhance sulfur oxidation in clouds via a catalytic cycle with a chain length of ~8. Finally, in presence of sea-salt aerosol, the uptake of NO3 leads to low ambient concentrations of NO3, efficient S(IV) oxidation, and possibly, to release of halogens from the aerosol. The results of this study imply that the multiphase reaction of NO3 may be important in the remote marine boundary layer. ¿ 1998 American Geophysical Union