Nighttime observations of NO3, O3, NO2, and aerosol distributions were made at Helgoland in October 1996. Frequently polluted air was observed. Air transport from the mainland over the sea takes several hours, and lifetimes for NO3 and N2O5 can be determined. Those lifetimes are compared with aerosol size distributions measured simultaneously. The lifetimes of N2O5 show clear correlations with total aerosol surface and mean aerosol size, indicating that the most likely sink of NO3 is hydrolysis of N2O5 on wet aerosols of varying composition. The range of possible uptake coefficients is calculated from the measurements and compared with literature data. ¿ 2000 American Geophysical Union