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Chemical composition and hygroscopicity closure of marine aerosol in high time resolution has not been yet achieved because of the difficulty in measuring refractory sea-salt concentration in near-real time. In this study, attempts were made to achieve a closure for marine aerosol based on a humidified tandem differential mobility analyser (HTDMA) and a high-resolution time-of-flight aerosol mass spectrometer (AMS) for wintertime aerosol at Mace Head, Ireland. The aerosol hygroscopicity was examined as a growth factor (GF) at 90 % relative humidity (RH). The corresponding GFs of 35, 50, 75, 110 and 165 nm particles were 1.54 ± 0.26, 1.60 ± 0.29, 1.66 ± 0.31, 1.72 ± 0.29 and 1.78 ± 0.30 (mean ± standard deviation), respectively. Two contrasting air masses (continental and marine) were selected to study the temporal variation in hygroscopicity and the results demonstrated a clear diurnal pattern in continental air masses, while no diurnal pattern was found in marine air masses. In addition, the winter time aerosol was observed to be largely externally mixed in both contrasting air masses. Concurrent high time resolution PM1 (particulate matter < 1 μm) chemical composition by combined AMS and MAAP measurements comprising of organic matter, non-sea-salt sulphate, nitrate, ammonium, sea-salt and black carbon (BC) were used in predicting aerosol hygroscopicity using the Zdanovskii-Stokes-Robinson (ZSR) mixing rule. A generally good agreement (r2 = 0.824, slope = 1.02) was found between HTDMA measured growth factor (GF_HDTMA) of 165 nm particles and AMS+MAAP bulk chemical composition derived growth factor (GF_AMS). Over 95 % of the estimated GF exhibited less than 10 % deviation for the whole dataset and the deviation was mostly attributed to the neglected mixing state as a result of bulk PM1 composition. [ABSTRACT FROM AUTHOR]