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A suite of aerosol physical and chemical measure- ments were made at the Mace Head Atmospheric Research Station, Co. Galway, Ireland, a coastal site on the eastern seaboard of the north Atlantic Ocean during NAMBLEX. The data have been used in this paper to show that over a wide range of aerosol sizes there is no impact of the inter- tidal zone or the surf zone on measurements made at 7 m above ground level or higher. During the measurement pe- riod a range of air mass types were observed. During anticy- clonic periods and conditions of continental outflow Aitken and accumulation mode were enhanced by a factor of 5 com- pared to the marine sector, whilst coarse mode particles were enhanced during westerly conditions. Baseline marine con- ditions were rarely met at Mace Head during NAMBLEX and high wind speeds were observed for brief periods only. The NAMBLEX experiment focussed on a detailed assess- ment of photochemistry in the marine environment, investi- gating the linkage between the HOx and the halogen radical cycles. Heterogeneous losses are important in both these cy- cles. In this paper loss rates of gaseous species to aerosol surfaces were calculated for a range of uptake coefficients. Even when the accommodation coefficient is unity, lifetimes due to heterogeneous loss of less than 10 s were never ob- served and rarely were they less than 500 s. Diffusional limi- tation to mass transfer is important in most conditions as the coarse mode is always significant. We calculate a minimum overestimate of 50% in the loss rate if this is neglected and so it should always be considered when calculating loss rates of gaseous species to particle surfaces. HO2 and HOI have ac- commodation coefficients of around 0.03 and hence we cal- culate lifetimes due to loss to particle surfaces of 2000 s or greater under the conditions experienced during NAMBLEX. Aerosol composition data collected during this experiment provide representative information on the input aerosol char- acteristics to western Europe. During NAMBLEX the sub- micron aerosol was predominately acidified sulphate and or- ganic material, which was most likely internally mixed. The remaining accumulation mode aerosol was sea salt. The or- ganic and sulphate fractions were approximately equally im- portant, though the mass ratio varies considerably between air masses. Mass spectral fingerprints of the organic fraction in polluted conditions are similar to those observed at other locations that are characterised by aged continental aerosol. In marine conditions, the background input of both sulphate and organic aerosol into Europe was observed to be between 0.5 and 1µg m 3 . Key differences in the mass spectra were observed during the few clean periods but were insufficient to ascertain whether these changes reflect differences in the source fingerprint of the organic aerosol. The coarse mode was composed of sea salt and showed significant displace- ment of chloride by nitrate and to a lesser extent sulphate in polluted conditions.

Chemical and physical characteristics of aerosol particles at a remote coastal location, Mace Head, Ireland, during NAMBLEX