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

Aerosol number concentrations and size distributions from 3 nm to 20 µm diameter were measured at the Mace Head Atmospheric Research Station, Co. Galway, Ireland, a coastal site on the eastern seaboard of the north Atlantic Ocean. Both on and offline size resolved aerosol composition measurements were also made using an Aerodyne Aerosol Mass Spectrometer (AMS) and ion chromatographic analysis of daily samples collected using a Micro-Orifice Uniform Deposit Impactor (MOUDI). Particle number concentrations, size distributions and AMS measurements were determined at 7 and 22 m above ground level to investigate local effects on the aerosol size distribution induced by the tidal zone. During periods of new particle formation ultrafine particle number concentrations are large and variable, however, outside these periods no variability in particle number was observed at any size, nor was the particle composition variable. Analysis of particle size distributions show that within each air mass observed particle number concentrations were very consistent. During anticyclonic periods and conditions of continental outflow Aitken and accumulation mode were enhanced by a factor of 5 compared to the marine sector, whilst coarse mode particles were enhanced during westerly conditions. Baseline marine conditions were rarely met at Mace Head during NAMBLEX and high wind speeds were observed for brief periods only. Loss rates of gaseous species to aerosol surfaces were calculated for a range of uptake coefficients. Even when the accommodation coefficient is unity, lifetimes of less than 100 s were never observed and rarely were lifetimes less than 500 s. Diffusional limitation 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 accommodation coefficients of around 0.03 and hence we calculate lifetimes due to loss to particle surfaces of 2000 s or greater. Aerosol composition measurements using the AMS show accumulation mass modes of acidified sulphate and organic material, both of which have the same shape and are centred at around 350 nm vacuum aerodynamic diameter, implying an internal mixture. The organic and sulphate are approximately equally important, 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. Even in marine conditions a background concentration of between 0.5 and 1 µg m−3 of both organic and sulphate was observed. 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. However, in an accompanying paper (Dall'Osto et al., 2005) periods of organic dominated aerosol particles were also observed and could be separated from the aged continental aerosol. The coarse mode was composed of sea salt and showed significant displacement of chloride by nitrate and to a lesser extent sulphate in polluted conditions.