Hygroscopic properties of aerosol particles in the northeastern Atlantic during ACE-2

Measurements of the hygroscopic properties of sub-micrometer atmospheric ae rosol particles were performed with hygroscopic tandem differential mobilit y analysers (H-TDMA) at 5 sites in the subtropical north-eastern Atlantic d uring the second Aerosol Characterization Experiment (ACE-2) from 16 June t o 25 July 1997. Four of the sites were in the marine boundary layer and one was, at least occasionally, in the lower free troposphere. The hygroscopic diameter growth factors of individual aerosol particles in the dry particl e diameter range 10-440 nm were generally measured for changes in relative humidity (RH) from <10% to 90%. In the marine boundary layer. growth factor s at 90% Rn were dependent on location, air mass type and particle size. Th e data was dominated by a unimodal growth distribution of more-hygroscopic particles, although a bimodal growth distribution including less-hygroscopi c particles was observed at times, most often in the more polluted air mass es. In clean marine air masses the more-hygroscopic growth factors ranged f rom about 1.6 to 1.8 with a consistent increase in growth factor with incre asing particle size. There was also a tendency toward higher growth factors as sodium to sulphate molar ratio increased with increasing sea-salt contr ibution at higher wind speeds. During outbreaks of European pollution in th e ACE-2 region, the growth Factors of the largest particles were reduced, b ut only slightly. Growth factors at all sizes in both clean and polluted ai r masses were markedly lower at the Sagres, Portugal site due to more proxi mate continental influences. The frequency of occurrence of less-hygroscopi c particles with a growth factor of ca. 1.15 was greatest during polluted c onditions at Sagres. The free tropospheric 50 nm particles were predominate ly less-hygroscopic, with an intermediate growth Factor of 1.4, but more-hy groscopic particles with growth factors of about 1.6 were also frequent. Wh ile these particles probably originate from within the marine boundary laye r, the less-hygroscopic particles are probably more characteristic of lower free tropospheric air masses. For those occasions when measurements were m ade at 90% and an intermediate 60% or 70% RH, the growth factor G(RH) of th e more-hygroscopic particles could be modelled empirically by a power law e xpression. For the ubiquitous more-hygroscopic particles, the expressions G (RH) = (1 - RH/100)(-0.210) for 50 nm Aitken mode particles and G(RH) = (1 - RH/100)(-0.233) for 166 nm accumulation mode particles are recommended fo r clean marine air masses in the north-eastern Atlantic within the range 0 < RH < 95%, and for wind speeds for which the local sea-salt production is small (<ca. 8 m s(-1)).