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A Hygroscopicity Tandem Differential Mobility Analyser (HTDMA) was used toevaluate the hygroscopic properties of aerosol particles measured at the Puyde Dôme research station in central France, periodically from September2008 to January 2010, and almost continuously from October 2010 to December2012. This high-altitude site is ideally situated to allow for both theupper part of the planetary boundary layer and the lower free troposphere tobe sampled. The aim of the study is to investigate both the influence ofyear-to-year, seasonal and diurnal cycles, as well as the influence of airmass type on particle hygroscopicity and mixing state.  Results show that particle hygroscopicity increases with particle size anddepends both on air mass type and on season. Average growth factor values,GFs, are lowest in winter (1.21 ± 0.13, 1.23 ± 0.18 and1.38 ± 0.25 for 25, 50 and 165 nm particles, respectively) and highestin autumn (1.27 ± 0.11, 1.32 ± 0.12 and 1.49 ± 0.15 for 25,50 and 165 nm particles, respectively). Particles are generally morehygroscopic at night than during the day. The seasonal and diurnal variationsare likely to be strongly influenced by boundary layer dynamics. Furthermore,particles originating from oceanic and continental regions tend to be morehygroscopic than those measured in African and local air masses. The highhygroscopicity of oceanic aerosol can be explained by large proportions ofinorganic aerosol and sea salts.  Aerosols measured at the Puy de Dôme display a high degree of externalmixing, and hygroscopic growth spectra can be divided into three differenthygroscopic modes: a less-hygroscopic mode (GF < 1.3), ahygroscopic mode (GF~1.3–1.7) and a more-hygroscopic mode(GF > 1.7). The majority of particles measured can be classifiedas being in either the less-hygroscopic mode or the hygroscopic mode, andonly few of them have more-hygroscopic properties. The degree of externalmixing, evaluated as the fraction of time when the aerosol is found with twoor more aerosol populations with different hygroscopic properties, increaseswith particle size (average yearly values are 20, 28 and 45 {%} for 25, 50,and 165 nm particles, respectively). The degree of external mixing is moresensitive to season than to air mass type, and it is higher in the coldseasons than in the warm seasons.  With more than two years of nearly continuous measurements, this studygathers the results from one of the longest data sets of hygroscopic growthfactor measurements to date, allowing a statistically relevant hygroscopicgrowth parameterization to be determined as a function of both air mass typeand season

Hygroscopic properties and mixing state of aerosol measured at the high-altitude site Puy de Dôme (1465 m a.s.l.), France