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

A Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) was used to evaluate the hygroscopic properties of aerosol particles measured at the Puy de Dôme research station in central France from September 2008 to December 2012. This high-altitude site is ideally situated to allow for both the upper part of the planetary boundary layer and the lower free troposphere to be sampled. The aim of the study is to investigate both the influence of year-to-year, seasonal, and diurnal cycles, as well as the influence of air mass type on particle hygroscopicity and mixing state. Results show that particle hygroscopicity increases with particle size and depends both on air mass type and on season. Average growth factor values are lowest in winter (1.21±0.13, 1.23±0.18 and 1.38±0.25 for 25, 50 and 165nm particles, respectively) and highest in autumn (1.27±0.11, 1.32±0.12 and 1.49±0.15 for 25, 50 and 165nm particles, respectively). Particles are generally more hygroscopic at night than during the day. The seasonal and diurnal variations are likely to be strongly in fluenced by boundary layer dynamics. Furthermore, particles originating from oceanic and continental regions tend to be more hygroscopic than those measured in African and local air masses. The high hygroscopicity of marine aerosol may be explained by large proportions of inorganic aerosol and sea salts, and it is speculated that continental particles are more hygroscopic than local and African ones due to ageing of fresh combustion aerosol. Aerosol measured at the Puy de Dôme display a high degree of external mixing, and hygroscopic growth spectra can be divided into three different hygroscopic modes: a less hygroscopic mode (GF< 1.3), a hygroscopic mode (GF 1.3-1.7) and a more hygroscopic mode (GF> 1.7). The majority of particles measured can be classified 25 as being in either the less hygroscopic mode or the hygroscopic mode, and only few of them have more hygroscopic properties. The degree of external mixing, evaluated as the fraction of time when the aerosol is found with two or more populations with different hygroscopic properties, is found to increase with particle size (average yearly values are 22, 33 and 49% for 25, 50, and 165nm particles, respectively). The degree of external mixing is more sensitive to season than to air mass type, and it is higher in the cold seasons than in the warm seasons. This study gathers the results from one of the longest data sets of hygroscopic growth factor measurements to date, allowing a statistically relevant hygroscopic growth parameterization to be determined as a function of both air mass type and season. [ABSTRACT FROM AUTHOR]