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. High altitude stations are often emphasized as free troposphericmeasuring sites but they remain influenced by atmospheric boundary layer(ABL) air masses due to convective transport processes. The local andmeso-scale topographical features around the station are involved in theconvective boundary layer development and in the formation of thermallyinduced winds leading to ABL air lifting. The station altitude alone is nota sufficient parameter to characterize the ABL influence. In this study, atopography analysis is performed allowing calculation of a newly definedindex called ABL-TopoIndex. The ABL-TopoIndex is constructed in order tocorrelate with the ABL influence at the high altitude stations and long-termaerosol time series are used to assess its validity. Topography data fromthe global digital elevation model GTopo30 were used to calculate fiveparameters for 43 high and 3 middle altitude stations situated on fivecontinents. The geometric mean of these five parameters determines a topographybased index called ABL-TopoIndex, which can be used to rank the high altitudestations as a function of the ABL influence. To construct the ABL-TopoIndex,we rely on the criteria that the ABL influence will be low if the station isone of the highest points in the mountainous massif, if there is a largealtitude difference between the station and the valleys or high plains, ifthe slopes around the station are steep, and finally if the inverse drainagebasin potentially reflecting the source area for thermally lifted pollutantsto reach the site is small. All stations on volcanic islands exhibit a lowABL-TopoIndex, whereas stations in the Himalayas and the Tibetan Plateau havehigh ABL-TopoIndex values. Spearman's rank correlation between aerosoloptical properties and number concentration from 28 stations and theABL-TopoIndex, the altitude and the latitude are used to validate thistopographical approach. Statistically significant (SS) correlations arefound between the 5th and 50th percentiles of all aerosol parameters and theABL-TopoIndex, whereas no SS correlation is found with the stationaltitude. The diurnal cycles of aerosol parameters seem to be best explainedby the station latitude although a SS correlation is found between theamplitude of the diurnal cycles of the absorption coefficient and theABL-TopoIndex.

Identification of topographic features influencing aerosol observations at high altitude stations