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Context. It is known that the Canary Islands are normally affected by dominant winds flowing from north-northeast, that in some meteorological conditions, can transport sand from the Sahara desert at high altitude. The dust may affect the efficiency of the telescopes and decrease the transparency of the sky.

Aims. To maximize the scientific return of the telescopes located at the Observatorio del Roque de Los Muchachos (ORM), we present an analysis of the atmospheric dust content and its effects on astronomical observations. Than B, V and I dust aerosol astronomical extinction are derived.

Methods. Using a 5-year series database of data taken from a dust monitor located inside the Telescopio Nazionale Galileo (TNG) dome, we computed mean hourly and daily values of the dust content as measured with a four-channel dust monitor.

Results. We detected particles of 0.3, 0.5, 1.0, and 5.0 mu m. Furthermore, using a power law we derived the content of 10.0 mu m particles. We found a typical local dust concentration ranging from 3 x 10(6) particles per cubic metre at 0.3 mu m, to 10(3) at 5.0 mu m and 10 at 10.0 mu m, increasing up to 3 orders of magnitudes during the dust storms, with a relatively higher increase of 1.0, 5.0, and 10.0 mu m particles. The number of local dust storm events is the same in the local winter and summer, but the average background and storm-related increases in the dust concentration in summer are significantly higher than in winter. In a uniform approximation, during the dust storms, an average height of the dust layer of 2.5 km above the telescope is inferred.

Conclusions. During the sand storms, La Palma Island is affected by an almost uniform layer extending up to 5 km above the sea level. The visible extinction is dominated by particles at 1.0, 5.0 and 10.0 mu m. In agreement with the results from Carlsberg Automatic Meridian Circle (CAMC), we find a typical extinction of about 0.2 mag airmass(-1) during dust storms.