Instrument assessment and atmospheric phenomena in relation to the night sky brightness time series

Several networks of Sky Quality Meters show long-term trends in measuring the night sky brightness. Causes of this variation could be ageing of the instruments and variations of the sky emission. The data from Ekar Observatory (Italy) are here considered. No significant ageing effect was detected on 9-year-old housing windows. A 9-year-old IR blocking filter shows a variation of its spectral transmittance up to 80%. No trend on the SQM output was found due to only the variation of the spectral distribution of the night sky emission. Together with ageing of the IR blocking filter, it causes a reduction of the SQM output of about 0.01 magSQM arcsec-2 per year. Slow trend of the aerosol optical depth suggests the air near the considered site is changing towards less polluted conditions. It reduces scattering of the artificial light and consequently sky glow in the light polluted considered site.