  Nighttime monitoring of the aerosol content of the lower atmosphere is a
challenging task, because appropriate reference natural light sources are
lacking. Here we show that the anthropogenic night sky brightness due to city
lights can be successfully used for estimating the aerosol optical depth of
arbitrarily thick atmospheric layers. This method requires measuring the zenith
night sky brightness with two detectors located at the limiting layer
altitudes. Combined with an estimate of the overall atmospheric optical depth
(available from ground-based measurements or specific satellite products), the
ratio of these radiances provides a direct estimate of the differential aerosol
optical depth of the air column between these two altitudes. These measurements
can be made with single-channel low-cost radiance detectors widely used by the
light pollution research community.
