Abstract. The majority of aerosol data are obtained from daytime measurements, and
there are few datasets available for studying nighttime aerosol
characteristics. In order to estimate the aerosol optical depth (AOD) and
the precipitable water vapor (PWV) during the nighttime using the moon as a
light source, a sky radiometer (POM-02, Prede Ltd., Japan) was modified. The
amplifier was adjusted so that POM-02 could measure lower levels of input
irradiance. In order to track the moon based on the calculated values, a
simplified formula was incorporated into the firmware. A new position sensor
with a four-quadrant detector to adjust the tracking of the Sun and moon was
also developed. The calibration constant, which is the sensor output for the
extraterrestrial solar and lunar irradiance at the mean Earth–Sun distance,
was determined by using the Langley method. The measurements for the Langley
calibration were conducted at the National Oceanic and Atmospheric
Administration/Mauna Loa Observatory (NOAA/MLO) from 28 September 2017 to
7 November 2017. By assuming that the correct reflectance is proportional to the
reflectance estimated by the Robotic Lunar Observatory (ROLO) irradiance
model, the calibration constant for the lunar direct irradiance was
successfully determined using the Langley method. The ratio of the
calibration constant for the moon to that of the Sun was often greater than
1; the value of the ratio was 0.95 to 1.18 in the visible and near-infrared
wavelength regions. This indicates that the ROLO model often underestimates
the reflectance. In addition, this ratio depended on the phase angle. In
this study, this ratio was approximated by a quadratic equation of the phase
angle. By using this approximation, the reflectance of the moon can be
calculated to within an accuracy of 1 % or less. In order to validate the estimates of the AOD and PWV, continuous
measurements with POM-02 were conducted at the Japan Meteorological
Agency/Meteorological Research Institute (JMA/MRI) from January 2018 to May 2018,
and the AOD and PWV were estimated. The results were compared with the
AOD and PWV obtained by independent methods. The AOD was compared with that
estimated by the National Institute for Environmental Studies (NIES) High
Spectral Resolution Lidar measurements (wavelength: 532 nm), and the PWV was
compared with the PWV obtained from a radiosonde and the Global Positioning
System. In addition, the continuity of the AOD (PWV) before and after
sunrise and sunset in Tsukuba was examined, and the AOD (PWV) of AERONET and
that of POM-02 at MLO were compared. In the results, the daytime and
nighttime AOD (PWV) measurements are shown to be statistically almost
equivalent. The AODs (PWVs) during the daytime and nighttime for POM-02 are
presumed to have the same degree of precision and accuracy within the
measurement uncertainty.
Sunlight initiated atmospheric photochemical reactions
