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Abstract. The majority of aerosol data are obtained from daytime measurements, andthere are few datasets available for studying nighttime aerosolcharacteristics. In order to estimate the aerosol optical depth (AOD) andthe precipitable water vapor (PWV) during the nighttime using the moon as alight source, a sky radiometer (POM-02, Prede Ltd., Japan) was modified. Theamplifier was adjusted so that POM-02 could measure lower levels of inputirradiance. In order to track the moon based on the calculated values, asimplified formula was incorporated into the firmware. A new position sensorwith a four-quadrant detector to adjust the tracking of the Sun and moon wasalso developed. The calibration constant, which is the sensor output for theextraterrestrial solar and lunar irradiance at the mean Earth–Sun distance,was determined by using the Langley method. The measurements for the Langleycalibration were conducted at the National Oceanic and AtmosphericAdministration/Mauna Loa Observatory (NOAA/MLO) from 28 September 2017 to7 November 2017. By assuming that the correct reflectance is proportional to thereflectance estimated by the Robotic Lunar Observatory (ROLO) irradiancemodel, the calibration constant for the lunar direct irradiance wassuccessfully determined using the Langley method. The ratio of thecalibration constant for the moon to that of the Sun was often greater than1; the value of the ratio was 0.95 to 1.18 in the visible and near-infraredwavelength regions. This indicates that the ROLO model often underestimatesthe reflectance. In addition, this ratio depended on the phase angle. Inthis study, this ratio was approximated by a quadratic equation of the phaseangle. By using this approximation, the reflectance of the moon can becalculated to within an accuracy of 1 % or less. In order to validate the estimates of the AOD and PWV, continuousmeasurements with POM-02 were conducted at the Japan MeteorologicalAgency/Meteorological Research Institute (JMA/MRI) from January 2018 to May 2018,and the AOD and PWV were estimated. The results were compared with theAOD and PWV obtained by independent methods. The AOD was compared with thatestimated by the National Institute for Environmental Studies (NIES) HighSpectral Resolution Lidar measurements (wavelength: 532 nm), and the PWV wascompared with the PWV obtained from a radiosonde and the Global PositioningSystem. In addition, the continuity of the AOD (PWV) before and aftersunrise and sunset in Tsukuba was examined, and the AOD (PWV) of AERONET andthat of POM-02 at MLO were compared. In the results, the daytime andnighttime AOD (PWV) measurements are shown to be statistically almostequivalent. The AODs (PWVs) during the daytime and nighttime for POM-02 arepresumed to have the same degree of precision and accuracy within themeasurement uncertainty.

Nocturnal aerosol optical depth measurements with modified sky radiometer POM-02 using the moon as a light source