Validation of Aerosol Optical Depth and ngstrm Exponent retrieved by solar and lunar photometry techniques in Burjassot (Valencia)

Among the many existing aerosol detection techniques, lunar photometry is particularly appealing, since its combination with sun-photometer retrievals could allow a quasi-continuous monitoring of aerosol properties. Great efforts have therefore been devoted to improve the instrumentation and especially the calibration methods, so as to sort out the inherent difficulties in the computation of the -not constant- extraterrestrial lunar irradiance. Thus, assessing the coherence between solar and lunar retrievals is of utmost importance. In this work, the agreement between Level 1.5 solar and lunar retrievals of Aerosol Optical Depth (AOD) and the ngstrm Exponent () in AERONETs Burjassot (Valencia) site from July 2015 to January 2024 are evaluated. Then, inter- and intra-annual monthly means are compared and validated through a correlation study. In agreement with previous analyses of solar measurements over this region, higher values of AOD are observed in summer months in comparison with the rest of the year. This could be related to factors such as particle stagnation, hygroscopic processes, secondary aerosol formation (favoured by higher irradiation levels), or dust outbreaks, which are more frequent at this time of the year. Indeed, the latter could be responsible for a secondary AOD maximum found in April. On the other hand, hourly means from both data sets will be also jointly analyzed in order to study the consistency of retrievals in the transition from day to night and vice versa. In these cases, factors such as the influence of the Moon phase and the interval of time between the last and first Sun and Moon measurements, respectively, will be taken into account. Finally, after checking the agreement between solar and lunar data, hence justifying their joint use, AOD temporal evolution from solar and lunar data are examined, concluding that AOD tends to decrease along the years. This result is not unexpected, taking into account the decreasing AOD trends over Europe and North America that have been reported by several authors.