Diurnal and Nocturnal Dust Aerosol Characteristics at Ayvaj of Southwest Tajikistan Determined From Sun-Sky-Lunar Photometric Measurements

Based on CE318-T sun-sky-lunar photometric measurements, this study examined the diurnal and nocturnal variations of dust aerosol characteristics from June to October 2023 at Ayvaj of southwest Tajikistan for the first time. The results indicated that the ROLO-RCF (RObotic Lunar Observatory model with correction factor)-modified method was capable of reliably calculating nocturnal aerosol optical depths (AODs) and the Ångström exponent (AE_{440–870 nm}) measured from the CE318-T photometer, which was validated with synchronous observations of a lidar. The AOD day/night transition coherence test has firmly demonstrated the reliability of nighttime AODs obtained from a photometer at night-to-day and day-to-night time intervals. Both spectral AOD values and AE_{440–870 nm} exhibited prominent diurnal variations, and nocturnal AODs showed a good continuity with daytime AODs in a wide range of moon's illuminations. The maximal AOD_{500 nm}, PM_2.5 and PM₁₀ mass concentrations under a heavy dust event on 9–10 June, 2023 were 3.07, 2,800 and 9,400 μg/m³, respectively, and corresponding AE_{440–870 nm}, fine-mode fraction, and ratio of PM_2.5/PM₁₀ were 0.01, 0.17 and 0.24, respectively, implying the dominance of large desert dust particles in Ayvaj. The percentage departures of AOD_{500 nm} showed systematically negative values (−22% ∼ −5%) during daytime and positive values (+6% ∼ +25%) in nighttime and corresponding AE_{440–870 nm} ranged within ±25%, attributable to the frequent intrusion of dust events. Both daytime and nighttime AODs showed a similar monthly variation characteristic, and average AOD values (AOD_{440 nm} of 0.25 ∼ 0.51) in prevailing dust seasons were generally greater than the ones (AOD_{440 nm} of 0.16 ∼ 0.45) in September and October. In contrast, the monthly average AE_{440–870 nm} exhibited an opposite pattern. The overall average daytime and nighttime AOD_{440 nm} values were 0.327 ± 0.187 and 0.381 ± 0.135, respectively, and corresponding AE_{440–870 nm} were 0.472 ± 0.192 and 0.467 ± 0.200, respectively, suggesting that coarse-mode dust particles were the main contributors to total aerosol loading in Ayvaj. In summary, this paper has verified that the CE318-T photometer had a unique capability to measure nocturnal aerosol optical parameters under a wide range of lunar phase cycles in Ayvaj. Such significant diurnal variability of dust AODs and AE_{440–870 nm} disparity should be included in regional climate models to simulate its diurnal radiative effect in the future work.