Radiative Effect and Mixing Processes of a Long-Lasting Dust Event over Athens, Greece, during the COVID-19 Period

Abstract

We report on a long-lasting (10 days) Saharan dust event affecting large sections of South-Eastern Europe by using a synergy of lidar, satellite, in-situ observations and model simulations over Athens, Greece. The dust measurements (11–20 May 2020), performed during the confinement period due to the COVID-19 pandemic, revealed interesting features of the aerosol dust properties in the absence of important air pollution sources over the European continent. During the event, moderate aerosol optical depth (AOD) values (0.3–0.4) were observed inside the dust layer by the ground-based lidar measurements (at 532 nm). Vertical profiles of the lidar ratio and the particle linear depolarization ratio (at 355 nm) showed mean layer values of the order of 47 ± 9 sr and 28 ± 5%, respectively, revealing the coarse non-spherical mode of the probed plume. The values reported here are very close to pure dust measurements performed during dedicated campaigns in the African continent. By utilizing Libradtran simulations for two scenarios (one for typical midlatitude atmospheric conditions and one having reduced atmospheric pollutants due to COVID-19 restrictions, both affected by a free tropospheric dust layer), we revealed negligible differences in terms of radiative effect, of the order of +2.6% (SWBOA, cooling behavior) and +1.9% (LWBOA, heating behavior). Moreover, the net heating rate (HR) at the bottom of the atmosphere (BOA) was equal to +0.156 K/d and equal to +2.543 K/d within 1–6 km due to the presence of the dust layer at that height. On the contrary, the reduction in atmospheric pollutants could lead to a negative HR (−0.036 K/d) at the bottom of the atmosphere (BOA) if dust aerosols were absent, while typical atmospheric conditions are estimated to have an almost zero net HR value (+0.006 K/d). The NMMB-BSC forecast model provided the dust mass concentration over Athens, while the air mass advection from the African to the European continent was simulated by the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model.

Increased Shamal winds and dust activity over the Arabian Peninsula during the COVID-19 lockdown period in 2020

“…This is consistent with a reduced clockwise shift and strengthening of the Shamal winds in June with respect to April and May ( Bou Karam Francis et al, 2017 ), with the dust plumes extending further to the east. The increase in aerosol loading during the COVID-19 lockdown due to non-anthropogenic factors such as dust events reported here has also been noted elsewhere in particular over India ( Dumka et al, 2021 ), Greece ( Kaskaoutis et al, 2021 , Kokkalis et al, 2021 ) and Australia ( Zhao, 2020 ), in the latter in association with the bushfires in the 2019/2020 austral summer season.

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“…In fact, in Morocco the decrease in PM 10 by more than half due to reduced local emissions was offset by long-range transported aerosols ( Otmani et al, 2020 ), highlighting the crucial role of non-local emissions. The increase in aerosol loading during the COVID-19 lockdown due to natural causes such as dust advection and bushfires has also been reported over India ( Dumka et al, 2021 ), Greece ( Kaskaoutis et al, 2021 , Kokkalis et al, 2021 ) and Australia ( Zhao, 2020 ). Being one of the largest sources of mineral dust on Earth ( Kok et al, 2021 ), and with mineral dust potentially accounting for >40 % of the PM 10 levels ( e.g ., Guan et al, 2019 ), it is important to understand the impact of the large-scale circulation on dust activity over the Arabian Peninsula (e.g., Francis et al, 2019 ) and hence its effects on the PM 10 concentrations during the lockdown period.…”

Impact of COVID-19 induced lockdown on land surface temperature, aerosol, and urban heat in Europe and North America

“…The key findings revealed a notable reduction in AOD during the period March–May in the pandemic year (2020) compared to the mean of non-pandemic years (2015–19). However, several regions exhibited positive AOD anomalies, which could be attributed to the contribution of natural phenomena like dust events, biomass burning that may increase significantly the AODs during the lockdown period ( Kanniah et al, 2020 ; Kokkalis et al, 2021 ). Positive omega values could be also associated with positive AOD since a downward airflow was found over some areas in 2020, which usually preventing an efficient dispersion of air pollution ( Acharya et al, 2021 ; Ogen, 2020 ).…”

Optical and Microphysical Properties of Aged Biomass Burning Aerosols and Mixtures, Based on 9-Year Multiwavelength Raman Lidar Observations in Athens, Greece

“…The lidar data used in this study were obtained over the city of Athens at the suburban site of the National and Technical University of Athens (NTUA; 37.96 • N, 23.78 • E, elevation 220 m asl.) using the multi-wavelength elastic-Raman lidar system aerosol and ozone lidar system (EOLE; [42]) of the Laser Remote Sensing Unit (LRSU; http://lrsu.physics.ntua.gr/; last access: 26 March 2021) during the time period 2011-2019. During this period, the EOLE performed systematic measurements twice per week, every Monday and Thursday, except on rainy days, according to the European Aerosol Lidar Network (EARLINET) protocol [43].…”