The Combined Effect of Ozone and Aerosols on Erythemal Irradiance in an Extremely Low Ozone Event during May 2020

Increasing solar UV radiation in Dortmund, Germany: data and trend analyses and comparison to Uccle, Belgium

Increasing solar ultraviolet radiation (UVR) can raise human exposure to UVR and adversely affect the environment. Precise measurements of ground-level solar UVR and long-term data series are crucial for evaluating time trends in UVR. This study focuses on spectrally resolved data from a UVR measuring station in Dortmund, Germany (51.5° N, 7.5° E, 130 m a.s.l.). After a strict quality assessment, UV data, such as the daily maximum UV Index (UVImax) and daily erythemal radiant exposure (Her,day) values, were analyzed concerning monthly and annual distribution, frequency, occurrence of highest values and their influencing factors. An advanced linear trend model with a flexible covariance matrix was utilized and applied to monthly mean values. Missing values were estimated by a validated imputation method. Findings were compared to those from a station in Uccle, Belgium (50.8° N, 4.3° E, 100 m a.s.l.). Parameters possibly influencing trends in both UVR and global radiation, such as ozone and sunshine duration, were additionally evaluated. The 1997-2022 trend results show a statistically significant increase in monthly mean of Her,day (4.9% p. decade) and UVImax (3.2% p. decade) in Dortmund and Her,day (7.5% p. decade) and UVImax (5.8% p. decade) in Uccle. Total column ozone shows a slight decrease in the summer months. Global radiation increases similarly to the UV data, and sunshine duration in Dortmund increases about twice as much as global radiation, suggesting a strong influence of change in cloud cover. To address health-related consequences effectively, future adaptation and prevention strategies to climate change must consider the observed trends.

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

The outbreak of SARS CoV-2 (COVID-19) has posed a serious threat to human beings, society, and economic activities all over the world. Worldwide rigorous containment measures for limiting the spread of the virus have several beneficial environmental implications due to decreased anthropogenic emissions and air pollutants, which provide a unique opportunity to understand and quantify the human impact on atmospheric environment. In the present study, the associated changes in Land Surface Temperature (LST), aerosol, and atmospheric water vapor content were investigated over highly COVID-19 impacted areas, namely, Europe and North America. The key findings revealed a large-scale negative standardized LST anomaly during nighttime across Europe (-0.11 °C to -2.6 °C), USA (-0.70 °C) and Canada (-0.27 °C) in March-May of the pandemic year 2020 compared to the mean of 2015-2019, which can be partly ascribed to the lockdown effect. The reduced LST was corroborated with the negative anomaly of air temperature measured at meteorological stations (i.e. -0.46 °C to -0.96 °C). A larger decrease in nighttime LST was also seen in urban areas (by ∼1-2 °C) compared to rural landscapes, which suggests a weakness of the urban heat island effect during the lockdown period due to large decrease in absorbing aerosols and air pollutants. On the contrary, daytime LST increased over most parts of Europe due to less attenuation of solar radiation by atmospheric aerosols. Synoptic meteorological variability and several surface-related factors may mask these changes and significantly affect the variations in LST, aerosols and water vapor content. The changes in LST may be a temporary phenomenon during the lockdown but provides an excellent opportunity to investigate the effects of various forcing controlling factors in urban microclimate and a strong evidence base for potential environmental benefits through urban planning and policy implementation.

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

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.