The spring of 2020 witnessed an unprecedented surge in solar irradiance across Western Europe, shattering previous sunshine duration records in several countries, including the United Kingdom, Belgium, Germany, and notably, the Netherlands. The Netherlands recorded its highest surface irradiance since 1928, surpassing the 2011 record by a significant 13%, and exhibiting a record-low diffuse irradiance fraction of only 38%. This exceptional event occurred concurrently with the first European wave of the COVID-19 pandemic, during which widespread lockdowns led to a substantial decrease in anthropogenic pollution across the continent. This prompted the research question: Did the reduction in anthropogenic aerosols and contrails, resulting from the COVID-19 lockdown measures, play a significant role in the observed record-breaking irradiance? Understanding the relative contributions of meteorological factors and aerosol reduction is crucial for several reasons. First, it helps to disentangle the complex interplay between weather patterns and air quality in shaping solar irradiance. Second, it informs future predictions of solar irradiance extremes and their potential impact on various sectors like energy production and agriculture. Third, it helps quantify the impact of human activities (such as air travel) on the global radiation balance. Although it was hypothesized that reduced anthropogenic aerosols and contrails due to the lockdown would contribute, Western Europe's already relatively clean air suggested that the contribution of exceptional weather would be more significant. The aim of this study, therefore, was to precisely quantify the individual contributions of weather patterns and aerosol reduction to the extreme solar irradiance levels observed during spring 2020.

Previous research has documented both "global dimming" and subsequently "global brightening," highlighting long-term trends in surface solar radiation. These trends are primarily attributed to changes in aerosol concentrations. Studies on the impact of weather patterns on solar irradiance variability, particularly in Northern Europe, have established a clear link between synoptic conditions and surface irradiance levels. However, the existence of trends in European weather patterns remains a subject of debate, as the sensitivity of analyses varies depending on classification methods, geographical domain, and temporal scales considered. While some research indicates increases in spring blocking highs and decreases in cyclonic activity, other studies report an absence of significant trends in the frequency and persistence of blocking events during spring. This uncertainty underscores the complexity of attributing changes in solar irradiance solely to weather patterns, highlighting the need for detailed analysis of specific events like the 2020 spring.

To investigate the factors contributing to the 2020 spring irradiance extreme, the researchers employed a multi-pronged approach combining observational data analysis with radiative transfer modeling. The study analyzed data from the Baseline Surface Radiation Network (BSRN) measurement station in Cabauw, Netherlands, a location central to the regions reporting record sunshine durations. This station provided time series of irradiance, aerosol optical depth (AOD), and precipitable water vapor. Flight activity data from the OpenSkyNetwork was used to assess the impact of reduced air traffic on contrail formation. The researchers used ERA5 reanalysis data to analyze anomalies in atmospheric circulation patterns, precipitable water, cloud cover, and AOD, comparing 2020 to its climatology (2004–2020). To account for potential bias in separating aerosols from optically thin cirrus, hourly AOD values from the CAMS aerosol product were utilized, providing a longer-term statistical perspective and validating against ground-based observations. Contrail-cirrus effects were estimated by comparing 2020 to 2011 and 2015, years with high and low AOD respectively, while considering meteorological conditions for contrail formation. Manual inspection of high-resolution satellite images from NASA Worldview was conducted to assess cirrus and contrail occurrence in the Netherlands. Finally, a radiative transfer model (RTE+RRTMGP) was used to simulate surface irradiance under various conditions (with/without aerosols, clouds, and water vapor), allowing for the quantitative assessment of each factor's contribution to the observed irradiance extreme. By comparing model simulations with observations, the researchers could quantify the relative contributions of clouds, aerosols, and water vapor to the surface irradiance levels. The model used ERA5 reanalysis data for atmospheric profiles and CAMS McClear data to infer aerosol effects. The model output was validated against observations for a case study and the entire spring period, comparing against observations from Cabauw and Veenkampen stations.

The analysis revealed several key findings. First, the onset of the prolonged period of fair weather in late March coincided with a sharp decline in flight activity due to the COVID-19 lockdown. Second, spring 2020 exhibited exceptionally dry atmospheric conditions, with significantly lower precipitable water compared to previous years. Third, the total cloud cover during spring 2020 was exceptionally low, with the 95th percentile, mean, median, and 5th percentile values being the lowest on record, indicating a significant reduction compared to previous years, including 2011, which held the previous irradiance record. Fourth, although spring 2020 had the lowest median hourly AOD since 2004, indicating cleaner air, the minimum and 5th percentile AOD values were not unusually low. Fifth, manual inspection of satellite imagery revealed that 2011 and 2015 showed significantly more cirrus clouds and contrail contamination compared to 2020, supporting the notion of reduced contrail-cirrus effects due to decreased aviation. Sixth, radiative transfer modeling estimates revealed that the reduction in cloud cover contributed a significant +30.7 W m⁻² increase in surface irradiance compared to the 2010–2019 mean, significantly outweighing the increase due to lower aerosols (+2.3 W m⁻²). The impact of water vapor was relatively small (+1.5 W m⁻²). In summary, the exceptionally dry and cloud-free weather played the dominant role in setting the 2020 record in surface irradiance, with the reduced emissions of anthropogenic aerosols having a far smaller impact. This result is further illustrated in Figure 1, showing the top ten years of daily mean integrated global horizontal irradiance (GHI) since 1928 for the Veenkampen station in the Netherlands, clearly demonstrating that 2020 significantly exceeded previous records. Figure 4a illustrates the daily variation of irradiance, and Figure 4b shows the comparative irradiance for the previous ten years. These figures clearly demonstrate the exceptional nature of the 2020 event, far exceeding the irradiance levels of previous years.

The findings directly address the research question by quantifying the relative contributions of weather and aerosol reduction to the record-breaking irradiance of spring 2020. The results clearly demonstrate that the exceptionally dry and cloud-free weather conditions, rather than reduced aerosol emissions due to the COVID-19 lockdown, were the primary driver of the irradiance extreme. While the reduced aerosol loading contributed a small positive effect, its magnitude was an order of magnitude smaller than the impact of the favorable weather. This emphasizes the dominant role of meteorological factors in shaping extreme irradiance events, even in the context of significant changes in air quality. The study highlights that despite the well-documented downward trend in aerosol concentrations over Western Europe and the observed decrease in AOD during spring 2020, the exceptionally favorable meteorological conditions were the decisive factor in establishing the new irradiance record. The observed low diffuse fraction further supports this conclusion, indicating a considerable reduction in light scattering due to the exceptionally cloud-free conditions. This has significant implications for forecasting future irradiance extremes and for understanding the relative importance of weather and air quality in determining solar energy resources.

This study demonstrates that the record-high solar irradiance in Western Europe during the first COVID-19 lockdown was primarily caused by exceptional weather conditions, specifically unusually dry and cloud-free skies. The reduction in anthropogenic aerosols due to lockdown measures played a minor role compared to the impact of weather. The findings highlight the dominant influence of meteorology on solar irradiance extremes and underscore the need for improved understanding of weather patterns and their influence on future irradiance projections. Future research should investigate the potential trends in weather patterns that could be contributing to more frequent occurrence of cloud-free and dry periods.

The study focused on a single spring, limiting the generalizability of the findings to other years or seasons. While the radiative transfer model accounted for various atmospheric components, other potential aerosol effects, such as rapid cloud adjustments to temperature changes and aerosol influence on cloud microphysical processes, could not be fully quantified. Additionally, the manual inspection of satellite imagery for contrail-cirrus assessment introduces subjective judgment. Further analysis with longer-term data and improved modeling techniques could address these limitations.