Effect of pollution on lightning incidence during the COVID-19 pandemic

The correlation between anthropogenic aerosol pollution and lightning activity has been known for some time, although the mechanism connecting the two has not been identified yet. The lockdowns enacted to contain the COVID-19 pandemic were an unprecedented opportunity to further explore this relationship.

Pre-2020 studies on the effect of pollution on lightning incidence show a positive correlation of anthropogenic aerosols with increased lightning activity in the area1–5. The relationship between the two factors is complex, non-linear, and depends on the concentration and type of aerosols6. On the one hand, aerosols can act as cloud condensation nuclei7, contributing to droplet formation, cloud electrification and lightning generation8. However, high concentrations of these aerosols could lead to a reduction of lightning events due to radiative effects9,10 In addition, the lightning incidence is weather-dependent, which distorts the effect associated with aerosols11.

Following the spread of SARS-CoV-2 coronavirus infections, which causes the respiratory disease COVID-19, the WHO (World Health Organisation) recognised this outbreak as a pandemic in March 2020. Several countries decreed lockdowns to ensure social distancing.

As a direct consequence of mobility restrictions and the shutdown of non-essential economic activity, there was a decrease in emissions of air pollutants, including PM2.5 (particles with a diameter of 2.5 micrometres or less) and PM10 (particles with a diameter between 10 and 2.5 micrometres)11–16. This unique opportunity allowed us to analyse in detail the relationship between aerosols and lightning strikes in Italy11 and Brazil12. In this article, we will briefly discuss both publications.

Lightning activity over São Paulo and Belo Horizonte (Brazil)

On 19 March 2020, closures began in several large cities in Brazil, such as São Paulo and Belo Horizonte. Therefore, Pinto Neto and others12 chose a period of analysis from 20 March to 2 April 2020, which was compared with data obtained from 2015 to 2020.

The authors found a considerable reduction in pollution in 2020 compared to previous years. In São Paulo, the percentage of cloud-to-ground lightning was 4%, which is significantly lower than values from other years. The average peak current for negative cloud-to-ground lightning was also lower than in 2018, 2016 and 2015. In contrast, in the city of Belo Horizonte, the percentage of positive cloud-to-ground lightning is significantly higher than previous years’ values. In any case, all these detected aspects show a strong influence of pollution abatement on lightning characteristics. Moreover, this influence could be different in each city depending on the differences in pollutant concentration.

The results of Pinto Neto and others12 on the variation in cloud-to-ground and cloud-to-ground positive lightning percentage are in agreement with those reported by Liu and others17. In this study, polluted and unpolluted oceanic regions of the South China Sea were compared. Since there is no different thermodynamic contrast in adjacent ocean regions, the lightning produced in the ocean can serve as an indicator of aerosol effects. This work found an increase in average lightning density (by 3.7 times), intra-cloud lightning density (by 5 times), cloud-to-ground lightning density (by 2.5 times), and positive lightning density in both intra-cloud (by 6.0 times) and cloud-to-ground (by 14.7 times) for the polluted region compared to the unpolluted region17.

Therefore, the results of the study by Pinto Neto and others12 during confinement in Brazil support previous work where lightning incidence is considered to be non-linearly related to the concentration and type of contamination.

Influence of confinement on lightning strikes in the Po Valley

The Po valley is a highly industrialised region in northern Italy with high lightning activity due to certain meteorological characteristics (proximity to mountains, humidity flow from the Adriatic Sea, convergence of cold and warm air masses). Moreover, in this valley, ventilation conditions are poor and, together with the low temperature, promote the permanence of aerosols for long periods of time11.

The lockdown decreed in Italy for the COVID-19 pandemic lasted from 9 March to 18 May 2020, although this was followed by a long period of de-escalation of the measures. As a result, there was a drastic reduction in industrial activity, traffic and rail transport which led to a decrease in the concentration of PM2.5 and other pollutants11.

This period of confinement and de-escalation coincided with a 70% decrease in lightning activity, as well as a 15% drop in PM2.5 concentration on storm days when it is compared to the 2017-2020 valley climatological average.

The authors took into account meteorological changes that also influence the aerosol concentration, but are independent of confinement. Therefore, they used three meteorology-based lightning parameterisations to estimate the effect of PM2.5 concentration reduction on lightning activity. The calculations showed that about 64% of the lightning decrease could be attributed to meteorology, while about 36% came from the reduction in aerosol emission11.

Both the Brazilian and Italian studies support the theory that anthropogenic aerosols impact on lightning activity. The results of these studies are highly interesting to be applied to the optimisation of lightning prediction according to global climate models12 and the prediction of forest fires caused by lightning11,18.

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