The Impact of COVID-19-Related Measures on the Solar Resource in Areas with High Levels of Air Pollution

Summary

Restrictions enacted to reduce the spreading of COVID-19 have resulted in notably clearer skies around the world. In this study, we confirm that reduced levels of air pollution correlate with unusually high levels of clear-sky insolation in Delhi, India. Restrictions here were announced on March 19^th, with the nation going into lockdown on March 24^th. Comparing insolation data before and after these dates with insolation from previous years (2017 to 2019), we observe an 8.3% ± 1.7% higher irradiance than usual in late March and a 5.9% ± 1.6% higher one in April, while we find no significant differences in values from previous years in February or early March. Using results from a previous study, we calculated the expected increase in insolation based on measured PM2.5 concentration levels. Measurements and calculations agree within confidence intervals, suggesting that reduced pollution levels are a major cause for the observed increase in insolation.

Highlights

• •Unusually high insolation levels appeared in Delhi after COVID-19 lockdown
• •The increase in insolation coincides with an ∼50% reduction in air pollution
• •In late March, 8.3% more sunlight reached the city than in previous years
• •Elevated insolation is consistent with expectations from reduced pollution

Context & Scale

A broadly noticed consequence of the restrictions enacted to counter the spreading of COVID-19 was unusually clear skies in many regions of the world. Better air quality has an impact on solar power generation, as fewer pollution particles in the air means that more sunlight will reach solar panels on the ground. In this study, we explore how air pollution in Delhi has developed following the lockdown on March 24^th and how this reduction has affected how much sunlight the city received. First, we looked at measured particle concentration levels and noticed that after lockdown, levels dropped to about half of where they had been in previous years. Then, we looked at how much sunlight was received by solar panels. Before lockdown, insolation was similar to previous years. Yet, after lockdown, there was a clear and significant increase. In late March, insolation was up by 8.3% compared with levels in previous years. This is comparable to moving a solar panel from Toronto to Houston.

Following restrictions to counter the spread of COVID-19, reports about unusually clear skies appeared in many regions of the world. In this study, we explore how air pollution in Delhi has developed following the lockdown on March 24^th and how this reduction has affected how much sunlight the city received. Until early April, pollution levels had dropped to half of their usual levels. The cleaner air allowed more sunlight to pass, increasing insolation by more than 8%.

Introduction

A very noticeable effect of the restrictions enacted to counter COVID-19 are clearer skies. Satellite data showed a reduction in air pollution levels in many areas of the world. On March 19^th, the European Space Agency (ESA) reported a decline in nitrogen dioxide levels over China.¹ On April 9^th 2020, NASA reported a 30% drop in air pollution over the Northeastern US.² On April 26^th, ESA reported pollution levels fell by 45%–50% compared with those in the same period last year in some European cities.³ In previous reports, we discussed the impact of haze^{4 , 5} and air pollution⁶ on the performance of photovoltaic (PV) systems and provided a quantitative relation between PM2.5 (particle matter) concentrations and solar resource in India and Singapore. Given the recent low levels of air pollution, we set out to explore whether there wass a noticeable impact on the available solar resource that could have lead to unusually high electricity generation from PV installations. An article in Green Tech Media (GTM) from April 22^nd already mentioned that “reduced air pollution from the lockdown has contributed to new records (in PV electricity generation) in Germany and the UK.”⁷

To quantify a possible impact on solar resource, we used recent air quality and weather data from Delhi, one of the most polluted cities on the planet,⁸ for the years 2017 to 2020. We follow the approach described in Peters et al.⁶ to quantify clear-sky insolation. Comparing air pollution and insolation characteristics from 2020 with those of previous years, we identified and quantified anomalies for the periods before and after measures were taken in India.

Results

COVID-19-Related Measures in Delhi

The first case of COVID-19 in India was confirmed on January 30^th, 2020. As in other countries, confirmed case numbers only rose slowly, initially, yet on March 19^th, more than 500 cases were confirmed, and further measures were decided by the government. The same day, Prime Minister Modi asked all citizens to observe a Janata curfew (people’s curfew) on Sunday, March 22^nd.⁹ The curfew lasted for 14 h and affected everyone not pursuing an “essential” profession. At the end of the curfew, a lockdown was announced beginning midnight, March 24^th, for a duration of 21 days.¹⁰ On April 14^th, the lockdown was extended until May 3^rd, with relaxations in little-affected areas being possible after April 20^th.¹¹ As this article is written, the lockdown has been further extended through May 17^{th 12} with additional relaxations being in place.

Timeline of PM2.5 Particle Concentration

PM2.5 particle concentration measured at the US embassy in Delhi¹³ once every hour for the months of February to April and years 2016 to 2020 is shown in Figure 1 . Note that no air pollution data were available between February 11^th and March 14^th, 2020. While a reduction in air pollution is visible for all years as winter gives way to spring, concentrations in March and April in 2020 reach lower levels than usual. This reduction becomes especially clear when looking at relative concentration deviations, defined here as the 2020 value divided by the average value from the previous years minus one, shown in the lower part of Figure 1. Before the Janata curfew, 2020 showed a particle concentration signature that is similar to that of previous years, i.e., values around 0%. Just before the Janata curfew, a notable reduction in particle concentration begins with levels decreasing to −50% of those in previous years at the end of March—these findings are similar to those shown by the European Space Agency³ for other cities. Low PM2.5 concentration levels are maintained, mostly, throughout April.

Figure 1.

Timeline of PM2.5 Concentration in Delhi

PM2.5 particle concentration measured at the US Embassy in Delhi between February 1^st and April 30^th for the years 2017 to 2020. The graph on the bottom shows the relative deviation of 2020 values from those of previous years (2017 to 2019). Solid lines are running averages over 1 week.

Measured Clear-Sky Irradiance before and after Lockdown

To explore a potential impact on solar resource, we analyzed irradiance data captured with a pyranometer close to a commercial PV installation in Paschim Vihar, a residential area in Delhi. Data were analyzed for four time periods: (1) the month of February; (2) the period between March 1^st and March 20^th, which marks the period before the curfew in 2020; (3) March 21^st to March 31^st; and (4) the month of April. To reconstruct clear-sky conditions, we used the approach published in Peters et al.,⁶ and for each time stamp, the 75% percentile value of all available data was calculated and plotted. The resulting clear-sky insolation curves are shown in the upper half of Figure 2 . Curves for 2017 to 2019 are shown in blue, the curve for 2020 in red. Before March 20^th (periods 1 and 2), there was no visible difference in insolation between 2020 and the previous years; after March 20^th (periods 3 and 4), the insolation curve for 2020 had a notably greater amplitude than those of previous years.

Figure 2.

Reconstructed Clear-Sky Insolation and Insolation Anomaly

Upper half shows reconstructed clear-sky insolation curves for four investigated periods in the years 2017 to 2020. The value for 2020 is plotted in red, pervious years in different shades of blue. Lower half shows insolation anomaly. Here, the difference between the 2020 value and the average value of previous years is shown above. The dark green line is a guide to the eye, showing the 3 h adjacent average.

In addition to the reconstructed clear-sky insolation curve, we also show the insolation anomaly in the lower half of Figure 2. Insolation anomaly was calculated as the difference between the 2020 clear-sky insolation and the average of previous years for each time stamp. Before March 20^th, we observed little to no deviation to the behavior of previous years. After March 20^th, and especially in period 3, insolation levels were higher than normal. To test whether these deviations were significant, we compared the insolation anomaly with typical insolation variations from previous years over the entire observed period (February 1^st to April 30^th). Typical insolation variations were calculated as the weighted average of the standard deviation of clear-sky insolation values for 2017 to 2019 (blue curves in Figure 2) for the four considered periods. The obtained value was 4.6 W/m². Results are summarized in Table 1 . Comparing the average anomalies with typical variations, represented by the uncertainties, shows that high insolation values after March 20^th, 2020, are statistically significantly abnormal, whereas calculated anomalies before March 20^th are within typical fluctuations.

Table 1.

Average Insolation Anomaly Values

Period/Quantity	February	March 1–20	March 21–30	April
Average Anomaly	0.3 ± 4.6 W/m2	4.6 ± 4.6 W/m2	22.6 ± 4.6 W/m2	16.9 ± 4.6 W/m2
Relative Average Anomaly	0.14 ± 2.3%	1.8 ± 1.8%	8.3 ± 1.7%	5.9 ± 1.6%

PM2.5 Concentration as Indicator for Insolation

In Peters et al.,⁶ we presented an analysis of the correlation between PM2.5 concentration and solar resource in Delhi. We showed that insolation could be predicted using the exponential relation $\frac{I}{I_0}=Exp\left[-\frac{PM2.5}{750\pm 90}\right]$. The assumption behind this relation is that the PM2.5 concentration measured on the ground can be used as an indicator for the overall atmospheric optical depth. We used this relation to calculate the expected insolation anomaly at noon during the four considered periods. A perfect quantification should not be expected as the relation used describes average behavior over multiple years. The results of these calculations together with the average measured relative anomaly are shown in Figure 3 . We observe that calculated and measured results generally agree, with measured values in late March being especially high compared to expectations. This behavior suggests that the drop in air pollution levels is the main cause for the observed increase in insolation. It also suggests that the composition and optical characteristics of the air pollution in Delhi in 2020 is not profoundly different from that in previous years. In addition to reduced ground-level air pollution, reduced air pollution in higher atmospheric levels can be expected, due to reductions in pollutant emission from industry or air traffic. The corresponding change in atmospheric optical depth could be an additional contributor to the observed anomaly.

Figure 3.

Comparison of Measured and Calculated Anomaly

Measured (orange) and calculated (blue) relative insolation anomaly at noon for the four considered periods. Error bars mark uncertainties for the calculations and typical variations in irradiance for the measured values. Calculations were done using the method presented in Peters et al.⁶

Discussion

Following the outbreak of COVID-19, first a curfew on March 22^nd and then a lockdown were in effect all over India. Following these measures, unusually clear skies were observed in Delhi.^{14 , 15} When looking at particle concentration data recorded at the US embassy in Delhi, we found that air pollution levels were indeed lower than usual—in late March and April, values dropped to about half of what they had been in previous years over longer periods of time. It should be noted, though, that the air quality in Delhi in April, even with lockdown, was still worse than the typical air quality in other cities. Looking at histograms in Peters et al.,⁶ the closest resemblance we found was Hanoi 2017. Places like London, Los Angeles, or Jakarta, all had better air quality in 2017 overall than Delhi did in April 2020.

Given the unusually low levels of air pollution and the allegedly good visibility in Delhi, we set out to explore if the conditions for solar energy production were impacted by the COVID-19-related measures. For this purpose, we analyzed measured insolation data from a PV installation in Paschim Vihar, about 10 km from Delhi center. For more details about the implications of this distance, please refer to Peters et al.⁶ Reconstructing clear-sky data for this installation, we found that insolation levels after March 20^th were notably and significantly above those from previous years: 8.3% ± 1.7% in late March and 5.9% ± 1.6% on average in April. When looking at data from earlier in March or February, we observed no significant deviations from previous years. We also compared the measured insolation anomaly to predictions using the method outlined in Peters et al.⁶ Agreement between simulations and measurements indicate that the drop in air pollution is the root cause for the observed increase in insolation and that the optical characteristics of the air above Delhi are consistent with that of previous instances of low air pollution. We conclude: COVID-19-related measures have improved air quality, which has increased solar insolation in Delhi.

What Does This Mean for PV?

Results shown here paint a plausible picture: air pollution levels drop notably, and this drop results in clearer air that allows more sunlight to pass through the atmosphere, which increases the yield of PV installations. For Delhi, our analysis supports this picture. PV installations in March and April received more than 6% more light in total than in previous years and will continue to generate record amounts of electricity as long as air pollution levels stay low. We expect the same to be true for urban PV installations in other cities with high air pollution levels and COVID-19-related restrictions. Examples for such cities are Kolkata, Wuhan, Mumbai, Dhaka, Los Angeles, or London.

In many other cities and rural areas, however, the anthropogenic contribution to aerosols is much smaller. We performed a similar analysis, as shown in Figure 1, for data from an air-quality measurement station and a nearby PV plant in Northern Italy, the first European country to enact severe restrictions. We could not find any statistically significant effects here. Anthropogenic air pollution levels in this area are so low that the current reductions could only have a minute impact on PV power generation. Currently, we have no indications to confirm statements, like the one found in a recent GTM article,⁷ that “Reduced air pollution from the lockdown has contributed to new records in Germany and the U.K.” Cleaner air may have had a tiny influence, yet we attribute the high solar energy production, for the most part, to the very sunny weather in March and April. In both Germany and the UK, April 2020 was the sunniest April in recorded history.^{16 , 17}

Afterthought

Air pollution and solar energy interact in a self-reinforcing network. High levels of air pollution block sunlight and cause greater levels of soiling,^{18 , 19} either of which reduce solar panel performance. Replacing the sources of air pollution by solar-powered technology will result in cleaner air, which, in turn, will benefit solar-powered devices. Much greater benefits from cleaner air can be expected for human health.²⁰ One relevant aspect of this study is that air pollution may factor into the vulnerability of people to the current coronavirus.^{21 , 22} Reduced levels of air pollution triggered by restrictions to counteract the spreading of the virus could have a positive network effect and reduce harm from the disease.

Yet again, results here, like many others, only point in one direction: it is vitally important that we improve air quality, especially in urban areas with high pollution levels. Hopefully, solar energy can contribute to this goal.

Published: June 19, 2020