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Is Lower Air Pollution Increasing Received UV Radiation?

22nd April 2021

It’s been an exceptionally challenging 12-14 months for populations across the world. The war against COVID-19 has been a predominantly negative experience for many of us.

It makes a nice change, then, when a positive side-effect is identified. In this case, a substantial decrease in air pollution levels following the enactment of lockdowns far and wide. That’s included a huge drop in CO2 and NO2 levels.

With cleaner air, come healthier lives. Humans and animals alike breathe easier. The reduced daily assault of pollutants means are skin has a better time too. Meanwhile, reduced atmospheric haziness gives us a sharper view of the landscape around us – including the clearest views in decades of the Himalayas.

That clarity may, however, come with an undesirable side-effect: Increased ultraviolet (UV) radiation reaching the surface (as part of sunlight).

Airborne Pollutants Shield Us from UV Rays

Suspended in the air, pollutant particles intercept the sun’s rays and may either absorb or scatter them. It depends on the pollutant type – and that’s where the picture complicates greatly. Some have far more of a shielding effect than others.

Overall, though, research has identified a net drop in such particles and as such, reduced shielding of the surface is expected.

The effect can be profound. A detailed study of air pollution and UV levels in Beijing identified a drop of up to 50% in UV levels on high air pollution days. Interestingly, it was also found that the change in UV is about twice that of total solar radiation.

Granted, the vast city of Beijing is an extreme example, but the point stands: there’s a relationship.

…but this doesn’t guarantee that surface UV will increase whenever air pollution drops.

Complication: The Ozone Layer

You see, some of the pollutants have a nasty habit of interacting destructively with something known as the ozone layer. Situated in the lower stratosphere, this layer absorbs some of the UV radiation from the sun.

Importantly, it absorbs nearly all of a dangerous component named ‘UV-C’. This is the portion of UV radiation that has the highest energy, meaning it can have the fastest and strongest impact. It also blocks most of the ‘UV-B’ component, which is less energetic than ‘UV-C’ but more so than the remaining component, ‘UV-A’. That last one forms 95% of what reaches the surface.

The strength of the ozone layer determines how much of all three components makes it to the surface. So, the more it’s depleted by pollutants (mostly chlorofluorocarbons; CFCs), the more UV we’re subjected to.

The recent drops in pollution due to lockdowns has given the ozone layer a breather – but has it responded?

Long Lives the Pollutants

Well, taking a couple of reference points in the UK as examples, it appears not. For the time of year, measured lower-stratosphere ozone levels have been very similar in the past year to the preceding two. That is, once you look past the day-to-day variation (caused by airflows pooling or spreading out the ozone).

Graphs obtained via https://uk-air.defra.gov.uk/data/ozone-data.php. Sadly the images aren’t provided in great resolution but hopefully you can get the gist from the black line!

This is likely to be true on a much wider scale, too. Why can I be so presumptive? The reason is the ‘lifetime’ of pollutants in the atmosphere – specifically the CFCs. Depending on the specific type, they hang around for between 20 and 100 years!

That means we can’t expect to see any impact on the ozone layer from 12-14 months of intermittent lockdowns.

With this in mind, we can expect that for any given weather setup, UV radiation levels have become higher because of lockdown-driven pollution drops. Expecting is easy, proving is another matter…

Case Study: University of Reading

Take the observed UV radiation at Reading University, for example. Below, I’ve plotted the range of historical Jan-Apr observations 2005-2019, along with the measurements for 2020-2021 so far.

There’s a distinct lack of overall upward shift in UV here. There’s a right mix of high and low UV days, with just a handful setting daily records in 2020 and 2021 so far.

That variation’s a big deal, though. It represents the unsurprisingly strong relationship with cloud cover. This large ‘natural variability’ makes it difficult to identify responses to changing conditions without a sample spanning a few decades. For lockdown influence, we’ve only got 12-14 months.

The Longer View: Correlation Evident

But we can look beyond COVID-19 to consider longer timescales. At this point, it really matters where in the world you live. In some regions, particulate pollution has been trending higher in recent decades. In others, like the UK, it’s been heading down (see page 9).

Going back to Reading University, the table below shows analysis of daily maximum UV data covering 2005-present. Specifically, the frequency of record-high UV days each year.

The sum of percentages across 2005-2021 far exceeds 100% due to many instances of highest UV being shared across multiple years. The 2021 figure is up to April 21st.

Although the sample size is still a bit small, there’s an inescapable pattern here. Record-high UV days have become markedly more common since 2015.

It’s noteworthy that we see this despite a varying mix of weather patterns across the years. In time, we may see the ozone layer thicken and counteract this to some degree. It will depend on how global pollution levels change – especially emission of CFCs, which continue to occur illegally in some spots.

Broadly speaking, atmospheric pollution has either held steady or dropped slightly since 1990 in the Americas, Oceania, and south Africa. Eastern Europe and northern Asia have seen substantial reductions.

Meanwhile, considerable increases have occurred across much of Africa and southern Asia (explore here). In some areas, this is very visible, with dense haze almost obscuring the sun’s location at times.

The outlook is hopeful, though. The tide’s begun to turn recently in many cities, as new innovations pave the way to a cleaner future.

Here’s to breathing easier, while taking care with the sunlight, in the decades to come.

James Peacock MSc
Head Meteorologist at MetSwift

Cover Photo by Unknown Author is licensed under CC BY

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