Dr Laura Revell
When you say ‘aerosols,’ I think deodorant spray cans…
Yes and no. Aerosols – tiny particles or droplets suspended in the air – are the mist of liquid particles that come out of spray cans. But they also exist in the atmosphere on vast scales and are produced from various natural processes and human activities.
Natural processes include volcanic eruptions – when volcanic plumes reach the cold upper atmosphere, some of the gases released from the eruption condense and form aerosols. Winds in the upper atmosphere then spread the aerosols globally. Other natural aerosols include dust and sea salt. Soot and smoke aerosols, on the other hand, are produced from fossil fuel burning.
Why should I care about them?
I’m glad you asked. When sunlight enters the atmosphere and hits an aerosol particle, the light is scattered in all directions – just like if you shone a torch at a disco ball. We know that aerosols scatter some sunlight back into space, and we can learn a lot about their effects on climate if we have a closer look at what happens after a volcano erupts. For example, in 1991 the Mt. Pinatubo volcano erupted in the Philippines. This was such a major eruption that it affected surface temperatures across the planet over the following two years, and especially in northern Africa and southwestern Asia where the winters got colder on average.
It sounds like aerosols might partially offset global warming, then?
How perceptive of you. One recent study estimated that potential future aerosol reductions could contribute an extra 0.5 – 1.1 °C to global average temperature rise.
Why would we try to reduce aerosols in future if they help cool the Earth?
The problem is that aerosols are also air pollutants and can cause respiratory and cardiovascular problems in humans – not to mention visibility problems! Have you ever noticed how the air can look hazy in a big city when it hasn’t rained for a while? That haze is caused by aerosols. Because of their role as air pollutants, we expect that atmospheric aerosol abundances will decrease in the future in populated regions, as many countries move to clean up their air quality.
Indeed. On the other hand, there is still a lot that we don’t know about how aerosols behave in the atmosphere. In fact, the Intergovernmental Panel on Climate Change has identified aerosols as one of the biggest sources of uncertainty in constraining human impacts on climate. Part of the problem is that aerosols can contribute to cloud formation. In turn, clouds have their own (complicated) interactions with radiation. However, we don’t have a clear idea of the extent to which such aerosol-cloud interactions occur. As you can imagine, in very cloudy regions such as the Southern Ocean – one of the cloudiest regions on Earth – trying to figure out how aerosols and clouds influence Earth’s atmospheric energy balance can be very Challenging!
Challenging with a capital C?
Terrible pun, sorry. As part of the Deep South National Science Challenge, we’re trying to understand how aerosols and clouds behave over the Southern Ocean, and how we can better understand their effects on the atmospheric energy balance with climate models. All of this is particularly important for understanding climate change in countries close to the Southern Ocean.
Such as New Zealand?
Again, you’re very perceptive. Yes, we need to get the Southern Ocean right so that we can be confident in our climate change projections for New Zealand.
So where do aerosols come from over the Southern Ocean? You mentioned sea salt earlier?
Yes, sea salt is an important aerosol over the Southern Ocean along with sulfate aerosol. When sea ice melts in spring and summer, algae living on the underside of sea ice release a compound called dimethyl sulfide, or DMS for short, into the surrounding ocean water. DMS can then be transferred to the atmosphere where it undergoes a series of chemical reactions which ultimately produce sulfate aerosol. That aerosol (along with other Southern Ocean aerosols, such as sea salt) will then scatter sunlight (like a whole bunch of tiny disco balls), contribute to cloud formation, and alter the atmospheric energy balance.
You mean algae play a role in the atmosphere’s energy balance? That is… amazing.
Fascinating world, isn’t it?
Laura Revell is an atmospheric scientist at Bodeker Scientific, lectures environmental physics at the University of Canterbury, and leads the Deep South Challenge project ‘Sulfate aerosols over the Southern Ocean.’