A matter of chemistry
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ImaginAIR: S-cars in the sky Image © Greta De Metsenaere
Do all gases matter for the environment?
Many of the gases in the air are not especially important in terms of chemistry. Some trace gases like carbon dioxide and nitrous oxide don’t react readily in the air, and for this reason they are categorised as long-lived gases. The main component of air, nitrogen, is also largely inert in the atmosphere. Long-lived trace gases are present at roughly the same concentrations all over the world. If you took a sample in the northern hemisphere and the southern hemisphere, there would not be much difference in terms of the amount of these gases in the air.
However, concentrations of other gases like sulphur dioxide, ammonia and the sunlight-sensitive oxidants such as ozone, are much more variable. These gases represent a threat to the environment and human health, and because they react so quickly in the atmosphere they don’t last long in their original form. They react quickly to form other compounds or are removed by deposition to the ground, and are referred to as short‑lived gases. They are therefore present close to the places they were emitted or formed by reaction. Remote sensing satellite imagery shows hotspots of these short‑lived gases in certain parts of the world, typically in industrialised areas.
How can these short-lived gases create problems for air quality and the environment?
Many of these short-lived gases are toxic to human health and vegetation. They are also readily transformed in the atmosphere to other pollutants, some by the action of sunlight. The sun’s energy is capable of splitting many of these reactive short-lived gases into new chemical compounds. Nitrogen dioxide is a good example. Nitrogen dioxide is produced mainly by burning fuel, whether in cars burning petrol, or electricity plants burning gas and coal. When nitrogen dioxide is exposed to sunlight, it is split into two new chemical compounds: nitric oxide and what chemists call atomic oxygen.
Atomic oxygen is simply a single atom of oxygen. The atomic oxygen reacts with molecular oxygen (two oxygen atoms combined as molecules O2) to form ozone (O3), which is toxic to ecosystems and human health, and is one of the most important pollutants in all industrialised countries.
But in the 1980s, did we not need ozone to protect us from too much radiation from the sun?
That’s correct. But the ozone in the ozone layer is in the stratosphere at altitudes between 10 km and 50 km above the surface, where it provides protection from UV radiation. However, the ozone at lower levels — commonly referred to as ground-level ozone — is a threat to human health, crops, and other sensitive vegetation.
Ozone is a powerful oxidant. It enters plants through small pores in the leaves. It is absorbed by the plant and generates free radicals — unstable molecules that damage membranes and proteins. Plants have sophisticated mechanisms to deal with free radicals. But if a plant has to devote some of the energy it harvests from sunlight and photosynthesis to repairing the cell damage caused by free radicals, it will have less energy to grow. So when crops are exposed to ozone, they are less productive. Across Europe, North America, and Asia, agricultural yields are reduced by ozone.
The chemistry of ozone in humans is quite similar to the chemistry of ozone in plants. But instead of entering through pores in the plant’s surface, ozone is absorbed through the lining of the lung. It creates free radicals in the lining of the lung and damages lung function. So the people most at risk from ozone are those with impaired breathing. If you look at the statistics, periods of high ozone show an increase in the daily death rate for humans.
Given that these gases are short lived, shouldn’t a drastic cut in nitrogen dioxide emissions lead to a quick decline in ozone levels?
In principle, yes. We could cut emissions and ozone levels would begin to fall. But ozone is created from very close to the earth’s surface all the way up to a height of about 10 km. So there is quite a lot of background ozone still up there. If we stopped emitting it all, it would take a month or so to be back down at natural levels of ozone.
But even if Europe took that action on emissions, it would not really reduce our exposure to ozone. Part of the ozone entering Europe comes from the ozone generated from European emissions. But Europe is also exposed to ozone transported from China, India, and North America. Nitrogen dioxide itself is a short-lived gas, but the ozone it creates can last longer and therefore has time to be carried by wind around the world. A unilateral EU decision would reduce some of the peaks of ozone production over Europe, but it would make only a small contribution to the global background, because Europe is just one contributor among many.
Europe, North America, China, India, and Japan all have an ozone problem. Even the rapidly developing countries such as Brazil (where biomass burning and vehicles release ozone precursor gases) have an ozone problem. The cleanest parts of the world in terms of ozone production are the remote ocean areas.
(c) Cesarino Leoni, ImaginAIR/EEA
Is ozone the only source of concern?
Aerosols are the other main pollutant and are more important than ozone. Aerosols in this sense aren’t what consumers typically think of as being aerosols, such as deodorants and furniture spray that can be bought in the supermarket. For chemists, aerosols are small particles in the atmosphere, also referred to as particulate matter (PM). They can be solid or liquid, and some of the particles become droplets in moist air and then return to solid particles as the air dries. Aerosols are associated with higher human mortality, and the people most at risk are those with respiratory problems. Particulate matter in the atmosphere causes larger health effects than ozone.
Many of the pollutants created by human activities are emitted as gases. For example, sulphur is usually emitted as sulphur dioxide (SO2) while nitrogen is emitted as nitrogen dioxide (NO2) and/or ammonia (NH3). But once they’re in the atmosphere, these gases are transformed into particles. This process turns sulphur dioxide into sulphate particles, which are no bigger than a fraction of a micron.
If there’s enough ammonia in the air, then that sulphate reacts to become ammonium sulphate. If you looked at the air over Europe 50 years ago, ammonium sulphate was a really dominant component. But we’ve greatly reduced sulphur emissions over Europe — by about 90 % since the 1970s.
But although we have reduced sulphur emissions, we haven’t reduced ammonia emissions by anywhere near as much. This means that the ammonia in the atmosphere reacts with other substances. For example, NO2 in the atmosphere transforms to nitric acid, and this nitric acid reacts with the ammonia to produce ammonium nitrate. Ammonium nitrate is very volatile. Higher in the atmosphere, ammonium nitrate is a particle or a droplet, but on a warm day and close to the surface, ammonium nitrate splits up into nitric acid and ammonia, both of which deposit on the earth’s surface very rapidly.
What happens if nitric acid is deposited on the earth’s surface?
Nitric acid provides an addition of nitrogen to the earth’s surface and effectively acts as a fertiliser on our plants. In this way, we are fertilising the natural environment of Europe from the atmosphere in the same way that farmers fertilise cropland. The additional nitrogen fertilising the natural landscape results in acidification and leads to enhanced nitrous oxide emission, but it also increases the growth of forests and so is both a threat and a benefit. The largest effect of the nitrogen deposited on the natural landscape is in providing additional nutrients to natural ecosystems. As a result, the nitrogen-hungry plants grow very quickly and flourish and out-compete the slow‑growing species. This leads to the loss of more specialist species, which have adapted to flourish in a low-nitrogen climate. We can already see a change in the biodiversity of flora across Europe as a result of our fertilising the continent from the atmosphere.
(c) Cesarino Leoni, ImaginAIR/EEA
"Each of us is trying to create in our environment the optimal conditions for our well-being. The quality of the air we breathe has a significant influence on our lives and our well-being."
Cesarino Leoni, Italy
We dealt with sulphur emissions and the ozone layer. Why haven’t we dealt with the ammonium problem?
Ammonia emissions come from the agricultural sector and especially the intensive dairy sector. Urine and manure from cows and sheep on the fields lead to emissions of ammonia to the atmosphere. It is very reactive and readily deposits on the landscape. It also forms ammonium nitrate and is an important contributor to particulate matter in the atmosphere, and to associated human health problems. Most of the ammonia we emit in Europe deposits in Europe. There has to be a stronger political will to introduce control measures to reduce ammonia emissions.
Interestingly, in the case of sulphur, the political will was absolutely there. I think this was partly due to a sense of moral obligation by the large emitter countries of Europe relative to the net receiver countries of Scandinavia, where the bulk of the acid deposition problems occurred.
Reducing ammonia emissions would mean targeting the agricultural sector, and agriculture lobbies are rather influential in political circles. It’s no different in North America. There’s also a large problem with ammonia emissions in North America and there is also no action to control it there either.
Professor David Fowler from the Centre for Ecology & Hydrology of the Natural Environment Research Council in the United Kingdom
On atmospheric chemistry: ESPERE Climate Encyclopaedia