The Important Role of the Tropics in the Self-Cleaning Capacity of the Atmosphere
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Transcript: English(auto-generated)
00:21
Thank you, Dr. Ulrich, for the introduction. Countess and dignitaries, whoever are left here, and of course, representative from the European Union, who we always can rely upon, most welcome. Ladies and gentlemen, and especially, of course,
00:41
a good word to the students. What I will speak about in this short introductory speech, and I hope we will have more discussions later on this topic, is basically how the atmosphere keeps itself clean. Because certainly, mankind is polluting the atmosphere.
01:04
And in some ways, also, natural processes puts lots of material in the atmosphere, which in some way has to disappear. Otherwise, the situation around us would not be very pleasant. The atmosphere is largely consisting of nitrogen, 78%,
01:26
almost 21% oxygen, and almost 1% argon. These are the main constituents, and they make up almost 100% of the atmosphere.
01:40
I'm saying almost, because there are a number of gases which are sort of the rest gases, starting with carbon dioxide, which at the present has a concentration in the atmosphere of about 360 molecules per million molecules of air. Carbon dioxide is of large importance, of course,
02:04
for life on Earth. The photosynthesis process is fundamental, and it is also a greenhouse gas. It plays a large role in climate. But in the chemistry of the atmosphere, it plays hardly any role at all.
02:22
Regarding the chemistry of the atmosphere, we start to become interested in gases with extremely low mixing ratios of the order, in the case of methane, 1.6 or 7 molecules per million air molecules. In the case of ozone, even less.
02:42
And we can go down to gases which have concentrations in the atmosphere, which are counted in a few molecules per 10 to the 12 molecules of air. These play a large role in the chemistry of the atmosphere.
03:01
One of these gases is ozone, which has a irregular distribution in the atmosphere. It's maximum concentrations one finds in the stratosphere at altitudes between about 15 and 25 kilometers.
03:21
There's very little ozone in the troposphere. And it's about this little amount of ozone in the troposphere about which I will speak to you mainly in this lecture. The amount is, on the average, about 40 molecules per 1,000 million air molecules. Very little amount, but of fundamental importance
03:42
for the photochemistry of the troposphere. Here also, you see a temperature profile in the atmosphere. In the troposphere, up to the tropopause, about 10 kilometers altitude temperatures decline. And in the stratosphere, temperatures are about constant with height.
04:01
And this has to do with the absorption of ultraviolet radiation by ozone, which is an energy source for the stratosphere. Well, time is too short to talk about the stratosphere, so I will concentrate on the troposphere. How do we get ozone in the troposphere?
04:21
Well, one explanation, and this was the traditional explanation until about 25 years ago, was that ozone is formed in the stratosphere in tropical regions between about 15 and 30 kilometers. And then, due to atmospheric meteorological processes,
04:42
is coming down at higher and mid and high latitudes into the troposphere. And the belief until 25 years ago was that then finally, ozone is destroyed at the ground and not much was happening in the troposphere. That was the belief until 25 years ago.
05:01
Well, we know now ozone is much more active than so. First, I want to show you here also another function of ozone, and that is filtering out of ultraviolet radiation. What is shown in this figure is the altitude to which radiation from the sun
05:22
can penetrate into the atmosphere. And you see that up to wavelengths of about 200 nanometers, the ultraviolet radiation is absorbed by mainly oxygen and nitrogen and related species in the atmosphere.
05:40
However, these compounds do not absorb longer wavelength radiation. And were it not for ozone, which absorbs especially strong in this wavelength band between 200 and about 300 nanometer, this radiation would come down to the Earth's surface and certainly make life as we know it now impossible.
06:03
So it's extremely important, this filtering action of ozone. But this action is not perfect. Some UVB radiation, radiation which can damage the biosphere, penetrates down to the Earth's surface starting at about 300 nanometer. And it is in a short wavelength band
06:22
that this radiation can do harm to life on Earth. And of course, life on Earth has developed some resistance to that radiation, but it's not perfect. For instance, if we are too long in the sun, we get a sunburn. And it would be strange if we white human beings
06:44
would be the only species on Earth which is affected by ultraviolet radiation. However, this ultraviolet radiation also has an extremely important function in cleaning the atmosphere. Because up to about in this wavelength band,
07:02
ozone photolysis with ultraviolet radiation can give rise to electronically excited so-called O-stingled D atoms, which have enough energy to react with water vapor to make OH radicals. And it's the OH radicals which really clean up the atmosphere.
07:21
We can call the OH radicals the detergent of the atmosphere. Of these OH radicals, which as repeated here, are formed by the interaction of ultraviolet radiation on ozone and the reaction with water vapor, these OH radicals in the atmosphere
07:41
have a mixing ratio of only roughly four molecules per 10 to the 14, four OH molecules per 10 to the 14 air molecules. So an exceedingly low concentration in the atmosphere. Of oxygen, we have 21%. But oxygen does not clean up in our environment.
08:04
It is the OH radical which does the job. And we see that very clearly reflected in the concentrations and variability of trace gases in the atmosphere. Some gases react only slowly with OH.
08:21
Methane, for instance, has a lifetime of eight years. That means we have a rather even distribution around the globe with a little more methane in the northern hemisphere than in the southern hemisphere. We have a whole list of gases. In fact, most gases which are emitted into the atmosphere by human activities or by nature
08:40
are removed by reactions with OH radicals. And we see here a number of examples of such gases. The OH radical does not react with everything in the atmosphere. For instance, it does not react with the CFC gases. And it does not react with nitrous oxide, N2O.
09:01
And it's these gases which go up in the stratosphere and can play a substantial role in the chemistry of the stratosphere. One thing which was not realized until about 25 years ago was that ozone can also be formed in the troposphere.
09:23
Can be destroyed in the troposphere, for instance, by the reaction which forms OH radicals. But it can also be destroyed. And the simplest example of such reactions is shown in this view graph, the oxidation of carbon monoxide. It's a very important process in the atmosphere.
09:43
It starts by reaction with OH. And depending on whether there is enough NO in the atmosphere, nitric oxide or not, CO is oxidized to CO2 either by producing ozone or by consuming ozone.
10:01
There's a chain of catalytic reactions in both cases in which the amount of NO plays a very large role. Very little NO, the HO2 radical which is formed here will form NO2 and then ozone in the next steps. If there's too little NO in the atmosphere, HO2 will actually react with ozone itself
10:23
and produce this net result. We see here a number of catalysts playing a role in these chain reactions, OH, HO2, NO, NO2. And they're being recycled all the time. And we get a very active chemistry.
10:43
Not only carbon monoxide, oxidation is involved in such reactions which produce ozone. Also, all hydrocarbons which are emitted into the atmosphere are broken down in similar ways, but the number of reactions rises very rapidly
11:02
with the number of carbon atoms in the hydrocarbon. So in the case of methane, we have already many more reactions which I will not bother you about. But the result is always the same. If there is enough NO in the atmosphere, there is a good chance to form ozone in the troposphere.
11:23
The amount of methane is increasing in the atmosphere. This we know from analysis of air bubbles enclosed in glaciers, we know that earlier there was about, in this warm period, there was about 700 parts per thousand million of methane in the atmosphere
11:43
and now we have two and a half times more methane in the atmosphere due to a number of human-related activities. Rice fields produce methane, cows produce methane, and also coal mines produce methane.
12:00
Nitrogen oxides also are, to a large degree, the emissions into the atmosphere are, to a large degree, influenced by human activities. If you look at the natural sources for NO in the atmosphere, soils, lightning, and flux from the stratosphere, then we talk about numbers in the order of seven
12:22
to 30 million ton of nitrogen per year which are emitted into the atmosphere. The anthropogenic source is fossil fuel burning, the burning of biomass in the tropics, and also aircraft emissions deliver about the same amount or even more NO in the atmosphere.
12:42
So methane is increasing. For a long time, carbon monoxide was increasing and the amount of NO put into the atmosphere is increasing and that means ozone concentrations in the troposphere, at least in the northern hemisphere, should go up and we see these, the clearest in measurements which were made at Holland-Peisenberg, not far from here,
13:03
which here is shown the average annual ozone profile in 1968 and in 1989, in a period of about 20 years, we see in much of the troposphere almost doubling of the amount of ozone in the stratosphere.
13:22
The opposite effect which is due to the emissions and the activities of the chlorofluorocarbons. There are many other places where we notice an increase of ozone in the atmosphere. And I want to emphasize in the rest of my talk
13:41
the tropics. If you look at the total amount of ozone which is located in a vertical column in the atmosphere, we see that in the tropics, we in general have a minimum in this amount. At higher latitudes, except for the ozone hole
14:02
which has developed here, there is much more protective ozone. That means the UVB radiation is especially penetrating through the atmosphere in the tropics. And because it's also very humid there, lots of water vapor, the amount of OH radicals
14:21
in the atmosphere is mainly produced in the tropics. And this we can estimate from model calculations. Here we see model estimates of the annual average amount of OH radicals in the troposphere
14:43
expressed in millions of molecules per cubic centimeter. And we see that in the tropics we have on the average about two times 10 to the sixth OH radicals per cubic centimeter and much less going towards higher latitudes. So this is the area of the atmosphere in which the photochemical activity,
15:02
the cleaning activity in the atmosphere is the biggest. And this is reflected in the removal rates of carbon monoxides, here the red bars, and of methane in the atmosphere. Even there we have a maximum in the tropics. So clearly the tropics play a very large role
15:21
in the chemistry of the atmosphere. How do we know that these OH radical calculations are correct? Well, we have some indications that they are correct because we can compare model calculations of methyl chloroform with measurements.
15:43
Methyl chloroform is a gas which has in the past only been produced by human activities. We know how much has been put into the atmosphere and we can measure at a number of places around the world how much of this gas is present at these places.
16:00
And we can do the same, we can simulate it with models. And you see that the model estimates and the actual measurements in five locations shown here around the world coincide very closely. So the OH radical calculations, which I showed you before, are roughly correct.
16:23
And this, here in this view graph I repeat again the results which I showed you earlier in the colored view graph, the breakdown of CO in the atmosphere as a function of latitude and of methane. We can do the same game for the pre-industrial atmosphere with this remodeling.
16:41
You see that these bars are much lower. So in this way we can estimate how much of methane and carbon monoxide has been put into the atmosphere by natural processes earlier in Earth history and by the sum of natural and anthropogenic process.
17:01
So we can say that the difference between these curves are the anthropogenic emissions of methane. And that is very helpful because regarding the budget of methane we need that sort of information. As the situation is now, the anthropogenic contribution
17:24
of methane by a number of activities, emissions by ruminants, animal manure, decay, rice fields, biomass burning, municipal landfills produce methane, and also natural gas leaks and coal mining, they all produce methane.
17:42
And we know roughly the sum of their emissions. They are of the order of 350 million ton of methane per year, which is larger or equal, let's say at least as large as the input by natural processes. So we very clearly see here the influence of human activities.
18:03
Things in our part of the world are getting better. The emissions of carbon monoxide in a number of industrial countries have been going down. And that is a good sign. However, this is not the case in the tropics. In the tropics, many of you might think
18:22
that the tropics are still a clean part of the atmosphere because there's no industry. But that doesn't apply because in the tropics there is one human activity happening mainly during the dry season, which is a major source for pollution, namely biomass burning.
18:44
There are a number of activities contributing to that shift in cultivation, deforestation, fires in savannas, mostly grasses, the burning of firewood, and also agricultural waste burning is done a lot in the tropics. And here are estimates of how much material
19:03
is burned in this way expressed in units of 10 to the 15 grams of carbon per year. And that adds up to about almost two to five times 10 to the 15 gram of carbon each year is burned in the tropics.
19:21
And the upper number here is approaching the burning of carbon by fossil fuel burning. Now this burning is not a very clean process. There are no emission controls. So many of the species which are put into the atmosphere, also by industrial burning,
19:42
are put into the atmosphere by biomass burning. And we see this very clearly in the measurements at the Earth's surface, for instance, a station in Brazil, in Mato Grosso, Cuyahua. What is shown here is the amount of smoke
20:02
in the atmosphere. Always in the month of September to October, we see, or no, even August to October, we see a maximum in the amount of smoke in the atmosphere. Maybe one year worse than another year, but it's always the same pattern. Lots of smoke in the atmosphere in the tropics,
20:20
mainly in rural areas, so not in urban areas. We see this also in the distribution of carbon monoxide, I'll show you a figure of that rather soon, and in the concentrations of ozone. In one particular year, this was 1988,
20:43
actually during the month of September in this same station in Brazil, half of the days, a level more than 80 nanomole per mole, that is 160 microgram per cubic meter of ozone was measured, which is a pollution episode.
21:02
It's a very regularly occurring phenomenon in this station and in many other rural stations in the subtropics and in the tropics. We see this also in the southern part of Africa, in Zimbabwe, what we see here are vertical profiles.
21:23
Here we see the height in meters. We see a maximum of smoke somewhere at two kilometer altitude, that's at the top of the mixing boundary layer. We see this coinciding with a maximum in carbon monoxide and carbon dioxide and in ozone,
21:42
all products of biomass burning, ozone by photochemical reactions. This is also seen, actually this pollution is very clearly seen from space. In 1994, on the space shuttle, measurements were made of the tropospheric amount
22:03
of CO in the atmosphere. In the month of April, when most of the burning takes place in the northern hemisphere, industrial and some biomass burning, we see a very clean southern hemisphere. However, half a year later, when the southern hemisphere
22:22
is in the phase of a dry period, we see red colors here, that means large amounts of carbon monoxide in the troposphere, which we clearly can see from space. So there's actually more pollution at this time of the year than six months earlier in the northern hemisphere.
22:42
And this is also reflected in measurements of ozone in the atmosphere, which during the dry season can be very high. In the tropics, it's not everywhere dirty. As we go over the Pacific Ocean, we see often situations in which we find
23:02
very little ozone in the atmosphere and expressed in less than 10 nanomole per mole. And in fact, in the upper troposphere in the tropics, we often see very almost unmeasurable amount of ozone just below the tropopause. This is the height in which we entered the stratosphere
23:24
and we see a rapid rise in ozone. But just below that, there may be zones of very little ozone, which is related to very heavy convection in these areas. And exactly how to explain that is not yet fully clear.
23:41
In these parts of the atmosphere, there's very little NO to make ozone and all chemical processes are aimed, so to say, at destroying ozone. So if we go to the tropics, we can enter in many situations. We can enter into situations with very little ozone.
24:02
We can enter into situations with very large amounts of ozone in biomass-burning regions or in between. And in fact, we have very little, yet very little information about the distribution of ozone in the tropics. That means we have great difficulties understanding quantitatively the chemistry
24:23
of the atmosphere in tropical regions because of lack of scientific activities in the past. So that's where we should be heading. I think the very important issue in the future in our research will be the things happening
24:41
in the subtropics and in the tropics. That's where most of the population of the world is residing and that's where most of the developments, population growth, but also industrial and agricultural growth will be happening. And I think that's where we will see the greatest changes
25:03
in the future of our atmosphere, chemical future of our atmosphere. So that's my recommendation, is to start looking there, do more work there, and collaborate with scientists in these countries, which of course need education.
25:22
Here's where I would like to stop. Time is, of course, very short here to explain everything about chemistry of the atmosphere and maybe I can make little advertisements for two books for those who would like to know more about this subject. And I thank you very much for your attention.