25 Years of Ozone Warnings: Record Highs Are Rare Today
Jülich, 26 July 2019 – Exactly 25 years ago today, on 26 July 1994, the temperatures and ozone levels were high. It was a Tuesday and the day when the red–green federal government in Hessen issued the first ozone alert in Germany. In the following interview, Dr. Franz Rohrer and Dr. Dieter Klemp from Jülich’s Institute of Energy and Climate Research explain why the record highs from back then of over 300 micrograms per cubic metre seldom occur today. They also show how, of all things, nitrogen oxides emitted from vehicles help to keep the ozone concentration in cities much lower today.
Klemp remembers the historic event well. At the time, he was busy measuring the levels of ozone and other pollutants in the air. Today, he still frequently works in his measurement laboratory – which has since become a mobile operation called MOBILAB – to ascertain the air quality in cities and conurbations.
What are your memories of that day 25 years ago?
Klemp: On that summer’s day, 26 July 1994, we were in Wuppertal conducting measurements with our laboratory equipment, which back then was not yet mobile. The laboratory was located in the city, close to the university. We observed an ozone episode, or “summer smog”, around this time: every day, we recorded an increase in ozone of 10–20 micrograms. The ozone levels ultimately rose to 260 micrograms per cubic metre. Such record highs are extremely rare today.
How has the situation changed since then?
Rohrer: Such extreme values are no longer observed today. The catalytic converters for cars and trucks that were introduced at the time are the main reason for this. They have massively reduced the emission of hydrocarbons, which are essentially a fuel for ozone formation. And since the concentration of hydrocarbons in the environment has reduced by more than a factor of 15 within the past 25 years, the ozone problem is much smaller today.
This effect is intensified by the fact that in spite of catalytic converters, nitrogen oxides have not reduced to the same extent, which is due to the diesel situation. The reason for this is that the overall proportion of diesel cars in the vehicle fleet has increased, almost compensating for the effect of massive nitrogen oxide decreases in petrol cars.
What is the connection between nitrogen oxides and ozone?
Rohrer: A special type of “local” situation occurs in areas close to dense road traffic. Nitric oxide (NO), which is emitted by traffic, very quickly reacts with ozone to nitrogen dioxide (NO2). It essentially takes ozone from the local air, which can lead to extremely low ozone concentration levels in cities. In fact, measurement stations along many roads no longer measure any ozone at all. The ozone levels there fell below the measurement limit a long time ago.
And conversely, if nitric oxide emissions are reduced, then less ozone will be removed from the air locally. This will lead to increased ozone levels – maybe not record highs, but higher average values. And this is the situation we are seeing in some cities at the moment.
Can you maybe provide an example?
Rohrer: It is easily explained using Forschungszentrum Jülich’s freely available web tool, JOIN. I used it to access data from the measurement station at Simmerath. This station is a “background station”, which means that emissions from traffic have hardly any influence on its measurements. The ozone concentration has remained largely constant there for the past 25 years. Outliers can be explained by years with high temperatures, where more ozone is produced due to accelerated photochemistry. This was the case in 2003, for example, and again last year, 2018. If you then look at a different station close to a city, such as in the Chorweiler district of Cologne, you can see that the ozone values have risen over the past few years.
Klemp: The reason for the increase in Chorweiler is not, however, that more ozone is being produced. This is evident from the background concentration in Simmerath: it is higher overall there and has hardly changed. The decisive factor is that nitrogen oxide emissions from traffic have decreased. At the Chorweiler station, nitrogen oxides have fallen by about 30 % since 1990. Accordingly, less ozone is converted, and the ozone concentration rises.
What this means becomes clear if you consider the nitrogen dioxide concentration in parallel to the ozone levels – the NO2 values essentially contain the ozone that was previously converted locally. This shows that the sum of ozone and nitrogen dioxide has not increased in Chorweiler over the years, just as it has remained constant at background stations such as Simmerath.
Where does the measured ozone come from?
Rohrer: A fact that is often overlooked is that ozone can remain in the lower layers of the atmosphere, known as the troposphere, for extremely long periods of time – up to a month. During this time, it circles the globe several times at our latitudes due to the typical west winds. The largest portion of this ozone is not formed here in Germany: rather, it flows over the Atlantic from the USA. This only takes a few days. Ozone also moves west to the USA from China and India, and then arrives in Germany. From here, of course, it then moves on towards the west, to Asia, and so on.
How is this ozone formed?
Rohrer: Ozone is formed from the photochemical degradation of hydrocarbons, which originate mostly from traffic, but also from industry and plants. Other important sources nowadays include perfumes, shampoos, and deodorants, by the way. These frequently make up the most important sources of ozone in American cities today because the emissions from traffic have decreased immensely.
Various processes play a role in this context. Some of the ozone that arrives here from the USA is degraded through contact with soil. In addition, the photochemical processes we already mentioned cause a slight increase in ozone. Overall, however, when you look at annual averages, I believe that these days more ozone is degraded in Europe than is produced.
This becomes clear when you see the ozone trend at Europe’s western coast, at the Mace Head Global Atmospheric Watch (GAW) station in Ireland. The NO2 concentration there is so low that it is virtually negligible. This means that we can detect the ozone that crosses the Atlantic and arrives in Europe from the USA. And this value has increased slightly at Mace Head over the past few years.
Why do we still have fairly high values on hot days?
Rohrer: When certain weather situations – which we call “stagnant high-pressure regions” – lead to a concentration of pollutants in one location over a period of time, it is possible that during this relatively short period of a few days, ozone is formed and increases from day to day. But in such situations, short-term vehicle bans do not help. All of the ingredients are already there, and the pot is already boiling. These record highs do not affect annual averages, by the way. They usually only last a few days. As soon as the stagnant high-pressure system dissolves and the air is mixed up again, the values normalize.
We are seeing the beginning of one of these high-temperature periods right now. The example below shows the increase in the current ozone concentration at the measurement station in Niederzier, which is very close to Forschungszentrum Jülich. For comparison, the graph also shows the situation in 1994 at the same station, which illustrates that values in the region were generally higher 25 years ago.
With today’s reduced hydrocarbon concentration, we seem to require temperatures about 3–5 °C higher in order to reach ozone values comparable to those recorded in 1994. And just like in 1994, you need a period of several days with very little wind and sufficiently high temperatures for the ozone concentration to increase from day to day. The current weather situation meets all of these conditions, with the result that we are currently observing high ozone concentrations. This has actually become a rare event in recent years.
Finally, let’s take a look into the future: will the ozone problem become more pressing again in the medium or long term?
Rohrer: I think that traffic emissions will reduce further in the long term, probably to a tiny fraction within the next 25 years. When this happens, I believe that we will no longer have an ozone problem. But this only applies if no other hydrocarbon source is added to the mix. This would directly cause values to increase again. The main factor will be how emissions change in other countries such as India and China. If the economy in India keeps developing the way it is – and a stronger economy today always means more energy production from fossil fuels – then this is likely to lead to more ozone. And this ozone will ultimately arrive in Germany, even if it has to travel around the globe to get here.
Klemp: On the other hand, it must also be mentioned that the past 25 years have seen significant improvements, thanks in particular to catalytic converters in vehicles – for ozone at least. In terms of nitrogen oxides, we have not achieved as much. Nevertheless, the introduction of catalytic converters has undoubtedly been a great success. And the future may bring further changes.
Dr. Franz Rohrer
Institute of Energy and Climate Research – Troposphere (IEK-8)
Tel: +49 2461 61-6511
Dr. Dieter Klemp
Institute of Energy and Climate Research – Troposphere (IEK-8)
Tel: +49 2461 61-6114
Press officer, Corporate Communications
Tel: +49 2461 61-4771