Better measurements of tropospheric ozone

Ozone is an important atmospheric trace gas. In the stratosphere, it protects us from solar ultraviolet radiation but in the troposphere – the lowest layers of the atmosphere – it is a pollutant. Here, it acts as a powerful oxidant and is harmful to human health and crops.

Although satellites can monitor ozone over large areas in the atmosphere, they cannot accurately distinguish between tropospheric and stratospheric ozone because about 90% of the total atmospheric ozone lies in the stratosphere. Now, Maxim Eremenko of the Université Paris-Est and Université Denis Diderot has shown that data from the infrared sounder IASI aboard the MetOp-A satellite are perfectly suited for measuring tropospheric ozone, thanks to the instrument's high sensitivity and wavelength resolution.

"Together with the fact that IASI is a metrological sounder (that will operate until 2020 and beyond), this means that we are able to monitor ozone in the troposphere on a daily basis for many years to come," Eremenko told environmentalresearchweb. "This is extremely important for current initiatives that aim to improve air quality and limit pollution events."

Since ozone is also a strong greenhouse gas, the technique will be useful for climate research. Indeed, the scientists will be able to follow tropospheric ozone transport worldwide with a spatial resolution of about 25 km.

IASI takes thousands of measurements over Europe each day. Eremenko and co-workers used this data together with software developed at the Institute of Meteorology and Climate Research (IMK) in Karlsruhe, Germany. The team developed a numerical method to extract tropospheric ozone from the IASI data and had to overcome two challenges: clouds and temperature. Clouds are not transparent, so the researchers eliminated all measurements taken when clouds were present. And temperature is important because the infrared signals measured by IASI strongly depend on vertical temperature profiles in the atmosphere.

Once they had obtained preliminary ozone amounts, the scientists focused on measurements taken during the 2007 Southern European heat wave. They also found that data from balloon sonde ozone measurements and a French chemical model used for air quality forecasting (CHIMERE) agreed well with the IASI data, although the CHIMERE model may now need improving, said Eremenko.

"In the future, we will provide this data as input for atmospheric models in order to improve predictions concerning air quality and climate – since ozone is important for both these aspects," he added. "The observations will also be very useful in monitoring and quantifying emissions in different countries and following air pollution transport across continents day by day, which is essential for pollution reduction policies. This will probably change the way we think about tropospheric ozone, air quality and pollution."

The team is now analysing data from summer 2007 to summer 2008 to identify seasonal trends. The researchers will also include their data in models like CHIMERE to improve predictions. "We have just started to study other highly polluted parts of the world and are working on preparing a geostationary satellite platform that would provide observations with a much higher frequency, such as every hour," revealed Eremenko.

The study, which was supported by the National Centre for Scientific Research (CNRS) and the French Space Agency (CNES), was reported in Geophysical Research Letters.