Ozone in the northern hemisphere has been growing for the past 20 years

In a first study using ozone data collected by commercial aircraft, researchers from the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder found that levels of the pollutant are in the lowest part of the Earth’s atmosphere. increased across the Northern Hemisphere over the past 20 years. That is even if closer controls on ozone emissions predecessors in some places have reduced ground level ozone, including North America and Europe.

Tropospheric ozone – ozone between the earth’s surface and 12 to 15 kilometers above the earth – is a greenhouse gas and air pollutant that, at high levels, can damage human lungs and damage plants.

In a study published today in the journal Science Advances, the team found an overall increase in ozone levels above the Northern Hemisphere. “That’s a great thing, because it means that if we try to limit our pollution locally, it may not work as well as we thought,” said Audrey Gaudel, a CIRES scientist working at NOAA Chemical Sciences Laboratory and the lead author of the study. She and her colleagues documented the largest ozone increases in the tropics, Gaudel said, noting that ozone exported from the tropics could drive increases above other areas of the Northern Hemisphere.

Gaudel and her co-authors, CIRES scientists at NOAA and international colleagues, also found the most striking increases in areas where ozone levels were once low: for example Malaysia / Indonesia, Southeast Asia and India. These regions had very low ozone values ​​between 1994-2004, and in recent years very high levels, between 2011-2016.

Earlier studies could not draw firm conclusions about northern hemisphere ozone trends, according to Gaudel, because there are too few long-term locations for surveillance and because new satellites with almost worldwide coverage have yielded conflicting results on ozone trends.

That is why the researchers accessed aircraft data from the European In-Service Aircraft for the Global Observing System (IAGOS) program. “Since 1994, IAGOS has been measuring ozone worldwide using the same instrument on every planet, allowing us to have consistent measurements over time and space from the earth’s surface to the upper troposphere,” Gaudel said. Between 1994 and 2016, commercial aircraft captured 34,600 ozone profiles, or about four profiles each day.

Gaudel and her colleagues used these measurements to calculate changes in tropospheric ozone from the mid-1990s to 2016 above 11 regions in the Northern Hemisphere. They found an overall increase in ozone in all regions where they were seen, including four in the latitude, two in the subtropics, two in the tropics and three equatorial regions. On average, median ozone values ​​had increased by 5% per decade.

In the so-called ‘lower troposphere’, which is closer to the earth’s surface, ozone has decreased above some mid-latitude regions, including Europe and the United States, where emissions of ozone predecessors have been reduced. The researchers found that reductions were offset by elevations higher in the troposphere – with the net result being an overall ozone increase from the surface to 12 km.

To understand what caused the observed ozone changes, the researchers looked at the emission inventories of one of the major ozone precursors – nitrogen oxides (NOx) – used as input for the global chemical transport model MERRA-2 GMI, which the IAGOS accurately reproduces measurements. The model showed that increased anthropogenic emissions in the tropics were likely to drive the observed increase in ozone in the Northern Hemisphere.

Next, Gaudel wants to take a closer look at ozone in the tropics. Africa could emerge as a global hotspot for forerunners of air pollution, and IAGOS data will allow it to delve deeper into the role of that continent in recent trends. It will also compare tropical ozone measurements from IAGOS, taken above polluted regions, with measurements from the NASA Atmospheric Tomography (ATom) field campaign, which measure trace gases and aerosol particles in more distant, less polluted regions, including the tropics. And it will look at measurements of TROPOMI, an instrument aboard a satellite of the European Space Agency that collects information on atmospheric composition.

“We want to understand the variability of ozone and its predecessors and the impact of polluted regions on remote regions,” Gaudel said. “That we use the best tools we have, including IAGOS, ATom data and TROPOMI data, to get profiles and columns of ozone and its predecessors from various types of human activities and natural resources.”