Ozone in the northern hemisphere has increased over the past 20 years

In a first-ever study using ozone data collected by commercial aircraft, researchers at the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado at Boulder found that pollutant levels in the lower end lower Earth’s atmosphere had risen across the Northern Hemisphere over the past 20 years. This is all the more so as tighter controls on emissions of ozone precursors have reduced ground-level ozone in some places, notably in North America and Europe.

Ground-level ozone – the 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 people’s lungs and damage plants.

In a study published today in the journal Scientific advances, the team saw an overall increase in ozone levels over the northern hemisphere. “This is a big problem because it means that if we try to limit our pollution locally, it might not work as well as we thought,” said Audrey Gaudel, CIRES scientist working at the chemical science laboratory of the NOAA and lead author of the study. She and her colleagues have documented the largest increases in ozone in the tropics, Gaudel said, noting that ozone exported from the tropics could lead to increases over other areas of the northern hemisphere.

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

Previous studies have not been able to draw firm conclusions about ozone trends in the Northern Hemisphere, according to Gaudel, because there are too few long-term monitoring sites and because new satellites with coverage almost global have provided conflicting results on ozone trends.

The researchers therefore turned to data from European in-service aircraft for the Global Observing System (IAGOS) program. “Since 1994, IAGOS has measured ozone around the world using the same instrument on all planes, giving us consistent measurements over time and space, from the Earth’s surface to the upper troposphere. “Said Gaudel. Between 1994 and 2016, commercial aircraft captured 34,600 ozone profiles, or about four profiles each day.

Gaudel and his colleagues used these measurements to calculate changes in ground-level ozone from the mid-1990s to 2016 over 11 regions of the northern hemisphere. They found an overall increase in ozone in all regions they looked at, including four in mid-latitudes, 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 over some mid-latitude regions, including Europe and the United States, where emissions ozone precursors have decreased. The researchers found that these reductions were offset by higher increases in the troposphere – the net result being an overall increase in surface ozone at 12 km.

To understand what was causing the observed changes in ozone, the researchers looked at emission inventories of one of the major precursors of ozone – nitrogen oxides (NOx) – used as an input for the transport model. of the MERRA-2 GMI global chemistry, which faithfully reproduces the IAGOS measurements. The model showed that the increase in anthropogenic emissions in the tropics was probably the cause of the observed increase in ozone in the northern hemisphere.

Next, Gaudel wants to take a closer look at ozone in the tropics. Africa may be emerging as a global hotspot for air pollution precursors, for example, and data from IAGOS will allow her to deepen the continent’s role in recent trends. It will also compare IAGOS’s tropical ozone measurements, taken over polluted regions, with measurements from NASA’s atmospheric tomography campaign (ATom), which measured traces of gas and airborne particles. aerosol in more remote and less polluted areas, including the tropics. And it will examine measurements from TROPOMI, an instrument aboard a European Space Agency satellite that collects information on atmospheric composition.

“We want to understand the variability of ozone and its precursors and the impact of polluted areas on remote areas,” Gaudel said. “So we are using the best tools we have, including IAGOS, ATom and TROPOMI data, to obtain profiles and columns of ozone and its precursors from different types of human activities and natural sources.