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Scientists Record Increased Ozone Levels Across Northern Hemisphere

Scientists from the Cooperative Institute for Research in Environmental Sciences (CIRES) have discovered, for the first time, that the levels of the pollutant ozone have increased in many parts of the Earth’s atmosphere over the Northern Hemisphere in the past two decades.

IAGOS makes global observations of atmospheric composition from commercial aircraft. Between 1994 and 2020, IAGOS collected measurements on more than 62,000 flights worldwide. Image Credit: In-Service Aircraft for the Global Observing System.

This observation was made using the ozone data obtained by commercial aircraft. That scenario persists even as more rigorous controls on ozone precursor emissions have decreased ground-level ozone in certain areas, including Europe and North America.

Tropospheric ozone—that is, ozone that exists between the Earth’s surface and 12 to 15 km above Earth—is an air pollutant and a greenhouse gas that, at increased concentrations, can damage plants and affect people’s lungs.

The researchers have discovered that ozone levels have increased considerably above the Northern Hemisphere. The study was recently published in the Science Advances journal.

That’s a big deal because it means that as we try to limit our pollution locally, it might not work as well as we thought.

Audrey Gaudel, Study Lead Author and Scientist, NOAA Chemical Sciences Laboratory, Cooperative Institute for Research in Environmental Sciences

Gaudel added that she and her collaborators recorded the highest increase in ozone levels in the tropics and noted that ozone exported from the tropics could be driving such increased ozone levels above other regions of the Northern Hemisphere.

Gaudel and the co-authors of the study, CIRES researchers in NOAA and international collaborators, also observed the most prominent increases in regions where ozone concentrations were lowest at one point in the past: for instance, India, Southeast Asia, and Indonesia/Malaysia.

These areas had extremely low ozone levels between 1994 and 2004, and in the recent past, they had extremely high ozone values between 2011 and 2016.

According to Gaudel, earlier studies could not reach a solid conclusion regarding ozone trends in the Northern Hemisphere. This was because there are not many long-term monitoring sites and the latest satellites with near-global coverage have given contradictory outcomes relating to ozone trends.

Therefore, the team turned to aircraft data collected from Europe’s In-Service Aircraft for the Global Observing System (IAGOS) program.

Since 1994, IAGOS has measured ozone worldwide using the same instrument on every plane, giving us consistent measurements over time and space from Earth’s surface to the upper troposphere.

Audrey Gaudel, Study Lead Author and Scientist, NOAA Chemical Sciences Laboratory, Cooperative Institute for Research in Environmental Sciences

From 1994 to 2016, as many as 34,600 ozone profiles, or around four profiles per day, were captured by commercial aircraft.

Along with her collaborators, Gaudel utilized these measurements to estimate the variations in the tropospheric ozone from the mid-1990s to 2016 above 11 areas in the Northern Hemisphere.

The team observed an overall increase in ozone levels in all the regions where they investigated, including two in the tropics, two in the subtropics, three equatorial areas, and four in the mid-latitudes. There was a 5% increase in median ozone values on average for every 10 years.

The researchers also observed that in the lower troposphere—that is, the atmospheric layer closest to the surface of the Earth—ozone levels had reduced above certain mid-latitude areas, such as the United States and Europe, where emissions of ozone precursors have reduced.

They noted that such reductions were compensated by increased ozone trends higher in the troposphere—with the net outcome being an overall increase in ozone levels from the surface to 12 km.

To infer the factors that were causing the observed ozone variations, the team looked at the emissions inventories of nitrogen oxides (NOx)—one of the major ozone precursors utilized as input for the global chemistry transport model called MERRA-2 GMI; this model precisely simulates the IAGOS measurements.

The model demonstrated that increased anthropogenic emissions in the tropic regions could be driving the increased ozone levels observed in the Northern Hemisphere.

Gaudel has now planned to take a closer look at the ozone levels in the tropic regions. For instance, Africa may be emerging as a universal hotspot for air pollution precursors, and IAGOS information will allow Gaudel to look deeper into the continent’s role in the latest ozone trends.

In addition, Gaudel will compare the tropical ozone measurements taken from IAGOS, collected above the polluted areas, with measurements made from the NASA Atmospheric Tomography (ATom) field campaign. This campaign quantified aerosol particles and trace gases in less polluted and more distant areas such as the tropics.

Gaudel will also look at the measurements made from TROPOMI—an instrument onboard a European Space Agency satellite collecting data on atmospheric composition.

We want to understand the variability of ozone and its precursors and the impact of polluted regions on remote regions. So we’re using the best tools we have, including IAGOS, ATom data, and TROPOMI data, to get profiles and columns of ozone and its precursors from different kinds of human activities and natural sources.

Audrey Gaudel, Study Lead Author and Scientist, NOAA Chemical Sciences Laboratory, Cooperative Institute for Research in Environmental Sciences

Journal Reference:

Gaudel, A., et al. (2020) Aircraft observations since the 1990s reveal increases of tropospheric ozone at multiple locations across the Northern Hemisphere. Science Advances.


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