Northern hemisphere saw 'bad ozone' levels rise in last 20 years despite emission cap in North America, Europe
In the lower part of the atmosphere, also called the troposphere, ozone is notorious for causing harm to humans and plants
Over the last two decades, the Northern hemisphere — which is home to about 88% of the global population — has witnessed an increase in ozone levels in the lower part of the atmosphere. The finding is from a new study that used commercial aircraft to track the pollutant.
Ozone has a dual role depending on where it is found. In the upper atmosphere, it shields the Earth from harmful UV rays. But in the lower part of the atmosphere, also called the troposphere, it is notorious for causing harm to humans and plants. Helping them form are the chemicals carbon monoxide, methane, and nitrous oxide, which escape from power plants and cars. The pollutant is a component of smog — air pollution that reduces visibility.
For long, experts have relied on satellites for information on ground-level ozone, which is also known as 'bad ozone' for its negative impact on human health. However, they could not detect if the pollutant was increasing or decreasing globally in the last decade. "That is concerning, given the impacts that ozone has on climate, health, and vegetation," Dr Audrey Gaudel, an atmospheric scientist working in the NOAA Chemical Sciences Laboratory and the study's lead author, told MEA WorldWide (MEAWW).
So the team turned to commercial aircraft data to assess the ozone levels in the lower atmosphere. "They give rather regional information. But if enough regions are covered, we can get a global picture," she said, adding they traced the pollutant across the Northern Hemisphere in the study.
The analysis of 20 years of data indicated something more concerning: ozone levels shot up globally despite a cap on emissions in North America and Europe. "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," Dr Gaudel, scientist from the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder, explained.
Of all the regions, the tropics — which includes Africa, Southeast Asia and India — showed the highest spike. They speculate that this may have driven a global rise. "Ozone and its precursors (carbon monoxide, methane and nitrous oxide) are transported over long distances," she explained. "Declines of ozone in the lower troposphere above North America and Europe have been offset by polluted air from the tropics."
The team carried a single instrument to detect levels of the pollutant between 1994 and 2016 to avoid inconsistent results. They gathered data from 11 regions in the Northern Hemisphere and concluded that the median ozone values have increased by 5% per decade, on average. Next, modeling studies found that human emissions could be the culprit behind the rise.
What does the study say about the future? Dr Gaudel does not expect to see an increase in ozone pollution in the future. "The Paris Agreement signed in 2016, which encourages countries around the world to develop a green economy in order to reduce greenhouse gas emissions, also helps reduce emissions of ozone precursors," she noted.
The study has opened avenues for further investigation. Dr Gaudel said the findings have suggested that tropics could play a crucial role in driving ground-ozone levels globally. Many places in the region may lack regulations to check the pollutant and its precursors, she added.
The study is published in Science Advances.