Harmful ozone levels have decreased over past 2 decades in Nova Scotia, says study

Levels of a potentially dangerous air pollutant have decreased in Nova Scotia over the past two decades, according to the first long-term analysis of surface ozone in the province.

In the upper atmosphere, naturally occurring ozone is a good thing because it blocks harmful ultraviolet rays.

But at ground (or surface) level, it's a harmful pollutant that breaks down lung tissue and damages crops. Pollutants emitted by power plants, cars, refineries and other sources react in sunlight and heat are created by a chemical reaction between nitrogen oxide and volatile organic compounds.

"We had a strong decreasing trend of ozone over the long-term period," said Morgan Mitchell, a researcher at Saint Mary's University in Halifax.

She's the lead author of a recent paper published in the journal Atmospheric Environment, which documents her master of applied science work in the Atmospheric Research Group of professor Aldona Wiacek. The study looked at hourly ozone levels and the so-called "precursor gas" levels between 2000 and 2018 at three monitoring stations in Nova Scotia.

The key findings are that locally-created ozone has decreased, and so has the amount of ozone pollutants blown into Nova Scotia from the northeastern United States. The latter finding is especially notable because ozone can be transported long distances and Nova Scotia is considered at the tailpipe of North America.

The study found this "transported" pollution now makes up a greater proportion of the surface ozone, accounting for 45 to 63 per cent of ozone levels in Halifax.

Mitchell attributes the overall decrease in ozone levels to regulations that cut emissions from sources like power generation and vehicle exhaust.

What Nova Scotia Power is doing

Nova Scotia Power has reduced its nitrogen oxide emissions by 58 per cent since 2005, said spokesperson Jacqueline Foster in a statement. She said the utility has installed low nitrogen oxide emissions systems on some of its thermal generating stations. Foster said further reductions will be seen as the utility weans itself off coal by 2030.

The study found precursor emissions in the Halifax area spike during morning rush hour and dissipate.

Mitchell said individuals can help help reduce the amount of locally-produced ozone.

"We can drive less cars, we can shop more locally, so less goods are being transported in the city from elsewhere and that will improve our air quality," she said.

More research being carried out

In the last five years of the study period, there was a reduction in the rate of the ozone decrease. Mitchell does not know why, saying it could be population growth or more people driving.

She said 70 per cent of the pollutants from the United States pass over Nova Scotia and never drop to the surface. Why that's the case is being researched.

"We really want to be able to better predict these events of high ozone so that we can warn vulnerable populations," said Mitchell.

"And to do that, we have to better understand when these transport events are happening and what is bringing that pollution from up above down to the surface."