A cloud of noxious particles brewing in the air above the Alberta oil sands is one of the most prolific sources of air pollution in North America, often exceeding the total emissions from Canada's largest city, federal scientists have discovered.

The finding marks the first time researchers have quantified the role of oil sands operations in generating secondary organic aerosols, a poorly understood class of pollutants that have been linked to a range of adverse health effects.

The result adds to the known impact of the oil sands, including as a source of carbon emissions that contribute to climate change. It also comes on the same day that the Bank of Canada delivered a sobering message about the country's economy, saying the devastating Alberta wildfires that hit Fort McMurray – leading to production cuts in the oil industry and the destruction of thousands of buildings – will cause a drop in Canada's gross domestic product in the second quarter.

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Given the economic circumstances and the political sensitivities currently surrounding the oil sands, the air pollutant study, published Wednesday in the journal Nature, offered the strongest test yet of the Trudeau government's promise to allow scientists in federal labs to speak freely with journalists about their results.

The pollutants the scientist measured are minute particles that are created when chemical-laden vapours from the mining and processing of bitumen react with oxygen in the atmosphere and are transformed into solids that can drift on the wind for days.

While researchers have long thought that the oil sands must be a source of such particles, the new results show that their impact on air quality is significant and of potential concern to communities that are downwind.

"It's another aspect that can and probably should be considered" in assessing the oil sands' environmental footprint, said John Liggio, an atmospheric chemist with Environment and Climate Change Canada and lead author of the study.

Using an aircraft bristling with sophisticated sensors, Dr. Liggio and his colleagues flew back and forth repeatedly through the largely invisible plume of emissions that extends from the oil sands in order to record the concentrations of a wide range of pollutants. The measurements were made in the summer of 2013, and gathered during nearly 100 hours of flying time over the oil sands and adjacent boreal forest.

"It's not for the faint of heart – or stomach," Dr. Liggio said of the low-level flights he and his colleagues endured during the study.

The airborne data, supported by further work with computer models and laboratory experiments, show that 45 to 84 tonnes of secondary organic aerosols are formed by the oil sands a day. By comparison, Canada's largest urban area, which includes Toronto and surrounding municipalities, generates 67 tonnes a day, much of it derived from car and truck exhaust.

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"The take-away is that there's more that's emitted into the atmosphere than we've fully appreciated," said Jeffrey Brook, an air-quality researcher with Environment and Climate Change Canada who participated in the oil sands study.

In 2014, the federal and provincial governments jointly issued standards for long-term average exposure to fine particulate matter. The emissions of secondary organic aerosols measured from the oil sands do not appear to exceed those long-term standards, but they do suggest that people living within reach of the emissions are experiencing elevated levels of fine particles in the air they breathe.

Scientists are still trying to understand the complex health effects those particles can trigger when inhaled, but they have been linked in previous studies to lung cancer, cardiovascular disease and diabetes.

The oil sands aerosols are also similar in abundance to those that U.S. researchers recorded rising from the massive oil spill caused by the Deepwater Horizon drilling-rig disaster in the Gulf of Mexico in 2010.

"However, the oil spill lasted a few months. The Alberta oil sand operations are an ongoing industrial activity," said Joost de Gouw, a research physicist with the U.S. National Oceanic and Atmospheric Administration who led the oil spill measurements.

Dr. de Gouw called the Canadian team's work "convincing," and added that air-quality researchers were becoming increasingly interested in the formation and effects of secondary organic aerosols, which constitute a growing fraction of the air pollution generated in North America and Europe from industrial sources as sulphur emissions decrease.

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Terry Abel, the director of oil sands at the Canadian Association of Petroleum Producers, said the new study will help the energy industry better understand the origins of the particles.

"We already know there are particles in the atmosphere. This is about where did they actually come from and how were they formed," he said in an interview. "If we're seeing a particular effect from particulate matter, this now gives us some clue as to what we would need to do to reduce those emissions or mitigate [them]."

Alberta Premier Rachel Notley's government says it supports the oil sands industry and argues that the province will have an advantage if it can better prove it takes environmental side effects seriously.

The Nature study is one of the most high-profile scientific papers to come out of Environment Canada's air-quality research division in some years. Dr. Liggio and his colleagues responded directly to questions from The Globe and Mail without a lengthy waiting period for permission to conduct interviews and without government officials monitoring their calls. Such practices were routine under the former Conservative government whenever journalists asked to speak with federal scientists about their published research.

With a report from Carrie Tait