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Where does urban mercury come from?

07 Aug 2018

Researchers in Canada have produced the first concurrent spatial assessment of gaseous mercury concentrations across a city. Their study of the Greater Toronto Area used cheap passive air samplers to identify minor sources of leaked urban mercury, termed fugitive emissions.

Map of mercury concentrations in Toronto

Urban areas emit mercury from fossil fuel combustion, metal manufacturing, cement and caustic soda production, medical and industrial waste, and cremations. Other sources may be significant too but have been hard to quantify. The UN’s Minamata Convention on Mercury adopted in 2013 calls for complete emissions inventories and better monitoring networks for mercury.

“Up to now do we really have a great handle on what these fugitive emissions could be in a city? Not really,” says Carl Mitchell of the University of Toronto Scarborough, Canada. “The assumption is that the vast majority of emissions to the environment are from the major known point sources.” Sources like coal-fired electricity plants and factories.

Such emissions are of serious concern because exposure to mercury causes severe neurological damage. Mercury itself is highly volatile, meaning it evaporates like water into the atmosphere. Once there it travels the globe; pollution from Europe, for example, can contaminate food chains in Canada.

To understand the role of fugitive emissions, Mitchell and his colleagues deployed 145 small passive air samplers across the Greater Toronto Area in July and August 2016. Each sampler cost about $10, didn’t require an energy source or compressed gases, and provided time-averaged concentration data.

A total of 58 staff and students of the University of Toronto Scarborough put a sampler at their home; at other sites the team attached samplers to a utility pole or tree. 43 of the samplers were near potential mercury sources – waste/recycling centres accepting mercury-containing products, crematoria, which may release mercury from dental amalgams, and hospital/dental facilities.

The team also deployed samplers in the summer of 2016 or 2017 near known sources of mercury – facilities within 100 km of downtown Toronto that report their atmospheric mercury emissions to Canada’s National Pollutant Release Inventory, including a waste facility, two steel plants, a cement producer and a wastewater treatment plant.

In this way, the team created a more detailed map of the emission sources in the Greater Toronto Area, and identified several small fugitive emitters in the city, including waste and recycling, and hospital/dental facilities.

Some results were surprising. A couple of sites reported by government sources to be high mercury emitters were, according to the samplers, low emitters. And other presumed low emitters measured higher concentrations of gaseous mercury than expected. Mitchell believes this speaks to the need for better methods and accuracy in government reporting on atmospheric mercury. The good news, according to the researchers, is that the passive samplers the team employed may be the perfect solution.

“Without this instrument, I feel, it is really difficult to otherwise get this information,” said Mitchell. Traditionally, scientists have deployed more expensive, active instruments at just a few sites in urban areas to monitor atmospheric mercury over the long term.

The samplers are cheap and easy to use while still providing accurate measurements. To begin to address fugitive emissions, explains Mitchell, “you need to be able to tell the difference between two relatively small numbers” – the source emission level and the background level – “and the really powerful thing we observed was we could statistically see those differences”.

Comparing the team’s findings with previously published studies, Mitchell says, “we’ve seen a definite reduction in the amount of mercury in the air over the past ten years or more”. He credits this to mitigation efforts already in place in Canada, such as the elimination of all coal-fired power in Toronto’s home province of Ontario.

According to Mitchell this work also provides an interesting case study for the use of passive samplers in the challenging urban environments of India and China, where they use more mercury in industry. “You could do a similar study there,” he says, “and it might actually be quite eye-opening.”

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