Baseline levels of ozone are continuing to creep up in the Lower Mainland despite reduced emissions of the key air pollutants that combine to cause smog.
Metro Vancouver officials say their staff and scientists from UBC and Environment Canada are trying to understand the discrepancy and why average levels of ground-level ozone haven't also declined over the past 10 years.
"This is a disturbing trend," said Roger Quan, Metro's air quality policy manager. "There isn't a good explanation."
He said rising levels of background ozone wafting here from across the Pacific Ocean may be the cause.
The average increase being observed does not apply to peak ozone levels measured on the worst air quality days each summer – those highest readings have steadily trended downward for 20 years.
Metro has just released a report titled Caring For Our Air that paints a generally optimistic picture of the region's trend of improving air quality over the past two decades, much of it due to cleaner vehicles, less sulphur in fuels and initiatives like AirCare.
The report shows ambient levels of fine particulate, sulphur dioxide, nitrogen dioxide and carbon monoxide levels have all declined by at least 20 per cent since 2001, while ozone is up nearly 20 per cent.
Ground-level ozone is caused by the reaction of nitrogen dioxide and volatile organic compounds, typically from solvents and fuels, in the presence of sunlight.
Emissions have been steadily declining from the three biggest sources of smog precursors – light vehicles, non-road engines and solvents.
But growth in population and traffic through the region mean the recent gains are expected to recede and air quality could worsen from 2020 through 2030.
Abbotsford Mayor Bruce Banman said Fraser Valley residents who once couldn't see Mount Baker on a clear day know the smog levels have improved.
"We can see it," he said. "It's clearly getting better."
But he said the gains have been hard-won and must be maintained.
Banman chastised Metro staff for failing to include the region's plans for a new garbage incinerator in the report, calling it a project that must be subjected to the highest level of scrutiny.
"The science has to be iron clad as to whether or not a waste-to-energy plant is a good move or not. I am not so sure yet that we have got good science."
He said air quality is important for reasons beyond human health, pointing to damage to agricultural crops from ozone and to tourism.
"If tourists can't see the mountains, they go somewhere else," Banman said.
There were no air quality advisories due to smog last year.
But Metro officials say the 2011 numbers should be taken with a grain of salt, because there were no major air quality inversion to trap ozone in the Fraser Valley and fuel smog formation.
Nor were there major wildfires that sent smoke here from elsewhere in the province.
Metro is forecasting further emissions reductions will come with the enforcement of more stringent federal regulations for large ships.
The regional district has also begun enforcing a system of permits and hefty fees to prod operators of backhoes, forklifts and other non-road diesel engines to upgrade old heavily polluting models.
Maps courtesy of Metro Vancouver Caring for the Air report