The dramatic emptying of Auckland's motorways over the first lockdown has allowed scientists an unprecedented look at the smoggy footprint of different vehicles on city roads.
While car traffic on city roads was cut by 80 per cent after the country moved to alert level 4 on March 27, pollution didn't fall by the same amount because of outsized emissions from trucks and buses that kept running.
The first lockdown period proved a golden opportunity for scientists to measure air quality around a city suddenly freed of vehicle pollution.
University of Auckland researchers got out and recorded levels of the five main pollutants: fine and coarse particulate (PM2.5 and PM10), black carbon (BC), ozone (O3) and nitrogen dioxide (NO2).
PhD candidate Hamesh Patel and Associate Professor Jenny Salmond gathered readings from central Auckland, at Queen St and Customs St, suburban Henderson and rural Patumahoe.
They also compared meteorological conditions during the month to past years and found wind levels to be similar to previous years, although rainfall was historically low for Auckland.
Their just-published figures show that, during level 4, light vehicle traffic volume dropped by an average of 80 per cent per day compared to the previous year.
There was a reduction in the number of heavy vehicles – mainly diesel buses and trucks - of 60 per cent.
Overall, they found a marked reduction in recorded levels of ambient air pollution during lockdown.
NO2 levels, mostly from the combustion of fossil fuels from motor vehicles, fell by 34.1 per cent in central Auckland, 56.9 per cent at Henderson and 35.6 per cent at Patumahoe.
One reason for the smaller reduction in NO2, particularly at the Auckland central site, was thought to be that buses kept running, as did trucks moving to and from the port.
Salmond said this showed it mattered which type of vehicles were taken off the roads.
"Although we reduced traffic by up to 80 per cent, pollution wasn't cut by the same amount and that's because buses and trucks account for a higher proportion of NO2 emissions," she said.
"This is important when make decisions about how to manage air quality in urban areas. If lockdown showed us one thing, it was that we can make significant improvements in air quality by reducing traffic flows.
"But these results suggest that gains in air quality can also be made by opting for electric buses and lower-emission buses and trucks."
Patel said the research also highlights a lack of detail in previous emissions inventories, with some pollutant and emissions sources potentially misrepresented or under-reported.
"This meant that what was thought to be known about pollutant and emissions sources didn't align with what was observed during lockdown," he said.
"Increased monitoring of the ambient air is therefore crucial in improving our knowledge and understanding and so regulators can make better informed decisions to provide safer and healthier air for us all to breathe."
Earlier, Niwa air quality scientist Dr Ian Longley estimated traffic pollution in Auckland, Wellington and Christchurch fell away to just 25 to 35 per cent of normal over level 4.
"Lockdown has provided vivid confirmation of how in New Zealand cities, isolated from each other and international neighbours, and where heavy industry is largely absent, many pollutants can be made to almost disappear overnight," he said.
"But although air quality changed dramatically across the cities we monitored - and probably all other towns and cities too - the benefits would not have been experienced equally."
He estimated while pollution was down by three quarters on average, at least a third of Aucklanders reduced their exposure to traffic pollution by 90 per cent during lockdown.
"This gain could have been extended to a few hundred thousand more people if diesel trucks and buses had been removed from the city centre."
Unless the way we worked and travelled changed, he said, it was likely it would be at least 15 to 20 years before New Zealanders experience the same levels of clean air as those achieved during lockdown.
Previous analysis showed that "business as usual" improvements in vehicle emissions technology means we may achieve similar air quality as during lockdown, some time in the late 2030s "if at all".