Scientists have revealed the complete picture of how last year's lockdown temporarily cut pollution – but not to the air-clearing levels perhaps expected.
While our bold response to Covid-19 also put a dent in traffic smog by largely bringing human movement in our towns and cities to a weeks-long halt, it also showed the potential environmental impact of moving most workers to their homes.
Nonetheless, it offered scientists a fascinating opportunity to see a New Zealand with the veils of everyday air pollution pulled back.
Last year, one Niwa scientist said it would likely be least 15 to 20 years before Kiwis breathed the same levels of cleaner air as they did during lockdown.
"The fact that there was a massive shift in everyone's day-to-day activities offered the opportunity to investigate what air quality is like when cars are off the road, when activities slow down," said Dr Nancy Golubiewski, a principal scientist at the Ministry for the Environment (MfE).
"Similar to so many places around the world, it was a moment that called for investigation – to make sure we analysed ongoing measurements in the context of the alert levels for whatever insights or lessons there were to learn."
A just-published study, led by Golubiewski and colleague Dr Nick Talbot, showed how levels of pollution shifted with each level of lockdown, teasing out the observed drops in key pollutants like fine and coarse particulate (PM2.5 and PM10), nitrogen dioxide (NO2) and black carbon.
"Part of MfE's remit is to understand environment across the motu, and we have ongoing work in pulling together national data sets," Golubiewski said.
"So, we paired our work in pulling together national data sets with our curiosity and interest in understanding the pandemic's effect on the environment, to learn what the implications of that may be, with a specific focus on air quality for this project."
Talbot said the selected testing sites monitored a range of different pollutants, such as smoke from domestic fires and roadside emissions from vehicle exhausts.
"We then compared the pollution levels that were observed during alert levels to estimates of what pollution levels would have been during a 'normal' year."
That included making comparisons with the average over the previous five years, and also using machine learning estimates that could account for differences in meteorology, such as temperature and wind, which brought their own influences.
Soon after alert level 4 kicked in and cleared traffic, the scientists found NO2 levels halved, falling to 48 to 55 per cent of their long-term mean.
When restrictions were relaxed slightly, mean NO2 concentrations moved again, shifting to around 45 per cent below normal levels across Auckland – but only to about 4 per cent below normal across the lower North Island.
And after New Zealand moved to level 2 on May 25, the return of traffic was marked with NO2 levels climbing to about 20 per cent below normal in the upper north.
Interestingly, in Auckland's Queen St, NO2 didn't rebound so quickly – something that could have been in response to expanding pavement space along the street.
Talbot said this would have moved vehicles – and therefore pollutants - further away from the shops, pavement and monitoring equipment, "allowing for increased dilution and dispersion and reducing measured concentrations".
Levels of black carbon – ultra-fine carbon particles associated with a range of health problems – similarly fell away over lockdown, but also not to the extent expected.
That there was much NO2 and black carbon still about, even as cars sat in driveways, could have owed to the fact that buses and delivery vehicles were still operating, Talbot said.
"This meant that reductions in NO2 and black carbon were not as great as may be expected, as diesel vehicles emit proportionally more NO2 and a lot more black carbon particulate than petrol vehicles."
As for levels of PM2.5 and PM10, the researchers didn't find any especially striking falls.
For instance, level 4 brought drops in PM10 which, across the country, ranged only between 11 and 34 per cent – and the reductions below normal were even smaller under level 3.
Further, they found levels rose considerably more than typical over the two weeks Kiwis spent under level 2.
"Even though weather conditions were mild, this could be in part due to more people lighting fires, simply because they were at home more," Talbot said.
But he also pointed out that PM10 concentration changes weren't as dependent on human behaviour anyhow.
"This is because there are many natural sources of PM10, such as sea salt and dust, that are more dependent on other factors, such as weather and wave size.
"However, smoke from solid fuel burning for heating greatly impacts PM2.5 - and PM10 - concentrations in many towns across New Zealand."
He said the data appeared to indicate there could have been more fires lit when more people were working from home.
"If so, this would be a significant finding if the 'new normal' of working remotely persists post Covid-19."
The study, published in the journal Atmospheric Environment, wasn't the first to look at the lockdown's effect on air quality.
Another University of Auckland paper also found that an 80 per cent drop in car traffic on city roads didn't force down pollution by the same amount, because of the outsized emissions from trucks and buses that kept running.
StatsNZ data has also shown how lockdown put a slight dent in greenhouse gas emissions last year, with a 4.8 per cent drop across 2020 - and a "citizen science" project even used Kiwis' backyard grass clippings to measure the carbon cut.