The improved air quality of the Covid-19 lockdown has gone.
The most recent Waikato Regional Council monitoring results show a return to normal concentrations.
For Waikato Regional Council chairman Russ Rimmington, the lockdown results demonstrate that a move to low-emission vehicles will make "a huge difference" to air quality.
"It also shows that our continued support and encouragement for industry to improve their emissions controls is quite justified when it comes to the health of our people and environment."
The council monitors air quality in airsheds in Hamilton, Tokoroa, Te Kūiti, Taupō, Putaruru, Morrinsville and Thames.
Improvement in air quality (PM10 levels) was seen in three out of the seven airsheds over the Covid-19 Alert Level 4 lockdown period.
A 21 per cent PM10 reduction in Hamilton and 11 per cent drop in Morrinsville is likely because of a reduction in emissions from traffic and also potentially a reduction in emissions from industry, according to council scientists. The average PM10 concentrations in both towns over levels 3 and 2 are back to expected levels for this time of year.
Te Kūiti's PM10 concentrations dropped 32 per cent during the lockdown, probably because of a reduction in industry emissions and a minor contribution from less traffic. The average PM10 concentrations over combined levels 3 and 2 are back to normal.
No statistically significant changes in PM10 concentrations were observed at the other airsheds.
Council senior scientist Jonathan Caldwell said the results could have been different had lockdown been during winter.
"With people spending more time inside and temperatures starting to cool towards the end of lockdown, woodburner use was increasing, which can confound the assessment of the impact of a reduction in traffic emissions."
Caldwell compared average PM10 concentrations in all seven airsheds over level 4 lockdown with the 33-day average for the same period over the past four years. He then did the same for the 33 days spent in levels 3 and 2.
"This is a more valid approach than comparing air quality directly prior to lockdown with air quality during lockdown, as air quality can be very variable from one day to the next. This is due to differences in meteorology such as air temperature and wind speed and also differences in emissions from vehicles during a public holiday, for example.
"In New Zealand, air quality varies strongly with season, with the worst air quality experienced during the winter months when it gets colder and we experience more calm inversion type conditions coupled with a big increase in the number of woodburners operating," Caldwell said.