The devastating Australian wildfires at the beginning of 2020 "impacted the atmosphere in a way that's never been seen before", according to NIWA, and potentially damaged Earth's protective ozone layer.
The Black Summer fires of 2019-20 burnt approximately 14.3 million hectares of land, destroyed 3000 homes, killed at least 34 people and up to 3 billion animals - and produced previously unseen changes in the stratosphere, which is roughly 16-20 km above the Earth's surface and where one third of the ozone layer resides.
The main changes in the months following the bushfires involved several chlorine-containing gases, including a significant drop in hydrogen chloride (HCI) and a rise in chlorine monoxide (CIO), NIWA said in a statement on Tuesday.
HCI "can be converted into a reactive form that destroys ozone", the statement added, and these gases "usually do not change much in this region (the stratosphere)".
Dr Susan Strahan, the study's lead, said "this is worrying because anything that messes with the chlorine family of gases - like HCI - has the potential to harm the ozone".
"We were unable to replicate what we were seeing in any models, meaning that the reactions taking place in the stratosphere on these smoke aerosols are unknown. We don't know what they are, and we can't calculate their effects on ozone.
"Just one atom of chlorine can destroy over 100,000 ozone molecules before it is removed from the atmosphere. From the observations of elevated chlorine monoxide (ClO), we think that ozone in the mid latitudes experienced some depletion following the 2020 fires."
Dan Smale, a NIWA researcher, said "the Australian bushfires were the biggest and most destructive ever recorded and seem to have caused unknown reactions affecting ozone chemistry, which is both tantalising from a science point of view and worrying from an environmental point of view".
"Our findings identify a knowledge gap in the processes that control ozone. With wildfires predicted to become more frequent and intense as the planet warms, the likelihood of ozone depletion is increased.
"Laboratory studies on the chemical reactivity of wildfire smoke particles are urgently needed."