"Our goal is to provide the information that cities need to assess their emissions and to help them evaluate whether their mitigation actions are effective," she said.
"For example, many Wellingtonians fill up on the Kapiti Coast because petrol is cheaper there, but they drive their cars and emit greenhouse gases mostly in Wellington where they live and work."
Previously, scientists guessed they knew city emissions with about 50 per cent uncertainty, which Turnbull acknowledged wasn't terribly useful in most cases.
A recent study she co-authored proved the first to actually determine how well we could measure emissions from cities.
In it, the researchers compared three largely independent methods: the traditional industry reporting method, and two different methods that use measurements of greenhouse gases in the atmosphere to determine the emission rate.
By comparing the three methods, they were able to show that they could measure emissions for an individual city with an accuracy of better than 10 per cent.
"That is enough to detect the emissions reductions that many cities have committed to."
Now, scientists are keen to understand precisely how much fossil fuel derived-CO2 was being absorbed by the land carbon sink within the city.
This sink worked through the balance of absorption of CO2 by plants using photosynthesis and CO2 release by respiration, but Turnbull said it was difficult to get a clear picture of exactly how it was functioning.
Only plants photosynthesised, but respiration came from animals and soils as well, and was harder to measure.
"We know a fair bit about the land carbon sink over the world as a whole, but there has been very little research to understand what goes on in cities anywhere in the world," she said.
"Until recently, most people either assumed that cities have no plants and therefore no land carbon sink, or that the urban ecosystem behaves the same as the surrounding rural areas.
"But of course, those are clearly wrong."
It was known, however, that Auckland was different than the other cities around the world where greenhouse gas emissions had been assessed in detail.
That was because Auckland's subtropical climate meant that it was green and growing all year round.
"We don't yet know whether that translates to larger uptake of CO2 by the urban ecosystem."
In Auckland, GNS Science is leading a pilot Auckland Carbon Emissions (ACE) study to develop a full carbon budget for Auckland City.
"The good news is Auckland's carbon sink may be offsetting more than we first anticipated," she said.
"This will be one of the few studies in the world using very precise radiocarbon measurements to quantify an urban carbon budget."
Every few months, scientists have been gathering air samples from around 25 sites around the city, measuring the CO2 concentration, and then radio-carbon dating the collected CO2.
That allowed them to determine how much of the CO2 stemmed from fossil fuels, and how much was coming from land carbon.
At the same time, they've been developing detailed models of Auckland's fossil fuel emissions and of the land carbon exchange.
"We're working with international researchers to bring the state of the art models to New Zealand," Turnbull said.
"Then we compare what we measure in the atmosphere with what the models tell us."
Where they find differences, they can go back and refine the models until they get agreement with the measurements.
And by combining the measurements and models, they'll get the best information possible.
"The atmospheric measurements tell us the net total of what ends up in the atmosphere, but they give only limited information about how it got into the atmosphere," she said.
"The models have lots of detail about the processes that produce the emissions, but they can miss sources or make incorrect assumptions in scaling up processes to the city scale.
"Together, we are able to overcome many of the issues with each method and get an accurate picture of not only total emissions, but where and when they come from."
The results so far suggested that there was indeed a land carbon sink in Auckland, and the team expected to provide a first estimate of the size of the land carbon sink later this year.
Meanwhile, GNS Science has joined Niwa, Landcare Research and the University of Waikato, for a wider project to determine the full carbon budget for the entire country.
Called CarbonWatch-NZ, the project evaluated CO2 and methane emissions as a whole, and focused on urban areas, farmlands and forests.
