Here’s a dirty little secret about those greenhouse gas emissions measurements that cities are gathering up in their quests for green credibility: they’re not necessarily that accurate.

That’s because they’re based on a long list of fuzzy statistics like fuel consumption, electricity usage, commute miles traveled and the like, all compiled into spreadsheets that use a bewildering cast of calculations to come up with a single number. Because they rely on data that’s collected annually, they’re also usually about a year or so behind the times.

Mike Woelk, CEO of Sunnyvale, Calif.-based startup Picarro, thinks it would be better to actually measure the city’s air itself, to see how many parts per million of CO2 are in it over the course of real time. As for how many sensors it takes to capture an entire city’s worth of atmosphere and its real-time carbon load, he thinks six will do to count London’s CO2 footprint during the 2012 Olympic Games -- as long as at least one of them is on a plane and another on a bus.

That’s the gist of a research project announced Thursday that’s seeking to put a new level of scientific certainty to the business of counting carbon. Under the partnership, Picarro is supplying its cavity ring-down spectroscopy (CRDS) instruments to EADS Astrium, a division of the European conglomerate that also owns Airbus, in a project that will seek to capture real-world GHG measurements before, during and after the Olympics.

It’s all a test of Astrium EADS’ new Emissions Measurement Service (EMS), which combines stationary, mobile (bus) and airborne (plane) sensors to capture the inputs and outputs of CO2 from a specified area -- in this case, London’s city limits. That may seem like a hopelessly complex task, but Woelk said it can be done, if you factor in the wind.

Think of a city’s airspace as a box without walls, he said. To define boundaries to this entity you’re trying to measure, a sensor network has to figure out how the wind moves the carbon molecules through that airspace, he said. Catching the input and the output makes determining what’s in the middle easier.

“If you understand how wind is moving in and out of cities,” he said in a Wednesday interview, “you can understand what those boundary conditions are, and the rate of transport of molecules.” In that sense, the wind is “a delivery mechanism that brings those emissions to our analyzers,” he said.

Picarro’s CRDS instruments, which cost about $70,000 apiece, take constant readings of carbon dioxide (CO2), carbon monoxide (CO), and methane (CH4) concentrations at their strategic locations, and that data is then turned into a model of the city’s global warming picture, with results that should be far more accurate than today’s models, he said.

Indeed, sometimes those real-air emissions can conflict with official statistics from old-school methodologies. During a project deploying sensors during this January's World Economic Forum in Davos, Switzerland, Picarro found that the city’s carbon inventory mechanisms were underreporting emissions by about 30 percent.

Cities are to account for about 70 percent of manmade global warming gases, making them a natural target for measurement. Whether cities are willing to pay extra for sensors to get a true measurement of their carbon footprints remains to be seen, though Woelk noted that the benefits of an always-on source of real-time atmospheric measurements add up over time, in terms of fewer highly paid engineers spending frustrating hours compiling compliance figures from year-old data.

Picarro, founded in 1998, has raised $48 million in venture capital investment, with investors including Benchmark Capital, Greylock Partners, SkyMoon Ventures, Staenberg Venture Partners and Weston Presidio Capital. Woelk said in January that Picarro was profitable and selling equipment globally.

Customers include the California Air Resources Board, Stanford University, the United Nations' World Meteorological Organization and Waste Management. Announced deals include a $5 million contract in 2011 with Earth Networks, formerly AWS Convergence Technologies, which owns the forecasting service.

Mobile sensors are something Picarro specializes in. Utility Pacific Gas & Electric is mounting the company’s devices on cars, including a few Chevy Volts, to sniff for gas leaks, for example. Clearly, having a plane-mounted sensor is critical for measuring atmospheric gas concentrations. To match moving sensors to data models, Picarro uses GPS to take second-by-second snapshots of gases detected within up to one meter accuracy, then calculates for wind speed to home in on the source.

In the case of the London project, scientific partners including France’s Laboratoire des Sciences du Climat et de l’Environnement (LSCE) and the U.K.’s National Centre for Earth Observation and National Physical Laboratory (NPL) Centre for Carbon Measurement, are helping to crunch the data and vet the results. Woelk wouldn’t say whether or not Picarro was talking to Astrium EADS about further development of the technology for commercial purposes.