High Nitrous Oxide Levels in California

With a new method for estimating greenhouse gases, researchers have found that the levels of nitrous oxide, a potent greenhouse gas, are much higher than previous predictions.

Lawrence Berkeley National Laboratory (Berkeley Lab) researchers estimated greenhouse gases by using a new model that combines atmospheric measurements with model predictions. According to the study, nitrous oxide levels in California may 2.5 to 3 times greater than previously expected. At that level, total N2O emissions would account for about 8 percent of California's total greenhouse gas emissions.

"If our results are accurate, then it suggests that N2O makes up not 3 percent of California's total effective greenhouse gases but closer to 10 percent," said Marc Fischer, lead researcher on both studies. "And taken together with our previous estimates of methane emissions, that suggests those two gases may make up 20 to 25 percent of California's total emissions. That's starting to become roughly comparable to emissions from fossil fuel CO2."

Worldwide levels of N2O have been rising rapidly for decades, and the major culprit was recently confirmed to be the heavy use of nitrogen fertilizers to grow the world's food. Other sources of N2O emissions include wetlands, animal and industrial waste and automobiles, but give off a less significant amount of emissions.

The standard method for estimating emissions levels has been to do what is called a "bottom-up inventory. This process involves listing all the activities that emit N2O, assigning an emission factor for each activity, and then tallying up the emissions.

"The biogeochemical processes that produce N2O are sensitive to environmental conditions and very small changes in things like temperature, moisture, the type of soil and when the fertilizer is applied," Fischer said. "All those factors can result in big differences in the amount of N2O that's produced. If you try to use a single number for a given patch of land, you're almost certainly going to get a variable result."

As a way to verify the method, Fischer and his team are currently comparing measured and predicted fossil fuel CO2 signals at the tower since there is a much better understanding of how much fossil fuel is burned in California. If the predicted signal and measured signal are close, then that would be a good indication that the method is sound.

"Initial comparisons of measured and predicted fossil fuel signals agree at the 10 to 20 percent level, suggesting that it is unlikely there are much larger errors in the transport model," he said. "But we haven't ruled it out."

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