As the United States prepares to withdraw from the Paris Climate Accord, cities and states across the country have taken the lead in atmospheric CO2 reduction efforts. Therefore, accurate measures of anthropogenic and biogenic carbon emissions are imperative for tracking the progress of municipalities in achieving climate targets.

Boston, Massachusetts, has adopted reduction targets that include reducing emissions 25 percent below 2005 levels by 2020, and total carbon neutrality by 2050, when the world's population is expected to reach 6 billion. A collaborative of U.S. researchers has developed a model using Boston emissions data that accurately quantifies emissions in the Boston urban region for a period of 16 months, and they report that it is capable of detecting fluxes greater than 18 percent. They have published their results in the Proceedings of the National Academy of Sciences.

Emissions are highest in cities, which have the largest concentrations of vehicular use and carbon-based power generation, and cities are therefore ideal testbeds for models of greenhouse gas monitoring. However, although cities are leading efforts to reduce emissions, most have not adopted detailed inventories of atmospheric carbon sources.

The researchers note that emissions are largely assessed by two means: a "bottom-up" approach that derives fossil fuel use from various sources combined with the carbon content of the fuel; and a "top-down" approach that quantifies emissions based on greenhouse gas (GHG) emissions measured in the atmosphere.

The methodologies and input data for bottom-up monitoring can change between assessments in different years, creating uncertainty about flux. The researchers therefore present a new top-down method based on inverse modeling, which they report can quantify uncertainties in emissions to detect trends caused by policy, economic changes and regulations. The inverse model can also be used to monitor bottom-up inventories of carbon fuels.

The researchers write, "This study used 16 months of CO2 measurements at two sites in Boston and three boundary sites outside the city with a high-resolution modeling framework to quantify average anthropogenic emissions in the region to be 0.92 kg C•m^-2•y^-1 (95 percent CI:079 to 1.06), which is 14 percent higher than that calculated by the ACES inventory."

ACES is the first regional bottom-up inventory of hourly CO2 emissions from fossil fuels across carbon-emitting sectors. The new top-down model has a couple of advantages. One is a so-called CO2 concentration curtain that is tied to surface measurements but varying in altitude—the researchers report that this representation improved the agreement between the average they modeled and actual atmospheric measurements by 40 percent.

Secondly, the model uses a detailed representation of urban biological fluxes. Without this representation, it is not possible to generate an accurate model during the five-month growing season.

The researchers note that improving transport models and and remote sensing data will further narrow uncertainties, and that deploying similar modelling frameworks in other cities around the world would verify the achievement of emissions reduction goals.

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