How different is the air’s CO2 concentration in Boston compared with rural forest despite their very different emissions?

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Urban areas are directly or indirectly responsible for the majority of human-caused CO2 emissions. This observational study characterized measured atmospheric CO2 mixing ratios and estimated CO2 fluxes at three sites along an urban-to-rural gradient in Boston, Massachusetts, USA. Although CO2 is a well-mixed greenhouse gas, the researchers found significant differences along this gradient in how, where, and when it was exchanged.

Total human-caused emissions were estimated using an emissions inventory and ranged from 1.5 to 37.3 mg·C·ha−1·yr−1 between rural Harvard Forest and urban Boston. Despite this large increase in emissions, the mean annual difference in atmospheric CO2 between sites was approximately 5% (20.6 ± 0.4 ppm).

Vegetation’s influence was also visible along the gradient. Green-up occurred near day of year 126 in Boston, 136 in Worcester, and 141 in Harvard Forest, highlighting differences in growing-season length. In Boston, gross primary production—estimated by scaling productivity by canopy cover—was approximately 75% lower than at Harvard Forest, but still represented a significant local flux of 3.8 mg·C·ha−1·yr−1.

The researchers concluded that reducing greenhouse gas emissions requires a better understanding of spatial and temporal variations in urban carbon fluxes and their underlying drivers.