Expanding the Boston Region Carbon Monitoring System: First 18 Months of Regular Total-Column Observations

The city of Boston and the State of Massachusetts have ambitious goals to reduce greenhouse gas emissions. Assessing the efficacy of mitigation policies aimed to meet these goals requires traceable methods for tracking actual changes in emissions. Eastern MA is one of the best instrumented and closely studied regions for CO₂ and CH₄ on the globe with a network of ground stations running continuously since 2013. Starting in 2014 two solar-observing Bruker EM27/SUN Fourier transform spectrometers were added to our carbon monitoring system to provide information on the total atmospheric columns of XCO₂, XCH₄, and XCO. Designed to be compact and robust, the EM27/SUNs are well suited to field deployment and were primarily used in campaign mode until May 2018 when they were semi-permanently installed in fixed locations. Remotely-operated enclosures enable us to continually observe column-averaged concentrations of GHGs both inside the urban core (at Cambridge, MA) and in the air entering the region (at Harvard Forest; Petersham, MA) during sunlit hours. Total column data are less sensitive to model errors in vertical mixing, and more directly comparable to the data obtained by the new generation of satellite sensors including TROPOMI and OCO-3. Summertime values of XCO₂ at Harvard Forest show a clear diel cycle in the growing season: high morning values reflect accumulation of respired CO₂ overnight, decreasing during the day due to photosynthetic uptake averaging -0.3 (as much as -0.5) ppm/hour. This uptake drives a diel cycle in the urban-rural gradient of XCO₂ which must be quantified in order to accurately separate out anthropogenic emissions. Column-averaged gradients of XCH₄ ranging from 2 – 10 ppb are regularly observed between Harvard Forest and Cambridge during westerly wind conditions. These column data are currently being added to our high-resolution inverse model framework along with tower measurements, Lidar observations, and, critically, a carefully constructed spatially-resolved prior model of emissions, to constrain fluxes of CH₄ and CO₂ in greater Boston.