Seasonal Variability in Local Carbon Dioxide Biomass Burning Sources Over Central and Eastern US Using Airborne In Situ Enhancement Ratios

We present observations of local enhancements in carbon dioxide (CO2) from local emissions sources over three eastern US regions during four deployments of the Atmospheric Carbon Transport-America (ACT-America) campaign between summer 2016 and spring 2018. Local CO2 emissions were characterized by carbon monoxide (CO) to CO2 enhancement ratios (i.e., Delta CO/Delta CO2) in air mass mixing observed during aircraft transects within the planetary boundary layer. By analyzing regional-scale variability of CO2 enhancements as a function of Delta CO/Delta CO2 enhancement ratios, observed relative contributions to CO2 emissions were separated into fossil fuel and biomass burning (BB) regimes across regions and seasons. CO2 emission contributions attributed to biomass burning (Delta CO/Delta CO2 > 4%) were negligible during summer and fall in all regions but climbed to similar to 9%-11% of observed combustion contributions in the South during winter and spring. Relative CO2 fire emission trends matched observed winter and spring BB contributions, but conflictingly predicted similar levels of BB during the fall. Satellite fire data from MODIS and VIIRS suggested the use of higher spatial resolution fire data that might improve modeled BB emissions but were not able to explain the bulk of the discrepancy.