Carbon Dioxide Distribution, Origins, and Transport Along a Frontal Boundary During Summer in Mid-Latitudes

Synoptic weather systems are a major driver of spatial gradients in atmospheric CO₂ mole fractions. During frontal passages, air masses from different regions meet at the frontal boundary creating significant gradients in CO₂ mole fractions. We quantitatively describe the atmospheric transport of CO₂ mole fractions during a mid-latitude cold front passage and explore the impact of various sources of CO₂. We focus here on a cold front passage over Lincoln, Nebraska on August 4th, 2016 observed by aircraft during the Atmospheric Carbon and Transport-America campaign. A band of air with elevated CO₂ was located along the frontal boundary. Observed and simulated differences in CO₂ across the front were as high as 25 ppm. Numerical simulations using Weather Research and Forecasting Model with Chemistry at cloud resolving resolutions (3 km), coupled with CO₂ surface fluxes and boundary conditions from CarbonTracker (CT-NRTv2017x), were performed to explore atmospheric transport at the front. Model results demonstrate that the frontal CO₂ difference in the upper troposphere can be explained largely by inflow from outside of North America. This difference is modified in the atmospheric boundary layer and lower troposphere by continental surface fluxes, dominated in this case by biogenic and fossil fuel fluxes. Horizontal and vertical advection are found to be responsible for the transport of CO₂ mole fractions along the frontal boundary. We show that cold front passages lead to large CO₂ transport events including a significant contribution from vertical advection, and that midcontinent frontal boundaries are formed from a complex mixture of CO₂ sources.