Greenhouse Gas Variability across Fronts over the Eastern United States during an Early versus a Late Summer Campaign

Synoptic-scale weather events like frontal passages and quasi-periodic movement of cyclonic and anti-cyclonic weather systems over land play an important role in distributing greenhouse gases (GHG, e.g., CO₂, CH₄) in the atmosphere. However, our knowledge and observational evidence of GHG variations across frontal boundaries are limited. Within this contribution, we report a synthesis of front-relative GHG horizontal and vertical distributions observed during a late summer (2016) versus an early summer (2019) field campaign of the NASA Atmospheric Carbon and Transport – America (ACT-America) project. These field campaigns documented GHG gradients across nine and seven frontal systems, respectively, in three regions of the US, namely, Mid-Atlantic, Upper Midwest, and South. The South and Midwest region flights in summer-2019 were around 2 months and 1 month earlier than in the summer-2016 campaign, respectively, while the Mid-Atlantic flights were performed in almost the same summer phase. Additionally, in 2019 the Missouri and Mississippi River flooding, resulting from excessive precipitation over the Plains in late spring (i.e., just before the summer-2019 campaign started), created anomalous soil-moisture and greenness conditions triggering delayed planting of agricultural fields and significant perturbations in phenology.

High-resolution remote and in-situ airborne observations were collected with two NASA aircraft: the C-130 and B-200. Using both active remote sensing and in-situ observations, we will discuss the magnitude of GHG frontal gradients in the atmospheric boundary layer (ABL) and free troposphere (FT) and how they varied between the two summers. Preliminary analyses indicate larger front-related CO₂ gradients in the ABL in summer-2016 compared to summer-2019. Additionally, from the summer-2019 data, we found that observed ABL CO₂ mole fractions in the South were depleted relative to the Gulf and the overlying free troposphere, providing more evidence of net carbon sink through photosynthesis in the region, in contrast to the summer-2016 deployment. GHG gradients in the free troposphere across fronts were smaller in 2019 than in 2016. Using observations and simulations for both summer campaigns, we will show how CO₂ and CH₄ gradients across frontal boundaries changed at different altitudes and how model-data syntheses was used to quantify differences in front-relative contrasts between the two summers.