Fossil vs. non-fossil CO sources in the US: New airborne constraints from ACT-America and GEM

Abstract:

Carbon monoxide (CO) has a strong influence on the oxidative capacity of the atmosphere and is commonly used as a combustion tracer to partition sources of CO₂ and other species. However, vehicular CO emissions have declined substantially in recent years, so that fossil fuels may no longer be the largest CO source for many times and locations in the US. This research applies new airborne measurements from the ACT-America and GEM campaigns with a high-resolution chemical transport model (GEOS-Chem CTM at 0.25°´0.3125°) to develop new constraints on CO sources from the Eastern US for years 2016-2019. Model runs employ tagged tracers for source tracking, with emissions based on the 2016 EPA NEI (anthropogenic), MEGANv2.1 (biogenic), and QFED (fires). Based on the prior simulations, summertime Eastern US CO enhancements (within the boundary layer) over the global background include mean contributions of 59% from photochemical production, 35% from direct anthropogenic emissions, and 6% from fires. The photochemical CO source at this time of year is dominated by biogenic VOCs (73%). In the model, fossil emissions thus account for only 47% of the mean CO enhancements in the Eastern US boundary layer during summer. This increases to 75% during spring and fall and to 93% during winter. We use the ACT-America (summer 2016, winter 2017, fall 2017, spring 2018, summer 2019) and GEM (summer 2017, winter 2018) datasets to evaluate these model predictions, and interpret the model-measurement comparisons in terms of implications for primary vs. secondary and biogenic vs. anthropogenic CO sources in the US.