The Global Budget of Atmospheric Ethanol: New Constraints from Remote Measurements

Ethanol is ubiquitous in the troposphere, emitted both by plants and from anthropogenic activity (including industry, solvent use, fuel, and agriculture). As a precursor of acetaldehyde and in turn peroxyacetyl nitrate (PAN), ethanol contributes to the long-range transport of NOx and increased tropospheric ozone production, and its high mixing ratios in cities can be a major sink of OH. Here, we reassess the global budget of tropospheric ethanol by comparing measurements from the Atmospheric Tomography campaign (ATom) to simulations using the global atmospheric chemistry model GEOS-Chem. We find that known sources already in the model underestimate observed ethanol mixing ratios by over 90%. To reduce the simulated ethanol bias, we add to GEOS-Chem a range of new ethanol emission sources informed by recent experimental and observational studies, including oceanic emissions, secondary production from the CH₃CH₂OO + OH reaction, and increased emissions from agriculture and from the use of ethanol in vehicular fuels. Most notably, we substantially expand anthropogenic ethanol emissions by incorporating emissions from volatile chemical products (VCPs). We find that the updates substantially improve the comparison of simulated ethanol mixing ratios to ATom measurements, reducing the normalized mean bias across all points from -91% to -23%. VCPs become the dominant atmospheric ethanol source, contributing slightly more than the combined growth and decay sources from plants (24 Tg/y vs. 22 Tg/y). The remaining negative model bias after our updates correlates strongly with the VCP and fuel sources, suggesting that anthropogenic emissions are still underestimated. This is particularly true over the South Atlantic, where model bias remains high (-70%) and where ATom sampled strong influence from Brazil, which has a large domestic bioethanol industry and high ethanol fuel use. We find that the increased ethanol sources reduce global tropospheric OH by 2.6% and increase tropospheric ozone by 1.7%, with the strongest effects (up to -18% OH, +6 ppb ozone) in South and East Asia. The new ethanol sources also lead to increased acetaldehyde and, in turn, PAN (+24%), which leads to strengthened redistribution of NOx from continental to remote tropospheric regions.