Atmospheric Acetaldehyde: Importance of Air‐Sea Exchange and a Missing Source in the Remote Troposphere

We report airborne measurements of acetaldehyde (CH₃CHO) during the first and second deployments of the National Aeronautics and Space Administration (NASA) Atmospheric Tomography Mission (ATom). The budget of CH₃CHO is examined using the Community Atmospheric Model with chemistry (CAM-chem), with a newly-developed online air-sea exchange module. The upper limit of the global ocean net emission of CH₃CHO is estimated to be 34 Tg a^-1 (42 Tg a^-1 if considering bubble-mediated transfer), and the ocean impacts on tropospheric CH₃CHO are mostly confined to the marine boundary layer. Our analysis suggests that there is an unaccounted CH₃CHO source in the remote troposphere and that organic aerosols can only provide a fraction of this missing source. We propose that peroxyacetic acid (PAA) is an ideal indicator of the rapid CH₃CHO production in the remote troposphere. The higher-than-expected CH₃CHO measurements represent a missing sink of hydroxyl radicals (and halogen radical) in current chemistry-climate models.