One‐dimensional photochemical study of H₂O₂, CH₃OOH, and HCHO in the marine boundary layer during Pacific Exploratory Mission in the Tropics (PEM‐Tropics) B

Abstract

This study was motivated by the discrepancies found between measurements made in the marine boundary layer during several large‐scale field campaigns and photochemical model simulations appropriate for these campaigns. Particularly for H 2 O 2 , CH 3 OOH, and HCHO, gaps between theories and measurements remain unresolved. The temporal and spatial distributions and behavior of these species indicate that they are subject to chemical and physical processes such as gas‐phase chemistry, scavenging by sea‐salt particles, sea‐salt aerosol chemistry involving halogen species, air‐sea gas exchange, and free troposphere‐marine boundary layer (FT‐MBL) exchange. We attempted to account for the previous disagreements and to identify dominant processes controlling the species by incorporating these processes into a single model. For this purpose, a one‐dimensional photochemical model was developed and verified against observational data from Pacific Exploratory Mission in the Tropics B (PEM‐Tropics B). A series of model simulations were conducted with varied chemical schemes and material boundary flux conditions. The results highlighted that two processes, FT‐to‐MBL O 3 transport and dry deposition, were critical in determining the abundance of H 2 O 2 , CH 3 OOH, and HCHO. This study also demonstrated that gas‐phase chemistry alone with rationalized fluxes was capable of capturing the behavior of CH 3 OOH and HCHO. Other processes, including scavenging by sea‐salt particles and sea‐salt aerosol chemistry involving halogen species, were found to be negligible under pristine conditions of this study. There was a general agreement between the observations and calculations for O 3 , OH, CH 3 OOH, and NO x . However, there remain gaps for H 2 O 2 and HCHO. By taking into account wet removal, the model reproduced the observation reasonably well.