P1.16

Vertical profiles of free radicals in the polluted nocturnal boundary layer: A one-dimensional model study

Andreas J. Geyer, University of California, Los Angeles, CA; and S. Wang and J. Stutz

The oxidation of anthropogenic and biogenic VOCs and NO_x in the polluted nighttime boundary layer (NBL) is primarily controlled by NO₃ radicals. NO ₃ oxidation can also lead to the formation of secondary peroxy and even hydroxyl radicals. Calculations of the oxidation capacity of the NBL from in-situ or long-path measurements of NO₃ and other radicals are, however, very difficult because the assumption of a well mixed boundary layer is often not valid during night. Since VOCs and NO_x are emitted near the ground while NO₃ radicals are formed at all altitudes, we can expect unique vertical profiles of these species. Recent measurements and model studies show in fact pronounced vertical profiles of NO₃ in the NBL. Here we present results from a 1D chemical box model of the nocturnal radical chemistry. The model includes vertical transport based on measured micrometeorological data, deposition and emission, and a simplified NO₃ and RO_x chemistry module. The model reproduced vertical profiles of NO₃, NO₂, and O₃ observed near Houston, where up to 180 ppt NO₃ at the top of the NBL and 40 ppt at the ground were found. It was found that vertical transport cannot be neglected at night. At certain altitudes, vertical transport of NO₃ even is a more important NO₃ source than its chemical production. The results also show that steady state calculations so far used in the investigation of NO₃ and N₂O₅ chemistry are not representative in all cases. The model predicts high values of OH radicals close to the ground which are caused by downward transport of RO₂, formed by the reaction of NO₃ + VOCs in heights above 10 m, towards the ground, where RO₂ is converted into HO₂ and OH by the emissions of NO. The dependence of the nocturnal radical chemistry on vertical stability, NO, and VOC emission fluxes will be discussed.

  Extended Abstract (340K)

Poster Session 1, General Poster Session
Monday, 10 February 2003, 2:30 PM-4:00 PM

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