Monday, 10 February 2003
Vertical profiles of free radicals in the polluted nocturnal boundary layer: A one-dimensional model study
The oxidation of anthropogenic and biogenic VOCs and NOx 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 NOx 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 ROx 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.