Modeling and analytical studies of the hydroxyl radical during MINOS:
Comparison with observations
Evangelos Baboukas1, Harald Berresheim2, Christian
Plass-Dülmer2, Mark Lawrence1 and Jos Lelieveld1
1 Max-Planck-Institute for Chemistry, Mainz, Germany
2 German Weather Service, MOHp, Hohenpeissenberg, Germany
The hydroxyl radical (OH) is the principal oxidant in the troposphere. We have studied
the photochemistry that controls OH levels over the eastern Mediterranean by analyzing
observations with a photochemical box model. The measurements were performed in August
2001 at the Finokalia ground station in Crete during the Mediterranean Intensive Oxidant
Study (MINOS). OH levels showed strong diurnal variability and the diurnal mean OH was
quite high during the entire study period (for details on the experimental results see
the complementary presentation of Berresheim et. al. in this session). Two distinct periods,
before and after August 12, were clearly distinguished with mean 24-hour concentrations of
approximately 4.5 and 6.5×106 molecules cm-3, respectively. The results from a 3-dimensional
global model including condensed NMHC chemistry (MATCH-MPIC) are in reasonable agreement for
the first period but they show a decrease during the second period, contrary to the
observations. Sensitivity analyses with a box model including more detailed NMHC chemistry,
constrained by the available observations, have been performed to study the variability of OH.
The construction of the model, which is a subset of a comprehensive master chemical mechanism
(MCM), as well as the underlying approach is described. In order to further understand the
chemistry, the model has been reduced using sensitivity analysis on both a clean and a
semipolluted day. Finally, the diurnal resolved budgets of production and loss of the OH
and for the relative contribution of the different source and sink terms are discussed.