July and August (Fig. 2). During summer, O3 photolysis is at its
highest and the humidity is increased, so the photolytic reactions
involving the resultant OH radical lead to the formation of high
concentrations of RO2. Minimum values are seen during the winter
months, typically in December and January when the solar intensity
is diminished. Data obtained from the Southern hemisphere
station, Cape Grim, demonstrates an opposite but analogous seasonalThe modelled data were monthly averages for the year 1998
which were compared with the selected field measurement data
recorded in the equivalent months of their respective campaign
years. Most of the measurement data were published as a combination
of both HO2 and RO2, so we calculated the concentrations of
HO2 þ RO2 from STOCHEM-CRI and then compared them with the
measured HO2 þ RO2 data (Fig. 2). In general, the field campaigns
yield higher peroxy radical concentrations than the model, which is
not surprising given that the model contains only a limited number
of the peroxy radical precursors (VOCs).
trend. Maximum and minimum values are recorded in June
and January, respectively (Fig. 2).