A good correlation between model and field measurements for total RO2 for
most of the selected stations suggests that they are appropriate background sites as the atmospheric
processes occurring at these stations are representative of the chemistry taking place within the entire
model grid square inwhich they are located. The seasonality exhibited by RO2 has been studied, with welldefined
cycles (highest in summer and lowest in winter) seen in both hemispheres. Peroxy radical-water
complexes, whilst not represented using Chemical Transport Models (CTMs) previously, are postulated
to perturb RO2 chemistry. The significance ofwater clusters (RO2.H2O) is investigated using the STOCHEMCRI
model and reveals that at 300 K the proportion of RO2 participating in complexation with water is
approximately 12% in the tropics.