a b s t r a c t
The importance of peroxy radical (RO2) chemistry in the troposphere is investigated using the STOCHEMCRI
global chemistry and transport model. The oxidation of VOCs leads to the formation of RO2 radicals
which are dominated by CH3O2 (83%), RCO3 (5%), isoprene derived peroxy radicals (6%), and terpene
derived peroxy radicals (1%). 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. Isoprene derived radicals are the most strongly bound of RO2 species
investigated and their degree of complexation at approx. 300 K far surpasses that of the generic peroxy
radicals by 3e5%. At higher altitudes (approx. 8 km) characterized by sub-ambient temperatures, the
fraction of RO2.H2Ocomplex that can exist is approximately 17% in the upper troposphere aboveMace Head
(Northern Hemisphere),14% above Cape Grim (Southern Hemisphere), and 8% aboveMauna Loa (Tropics).
© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license