The oxidation capacity of the highly polluted urban area of Santiago de Chile has been evaluated during a winter measurement campaign from May 25 to June 07 2005 with the results compared and contrasted with those previously evaluated during a summer campaign from March 8 to 20 2005 The OH radical budget was evaluated in both campaigns employing a simple quasi-photostationary state model (PSS) constrained with simultaneous measurements of HONO HCHO O-3 NO NO2 j((OD)-D-1) j(NO2) 13 alkenes and meteorological parameters In addition a zero dimensional photochemical box model based on the Master Chemical Mechanism (MCMv3 1) has been used for the analysis of the radical budgets and concentrations of OH HO2 and RO2 Besides the above parameters the MCM model has been constrained by the measured CO and other volatile organic compounds (VOCs) including alkanes and aromatics Total production and destruction rates of OH and HO2 in winter are about two times lower than that during summer Simulated OH levels by both PSS and MCM models are similar during the daytime for both winter and summer indicating that the primary OH sources and sinks included in the simple PSS model are predominant On a 24 h basis HONO photolysis was shown to be the most important primary OH radical source comprising 81% and 52% of the OH initiation rate during winter and summer respectively followed by alkene ozonolysis (125% and 29%) photolysis of HCHO (61% and 15%) and photolysis of O-3 (<1% and 4%) respectively During both winter and summer there was a balance between the OH secondary production (HO2 + NO) and destruction (OH + VOCs) showing that initiation sources of RO2 and HO2 are no net OH initiation sources This result was found to be fulfilled also for all other studies investigated Seasonal Impacts on the radical budgets are also discussed (c) 2009 Elsevier Ltd All rights reserved