1Physikalische Chemie, FB C, Bergische Universität Wuppertal, Gaußstraße 20, 42119 Wuppertal, Germany
2Environmental Sciences Division, National Research Center, Dokki, Giza, Egypt
3Faculty of Chemistry and Biology, University of Santiago de Chile, USACH, Alameda L. Bernardo O'Higgins 3363, Santiago, Chile
4Physics Department, Faculty of Science, University of Santiago de Chile, Alameda L. Bernardo O'Higgins 3363, Santiago, Chile
5National Centre for Atmospheric Science, University of Leeds, Leeds, UK
6School of Chemistry, University of Leeds, Leeds, UK
Abstract. The oxidation capacity of the highly polluted urban area of Santiago, Chile has been evaluated during an extensive summer measurement campaign carried out from 8–20 March 2005. The hydroxyl (OH) radical budget was evaluated employing a simple quasi-photostationary-state model (PSS) constrained with simultaneous measurements of HONO, HCHO, O3, NO, NO2, j(O1D), 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 to estimate production rates and total free radical budgets, including OH, HO2 and RO2. Besides the above parameters, the MCM model has been constrained by the measured CO and volatile organic compounds (VOCs) including alkanes and aromatics. Both models showed the same OH radical budget during daytime which indicates that the primary OH sources and sinks included in the simple PSS model are predominant. Mixing ratios of the main OH radical precursors were found to be in the range 0.8–7 ppbv (HONO), 0.9–11 ppbv (HCHO) and 0–125 ppbv (O3). The alkenes average mixing ratio was ~58 ppbC accounting for ~12% of the total identified non-methane hydrocarbons (NMHCs). During the day time (8 h–19 h), HONO photolysis was shown to be the most important primary OH radical source comprising alone more than ~55% of the total initial production rate, followed by alkene ozonolysis (~24%) and photolysis of HCHO (~16%) and O3 (~5%). The calculated average and maximum daytime OH production rates from HONO photolysis was 1.7 ppbv h−1 and 3.1 ppbv h−1, respectively. Based on the experimental results a strong photochemical daytime source of HONO is proposed. A detailed analysis of the sources of OH radical precursors has also been carried out.