Abstract. The oxidation capacity of the highly polluted urban area of Santiago, Chile has been evaluated during a 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, O 3 , NO, NO 2 , j (O 1 D), j (NO 2 ), 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, HO 2 and RO 2 . 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 simulate the same OH concentration during daytime indicating that the primary OH sources and sinks included in the simple PSS model predominate. 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 (O 3 ). The alkenes average mixing ratio was ∼58 ppbC accounting for ∼12% of the total identified non-methane hydrocarbons (NMHCs). During the daytime (08:00 h-19:00 h), HONO photolysis was shown to be the most important primary OH radical source comprising alone ∼55% of the total initial production rate, followed by alkene ozonolysis (∼24%) andCorrespondence to: J. Kleffmann (kleffman@uni-wuppertal.de) photolysis of HCHO (∼16%) and O 3 (∼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.

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The partitioning of laurdan and prodan between the external solvent and the microaggregates in AOT/ heptane/water water-in-oil microemulsions has been measured. We have also evaluated the dependence of the fluorescence spectra (as determined by the generalized polarization, GP) with the water content of the microemulsion and the characteristics of the emission and excitation GP spectra in order to assess the properties of the average microenvironments sensed by the microaggregate-bound probes. Partition constants (K, molar concentration units) were evaluated from the change in fluorescence intensity (at a fixed probe concentration) with the AOT concentration (at fixed W ) (water)/(AOT) values). For W values g10, a value K ) 1.0 ( 0.2 M -1 was obatined, both for laurdan and prodan. This result indicates that neither the length of the probe hydrophobic tail nor the size of the water pool significantly modifies the affinity of the probe for the microaggregates. Even lower values of K were obtained at lower W values. GP values strongly decrease for both probes when W increases up to ca. 10, a result that indicates an increase in water relaxation rates when the water/surfactant ratio increases up to the total surfactant head hydration. On the other hand, very small changes in GP are observed when W changes between 10 and 20. All these results indicate that probes associated to the microaggregates are located at the interface and that the properties of this region remain almost constant after total surfactant head hydration. GP values and GP spectra of laurdan molecules associated to the fully hydrated microaggregate interface are similar to those reported in phospholipid vesicles in the liquid crystalline state. As in this system, and despite their localization at the interface, laurdan molecules do not constitute a homogeneous population.

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