We present results, from Monte Carlo (MC) simulations, on polymer systems of freely jointed chains with spherical monomers interacting through the square well potential. Starting from athermal packings of chains of tangent hard spheres, we activate the square well potential under constant volume and temperature corresponding effectively to instantaneous quenching. We investigate how the intensity and range of pair-wise interactions affected the final morphologies by fixing polymer characteristics such as average chain length and tolerance in bond gaps. Due to attraction chains are brought closer together and they form clusters with distinct morphologies. A wide variety of structures is obtained as the model parameters are systematically varied: weak interactions lead to purely amorphous clusters followed by well-ordered ones. The latter include the whole spectrum of crystal morphologies: from virtually perfect hexagonal close packed (HCP) and face centered cubic (FCC) crystals, to random hexagonal close packed layers of single stacking direction of alternating HCP and FCC layers, to structures of mixed HCP/FCC character with multiple stacking directions and defects in the form of twins. Once critical values of interaction are met, fivefold-rich glassy clusters are formed. We discuss the similarities and differences between energy-driven crystal nucleation in thermal polymer systems as opposed to entropy-driven phase transition in athermal polymer packings. We further calculate the local density of each site, its dependence on the distance from the center of the cluster and its correlation with the crystallographic characteristics of the local environment. The short- and long-range conformations of chains are analyzed as a function of the established cluster morphologies.

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Secondary organic aerosol (SOA) formation remains a challenge for the air quality modelling community. Gaps in the current knowledge of certain processes involved in SOA formation and evolution, as well as the large number of uncertainties in many parameters involved, affect the accuracy of the simulation of atmospheric SOA concentrations. We present here an evaluation of several published SOA schemes by simulating experiments conducted in two outdoor chambers, EUPHORE (Ceam, Valencia, Spain) and UF (University of Florida, Gainesville, USA). The experiments focused on the oxidation of selected anthropogenic volatile organic compounds. We compare model estimates for one-step, two-step and multi-step gas-phase oxidation schemes with SOA concentrations measured in the chambers. For all schemes, some modifications were needed to obtain a better agreement between model simulations and observations.

RESUMENLos aerosoles orgánicos secundarios (AOS) constituyen una fracción de las partículas atmosféricas. Este tipo de partículas se forman como consecuencia de la reacción de oxidación de ciertos gases orgánicos, lo que conduce a la formación de compuestos de baja volatilidad. Del mismo modo que para otros contaminantes atmosféricos, los modelos de calidad del aire permiten la simulación de partículas, una herramienta muy útil en tareas de gestión de la calidad del aire. Sin embargo, el uso adecuado de estos modelos debe basarse en la validación de su capacidad para reproducir las concentraciones observadas. Las estaciones de monitoreo de la calidad del aire registran información sobre una amplia variedad de contaminantes atmosféricos. Desafortunadamente, no se dispone habitualmente de medidas de AOS, ya que la instrumentación que se tiene en dichas redes de monitoreo no permite la diferenciación de las fuentes primarias y secundarias de los aerosoles orgánicos. Este documento presenta una serie de experimentos de fotooxidación realizados en las cámaras de simulación del Fotorreactor Europeo (CEAM, España) en diferentes condiciones experimentales con objeto de obtener datos sobre la formación de AOS. El uso de este tipo de cámaras permite aislar los procesos químicos y de formación de aerosoles, por lo que los datos presentados en este estudio tienen un considerable valor para propósitos de evaluación de modelos de formación de AOS, al igual que para el estudio del comportamiento de este tipo de partículas. ABSTRACTSecondary organic aerosols (SOA) constitute a significant fraction of the atmospheric particulate matter. Theses particles are formed as a consequence of the oxidation reaction of certain organic gases that leads to the formation of low-volatility compounds. As for other pollutants, air quality models allow the simulation of particle levels and thus models constitute a powerful tool in air quality management. Nevertheless, the Atmósfera 26(1), 59-73 (2013) 60 M. G. Vivanco et al. accepted use of models must be based on the validation of its capacity to reproduce observed concentrations. Air monitoring sites provide measured information of a large variety of ambient pollutants. Unfortunately, measurements on SOA are not normally available, as current monitoring networks do not include instrumentation to distinguish primary from secondary sources of organic carbonaceous aerosol. This paper presents a set of photooxidation experiments performed in the European Photorreactor (EUPHORE) smog chamber (CEAM, Spain) under different experimental conditions to investigate SOA formation. The use of chambers allows the isolation of atmospheric chemistry and aerosol formation processes. Thus, although these measurements were obtained at initial precursor concentrations higher than those in atmospheric conditions, they constitute a valuable set of information for SOA model evaluation purposes.