Secondary organic aerosol 2. Thermodynamic model for gas/particle partitioning of molecular constituents

A model that predicts secondary organic aerosol (SOA) formation based on the thermodynamic equilibrium partitioning of secondary organic oxidation products has been developed for implementation into atmospheric models. Hydrophobic secondary products are assumed to partition to an absorbing organic aerosol consisting of primary organic aerosol (POA) and other secondary hydrophobic organics according to an equilibrium partitioning coefficient calculated iteratively for each secondary compound present. The hydrophobic module is evaluated by studying the partitioning of octadecanoic acid to surrogate POA species. As expected, the amount of octadecanoic acid predicted to be present in the aerosol phase increases as the total amount of absorbing material increases or as the total amount of acid present increases. Hydrophilic secondary compounds partition to an aqueous phase via Henry's law; the fraction of each compound's mass that partitions is determined by its Henry's law constant and its acid dissociation constant(s). The available liquid water content (LWC) of the aerosol is determined iteratively between an inorganic aerosol module and the hydrophilic module, which is evaluated by studying the partitioning of glyoxalic and malic acids. While glyoxalic acid tends to remain in the gas phase, malic acid partitions strongly to the aqueous phase, with ions being the dominant form in the aqueous phase. As expected, an increase in relative humidity increases the amount of water associated with the organics (ΔLWC), and a lower aerosol pH favors molecular solutes over ionized forms. Increasing pH results in higher effective Henry's law constants for the acids, yielding higher organic aerosol concentrations. Results also indicate that increasing ΔLWC induces additional partitioning of inorganics to the aqueous phase

Cellular location and activity of Escherichia coli RecG proteins shed light on the function of its structurally unresolved C-terminus

RecG is a DNA translocase encoded by most species of bacteria. The Escherichia coli protein targets branched DNA substrates and drives the unwinding and rewinding of DNA strands. Its ability to remodel replication forks and to genetically interact with PriA protein have led to the idea that it plays an important role in securing faithful genome duplication. Here we report that RecG co-localises with sites of DNA replication and identify conserved arginine and tryptophan residues near its C-terminus that are needed for this localisation. We establish that the extreme C-terminus, which is not resolved in the crystal structure, is vital for DNA unwinding but not for DNA binding. Substituting an alanine for a highly conserved tyrosine near the very end results in a substantial reduction in the ability to unwind replication fork and Holliday junction structures but has no effect on substrate affinity. Deleting or substituting the terminal alanine causes an even greater reduction in unwinding activity, which is somewhat surprising as this residue is not uniformly present in closely related RecG proteins. More significantly, the extreme C-terminal mutations have little effect on localisation. Mutations that do prevent localisation result in only a slight reduction in the capacity for DNA repair. © 2014 The Author(s)

Localization of an accessory helicase at the replisome is critical in sustaining efficient genome duplication

Genome duplication requires accessory helicases to displace proteins ahead of advancing replication forks. Escherichia coli contains three helicases, Rep, UvrD and DinG, that might promote replication of protein-bound DNA. One of these helicases, Rep, also interacts with the replicative helicase DnaB. We demonstrate that Rep is the only putative accessory helicase whose absence results in an increased chromosome duplication time. We show also that the interaction between Rep and DnaB is required for Rep to maintain rapid genome duplication. Furthermore, this Rep–DnaB interaction is critical in minimizing the need for both recombinational processing of blocked replication forks and replisome reassembly, indicating that colocalization of Rep and DnaB minimizes stalling and subsequent inactivation of replication forks. These data indicate that E. coli contains only one helicase that acts as an accessory motor at the fork in wild-type cells, that such an activity is critical for the maintenance of rapid genome duplication and that colocalization with the replisome is crucial for this function. Given that the only other characterized accessory motor, Saccharomyces cerevisiae Rrm3p, associates physically with the replisome, our demonstration of the functional importance of such an association indicates that colocalization may be a conserved feature of accessory replicative motors

Dynamic evaluation of a multi-year model simulation of particulate matter concentrations over Europe

International audienc

Multi-scale modeling of roadway air quality impacts: development and evaluation of a plume-in-grid model

International audienceEulerian three-dimensional (3D) grid-based models are widely used in air quality modeling. In such models, emissions are instantaneously diluted within the grid cells and, therefore, the near-source impacts of large point and line sources cannot be properly resolved. Plume-in-Grid models (PinG) use a subgrid-scale treatment to better represent local source contributions in an Eulerian grid-based simulation. PinG models already exist for point sources. However, modeling emissions from roadway traffic with point sources implies a very large computational burden. We present here a new PinG model that uses a Gaussian line source model, better suited than point sources to model roadway traffic emissions, embedded within an Eulerian model. The model is evaluated with a large dataset of nitrogen dioxide (NO2) concentrations over a 800 km road network. The PinG model leads to greater NO2 concentrations and shows better performance than the Eulerian model

Evaluation of a Plume-in-Grid Model for Line Sources with a Multiple Roadway Case Study

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Sensibilité des concentrations de PM2.5 au changement climatique sur l'Europe

National audienceLes concentrations de particules fines (PM2.5) sont fortement dépendantes à la fois des conditions météorologiques et de nombreux processus chimiques. Le but de cette étude est de présenter un modèle statistique permettant de caractériser ces concentrations à l'échelle européenne à partir des conditions météorologiques. Le modèle de chimie-transport Polyphemus/Polair3D a été utilisé pour simuler les concentrations de PM2.5 sur la période 2000-2008 en Europe. Chaque année de simulation a été évaluée de façon opérationnelle par comparaison avec les observations fournies par les réseaux de surveillance de la qualité de l'air. Par ailleurs, la longue durée de la simulation (9 ans) a permis d'effectuer une évaluation dynamique de la capacité du modèle à reproduire la variabilité des concentrations et de la composition chimique des PM2.5 en fonction de la météorologie (température, précipitations, vent). Les résultats de ces simulations sont utilisés par la suite en tant que pseudo-observations pour implémenter un modèle statistique visant à caractériser les concentrations de particules fines à l'échelle européenne. Ce modèle utilise uniquement les résultats de la simulation, des données climatiques de grande échelle (pression à la surface de la mer) et la température de surface. Après validation de ce modèle pour le climat présent, la méthode sera applicable pour évaluer les concentrations de PM2.5 à partir de projections climatique

A new line source model for air quality impacts of roadway traffic

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An atmospheric emission inventory of anthropogenic and biogenic sources for Lebanon

International audienceA temporally-resolved and spatially-distributed emission inventory was developed for Lebanon to provide quantitative information for air pollution studies as well as for use as input to air quality models. This inventory covers major anthropogenic and biogenic sources in the region with 5 km spatial resolution for Lebanon and 1 km spatial resolution for its capital city Beirut and its suburbs. The results obtained for CO, NOx, SO2, NMVOC, NH3, PM10 and PM2.5 for the year 2010 were 563, 75, 62, 115, 4,12, and 9 Gg, respectively. About 93% of CO emissions, 67% of NMVOC emissions and 52% of NOx emissions are calculated to originate from the on-road transport sector while 73% of SO2 emissions, 62% of PM10 emissions and 59% of PM2.5 emissions are calculated to originate from power plants and industrial sources. The spatial allocation of emissions shows that the city of Beirut and its suburbs encounter a large fraction of the emissions from the on-road transport sector while urban areas such as Zouk Mikael, Jieh, Chekka and Selaata are mostly affected by emissions originating from the industrial and energy production sectors. Temporal profiles were developed for several emission sectors. (C) 2012 Elsevier Ltd. All rights reserved