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List of Abstracts 229linked to know variations in the OH climate.Transformation 2.3 ID:4224 14:35Are megacity’s oxidizing environments changing?Erika Von Schneidemesser, Paul MonksUniversity of LeicesterContact: evonsch@gmail.comElevated levels of ozone, which are harmful to human health, have been observed in many urban areas andmegacities. The main precursors to ozone formation are volatile organic compounds (VOCs) and nitrogenoxides (NOx). Until now the reduction of ozone through the regulation of ozone precursors has been focusedon control of VOC emissions from road transport and petrol evaporation as these are the largest sources ofVOCs in urban areas. As air quality regulations further reduce anthropogenic emissions, one would expectthat the role that biogenic emissions play in the formation of ozone will become increasingly important.Isoprene, a tracer for biogenic emissions, has one of the highest photochemical ozone creation potentials of alarge number of volatile organic compounds (VOCs). Additionally, biogenic VOCs have been shown to havea significant impact on summer-time ozone formation not only in rural areas where the most vegetation ispresent, but also in urban areas. However, while their importance is increasing, their overall contribution toreactivity with respect to OH remains relatively small. Nitrogen dioxide emissions however, have beenincreasing in the past 10 years and their relative contribution to reactivity with respect to OH has also beenincreasing. That the oxidizing environment of a megacity (London) and the surrounding region has beenchanging is shown from the examination of the long-term trends in VOCs, carbon monoxide, methane, andNOx at a number of sites in and around London. The relative loss of these compounds with respect to theOH radical shows that the oxidizing balance of megacities and the surrounding regions have been and arechanging over time. Data from monitoring sites in and around London are used to compare the oxidativeenvironment in the city centre to urban background and semi-rural locations.Transformation 2.4 ID:4397 14:50A study of halogen chemistry in the tropical Atlantic Ocean boundary layerRoberto Sommariva, Roland Von GlasowUniversity of East AngliaContact: r.sommariva@uea.ac.ukThe role of halogenated species (chlorine, bromine, iodine) in the oxidative chemical processes occurring inthe marine boundary layer (MBL) has been investigated for several years, but many uncertainties remain inour knowledge of the sources, sinks and reactivities of these species. Measurements taken at the Cape VerdeAtmospheric Observatory (16°51'N, 24°52'W) from 2007 to 2009 and on the RRS Discovery during theRHaMBLe (Reactive Halogens in the Marine Boundary Layer) campaign in May-June 2007 have shown thatsignificant concentrations of bromine and iodine species are present in the tropical MBL.In this work, the one-dimensional model MISTRA (v7.4.1), which includes a detailed description ofmeteorological and microphysical processes and up-to-date gas and aqueous phase chemical mechanisms,was used to simulate the observations of bromine and iodine species in the region surrounding the CapeVerde islands under different scenarios. Sea-air fluxes derived from measurements of organic iodinecompounds during the RHaMBLe cruise were included in the model.Using the model results and the field observations we have investigated the sources of inorganic iodine iniCACGP-IGAC 2010 15 July, 2010