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Jenkin M.E.,Atmospheric Chemistry Services | Jenkin M.E.,University of Bristol | Young J.C.,University of Leeds | Rickard A.R.,University of York
Atmospheric Chemistry and Physics | Year: 2015

The chemistry of isoprene degradation in the Master Chemical Mechanism (MCM) has been systematically refined and updated to reflect recent advances in understanding, with these updates appearing in the latest version, MCM v3.3.1. The complete isoprene degradation mechanism in MCM v3.3.1 consists of 1926 reactions of 602 closed shell and free radical species, which treat the chemistry initiated by reaction with OH radicals, NO3 radicals and ozone (O3). A detailed overview of the updates is provided, within the context of reported kinetic and mechanistic information. The revisions mainly relate to the OH-initiated chemistry, which tends to dominate under atmospheric conditions, although these include updates to the chemistry of some products that are also generated from the O3-and NO3-initiated oxidation. The revisions have impacts in a number of key areas, including HOx recycling, NOx recycling and the formation of species reported to play a role in SOA (secondary organic aerosol)-formation mechanisms. The performance of the MCM v3.3.1 isoprene mechanism has been compared with those of earlier versions (MCM v3.1 and MCM v3.2) over a range of relevant conditions, using a box model of the tropical forested boundary layer. The results of these calculations are presented and discussed and are used to illustrate the impacts of the mechanistic updates in MCM v3.3.1. © 2015 Author(s).

Jenkin M.E.,Atmospheric Chemistry Services | Jenkin M.E.,University of Bristol
Atmospheric Environment | Year: 2014

The levels of ozone (O3) at a given location in the UK have been reported to be determined by a combination of global (northern hemispheric), regional and local-scale effects. These effects therefore also potentially influence values of the O3 exposure metric, AOT40 (the accumulated ozone exposure over a threshold of 40ppb). A corresponding background oxidant metric, AOXBT40, is defined, which quantifies the value of AOT40 corrected for the effects of local oxidant sources and the chemical coupling of O3 with emitted NOx (i.e. effectively a remote rural background AOT40 value). Data from the UK automatic monitoring network have been analysed to determine values of AOT40 and AOXBT40 at 39 UK sites for the example year of 2009. The spatial variation of AOXBT40 over the UK is consistent with contributions deriving both directly from the northern hemispheric baseline O3 level and from episodic events during which the baseline is supplemented by regional-scale oxidant formation; and the approximate relative importance of these inputs is estimated. The fractional decrement in AOT40 (AOT40/AOXBT40), shows a well-defined dependence on the average NOx level for the hours that contribute to AOXBT40, consistent with the expected impact of the local-scale chemical coupling of O3 and NOx. The AOXBT40 concept can therefore be used to help characterise the factors that control AOT40 on different spatial scales, and its potential applications in empirical O3 exposure modelling activities and model evaluation are briefly discussed. © 2014 Elsevier Ltd.

Jenkin M.E.,Atmospheric Chemistry Services | Jenkin M.E.,University of Bristol
Atmospheric Environment | Year: 2014

Empirical assessments of annual mean [NO2] have often made use of expressions describing its non-linear [NOx]-dependence, based on fitting to a large number of observations. These studies have also shown that annual mean [NO2] data possess a large amount of scatter, such that the target [NOx] that corresponds to the EU annual mean NO2 limit value (40μgm-3) can vary considerably from one site to another. A contribution to this scatter is shown to result from site-to-site variations in the [NOx]-averaging of the non-linear impact of the chemical coupling of NOx and O3 over the annual range of ambient conditions. The variability in the partitioning of oxidant (OX) into its component forms of NO2 and O3 is used as a diagnostic, with a particular focus on the impact of short-timescale [NOx] variability. Annual mean monitoring data from 95 UK sites reporting co-located measurements of O3 and NOx are used in the analysis, consisting of 906 site-years of annual mean concentrations measured over the period 1991-2011. The ratio of the upper and lower quartiles of the annual distribution of hourly mean [NOx] (the quartile ratio, QR) is used as a practical indicator of [NOx] variability. The data in a series of QR intervals are shown to deviate systematically from an idealised [NO2]/[OX] vs. [NOx] curve. The data are well-described by [NOx]-averaging the idealised curve, with QR being well correlated with the range of [NOx] over which the curve is averaged, providing strong evidence of a systematic impact of short-timescale [NOx] variability. A combined analysis of the data for sites nominally categorised as roadside, background and rural shows that (although scattered) the values of QR in each category apparently display systematic trends with annual mean [NOx], showing progressive increases in QR with decreasing [NOx] at roadside and polluted background sites. These dependences are used to interpolate between [NO2]/[OX] vs. [NOx] expressions based on constant QR to produce representative expressions for background and roadside sites. These may be appropriate for application in empirical policy models, with the high level of scatter for each site category being rationalised by the associated range in QR. © 2014 Elsevier Ltd.

Utembe S.R.,University of Melbourne | Jenkin M.E.,Atmospheric Chemistry Services | Shallcross D.E.,University of Bristol
Atmospheric Environment | Year: 2015

Ozone tagged labelling schemes have been implemented in a global Lagrangian chemistry-transport model to identify the intercontinental origins of surface ozone in Europe. Stratosphere-troposphere exchange gave rise to between 3 and 5ppb across Europe, whereas the mid-latitudes of the Middle East, Asia and the Pacific Ocean region contributed 6-8ppb. Surface ozone levels of 10-16ppb were associated with the mid-latitudes of North America and the North Atlantic Ocean regions. Appreciable intercontinental ozone production occurred downwind of continental regions and above the surface layer. Intercontinental ozone formation and transport from tropical regions contributed about 4ppb and was much less efficient compared with that from mid-latitudes. There were approaching 60 chemical processes driving intercontinental ozone formation, of which the HO2+NO, CH3O2+NO and CH3COO2+NO reactions were the most important. Ozone production appeared to be driven by OH oxidation of secondary reaction products rather than the oxidation of primary emitted VOCs. The largest intercontinental ozone contributions amounted to about 20ppb from North America to European baseline stations, 14ppb from Asia to North American baseline stations and 10ppb from Asia to European baseline stations. It is possible that changing intercontinental ozone production and transport could have led to seasonal ozone trends and shifts in seasonal cycles at northern hemisphere mid-latitude baseline ozone monitoring stations. © 2015 Elsevier Ltd.

Jenkin M.E.,Atmospheric Chemistry Services | Hurley M.D.,Ford Motor Company | Wallington T.J.,Ford Motor Company
Journal of Physical Chemistry A | Year: 2010

The reaction of CH3OCH2O2 with HO 2 has been investigated at 296 K and 700 Torr using long path FTIR spectroscopy, during photolysis of Cl2/CH3OCH 3/CH3OH/air mixtures. The branching ratio for the reaction channel forming CH3OCH2O, OH, and O2 has been determined from experiments in which OH radicals were scavenged by addition of benzene to the system, with subsequent formation of phenol used as the primary diagnostic for OH radical formation. The dependence of the phenol yield on the initial peroxy radical precursor reagent concentration ratio, [CH 3OH]0/[CH3OCH3]0, is consistent with prompt OH formation resulting mainly from the reaction of CH3OCH2O2 with HO2, such that the inferred prompt yield of OH is well-correlated with that of CH 3OCH2OOH, a well-established product of the CH 3OCH2O2 + HO2 reaction. The system was fully characterized by simulation, using a detailed chemical mechanism which included other established sources of OH in the system. This allowed a branching ratio of k2c/k2 = 0.19 ± 0.08 to be determined. The results therefore provide strong indirect evidence for the participation of the radical-forming channel of the title reaction. © 2010 American Chemical Society.

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