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Derwent R.G.,Rdscientific | Murrells T.P.,Ricardo PLC
Environmental Chemistry | Year: 2013

Environmental context Throughout the world there are many places where ozone levels are elevated above internationally accepted guidelines set to protect human health. Policy makers use air quality models to formulate emission control strategies to achieve these air quality goals for ozone. There are large uncertainties in these air quality models that mask the sensitivity of the model control strategies to chemical mechanism choice. Abstract Monte Carlo sampling of pre-specified parameter ranges has been used to replace a single ′best estimate′ photochemical trajectory model run with 11694 ′acceptable′ model runs that are each consistent with the observations of elevated O3 during the PUMA (Pollution in the Urban Midlands Atmosphere) campaign in the UK, West Midlands during 1999. These 11694 ′acceptable′ parameter sets were then used for probabilistic evaluation of photochemical oxidant control strategies, based on 30% reductions in volatile organic compounds and NOx precursor emissions and on precursor emission projections to 2020. The sensitivity of single ′best estimate′ model runs to chemical mechanism choice gave some indication of the robustness of photochemical oxidant control strategies. However, Monte Carlo parametric uncertainty analysis showed that sensitivity to mechanism choice failed to indicate the magnitudes of the likely uncertainty ranges in the O 3 responses to photochemical oxidant control strategies. Furthermore, Monte Carlo uncertainty analysis showed that there may be O3 air quality disbenefits from 30% NOx emission reduction that were not apparent from ′best estimate′ runs. © 2013 CSIRO.

Derwent R.G.,Rdscientific | Murrells T.P.,Ricardo PLC
Atmospheric Environment | Year: 2013

Monte Carlo sampling of pre-specified parameter ranges has been used to replace single 'best estimate' Photochemical Trajectory Model runs with 9006 'acceptable' model runs that were each consistent with the observations of elevated O3 during July 2006 at a rural location in southern England. These acceptable parameter sets were then used for probabilistic evaluation of policy relevant photochemical oxidant control strategies, based on 30% reductions in VOC and NOx precursor emissions and the geographic locations where the emission reductions were carried out. Indicator species ratios were used to complete a diagnostic evaluation of model performance. The chemical mechanism employed in the base case model was replaced with four other mechanisms from the literature. Probabilistic uncertainty analysis showed that the results of the policy-relevant scenarios showed little or no sensitivity to chemical mechanism choice. A large degree of error compensation could be achieved through the selection of acceptable parameter sets and this hid the differences between the five chemical mechanisms studied. © 2013 Elsevier Ltd.

Ebinghaus R.,Helmholtz Center Geesthacht | Jennings S.G.,National University of Ireland | Kock H.H.,Helmholtz Center Geesthacht | Derwent R.G.,Rdscientific | And 2 more authors.
Atmospheric Environment | Year: 2011

In this study, the concentrations of total gaseous mercury in baseline air masses arriving at Mace Head, Ireland after having traversed the thousands of kilometres uninterrupted fetch of the North Atlantic Ocean, have been used for the assessment of possible trends in the atmospheric mercury background concentration over a 14-year period (i.e., 1996-2009), a statistically significant negative (downwards) trend of -0.028 ± 0.01 ng m-3 yr-1, representing a trend of 1.6-2.0% per year, has been detected in the total gaseous mercury levels in these baseline air masses. These findings are set in the context of the available literature studies of atmospheric Hg trends. © 2011 Elsevier Ltd.

Derwent R.G.,Rdscientific | Manning A.J.,UK Met Office | Simmonds P.G.,University of Bristol | Spain T.G.,National University of Ireland | O'Doherty S.,University of Bristol
Atmospheric Environment | Year: 2013

Observations of surface ozone have been made at the Mace Head Atmospheric Research Station on the Atlantic Ocean coastline of Ireland from April 1987 through to December 2012. Using meteorological analyses and a sophisticated Lagrangian dispersion model, the hourly observations have been sorted into baseline northern hemisphere mid-latitude, European regionally-polluted and southerly sub-tropical maritime air masses. Baseline monthly average levels showed a pronounced seasonal cycle with spring maxima and summer minima. Baseline levels have shown an average annual increase of+0.25±0.09ppbyear-1 which has been stronger in the winter and spring and weaker in the summer months. The rate of this annual increase has slowed over the last decade to the extent that annual levels have been relatively constant through the 2000s. Annual mean O3 levels in European air masses have shown much reduced upwards trends compared with baseline air masses. European levels show a seasonal cycle with a spring maxima and a winter minima. Policy actions to reduce European regional-scale NOx emissions have led to an increase in wintertime O3 levels and a decrease in summertime peak levels. Levels in southerly sub-tropical maritime air masses have shown different seasonal cycles, different seasonal trends and systematically lower levels compared with baseline air masses. The baseline observations have been compared against a number of policy-relevant O3 metrics and these have demonstrated the potential importance of the growing northern hemisphere O3 baseline levels over the 25 year study period. © 2013 Elsevier Ltd.

Yang H.,University College London | Battarbee R.W.,University College London | Turner S.D.,University College London | Rose N.L.,University College London | And 2 more authors.
Environmental Science and Technology | Year: 2010

The Tibetan Plateau is described as the "Roof of the World" averaging over 4000 m above sea level; it is remote, isolated, and presumed to be a pristine region. In order to study the history of atmospheric mercury (Hg) pollution and its spatial variation across the Plateau, lakes were chosen from three areas forming a north to south transect. Sediment cores were taken from three sites in each area and dated using the radionuclides 210Pb and 137Cs. Analysis of the cores yielded the first comprehensive Hg reconstructions for the Plateau, showing clear Hg pollution at all sites. The first indication of Hg pollution is much earlier than the onset of the industrial revolution in Europe, but the most significant pollution increase is from the 1970s, followed by a further marked increase from the 1990s. The mean post-2000 atmospheric pollution Hg accumulation rates for the sampling sites were estimated at between 5.1 and 7.9 μg m2 yr1. The increase in Hg pollution over the last few decades is synchronous with the recent economic development in Asia (especially China and India), and pollution Hg levels continue to increase. Furthermore, contemporary sediment Hg accumulation rate data are in broad agreement with Hg deposition values derived from global models that attribute pollution to sources mainly within southeast Asia. As most of the sites are exceptionally remote and situated above the atmospheric boundary layer, these results underline the need to understand the local Hg cycle in both regional and global context. © 2010 American Chemical Society.

Manning A.J.,UK Met Office | O'Doherty S.,University of Bristol | Jones A.R.,UK Met Office | Simmonds P.G.,University of Bristol | Derwent R.G.,Rdscientific
Journal of Geophysical Research: Atmospheres | Year: 2011

Methane (CH4) and nitrous oxide (N2O) have strong radiative properties in the Earth's atmosphere and both are regulated through the United Nations Framework Convention on Climate Change. Through this convention the United Kingdom is obliged to report an inventory of annual emission estimates from 1990. This paper describes a methodology that estimates emissions of CH4 and N2O completely independent of the inventory values. Emissions have been estimated for each year 1990-2007 for the United Kingdom and for NW Europe. The methodology combines high-frequency observations from Mace Head, a monitoring site on the west coast of Ireland, with an atmospheric dispersion model and an inversion system. The sensitivities of the inversion method to the modeling assumptions are reported. The 20 year Northern Hemisphere midlatitude baseline mixing ratios, growth rates, and seasonal cycles of both gases are also presented. The results indicate reasonable agreement between the inventory and inversion results for the United Kingdom for N2O over the entire period. For CH4 the agreement is poor in the 1990s but good in the 2000s. The UK CH4 inventory reported reduction from 1990-1992 to 2005-2007 (over 50%) is dominated by changes to landfill and coal mine emissions and is more than double the corresponding drop in the inversion estimated emissions (24%). The inversion results suggest that the United Kingdom has met its Kyoto commitment (-12.5%) but by a smaller margin (-14.3%) than reported (-17.3%). The results for NW Europe with the United Kingdom removed show reasonable agreement in trend, on average the inversion results for N2O are 25% lower and for CH 4 21% higher. Copyright 2011 by the American Geophysical Union.

Archibald A.T.,University of Bristol | Archibald A.T.,University of Cambridge | Cooke M.C.,University of Bristol | Utembe S.R.,University of Bristol | And 3 more authors.
Atmospheric Chemistry and Physics | Year: 2010

Recently reported model-measurement discrepancies for the concentrations of the HOx radical species (OH and HO2) in locations characterized by high emission rates of isoprene have indicated possible deficiencies in the representation of OH recycling and formation in isoprene mechanisms currently employed in numerical models; particularly at low levels of NOx. Using version 3.1 of the Master Chemical Mechanism (MCM v3.1) as a base mechanism, the sensitivity of the system to a number of detailed mechanistic changes is examined for a wide range of NOx levels, using a simple box model. The studies consider sensitivity tests in relation to three general areas for which experimental and/or theoretical evidence has been reported in the peer-reviewed literature, as follows: (1) implementation of propagating channels for the reactions of HO2 with acyl and β-oxo peroxy radicals with HO2, with support from a number of studies; (2) implementation of the OH-catalysed conversion of isoprene-derived hydroperoxides to isomeric epoxydiols, as characterised by Paulot et al.∼(2009a); and (3) implementation of a mechanism involving respective 1,5 and 1,6 H atom shift isomerisation reactions of the β-hydroxyalkenyl and cis-δ- hydroxyalkenyl peroxy radical isomers, formed from the sequential addition of OH and O2 to isoprene, based on the theoretical study of Peeters et al. (2009). All the considered mechanistic changes lead to simulated increases in the concentrations of OH, with (1) and (2) resulting in respective increases of up to about 7% and 16%, depending on the level of NOx. (3) is found to have potentially much greater impacts, with enhancements in OH concentrations of up to a factor of about 3.3, depending on the level of NOx, provided the (crucial) rapid photolysis of the hydroperoxy-methyl-butenal products of the cis-δ-hydroxyalkenyl peroxy radical isomerisation reactions is represented, as also postulated by Peeters et al.∼(2009). Additional tests suggest that the mechanism with the reported parameters cannot be fully reconciled with atmospheric observations and existing laboratory data without some degree of parameter refinement and optimisation which would probably include a reduction in the peroxy radical isomerisation rates and a consequent reduction in the OH enhancement propensity. However, an order of magntitude reduction in the isomerisation rates is still found to yield notable enhancements in OH concentrations of up to a factor of about 2, with the maximum impact at the low end of the considered NOx range.

A parameterized representation of the mechanistic changes is optimized and implemented into a reduced variant of the Common Representative Intermediates mechanism (CRI v2-R5), for use in the STOCHEM global chemistry-transport model. The impacts of the modified chemistry in the global model are shown to be consistent with those observed in the box model sensitivity studies, and the results are illustrated and discussed with a particular focus on the tropical forested regions of the Amazon and Borneo where unexpectedly elevated concentrations of OH have recently been reported. © Author(s) 2010.

Utembe S.R.,University of Bristol | Cooke M.C.,University of Bristol | Archibald A.T.,University of Bristol | Shallcross D.E.,University of Bristol | And 2 more authors.
Atmospheric Environment | Year: 2011

A secondary organic aerosol (SOA) code, coupled to the reduced Common Representative Intermediates chemical mechanism (CRI v2-R5), has been used in the global 3-D chemistry-transport model, STOCHEM, to simulate the global distribution of organic aerosol (OA) mass loadings. The SOA code represents the gas-to-aerosol partitioning of products formed over several generations of oxidation of a variety of organic precursors emitted from anthropogenic, biogenic and biomass burning sources. The model also includes emissions of primary organic aerosol (POA), based on the AeroCom inventory and the Global Fire Emissions database (GFED). The calculated burdens for POA, 0.89Tg, and SOA, 0.23Tg, are well within the range of values that have been reported in previous modelling studies. The calculated SOA annual in-situ production of 22.5Tgyr-1 also falls within the 8-110Tgyr-1 range calculated by other models, but is somewhat lower than observationally-constrained top-down estimates which have been reported recently. The oxidation of biogenic precursors is found to account for about 90% of the global SOA burden, and this makes a substantial contribution to the highest annual mean surface OA concentrations (up to 8μgm-3), which are simulated in tropical forested regions. Comparison of the simulated OA mass loadings with surface observations from a variety of locations indicate a good description of the OA distribution, but with an average underestimation of about a factor of 3. Sustained formation of SOA into the free troposphere is simulated, with important contributions from second and third-generation products of terpene oxidation in the upper troposphere. Comparison of the simulated OA mass loadings with vertical profiles from the ACE-Asia campaign indicates a very good description of the relative variation of OA with altitude, but with consistent underestimation of about a factor of 5. Although the absolute magnitude of the global source strength is underestimated, this consistency nonetheless provides support for an important role for multigenerational oxidation chemistry as a large source of SOA in the free troposphere, and lends credence to the necessity of representing condensable material formed over several generations of VOC oxidation in global chemistry-transport models. © 2011.

Redington A.L.,UK Met Office | Derwent R.G.,Rdscientific
Atmospheric Environment | Year: 2013

The Lagrangian atmospheric dispersion model, NAME, has been used to model the formation and transport of anthropogenic and biogenic secondary organic aerosol (SOA) over North-West Europe in 2008. The model has been tested against daily organic carbon measurements at Harwell, a rural site in southern UK, where it was able to represent adequately the observed values in summer, with some under-prediction in winter. The model has been used to look at the contribution of SOA to total measured PM10 at four selected UK sites. The site with the greatest contribution (32%) of SOA to PM10 was Auchencorth, a rural site in Scotland and least (9%) at London Bloomsbury. The biogenic SOA (BSOA) dominated over the anthropogenic SOA (ASOA) in the UK and showed a strong seasonal cycle peaking in the summer. There was also a slight summer increase in ASOA. The model has been employed to provide source attribution between UK sources and sources in the rest of Europe. The contribution from Europe was generally small but varied considerably due to meteorology. The UK component showed a seasonal cycle, peaking in the summer months. On an annual basis, considering the four measurement sites, the percentage of SOA arriving from outside the UK was least at Auchencorth (9.8%) and most at London (28.4%). Total modelled SOA had a maximum contribution of 2-3 μg m-3 as a monthly average. (It should be noted that in addition there will be a small contribution from background SOA to these figures). Emission sensitivity studies revealed that the response of ASOA was highly non-linear, showing both positive and negative responses to a 30% reduction in all man-made NOx sources and the response was greater than 1:1 to a 30% reduction in all man-made VOC sources. BSOA showed only a small negative response to a 30% NOx reduction and no change to a 30% VOC reduction. © 2012.

Strong J.,Lancaster University | Whyatt J.D.,Lancaster University | Hewitt C.N.,Lancaster University | Derwent R.G.,rdscientific
Atmospheric Environment | Year: 2010

This paper describes the further development and application of the Edinburgh-Lancaster Model for Ozone (ELMO). We replace straight-line back-trajectories with trajectories and associated meteorology supplied by the US National Oceanic and Atmospheric Administration Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) service to allow more realistic modelling of specific UK ozone episodes. We call this ELMO-2. Model performance is rigorously tested against observed ozone concentrations for two episodes recorded across 14 rural UK monitoring stations during the spring and summer of 1995. For both episodes, the afternoon concentrations (usually coinciding with the daily maxima) are captured well by the model and the diurnal ozone cycle is reproduced, although the amplitude in concentrations is generally smaller than the observed. The summer episode is investigated further through indicator species analysis and source attribution, and found to be mainly VOC-limited. European emissions account for the majority of ozone production. We demonstrate how improved modelling leads to better understanding of regional and local ozone production across the UK under episodic conditions. © 2009 Elsevier Ltd. All rights reserved.

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