D. O. Topping

The University of Manchester, Manchester, England, United Kingdom

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Publications (65)210.71 Total impact

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    ABSTRACT: Laboratory measurements of vapour pressures for atmospherically relevant compounds were collated and used to assess the accuracy of vapour pressure estimates generated by seven estimation methods and impacts on predicted secondary organic aerosol. Of the vapour pressure estimation methods that were applicable to all the test set compounds, the Lee-Kesler [Reid et al., The Properties of Gases and Liquids, 1987] method showed the lowest mean absolute error and the Nannoolal et al. [Nannoonal et al., Fluid Phase Equilib., 2008, 269, 117-133] method showed the lowest mean bias error (when both used normal boiling points estimated using the Nannoolal et al. [Nannoolal et al., Fluid Phase Equilib., 2004, 226, 45-63] method). The effect of varying vapour pressure estimation methods on secondary organic aerosol (SOA) mass loading and composition was investigated using an absorptive partitioning equilibrium model. The Myrdal and Yalkowsky [Myrdal and Yalkowsky, Ind. Eng. Chem. Res., 1997, 36, 2494-2499] vapour pressure estimation method using the Nannoolal et al. [Nannoolal et al., Fluid Phase Equilib., 2004, 226, 45-63] normal boiling point gave the most accurate estimation of SOA loading despite not being the most accurate for vapour pressures alone.
    Physical Chemistry Chemical Physics 08/2014; · 4.20 Impact Factor
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    ABSTRACT: The growth, composition, and evolution of secondary organic aerosol (SOA) are governed by properties of individual compounds and ensemble mixtures that affect partitioning between the vapour and condensed phase. There has been considerable recent interest in the idea that SOA can form highly viscous particles where the diffusion of either water or semivolatile organics within the particle is sufficiently hindered to affect evaporation and growth. Despite numerous indirect inferences of viscous behaviour from SOA evaporation or 'bounce' within aerosol instruments, there have been no bulk measurements of the viscosity of well-constrained model aerosol systems of atmospheric significance. Here the viscous behaviour of a well-defined model system of 9 dicarboxylic acids is investigated directly with complimentary measurements and model predictions used to infer phase state. Results not only allow us to discuss the atmospheric implications for SOA formation through this representative mixture, but also the potential impact of current methodologies used for probing this affect in both the laboratory and from a modelling perspective. We show, quantitatively, that the physical state transformation from liquid-like to amorphous semisolid can substantially increase the importance of mass transfer limitations within particles by 7 orders of magnitude for 100 nm diameter particles. Recommendations for future research directions are given.
    Environmental science & technology. 07/2014;
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  • Faraday Discussions. 06/2013;
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    ABSTRACT: Clouds profoundly influence weather and climate. The brightness and lifetime of clouds is determined by cloud droplet number concentration, in turn dictated by the number of available seed particles. The formation of cloud droplets on non-volatile atmospheric particles is well understood. However, fine particulate matter in the atmosphere ranges widely in volatility. Co-condensation of semi-volatile compounds with water increases a particle's propensity for cloud droplet formation, with potential consequences for feedbacks between the terrestrial biosphere and climate. Here we systematically study cloud droplet formation, using a cloud parcel model extended to include co-condensation of semi-volatile organic compounds under a broad variety of realistic conditions. As an air parcel rises and cools, the concentration of organic vapour that it can hold declines. Thus, the simulated organic vapours become increasingly saturated as they ascend, and so condense on growing particles as they swell into cloud droplets. We show that condensation of increasingly volatile material adds to the soluble mass of these droplets and facilitates the uptake of additional water, which leads, in turn, to a substantial increase in the number of viable cloud droplets. We suggest that the co-condensation of semi-volatile organic compounds with water vapour has a substantial impact on the radiative properties of clouds.
    Nature Geoscience 06/2013; 6(6):443-446. · 11.67 Impact Factor
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    ABSTRACT: We present a parameterisation of aerosol activation, including co-condensation of semi-volatile organics, for warm clouds that has applications in large-scale numerical models. The scheme is based on previously developed parameterisations that are in the literature, but has two main modifications. The first is that the total aerosol mass is modified by the condensation of organic vapours entering cloud-base, whereas the second is that this addition of mass acts to modify the median diameter and the geometric standard deviation of the aerosol size distribution. It is found that the scheme is consistent with parcel model calculations of co-condensation under different regimes. Such a parameterisation may find use in evaluating important feedbacks in climate models.
    Atmospheric Chemistry and Physics 06/2013; 13(6):14447-14475. · 4.88 Impact Factor
  • David Owen Topping, Mark Barley, Gordon McFiggins
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    ABSTRACT: A simple approach to calculate liquid-liquid phase separation has been developed based on the derivation of partitioning coefficients between multiple liquid phases and included in a framework used to partition an arbitrary number of compounds between the vapour and particle phases in an atmospheric aerosol. The representation compares favourably with a more complex and expensive benchmark gas-particle thermodynamic model for simple well constrained sys tems. The model has then been applied to consider liquid phase separation in multicomponent particles formed by the equilibration of organic products generated by a near-explicit model of VOC oxidation. Inclusion of phase separation decreases the predicted mass of condensed organic material by 10 to 50%, dependent on the concentration of semi-volatile components and ambient conditions in the model scenario. The current study considers only two liquid phases, but the framework can readily accommodate an arbitrary number, though this is beyond the scope of the current work. Uncertainty introduced by the omission of phase separation is far lower than existing uncertainties in pure component vapour pressures, where orders of magnitude differences in predicted mass are found, though the bias introduced when choosing a particular method for estimating the saturation vapour pressure will influence the magnitude of phase separation. The proposed technique is the first to be used to practically deal with the many hundreds or thousands of components present in the ambient atmospheric aerosol.
    Faraday Discussions 05/2013; · 3.82 Impact Factor
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    ABSTRACT: Carbonyl oxides (“Criegee intermediates”), formed in the ozonolysis of alkenes, are key species in tropospheric oxidation of organic molecules and their decomposition provides a non-photolytic source of OH in the atmosphere.1-3 Recently it was shown that small Criegee intermediates, CIs, react far more rapidly with SO2 than typically represented in tropospheric models,4 which suggested that carbonyl oxides could have a substantial influence on the atmospheric oxidation of SO2. Oxidation of SO2 is the main atmospheric source of sulphuric acid (H2SO4), which is a critical contributor to aerosol formation, although questions remain about the fundamental nucleation mechanism.5-7 Non-absorbing atmospheric aerosols, by scattering incoming solar radiation and acting as cloud condensation nuclei, have a cooling effect on climate.8 Here we explore the effect of the Criegees on atmospheric chemistry, and demonstrate that ozonolysis of alkenes via the reaction of Criegee intermediates potentially has a large impact on atmospheric sulphuric acid concentrations and consequently the first steps in aerosol production. Reactions of Criegee intermediates with SO2 will compete with and in places dominate over the reaction of OH with SO2 (the only other known gas-phase source of H2SO4) in many areas of the Earth’s surface. In the case that the products of Criegee intermediate reactions predominantly result in H2SO4 formation, modelled particle nucleation rates can be substantially increased by the improved experimentally obtained estimates of the rate coefficients of Criegee intermediate reactions. Using both regional and global scale modelling, we show that this enhancement is likely to be highly variable spatially with local hot-spots in e.g. urban outflows. This conclusion is however contingent on a number of remaining uncertainties in Criegee intermediate chemistry
    Faraday Discussions 05/2013; · 3.82 Impact Factor
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    ABSTRACT: In order to model the properties and chemical composition of secondary organic aerosol (SOA) estimated physical property data for many thousands of organic compounds are required. Seven methods for estimating liquid density are assessed against experimental data for a test set of 56 multifunctional organic compounds. The group contribution method of Schroeder coupled with the Rackett equation using critical properties by Nannoolal was found to give the best liquid density values for this test set. During this work some problems with the representation of certain groups (aromatic amines and phenols) within the critical property estimation methods were identified, highlighting the importance (and difficulties) of deriving the parameters of group contribution methods from good quality experimental data. A selection of the estimation methods are applied to the 2742 compounds of an atmospheric chemistry database which showed that they provided consistent liquid density values for compounds with such atmospherically important (but poorly studied) functional groups as hydroperoxide, peroxide, peroxyacid and PAN. Estimated liquid density values are also presented for a selection of compounds predicted to be important in atmospheric SOA. Hygroscopic growth factor (a property expected to depend on liquid density) has been calculated for a wide range of particle compositions. A low sensitivity of the growth factor to liquid density was found and a single density value of 1350 kg.m$^{-3}$ could be used for all multicomponent SOA in the calculation of growth factors for comparison with experimentally measured values in the laboratory or the field without incurring significant error.
    The Journal of Physical Chemistry A 03/2013; · 2.77 Impact Factor
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    ABSTRACT: Carbonyl oxides ("Criegee intermediates"), formed in the ozonolysis of alkenes, are key species in tropospheric oxidation of organic molecules and their decomposition provides a non-photolytic source of OH in the atmosphere (Johnson and Marston, Chem. Soc. Rev., 2008, 37, 699, Harrison et al, Sci, Total Environ., 2006, 360, 5, Gäb et al., Nature, 1985, 316, 535, ref. 1-3). Recently it was shown that small Criegee intermediates, C.I.'s, react far more rapidly with SO2 than typically represented in tropospheric models, (Welz, Science, 2012, 335, 204, ref. 4) which suggested that carbonyl oxides could have a substantial influence on the atmospheric oxidation of SO2. Oxidation of 502 is the main atmospheric source of sulphuric acid (H2SO4), which is a critical contributor to aerosol formation, although questions remain about the fundamental nucleation mechanism (Sipilä et al., Science, 2010, 327, 1243, Metzger et al., Proc. Natl. Acad. Sci. U. S. A., 2010 107, 6646, Kirkby et al., Nature, 2011, 476, 429, ref. 5-7). Non-absorbing atmospheric aerosols, by scattering incoming solar radiation and acting as cloud condensation nuclei, have a cooling effect on climate (Intergovernmental Panel on Climate Change (IPCC), Climate Change 2007: The Physical Science Basis, Cambridge University Press, 2007, ref. 8). Here we explore the effect of the Criegees on atmospheric chemistry, and demonstrate that ozonolysis of alkenes via the reaction of Criegee intermediates potentially has a large impact on atmospheric sulphuric acid concentrations and consequently the first steps in aerosol production. Reactions of Criegee intermediates with SO2 will compete with and in places dominate over the reaction of OH with SO2 (the only other known gas-phase source of H2SO4) in many areas of the Earth's surface. In the case that the products of Criegee intermediate reactions predominantly result in H2SO4 formation, modelled particle nucleation rates can be substantially increased by the improved experimentally obtained estimates of the rate coefficients of Criegee intermediate reactions. Using both regional and global scale modelling, we show that this enhancement is likely to be highly variable spatially with local hot-spots in e.g. urban outflows. This conclusion is however contingent on a number of remaining uncertainties in Criegee intermediate chemistry.
    Faraday Discussions 01/2013; 165:45-73. · 3.82 Impact Factor
  • David Topping, Mark Barley, Gordon McFiggans
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    ABSTRACT: A simple approach to calculate liquid-liquid phase separation has been developed based on the derivation of partitioning coefficients between multiple liquid phases and inclusion in a framework used to partition an arbitrary number of compounds between the vapour and particle phases in an atmospheric aerosol. The representation compares favourably with a more complex and expensive benchmark gas-particle thermodynamic model for simple well-constrained systems. The model has then been applied to consider liquid phase separation in multicomponent particles formed by the equilibration of organic products generated by a near-explicit model of VOC oxidation. Inclusion of phase separation decreases the predicted mass of condensed organic material by -10 to -50%, dependent on the concentration of semi-volatile components and ambient conditions in the model scenario. The current study considers only two liquid phases, but the framework can readily accommodate an arbitrary number, though this is beyond the scope of the current work. Uncertainty introduced by the omission of phase separation is far lower than existing uncertainties in pure component vapour pressures, where orders of magnitude differences in predicted mass are found, though the bias introduced when choosing a particular method for estimating the saturation vapour pressure will influence the magnitude of phase separation. The proposed technique is the first to be used to practically deal with the many hundreds or thousands of components present in the ambient atmospheric aerosol.
    Faraday Discussions 01/2013; 165:273-88. · 3.82 Impact Factor
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    ABSTRACT: Knudsen Effusion Mass Spectrometry (KEMS) has been used to measure solid state equilibrium vapour pressures of several multifunctional aromatic compounds; phthalic, isophthalic, terephthalic, para-anisic, ortho-amino benzoic, meta-amino benzoic, para-amino benzoic, vanillic, syringic, 1,2,4-tricarboxylic benzoic, 3,5-dihydroxy-4-methyl benzoic and 4-methyl phthalic acids and nitrocatechol. Sub-cooled liquid vapour pressures were derived using Differential Scanning Calorimetry (DSC) measured thermochemical properties for the compounds measured here, as well as for other substituted benzoic acids using literature values. Unusual trends in the sub-cooled liquid vapour pressure, not represented by currently available vapour pressure estimation methods, are explained using a newly constructed Structure–Activity Relationship (SAR) with a combination of resonance and steric effects. This was then tested against further measurements of ortho-dimethyl amino benzoic and meta-dimethyl amino benzoic acids.
    RSC Advances 05/2012; 2(10):4430-4443. · 3.71 Impact Factor
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    D. O. Topping, G. McFiggans
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    ABSTRACT: The substantial uncertainty in the indirect effect of aerosol particles on radiative forcing in large part arises from the influences of atmospheric aerosol particles on (i) the brightness of clouds, exerting significant shortwave cooling with no appreciable compensation in the long wave, and on (ii) their ability to precipitate, with implications for cloud cover and lifetime. Predicting the ambient conditions at which aerosol particles may become cloud droplets is largely reliant on an equilibrium relationship derived by Köhler (1936). However, the theoretical basis of the relationship restricts its application to particles solely comprising involatile compounds and water, whereas a substantial fraction of particles in the real atmosphere will contain potentially thousands of semi-volatile organic compounds in addition to containing semi-volatile inorganic components such as ammonium nitrate. We show that equilibration of atmospherically reasonable concentrations of organic compounds with a growing particle as the ambient humidity increases has potentially larger implications on cloud droplet formation than any other equilibrium compositional dependence, owing to inextricable linkage between the aerosol composition, a particles size and concentration under ambient conditions. Whilst previous attempts to account for co-condensation of gases other than water vapour have been restricted to one inorganic condensate, our method demonstrates that accounting for the co-condensation of any number of organic compounds substantially decreases the saturation ratio of water vapour required for droplet activation. This effect is far greater than any other compositional dependence; more so even than the unphysical effect of surface tension reduction in aqueous organic mixtures, ignoring differences in bulk and surface surfactant concentrations.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 04/2012; 12(7):3253-3260. · 5.51 Impact Factor
  • Geophysical Research Letters 03/2012; 39. · 3.98 Impact Factor
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    ABSTRACT: In this paper we describe and summarize the main achievements of the European Aerosol Cloud Climate and Air Quality Interactions project (EUCAARI). EUCAARI started on 1 January 2007 and ended on 31 December 2010 leaving a rich legacy including: (a) a comprehensive database with a year of observations of the physical, chemical and optical properties of aerosol particles over Europe, (b) comprehensive aerosol measurements in four developing countries, (c) a database of airborne measurements of aerosols and clouds over Europe during May 2008, (d) comprehensive modeling tools to study aerosol processes fron nano to global scale and their effects on climate and air quality. In addition a new Pan-European aerosol emissions inventory was developed and evaluated, a new cluster spectrometer was built and tested in the field and several new aerosol parameterizations and computations modules for chemical transport and global climate models were developed and evaluated. These achievements and related studies have substantially improved our understanding and reduced the uncertainties of aerosol radiative forcing and air quality-climate interactions. The EUCAARI results can be utilized in European and global environmental policy to assess the aerosol impacts and the corresponding abatement strategies.
    Atmospheric Chemistry and Physics 12/2011; 11:13061-13143. · 4.88 Impact Factor
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    ABSTRACT: The primary and secondary contributions to the marine aerosol budget are subject to large uncertainty as are the contributory processes leading to the, often substantial, organic fraction. The resultant impacts on indirect radiative forcing through the effect of aerosol on marine cloud cover are consequently uncertain. Here we report in situ measurements of sub-micron aerosol composition, hygroscopicity and particle size-resolved cloud condensation nucleus behaviour from a number of open ocean and coastal experiments at different times of year. The ability to reconcile the measurements with each other and thereby to explain the aerosol properties in terms of particle size and composition will be discussed. We will attempt to place the ambient measurements in the context of primary seaspray bubbletank experiments to make inferences about the relative primary and secondary contributions to the properties of the ambient aerosol.
    AGU Fall Meeting Abstracts. 12/2011;
  • D. O. Topping, G. McFiggans
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    ABSTRACT: Atmospheric aerosol particles are comprised of both inorganic and organic compounds. Whilst the inorganic fraction is relatively well characterised, the organic fraction can comprise many thousands of largely unidentified compounds with a wide range of properties. This presents challenges when attempting to reconcile particulate composition and behaviour in the atmosphere. In addition, the size of an aerosol particle influences the equilibrium composition and phase state for a given set of ambient conditions and availability of semi-volatile material. Incorporating the influence of curvature in theoretical constructs can be complex. Unfortunately, basic absorptive equilibrium partitioning models largely neglect the influence of curvature, leading to errors in predicted composition and volatility for conditions in which size is likely to play an important role: new particle formation or cloud activation. In this study we present application of a partitioning model that explicitly accounts for impact of curvature on the equilibrium position for particles of any size and comprised of an unlimited number of compounds. Using this framework we are able to revisit predictions of cloud activation potential for organic aerosol with multiple semi-volatile fractions. Whilst the influence of curvature is explicit within the Kohler equation, this only accounts for the equilibration of water between and gaseous and particle phase. Direct use of the Kohler equation therefore does not implicitly account for the co-condensation/evaporation of semi-volatile components as the relative humidity changes. We show that equilibration of explicit consideration of gas to particle partitioning for atmospherically reasonable concentrations of any number of organic compounds with a growing particle as the ambient humidity increases has potentially larger implications on cloud activation than any other equilibrium compositional dependence, Replicating the true atmospheric behaviour of aerosol particles poses challenges for both the modelling and measurement community, our simulations suggesting it is impossible to uncouple composition and size in studying the role of aerosol particles in radiative forcing, as once previously thought. In addition to predictions of cloud activation potential allowing co-codensation of any number of organic compounds, we are able to explore generalised relationships between aerosol functionality and particle size. This allows us to probe the relationships between required abundance and volatility for those compounds expected to take part in the growth of freshly nucleated particles. A discussion of the caveats behind this approach is given, along with pitfalls of not explictly accounting for aerosol size in previously applied predictive frameworks with potential impacts of subsequent aerosol properties.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: We present a new and considerably extended parameterization of the thermodynamic activity coefficient model AIOMFAC (Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients) at room temperature. AIOMFAC combines a Pitzer-like electrolyte solution model with a UNIFAC-based group-contribution approach and explicitly accounts for interactions between organic functional groups and inorganic ions. Such interactions constitute the salt-effect, may cause liquid-liquid phase separation, and affect the gas-particle partitioning of aerosols. The previous AIOMFAC version was parameterized for alkyl and hydroxyl functional groups of alcohols and polyols. With the goal to describe a wide variety of organic compounds found in atmospheric aerosols, we extend here the parameterization of AIOMFAC to include the functional groups carboxyl, hydroxyl, ketone, aldehyde, ether, ester, alkenyl, alkyl, aromatic carbon-alcohol, and aromatic hydrocarbon. Thermodynamic equilibrium data of organic-inorganic systems from the literature are critically assessed and complemented with new measurements to establish a comprehensive database. The database is used to determine simultaneously the AIOMFAC parameters describing interactions of organic functional groups with the ions H+, Li+, Na+, K+, NH4+, Mg2+, Ca2+, Cl−, Br−, NO3−, HSO4−, and SO42−. Detailed descriptions of different types of thermodynamic data, such as vapor-liquid, solid-liquid, and liquid-liquid equilibria, and their use for the model parameterization are provided. Issues regarding deficiencies of the database, types and uncertainties of experimental data, and limitations of the model, are discussed. The challenging parameter optimization problem is solved with a novel combination of powerful global minimization algorithms. A number of exemplary calculations for systems containing atmospherically relevant aerosol components are shown. Amongst others, we discuss aqueous mixtures of ammonium sulfate with dicarboxylic acids and with levoglucosan. Overall, the new parameterization of AIOMFAC agrees well with a large number of experimental datasets. However, due to various reasons, for certain mixtures important deviations can occur. The new parameterization makes AIOMFAC a versatile thermodynamic tool. It enables the calculation of activity coefficients of thousands of different organic compounds in organic-inorganic mixtures of numerous components. Models based on AIOMFAC can be used to compute deliquescence relative humidities, liquid-liquid phase separations, and gas-particle partitioning of multicomponent mixtures of relevance for atmospheric chemistry or in other scientific fields.
    Atmospheric Chemistry and Physics 09/2011; 11:9155-9206. · 4.88 Impact Factor
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    D. Topping, D. Lowe, G. McFiggans
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    ABSTRACT: A flexible mixing rule is presented which allows the calculation of activity coefficients of organic compounds in a multi-component aqueous solution. Based on the same fitting methodology as a previously published inorganic model (Partial Differential Fitted Taylor series Expansion; PD-FiTE), organic PD-FiTE treats interactions between binary pairs of solutes with polynomials of varying order. Using 13 example compounds extracted from a recent sensitivity study, the framework is benchmarked against the UNIFAC model. For 1000 randomly derived concentration ranges and 10 relative humidities between 10 and 99 %, the average deviation in predicted activity coefficients was calculated to be 3.8 %. Whilst compound specific deviations are present, the median and inter-quartile values across all relative humidity range always fell within ±20 % of the UNIFAC value. Comparisons were made with the UNIFAC model by assuming interactions between solutes can be set to zero within PD-FiTE. In this case, deviations in activity coefficients as low as -40 % and as high as +70 % were found. Both the fully coupled and uncoupled organic PD-FiTE are upto a factor of ≈12 and ≈66 times more efficient than calling the UNIFAC model using the same water content, and ≈310 and ≈1800 times more efficient than an iterative model using UNIFAC. The use of PD-FiTE within a dynamical framework is presented, demonstrating the potential inaccuracy of prescribing fixed negative or positive deviations from ideality when modelling the evolving chemical composition of aerosol particles.
    Geoscientific Model Development Discussions 01/2011;
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    Atmospheric Chemistry and Physics 01/2011; 11(24):13061-13143. · 4.88 Impact Factor