Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Variations of bacteria and endotoxin concentrations in houses for fattening pigs and broilers within and between days = Binnen- en tussendagvariaties in bacterie- en endotoxineconcentraties in stallen voor vleesvarkens en -kuikens
    Aarnink, A.J.A. ; Hattum, T.G. van; Ogink, N.W.M. - \ 2015
    Wageningen : Wageningen UR Livestock Research (Livestock Research report 879) - 26
    bacteriën - endotoxinen - aërosolen - pluimveehokken - varkensstallen - huisvesting van kippen - vleeskuikens - varkenshouderij - pluimveehouderij - gezondheidsgevaren - gezondheid op het platteland - emissie - bacteria - endotoxins - aerosols - poultry housing - pig housing - chicken housing - broilers - pig farming - poultry farming - health hazards - rural health - emission
    In this study the diurnal variations in bacteria and endotoxin concentrations and emissions in houses for finishing pigs and broilers were determined.
    Measurement protocol for emissions of bio-aerosols from animal houses = Meetprotocol voor emissies van bio-aerosolen uit stallen
    Aarnink, A.J.A. ; Zhao, Y. ; Ogink, N.W.M. - \ 2015
    Wageningen : Wageningen UR Livestock Research (Livestock Research report 878) - 26
    aërosolen - emissie - meetsystemen - huisvesting, dieren - landbouwschuren - landbouw en milieu - volksgezondheid - stallen - emissiereductie - aerosols - emission - measurement systems - animal housing - barns - agriculture and environment - public health - stalls - emission reduction
    People living in the vicinity of newly build or enlarged livestock farms are concerned about possible effects on their health and their quality of life. This concern is mainly related to expected increasing concentrations of particulate matter (dust) in the environment and possible transmission of airborne diseases from animals to humans (zoonotic diseases). The objective of this study was to develop a measuring protocol for determining emissions of bio-aerosols from animal houses and to determine the efficiency of potential reduction systems.
    Technieken voor reductie van bio-aerosol emissies uit stallen
    Aarnink, A.J.A. ; Hagenaars, T.J. ; Ogink, N.W.M. - \ 2015
    Wageningen : Wageningen UR Livestock Research (Livestock Research rapport 828) - 31
    aërosolen - emissiereductie - broeikasgassen - stallen - veehouderij - bedrijfssystemen - intensieve veehouderij - gezondheidsgevaren - volksgezondheid - aerosols - emission reduction - greenhouse gases - stalls - livestock farming - farming systems - intensive livestock farming - health hazards - public health
    Sinds de uitbraken van besmettelijke veeziekten zoals de vogelgriep (AI) en vooral Q-koorts is er bij de Nederlandse bevolking een toegenomen bezorgdheid over de negatieve effecten van de veehouderij op de gezondheid van de mens. Gezondheidsproblemen kunnen ontstaan als gevolg van bepaalde micro-organismen, maar ook als gevolg van bepaalde componenten afkomstig van microorganismen. Deze kiemen en kiemcomponenten kunnen op zichzelf staande deeltjes in de lucht vormen, maar zijn vaak onderdeel van grotere stofdeeltjes. Stofdeeltjes die geheel of gedeeltelijk een biologische oorsprong hebben noemen we bio-aerosolen. Het doel van dit onderzoek is te inventariseren welke technieken potentie hebben om de emissie en/of de verspreiding van bio-aerosolen (met kiemen en kiemcomponenten) te reduceren en tevens te inventariseren welke daaruit direct beschikbaar zijn voor de praktijk. In dit rapport wordt vooral ingegaan op generieke methoden om de emissie van bio-aerosolen te reduceren.
    Processen en factoren die van invloed zijn op de emissie van bio-aerosolen uit stallen
    Aarnink, A.J.A. ; Yang Zhao, Yang ; Dekker, A. ; Ogink, N.W.M. - \ 2015
    Wageningen : Wageningen UR Livestock Research (Livestock Research rapport 829)
    aërosolen - emissie - volksgezondheid - gezondheidsbescherming - intensieve veehouderij - landbouwkundig onderzoek - rundvee - huisvesting van rundvee - nederland - aerosols - emission - public health - health protection - intensive livestock farming - agricultural research - cattle - cattle housing - netherlands
    In the Netherlands there are some regions with very high animal densities. From the sixties in the last century intensive livestock production has expanded largely, especially in the South and East of the Netherlands. The objective of this desk study was to describe the processes and factors that influences the formation and emission of bioaerosols. Based on this report priority and support can be given to those studies that help to prevent emission of pathogens from animal houses.
    Interactions between aerosal and convective boundary-layer dynamics over land
    Wilde Barbaro, E. - \ 2015
    Wageningen University. Promotor(en): Maarten Krol; Bert Holtslag, co-promotor(en): Jordi Vila-Guerau de Arellano. - Wageningen : Wageningen University - ISBN 9789462572652 - 182
    aërosolen - atmosferische grenslaag - grenslaagmeteorologie - aardoppervlak - modellen - simulatiemodellen - straling - atmosfeer - aerosols - atmospheric boundary-layer - boundary-layer meteorology - land surface - models - simulation models - radiation - atmosphere

    In this Section, we summarize the most important findings and relevant issues treated in detail in Chapters 2 to 5.

    The primary conclusion of this thesis is that it is necessary to take aerosols into account to accurately describe the convective atmospheric boundary-layer (CBL) dynamics and the land-surface processes. We reached this conclusion by systematically studying the land-CBL system and its couplings, and employed a hierarchy of models ranging from an eddy-resolving model (large-eddy simulation; LES) to non-eddy resolving models (mixed-layer model, and single column model). In addition to the numerical component, we used a complete observational data set to help us design and evaluate our numerical framework.

    Chapter 2 was devoted to the explanation of the radiative transfer code used in Chapters 4 and 5. We showed that despite the simplified treatment of solar radiation and its interactions with aerosols, our radiative code is in general agreement with a more sophisticated radiative transfer code, even for extreme aerosol loads. Moreover, our results reproduce observations of direct and diffuse radiation at the surface accordingly - as shown in Chapter 4.

    Regarding the longwave band, we showed that aerosols are not relevant for the estimation of the incoming longwave radiation at the surface. We concluded that Brunt's formula, depending only on screen level temperature and vapor pressure, is the most adequate to represent the incoming longwave radiation at the surface for the cases relevant for our studies.

    In Chapter 3 we investigated the impact of aerosol heat absorption on the dynamics of an idealized CBL with prescribed surface fluxes. We found that the structure and evolution of the CBL were influenced by the vertical distribution of the aerosols. Moreover, we showed that the aerosols influence the exchange of heat between the CBL and the free troposphere by (i) extinction of radiation and consequently reduced surface fluxes, and by (ii) deepening the entrainment zone depth. We highlighted the importance of high-resolution models to properly represent the effects of aerosol absorption of radiation on the dynamics of the CBL, especially in the entrainment zone. We demonstrated that, in addition to the properties of the aerosols, the vertical distribution is an important characteristic to properly describe the CBL height evolution and the dynamics of the upper part of the CBL. To further support the analysis of the LES results, we used a mixed-layer (MXL) model to calculate boundary-layer depth and the potential temperature jump at the inversion layer. In spite of the simplicity of this model, the mixed-layer results obtained for boundary-layer height and the inversion layer jump agreed well with the LES results.

    Extending the knowledge acquired with the academical prototypical experiments performed in Chapter 3, in Chapter 4 we quantified the effects of aerosol scattering and absorption of shortwave (SW) radiation both on the surface energy budget and on the CBL dynamics. To this end, we coupled our LES model and the MXL model to (i) a land-surface model and (ii) a broadband SW radiative transfer model, (described in Chapter 2). We successfully validated the results obtained with the LES model and MXL model using measurements of (thermo)dynamic variables and aerosol properties observed in Cabauw (the Netherlands). Our LES results showed that for Cabauw (over well-watered grassland) aerosols significantly alter the magnitude of the available energy at the surface and its partitioning. Under well-watered conditions, the sensible heat flux was more strongly reduced compared to the latent heat flux. Given the satisfactory agreement between the LES results and MXL model results, we further explored the sensitivity of the land-CBL system to a wide range of aerosol optical depths and single scattering albedos using the MXL model. Our results showed that higher loads of aerosols impose an energy restriction at the surface. As a result, we calculated a delay in the morning onset of the CBL and an advance in the CBL afternoon collapse. We also found that entrainment of aerosols from the residual layer plays a significant role in the development of the CBL dynamics during the day. An important aspect of Chapter 4 is the investigation of the different responses of the CBL dynamics depending on aerosol optical properties. Strongly absorbing aerosols deepened and warmed the CBL, while purely scattering aerosols shallowed and cooled the CBL.

    We highlighted that the results presented in Chapter 4 can be used as a benchmark to evaluate coupling and performance of the parametrizations for SW radiation, land-surface and boundary-layer schemes, implemented in mesoscale or global chemistry transport models.

    In Chapter 5 we increased the complexity of our land-CBL system representation by studying the formation and transport of ammonium nitrate aerosols. In doing so, we coupled in our LES radiation, chemistry, aerosols, CBL dynamics, and surface exchange processes of chemicals, heat and moisture. Our fully coupled LES model was again evaluated against observations of chemistry and aerosol fields and showed a good correspondence. In particular, our results showed a satisfactory agreement between the simulated and observed nitrate partitioning at the surface.

    We showed that gas-aerosol conversion of nitrate leads to highly non-linear profiles of nitrate concentrations and turbulent fluxes. Moreover, the shapes of the simulated profiles depended strongly on the time scale of gas-aerosol conversions. Note that the typical timescale of turbulent motions in the CBL is around 10-20 minutes. For shorter time scales of gas-aerosol conversion compared to the CBL dynamics timescale, we found that turbulent fluxes are larger and concentration profiles more tilted within the CBL. These results have a significant impact on the nitrate deposition flux at the surface. Our LES results confirmed that the large deposition velocities for aerosol nitrate close to the surface are actually due to outgassing of aerosol nitrate rather than a real deposition process.

    An important aspect discussed in Chapter 5 concerns the inability of non-eddy resolving models to accurately model the turbulent transport of nitrate within the CBL. Based on a detailed analysis of the flux budget equation, we showed that the exchange coefficient of heat used in our 1D model has to be increased to better account for the complex interaction between gas-aerosol conversion of nitrate and 3D turbulence within the CBL. Indeed, the new exchange coefficient also improved the comparison between gas-aerosol partitioning of nitrate calculated with our 1D model and surface observations.

    The results discussed in this thesis demonstrate the need for considering the influence of aerosols on the CBL dynamics. Specifically, aerosols influence important phenomena for the CBL evolution namely radiation, surface-atmosphere interactions, chemistry, and (thermo)dynamics. In addition to that, the availability of high-resolution numerical simulations is crucial to validate and evaluate results obtained by numerical models that do not explicitly resolve the turbulent field.

    Nanoemulsions obtained via bubble bursting at a compound interface
    Feng, J. ; Roche, M. ; Vigolo, D. ; Arnaudov, L.N. ; Stoyanov, S.D. ; Gurkov, T.D. ; Tsutsumanova, G.G. ; Stone, H.A. - \ 2014
    Nature physics 10 (2014). - ISSN 1745-2473 - p. 606 - 612.
    surfactant solutions - gas-bubbles - sea spray - alkanes - aerosols - water
    Bursting of bubbles at an air/liquid interface is a familiar occurrence relevant to foam stability, cell cultures in bioreactors and ocean–atmosphere mass transfer. In the latter case, bubble-bursting leads to the dispersal of sea-water aerosols in the surrounding air. Here we show that bubbles bursting at a compound air/oil/water-with-surfactant interface can disperse submicrometre oil droplets in water. Dispersal results from the detachment of an oil spray from the bottom of the bubble towards water during bubble collapse. We provide evidence that droplet size is selected by physicochemical interactions between oil molecules and the surfactants rather than by hydrodynamics. We demonstrate the unrecognized role that this dispersal mechanism may play in the fate of the sea surface microlayer and of pollutant spills by dispersing petroleum in the water column. Finally, our system provides an energy-efficient route, with potential upscalability, for applications in drug delivery, food production and materials science.
    Plant volatiles and the environment
    Loreto, F. ; Dicke, M. ; Schnitzler, J.P. ; Turlings, T.C.J. - \ 2014
    Plant, Cell & Environment 37 (2014)8. - ISSN 0140-7791 - p. 1905 - 1908.
    herbivorous insects - floral scent - isoprene - perception - evolution - emissions - aerosols - defense - enemies - stress
    Volatile organic compounds emitted by plants represent the largest part of biogenic volatile organic compounds (BVOCs) released into our atmosphere. Plant volatiles are formed through many biochemical pathways, constitutively and after stress induction. In recent years, our understanding of the functions of these molecules has made constant and rapid progress. From being considered in the past as a mere waste of carbon, BVOCs have now emerged as an essential element of an invisible language that is perceived and exploited by the plants' enemies, the enemies of plant enemies, and neighbouring plants. In addition, BVOCs have important functions in protecting plants from abiotic stresses. Recent advances in our understanding of the role of BVOC in direct and indirect defences are driving further attention to these emissions. This special issue gathers some of the latest and most original research that further expands our knowledge of BVOC. BVOC emissions and functions in (1) unexplored terrestrial (including the soil) and marine environments, (2) in changing climate conditions, and (3) under anthropic pressures, or (4) in complex trophic communities are comprehensively reviewed. Stepping up from scientific awareness, the presented information shows that the manipulation and exploitation of BVOC is a realistic and promising strategy for agricultural applications and biotechnological exploitations.
    Relatie tussen emissiereducties van bio-aerosolen en gezondheidsrisico's in de omgeving van veehouderijbedrijven: voorstudie.
    Hagenaars, T.J. ; Aarnink, A.J.A. ; Ogink, N.W.M. - \ 2013
    Lelystad : Central Veterinary Institute (CVI report 15/CVI0094) - 30
    emissiereductie - aërosolen - broeikasgassen - veehouderijbedrijven - duurzame veehouderij - gezondheidsgevaren - volksgezondheid - pathogenen - intensieve veehouderij - emission reduction - aerosols - greenhouse gases - livestock enterprises - sustainable animal husbandry - health hazards - public health - pathogens - intensive livestock farming
    This report describes what is presently known about possible health risks, to both humans and livestock, in the neighborhood of livestock farms, due to emission of bio-aerosols from these farms. Subsequently it answers the question to which extent such risks (if present) could be reduced by reducing the emission.
    Chemical and physical drivers of the evolution of organic aerosols over forests
    Janssen, R.H.H. - \ 2013
    Wageningen University. Promotor(en): Pavel Kabat, co-promotor(en): Jordi Vila-Guerau de Arellano; Laurens Ganzeveld. - [S.l. : S.n. - ISBN 9789461735102 - 148
    aërosolen - bossen - atmosferische grenslaag - meteorologische factoren - wolken - klimaat - aerosols - forests - atmospheric boundary-layer - meteorological factors - clouds - climate

    Diurnal evolution of organic aerosol over boreal and tropical forests

    The first research question of this thesis is: how do local surface forcings and large-scale meteorological forcings shape the evolution of organic aerosol over the boreal and tropical forest? This question is dealt with in Chapters 3 and 4 in case studies for the boreal and tropical forest, respectively. To answer this question a modeling tool (MXLCH-SOA) is developed, which represents land surface conditions and dynamical and chemical processes that influence the evolution of organic aerosol (OA) in a balanced way. The novelty of our approach is that it combines the dynamics of a convective boundary layer (BL) with a reduced gas-phase chemistry mechanism and a module for gas/particle-partitioning of semi-volatile organic species. The principles and governing equations of this modeling tool are described in Chapter 2 and in the subsequent chapters the simplified chemical reaction schemes are presented to calculate secondary organic aerosol (SOA) formation from terpenes (Chapter 3 and 4) and from isoprene (Chapter 4).

    Despite its simplicity, MXLCH-SOA is able to satisfactorily reproduce the main observed characteristics of dynamics, gas-phase chemistry and gas/particle partitioning for the two studied forest ecosystems and it enables us to explain the temporal variability of the concentrations of organic aerosol and its precursors as a function of the various processes. In short, the results show that the diurnal evolution of organic aerosol in a boreal and a tropical forest is the net result of land surface conditions, boundary layer dynamics, chemical transformations and gas/particle partitioning. In the case study for the boreal forest, the entrainment term of the background OA dominates the OA tendency, while in the tropical forest case it is the interplay of several local and large scale processes that shape the diurnal evolution of OA. A sensitivity analysis for the boreal forest case further shows that the OA concentration is sensitive to both volatile organic compound (VOC) emissions and the partitioning of the surface energy budget into a latent and a sensible heat flux. We have identified two regimes, based on which of the two studied land surface drivers dominates: one in which OA is mainly driven by SOA formation from the emitted VOCs and another in which dilution due to entrainment, as driven by the surface energy fluxes, determines the OA concentration. A background OA to fresh SOA ratio is introduced to facilitate the interpretation of this analysis and is used to quantify the contributions of both fresh and background components to the total OA concentration. One main difference between the two case studies is that in the boreal forest entrainment appears to dominate the diurnal cycle, which leads to a decreasing OA concentration during the day, while in the tropical forest the formation of SOA from both isoprene and terpenes leads to increasing OA concentrations during day time. The MXLCH-SOA framework therefore shows the need to represent all these biochemical and physical processes simultaneously in order to understand the diurnal evolution of OA.

    As the boundary layer dynamics-chemistry system is not a closed system, it is necessary to further study the influence of external forcings on the diurnal evolution of OA, besides the surface forcings. Two types of large-scale meteorological forcings and their effects on OA evolution through their impact on BL dynamics have been studied: subsidence due to the presence of a high pressure system and advection of relatively cool air. In Chapter 3 a theoretical sensitivity analysis is given of OA evolution to subsidence, which is applied to the tropical forest case study in Chapter 4. Subsidence has a rather counter-intuitive effect on OA concentrations: even though it suppresses the growth of the BL and consequently decreases the mixing volume for chemical species, it leads to decreased OA concentrations. The reason for this is that entrainment is strongly enhanced in case of subsidence due to thermodynamic effects, which results in a stronger dilution of OA. This knowledge is applied in the case study for the tropical forest in Chapter 4, since results from a large-scale model show subsiding air motions over the measurement site and surroundings at Borneo.

    In addition to subsidence, the advection of cool air is needed to reproduce the observed boundary layer dynamics at Borneo: only if subsidence and advection of relatively cool air are accounted for, the observed low BL height can be reconciled with the large observed surface sensible and latent heat flux. This cool air suppresses BL growth and entrainment. Consequently, the aerosol is trapped in a shallower layer, which leads to an increased concentration compared to the case without advection of cooler air. In conclusion, the large-scale meteorological forcings subsidence and advection of cool air have opposing effects on the diurnal evolution of OA, even though both suppress BL growth. These findings show the utility of our method in identifying effects that should be accounted for in large-scale chemistry transport models.

    The second research question is whether recently discovered pathways of isoprene chemistry are the key to closing the gap between measured and modeled organic aerosol concentrations in tropical forests and other high isoprene environments. To address this issue, several mechanisms which may affect SOA formation from isoprene are implemented in MXLCH-SOA and discussed in Chapter 4. The hydroxyl radical (OH), the main oxidant of isoprene, is thought to be regenerated in the oxidation of isoprene. We find that for the tropical forest case study, we cannot reconcile the modeled concentrations of VOCs, OH and OA with their observed concentrations and fluxes both for cases with and without OH recycling. Therefore, we conclude that the issue of recycling of the OH radical in the oxidation of isoprene has to be solved before its effect on SOA formation can be determined.

    The formation of SOA from isoprene involves multiple generations of oxidation and due to this complex chemistry there is no single mechanism which can explain SOA formation from isoprene under all conditions. To gain understanding in this issue, we have implemented different pathways through which isoprene SOA is known to form, although we do not explicitly account for the detailed isoprene oxidation chain. A central aspect of this branching approach is whether the isoprene peroxy radical chemistry follows the low- or the high-NOx pathway. We find that the latter channel dominates in our case study. For SOA formed through the high-NOx channel, we further account for the effect of the NO2/NO ratio on SOA yields. In the presented case study this has little effect as this ratio is low, it but could be more important in regions with slower photochemistry or higher emissions of anthropogenic pollution. In the low-NOx regime, isoprene epoxides (IEPOX) are important intermediate gas-phase species in the formation of isoprene SOA. Even though the low-NOx pathway is only a minor one here, the amount of IEPOX SOA formed is likely substantial, although a better understanding of the exact mechanisms for its formation is needed to confirm this. However, as in previous studies we systematically underestimate the organic aerosol concentration in a tropical forest even though we incorporate the state-of-the-art knowledge on isoprene SOA formation in MXLCH-SOA. Nevertheless, we advocate accounting for NOx regime specific chemical pathways when modeling isoprene SOA formation. As this field is rapidly evolving in terms of the development of new measurement techniques and the discovery of chemical mechanisms, we strongly recommend the intensive use of our modeling system to gain further understanding of the diurnal variability of OA and for testing new hypotheses under atmospheric conditions.

    Satellite observations of cloud droplet concentration over the boreal forest

    The final objective of this thesis is to understand how aerosols and meteorological factors influence cloud droplet concentration over the boreal forest. This is a first step in translating the process understanding such as addressed in the previous chapters to larger spatio-temporal scales. Since this objective considers different temporal and spatial scales, a different method is applied in Chapter 5 than in the foregoing chapters. Observations of cloud properties by the MODIS instrument onboard the Terra satellite are combined with a model that contains the microphysics and thermodynamics of a single-layered water cloud to obtain a seasonal cycle of cloud droplet number concentrations, averaged over 9 years of data. This seasonal cycle in cloud droplet concentration is compared with aerosol concentrations at the surface and meteorological fields from ECMWF reanalysis. We find that the cloud droplet number concentration is related to the potential temperature gradient in the boundary layer, a measure for the strength of convection, while it shows no clear relationship with the cloud active aerosol concentration at the surface. From this we conclude that the convective transport of the aerosols from the surface to cloud base is the limiting factor for their activation as cloud droplets. However, convection will also influence the formation of clouds from a thermodynamic perspective. Therefore, it is likely that convection, as driven by land surface conditions, regulates both transport of aerosols to cloud base and the height of the cloud base, defined as the height at which water vapor reaches its saturation pressure. To ultimately understand the effect of the boreal forest on cloud properties, the effects of aerosols and thermodynamics should be studied simultaneously.

    Aerosol processes relevant for the Netherlands
    Brugh, J.M.J. Aan de - \ 2013
    Wageningen University. Promotor(en): Maarten Krol, co-promotor(en): M. Schaap. - S.l. : s.n. - ISBN 9789461734211 - 172
    aërosolen - atmosfeer - simulatie - luchtverontreiniging - klimaat - simulatiemodellen - modellen - europa - nederland - aerosols - atmosphere - simulation - air pollution - climate - simulation models - models - europe - netherlands

    Particulate matter (or aerosols) are particles suspended in the atmosphere. Aerosols are believed to be the most important pollutant associated with increased human mortality and morbidity. Therefore, it is important to investigate the relationship between sources of aerosols (such as industry) and the concentration of harmful aerosols at ground level. Furthermore, aerosols influence the climate system by scattering and absorbing solar radiation and by influencing cloud properties. The total climate effect of aerosols is poorly understood compared to the climate effect of greenhouse gases. Therefore, climate studies also benefit from a better understanding of aerosols.

    The goal of this thesis is to investigate the spatial distribution of aerosols over Europe with focus on the Netherlands. The aerosol life cycle and effects are calculated with numerical simulations. Performing numerical simulations of aerosols is very challenging, because, in contrast to gas molecules, each individual aerosol differs in size, composition and microphysical properties. Without simplifications, a model has to track each individual particle, which would take far too much computational time, even for modern supercomputers. The challenge is to design simplifications in such a way that the life cycle of aerosols and the effects of aerosols on human health and climate are still properly represented.

    Many model studies are supported by measurements. Both the measurements and the models can have different purposes. Using the correct combination of different models and observations is key for studies on aerosols. A different combination of models and observations is required to accomplish the different sub goals of this thesis. These sub goals are:

    Investigation of the aerosol life cycle over Europe Improvement of the understanding of gas-aerosol phase transition of ammonium nitrate and aerosol optics Improvement of representation of aerosols and their effects in models

    The life cycle of aerosols in Europe is investigated in chapter 3. The full life cycle of aerosols has been implemented in a global transport model. It is concluded that Europe is a net source of anthropogenic (man-made) aerosols and a net sink of natural aerosols. The most important sink of anthropogenic aerosols is removal by clouds and rain, while natural aerosols are removed predominantly by dry deposition processes. By comparing model results with observations, it is concluded that the largest uncertainties are caused by the parameterisation of wet removal processes and by missing emissions.

    In the Netherlands, emissions of nitrogen oxides and ammonia are high because of the high population density and intensive agriculture. After oxidation of nitrogen oxides to nitric acid, ammonium nitrate aerosols can be formed. This aerosol is special, because it can evaporate under warm and dry conditions and condense back to the aerosol phase under cold and moist conditions. Like the case of clouds, the phase equilibrium changes with altitude as the atmospheric temperature decreases with altitude. The phase of ammonium nitrate is poorly detected by many measurement instruments, because the gas-aerosol partitioning can change inside the instrument. Partly due to the scarcity of reliable measurements, the phase transition of ammonium nitrate is poorly implemented in large-scale models.

    Because ammonium nitrate aerosol and its phase transition is important for the aerosol budget of the Netherlands, this process has further been investigated in case studies. The goal of case studies is to gain detailed insight in the aerosol processes and, ultimately, to develop better parameterisations for large-scale models. These case-studies are performed with more detailed small-scale models. In these models, not the full aerosol life cycle is simulated but only the processes that are being investigated. A large advantage, however, is that these models have a higher resolution both in the spatial and the temporal domain. As a result, the important processes can be resolved more precisely.

    Chapter 4 presents a case study where the interaction between ammonium nitrate phase transition and mixing in the lower atmosphere (boundary layer) is investigated for a warm day in spring. During an intensive measurement campaign near the Cabauw tower in the Netherlands, measurements of ammonium nitrate have been performed. Importantly, the gas and the aerosol phases have been separated with a special instrument so that both concentrations are measured without errors due to phase transition inside the instrument. It is shown that the observed partitioning between gas and aerosol ammonium nitrate deviates significantly from the thermodynamic equilibrium. The hypothesised explanation for this mismatch is that aerosol-rich air from higher altitudes (where the aerosol phase is preferred due to lower temperatures) is transported to the surface, increasing the aerosol-phase fraction of ammonium nitrate at the surface. This implies that the thermodynamic equilibrium is not instantaneously restored at the surface. A simulation of ammonium nitrate partitioning in the boundary layer has been performed with a simplified column model. The match between model results and observations improved drastically when applying a delay timescale up to two hours for the gas-aerosol equilibrium.

    The interaction between turbulence and ammonium nitrate partitioning is further investigated in a more detailed model study (chapter 5). In this model, turbulent motions are explicitly resolved. As highlighted above, downward motions are associated with higher aerosol concentrations, because the phase equilibrium of ammonium nitrate is shifted towards the aerosol phase at higher altitudes. Therefore, turbulent motions induce a fluctuating concentration of aerosol ammonium nitrate with updrafts containing lower aerosol ammonium nitrate concentrations and subsidence motions containing enhanced aerosol ammonium nitrate concentrations. It is discussed that these fluctuations in observations may provide information about the speed of gas-aerosol partitioning, which is very difficult to measure directly.

    Throughout chapters 3 to 5, several ideas for model improvements have been posed. These ideas originate both from knowledge gained in the studies and from further challenges that are discovered. One such improvement for models is a computationally efficient and adequate representation of the optical properties of aerosols. Implementation of aerosol optics has been quite challenging, because the physics of aerosol optics is very complicated. Chapter 6 presents a package that allows easy implementation of aerosol optics in atmospheric models that represent aerosols.

    Aerosol modelling is a very challenging task and can be developed much further. In this thesis, important steps have been taken to improve knowledge about aerosols. Future research should proceed by unravelling remaining aerosol mysteries, such as those presented in the final chapter (7) of this thesis.

    Refinement and application of a regional atmospheric model for climate scenario calculations of Western Europe
    Meijgaard, E. van; Ulft, L.H. van; Lenderink, G. ; Roode, S.R. de; Wipfler, E.L. ; Boers, R. ; Timmermans, R.M.A. van - \ 2012
    Wageningen : KVR (KVR report KVR 054/12) - ISBN 9789088150463 - 44
    klimaatverandering - scenario-analyse - temperatuur - neerslag - aërosolen - grenslaagmeteorologie - modellen - noordzee - nederland - climatic change - scenario analysis - temperature - precipitation - aerosols - boundary-layer meteorology - models - north sea - netherlands
    Het KNMI regionaal klimaat model RACMO wordt in toenemende mate gebruikt bij de detaillering van Klimaatscenario’s. Voorbeelden zijn de frequentie en intensiteit van hittegolven en de veranderingen daarin. Of te verwachten wijzigingen in het optreden van lokale neerslagextremen. In dit project zijn een aantal componenten van RACMO verder ontwikkeld. De bodemhydrologie van het model is verder verfijnd door ruimtelijke heterogeniteit in te voeren voor een aantal bodemparameters, zoals bodemtype en worteldiepte. Deze aanpassing resulteert in meer uitgesproken ruimtelijke structuren op regionale schaal.
    Airborne transmission of a highly pathogenic avian influenza strain H5N1 between groups of chickens quantified in an experimental setting.
    Spekreijse, D. ; Bouma, A. ; Koch, G. ; Stegeman, J.A. - \ 2011
    Veterinary Microbiology 152 (2011)1-2. - ISSN 0378-1135 - p. 88 - 95.
    respiratory syndrome virus - mycoplasma-hyopneumoniae - mouth-disease - pekin ducks - pigs - vaccination - transport - aerosols - h7n7
    Highly pathogenic avian influenza (HPAI) is a devastating viral disease of poultry and quick control of outbreaks is vital. Airborne transmission has often been suggested as a route of transmission between flocks, but knowledge of the rate of transmission via this route is sparse. In the current study, we quantified the rate of airborne transmission of an HPAI H5N1 virus strain between chickens under experimental conditions. In addition, we quantified viral load in air and dust samples. Sixteen trials were done, comprising a total of 160 chickens housed in cages, with three treatment groups. The first group was inoculated with strain A/turkey/Turkey/1/2005 H5N1, the second and third group were not inoculated, but housed at 0.2 and 1.1m distance of the first group, respectively. Tracheal and cloacal swabs were collected daily of each chicken to monitor virus transmission. Air and dust samples were taken daily to quantify virus load in the immediate surroundings of the birds. Samples were tested by quantitative RRT-PCR and virus isolation. In 4 out of 16 trials virus was transmitted from the experimentally inoculated chickens to the non-inoculated chickens. The transmission rate was 0.13 and 0.10 new infections per infectious bird at 0.2m and 1.1m, respectively. The difference between these estimates was, however, not significant. Two air samples tested positive in virus isolation, but none of these samples originated from the trials with successful transmission. Five dust samples were confirmed positive in virus isolation. The results of this study demonstrate that the rate of airborne transmission between chickens over short distances is low, suggesting that airborne transmission over a long distance is an unlikely route of spread. Whether or not this also applies to the field situation needs to be examined
    Effectiveness of multi-stage scrubbers in reducing emissions of air pollutants from pig houses
    Zhao, Y. ; Aarnink, A.J.A. ; Jong, M.C.M. de; Ogink, N.W.M. ; Groot Koerkamp, P.W.G. - \ 2011
    Transactions of the ASABE / American Society of Agricultural and Biological Engineers 54 (2011)1. - ISSN 2151-0032 - p. 285 - 293.
    livestock operations - bioaerosol samplers - ammonia emissions - efficiencies - microorganisms - transmission - buildings - reduction - aerosols - europe
    Emissions of air pollutants from livestock houses may raise environmental problems and pose hazards to public health. They can be reduced by scrubbers installed at the air outlets of livestock houses. In this study, three multi-stage scrubbers were evaluated in terms of their effectiveness in reducing emissions of airborne dust, total bacteria, ammonia, and CO2 from pig houses in winter. The three multi-stage scrubbers were one double-stage scrubber (acid stage+ bio-filter), one double-stage scrubber (acid stage + bio-scrubber), and one triple-stage scrubber (water stage + acid stage + bio-filter). Results showed that these scrubbers reduced concentrations of PM10 by 61% to 93%, concentrations of PM2.5 by 47% to 90%, concentrations of airborne total bacteria by 46% to 85%, and concentrations of ammonia by 70% to 100%. Concentrations of CO2 were not affected. Most of the airborne bacteria emitted from the pig houses were larger than 3.3 µm (73% to 95%). The multi-stage scrubbers removed 53% to 92% of them, compared with -42% to 20% removal effectiveness of the bacteria in the size range of 0.65 to 3.3 µm. The triple-stage scrubber was the most efficient in removing dust and ammonia. Compared to single-stage scrubbers, all three multi-stage scrubbers performed more consistently in reduction of PM10, PM2.5, total bacteria, and ammonia emissions from livestock houses and removed these pollutants more efficiently. It should be noted that all measurements were performed in winter at low ventilation rates, thus at low loadings of the multi-stage scrubbers
    Measuring airborne microorganisms and dust from livestock houses
    Yang Zhao, Yang - \ 2011
    Wageningen University. Promotor(en): Mart de Jong; Peter Groot Koerkamp, co-promotor(en): Andre Aarnink. - S.l. : s.n. - ISBN 9789085858676 - 205
    huisvesting, dieren - dierhouderij - vee - veehouderij - infectie door luchtdeeltjes - micro-organismen - bacteriën - virussen - bemonsteren - vleeskuikens - varkens - aërosolen - animal housing - animal husbandry - livestock - livestock farming - airborne infection - microorganisms - bacteria - viruses - sampling - broilers - pigs - aerosols

    Airborne transmission has been suspected to be responsible for epidemics of highly infectious disease in livestock production. In such transmission, the pathogenic microorganisms may associate with dust particles. However, the extent to which airborne transmission plays a role in the spread of diseases between farms, and the relationship between microorganisms and dust remain unclear. In order to better understand airborne transmission and to set up effective control techniques, this study investigated the performance of multi-stage air scrubbers on the reduction of emissions of microorganisms and dust from pig houses, and to evaluate the effectiveness and efficiency of different sampling devices for collecting microorganisms and dust.

    In winter, multi-stage scrubbers reduced emissions of airborne total bacteria by between 46% and 85%, PM10 by between 61% and 93%, PM2.5 by between 47% and 90%, and ammonia by between 70% and 100%. The EU reference dust sampler with an impaction pre-separator, which was designed for sampling dust in ambient air, could not be used to sample PM2.5 in livestock houses where dust concentrations were high, because overloading occurred. A sampler with a cyclone pre-separator was more tolerant of dust loads in livestock houses and was validated as a reference equivalent sampler. The method for evaluating the efficiency of bioaerosol samplers for airborne microorganisms was appropriate. It calculated the physical and biological sampling efficiencies separately, by excluding the viability losses in the non-sampling processes. The Andersen six-stage impactor, the All Glass Impinger (AGI-30) and the MD-8 had higher physical efficiencies than the OMNI-3000. The Andersen impactor and the AGI-30 had high (100%) biological efficiencies on sampling all five aerosolized microbial species (Enterococcus faecalis, Escherichia coli, Campylobacter jejuni, Mycoplasma synoviae and Gumboro vaccine virus). C. jejuni and Gumboro vaccine virus were inactivated by the OMNI-3000 during sampling, whereas E. coli and C. jejuni were inactivated by MD8. As a result, these two bioaerosol samplers had lower biological efficiencies. Although recipient broilers became infected, no culturable airborne Campylobacter were detected by the Andersen impactor, the AGI-30 and the OMNI-3000 in an airborne transmission of Campylobacter in broilers.

    From this study we concluded that installing multi-stage scrubbers at the air exhausts makes it possible to appreciably reduce aerial pollutant emissions from livestock production systems. As PM samplers with cyclone pre-separators are less vulnerable to high dust loadings, it is recommended that they are used to sample dust in livestock production systems. The efficiency of the bioaerosol samplers varies according to the microbial species sampled. Suitable samplers can be selected on the basis of their efficiencies and detection limits. Low aerial concentrations of specific microbial species are difficult to detect with current bioaerosol samplers. Knowledge gaps still exist throughout the process of long-distance airborne transmission of microorganisms, from suspension and transportation to deposition and infection.

    Surface and boundary layer exchanges of volatile organic compounds, nitrogen oxides and ozone during the GABRIEL campaign
    Ganzeveld, L.N. ; Eerdekens, G. ; Feig, G. ; Vilà-Guerau de Arellano, J. - \ 2008
    Atmospheric Chemistry and Physics 8 (2008). - ISSN 1680-7316 - p. 6223 - 6243.
    general-circulation model - chemistry-climate model - gaseous dry deposition - simple biosphere model - tropical rain-forest - coniferous forest - isoprene - emissions - amazonia - aerosols
    We present an evaluation of sources, sinks and turbulent transport of nitrogen oxides, ozone and volatile organic compounds (VOC) in the boundary layer over French Guyana and Suriname during the October 2005 GABRIEL campaign by simulating observations with a single-column chemistry and climate model (SCM) along a zonal transect. Simulated concentrations of O3 and NO as well as NO2 photolysis rates over the forest agree well with observations when a small soil-biogenic NO emission flux was applied. This suggests that the photochemical conditions observed during GABRIEL reflect a pristine tropical low-NOx regime. The SCM uses a compensation point approach to simulate nocturnal deposition and daytime emissions of acetone and methanol and produces daytime boundary layer mixing ratios in reasonable agreement with observations. The area average isoprene emission flux, inferred from the observed isoprene mixing ratios and boundary layer height, is about half the flux simulated with commonly applied emission algorithms. The SCM nevertheless simulates too high isoprene mixing ratios, whereas hydroxyl concentrations are strongly underestimated compared to observations, which can at least partly explain the discrepancy. Furthermore, the model substantially overestimates the isoprene oxidation products methlyl vinyl ketone (MVK) and methacrolein (MACR) partly due to a simulated nocturnal increase due to isoprene oxidation. This increase is most prominent in the residual layer whereas in the nocturnal inversion layer we simulate a decrease in MVK and MACR mixing ratios, assuming efficient removal of MVK and MACR. Entrainment of residual layer air masses, which are enhanced in MVK and MACR and other isoprene oxidation products, into the growing boundary layer poses an additional sink for OH which is thus not available for isoprene oxidation. Based on these findings, we suggest pursuing measurements of the tropical residual layer chemistry with a focus on the nocturnal depletion of isoprene and its oxidation products.
    Isoprene and monoterpene fluxes from Central Amazonian rainforest inferred from tower-based and airborne measurements, and implications on the atmospheric chemistry and the local carbon budget
    Kuhn, U. ; Andreae, M.O. ; Ammann, C. ; Araújo, A.C. ; Brancaleoni, E. ; Ciccioli, P. ; Dindorf, T. ; Frattoni, M. ; Gatti, L.V. ; Ganzeveld, L.N. ; Kruijt, B. ; Lelieveld, J. ; Lloyd, J. ; Meixner, F.X. ; Nobre, A.D. ; Pöschl, U. ; Spirig, C. ; Stefani, P. ; Thielmann, A. ; Valentini, R. ; Kesselmeier, J. - \ 2007
    Atmospheric Chemistry and Physics 7 (2007)11. - ISSN 1680-7316 - p. 2855 - 2879.
    volatile organic-compounds - boundary-layer - art. - emissions - aerosols - dependence - exchange - biomass - site - nox
    We estimated the isoprene and monoterpene source strengths of a pristine tropical forest north of Manaus in the central Amazon Basin using three different micrometeorological flux measurement approaches. During the early dry season campaign of the Cooperative LBA Airborne Regional Experiment (LBA-CLAIRE-2001), a tower-based surface layer gradient (SLG) technique was applied simultaneously with a relaxed eddy accumulation (REA) system. Airborne measurements of vertical profiles within and above the convective boundary layer (CBL) were used to estimate fluxes on a landscape scale by application of the mixed layer gradient (MLG) technique. The mean daytime fluxes of organic carbon measured by REA were 2.1 mg C m¿2 h¿1 for isoprene, 0.20 mg C m¿2 h¿1 for ¿-pinene, and 0.39 mg C m¿2 h¿1 for the sum of monoterpenes. These values are in reasonable agreement with fluxes determined with the SLG approach, which exhibited a higher scatter, as expected for the complex terrain investigated. The observed VOC fluxes are in good agreement with simulations using a single-column chemistry and climate model (SCM). In contrast, the model-derived mixing ratios of VOCs were by far higher than observed, indicating that chemical processes may not be adequately represented in the model. The observed vertical gradients of isoprene and its primary degradation products methyl vinyl ketone (MVK) and methacrolein (MACR) suggest that the oxidation capacity in the tropical CBL is much higher than previously assumed. A simple chemical kinetics model was used to infer OH radical concentrations from the vertical gradients of (MVK+MACR)/isoprene. The estimated range of OH concentrations during the daytime was 3¿8×106 molecules cm¿3, i.e., an order of magnitude higher than is estimated for the tropical CBL by current state-of-the-art atmospheric chemistry and transport models. The remarkably high OH concentrations were also supported by results of a simple budget analysis, based on the flux-to-lifetime relationship of isoprene within the CBL. Furthermore, VOC fluxes determined with the airborne MLG approach were only in reasonable agreement with those of the tower-based REA and SLG approaches after correction for chemical decay by OH radicals, applying a best estimate OH concentration of 5.5×106 molecules cm¿3. The SCM model calculations support relatively high OH concentration estimates after specifically being constrained by the mixing ratios of chemical constituents observed during the campaign. The relevance of the VOC fluxes for the local carbon budget of the tropical rainforest site during the measurements campaign was assessed by comparison with the concurrent CO2 fluxes, estimated by three different methods (eddy correlation, Lagrangian dispersion, and mass budget approach). Depending on the CO2 flux estimate, 1¿6% or more of the carbon gained by net ecosystem productivity appeared to be re-emitted through VOC emissions
    Water soluble inorganic trace gases and related aerosol compounds in the tropical boundary layer. An analysis based on real time measurements at a pasture site in the Amazon Basin
    Trebs, I. - \ 2005
    Wageningen University. Promotor(en): J. Slanina. - [S.l.] : S.n. - ISBN 9789085041733 - 144
    rivierwater - aërosolen - gassen - energiebalans - atmosfeer - amazonia - brazilië - grenslaag - river water - aerosols - gases - energy balance - atmosphere - amazonia - brazil - boundary layer

    This dissertation investigates the behavior of water-soluble inorganic trace gases and related aerosol species in the tropical boundary layer. Mixing ratios of ammonia (NH3), nitric acid (HNO3), nitrous acid (HONO), hydrochloric acid (HCl), sulfur dioxide (SO;,) and the corresponding water-soluble aerosol species, ammonium (NH,,1), nitrate (NO3 ), nitrite (NO,), chloride (CI) and sulfate (SO,") were measured at a pasture site in the Amazon Basin (Rondónia, Brazil). Sampling was performed from 12 Sep. to 14 Nov. 2002, covering the late dry (biomass burning) season, the transition period and the onset of the wet season (clean conditions) (LBA-SMOCC*). Measurements were made continuously using a wet-annular denuder (WAD) in combination with a Steam-Jet Aerosol Collector (SJAC) followed by on-line analysis. Real-time data were combined with measurements of the aerosol compounds sodium (Na+), potassium (K), calcium (Ga ), magnesium (Mg2) and low-molecular weight polar organic acids determined using integrated filter samples. Additionally, on-line measured mixing ratios of nitric oxide (NO), nitrogen dioxide (NTX) and ozone (O3) as well as (microj-meteorological quantities are considered, Gaseous NH3 was present in mixing ratios an order of magnitude higher than those of HN0:i, HONO, HCl and SO,. Thermodynamic equilibrium models are used to explore the impact of mineral cations (particularly pyrogenic K+) and LMW polar organic acids on the NH4+-NO3"-C1 -SO.," -H2O aerosol system. Mineral cations present in Amazonian fine mode aerosols significantly balanced aerosol NO.," and SO/ during daytime and (NH4)2SOj appeared to be only a minor aerosol component. Thermodynamic equilibrium permitted the formation of aqueous ??,,??;, and NH^Cl only during nighttime at RH>90 %. During daytime, excess NH3 neutralized LMW polar organic acids, forming aerosol NRy. Local dry and wet deposition rates of inorganic ? are presented. Dry ? deposition was inferred using the "big leaf multiple resistance approach" and a canopy compensation point model. Dry ? deposition is dominated by NH:i and NO3, which featured highest mixing ratios as a consequence of biomass burning activities during the dry season. The pasture site was likely to have a strong potential for daytime NH3 (re-) emission, owing to high canopy compensation points, which are related to high surface temperatures and to direct NH^ emissions from cattle excreta. Total (dry + wet) ? deposition was estimated to be 7.3 - 9.8 kgN ha"1 yr"\ which exceeds predictions for the Amazon region by global chemistry and transport models by at least factor of two.
    Guidelines to use the RespiCon unit for dust concentration measurements in practice
    Mosquera, J. ; Wagemans, M.J.M. ; Aarnink, A.J.A. ; Huynh Thi Thanh Thuy, ; Gijsel, P. de; Hol, J.M.G. - \ 2004
    Wageningen : Agrotechnology & Food Innovations (Rapport / Agrotechnology & Food Innovations nr. 293) - ISBN 9789067548533 - 32
    luchtverontreiniging - aërosolen - stof - deeltjesgrootte - gezondheidsgevaren - protocollen - air pollution - aerosols - dust - particle size - health hazards - protocols
    Estimating the probability of recontamination via the air using Monte Carlo simulations
    Asselt-den Aantrekker, E.D. van; Beumer, R.R. ; Gerwen, S.J.C. van; Zwietering, M.H. ; Schothorst, M. van; Boom, R.M. - \ 2003
    International Journal of Food Microbiology 87 (2003)1-2. - ISSN 0168-1605 - p. 1 - 15.
    quantitative risk-assessment - microbial food safety - plants - milk - contamination - aerosols - model - fluid
    Recontamination of food products can cause foodborne illnesses or spoilage of foods. It is therefore useful to quantify this recontamination so that it can be incorporated in microbiological risk assessments (MRA). This paper describes a first attempt to quantify one of the recontamination routes: via the air. Data on the number of airborne microorganisms were collected from literature and industries. The settling velocities of different microorganisms were calculated for different products by combining the data on aerial concentrations with sedimentation counts assuming that settling is under the influence of gravity only. Air movement is not explicitly considered in this study. Statistical analyses were performed to clarify the effect of different products and seasons on the number of airborne microorganisms and the settling velocity. For both bacteria and moulds, three significantly different product categories with regard to the level of airborne organisms were identified. The statistical distribution in these categories was described by a lognormal distribution. The settling velocity did not depend on the product, the season of sampling or the type of microorganism, and had a geometrical mean value of 2.7 mm/s. The statistical distribution of the settling velocity was described by a lognormal distribution as well. The probability of recontamination via the air was estimated by the product of the number of bacteria in the air, the settling velocity, and the exposed area and time of the product. For three example products, the contamination level as a result of airborne recontamination was estimated using Monte Carlo simulations. What-if scenarios were used to exemplify determination of design criteria to control a specified contamination level. (C) 2003 Elsevier Science B.V. All rights reserved.
    Simulation of global sulfate distribution and the influence of effective cloud drop radii with a coupled photochemistry-sulfur cycle model
    Roelofs, G.J. ; Lelieveld, J. ; Ganzeveld, L.N. - \ 1998
    Tellus Series B: Chemical and Physical Meteorology 50 (1998)3. - ISSN 0280-6509 - p. 224 - 242.
    general-circulation model - climate model - parameterization - aerosols - methanesulfonate - chemistry - troposphere - atmosphere - gases - sensitivity
    A sulfur cycle model is coupled to a global chemistry-climate model. The simulated surface sulfate concentrations are generally within a factor of 2 of observed concentrations, and display a realistic seasonality for most background locations. However, the model tends to underestimate sulfate and overestimate surface SO2 at relatively polluted locations. A possible explanation for this is that additional oxidation reactions not considered in the model, may be important. Calculated tropospheric sulfate column abundances exceed those of previous studies, which is predominantly associated with a less efficient nucleation scavenging in wet convective updrafts. Through the simultaneous calculation of the sulfur cycle and tropospheric photochemistry, simulated H2O2 and SO2 concentrations are strongly linked, especially in polluted areas. The coupled model simulates a stronger oxidant limitation and, consequently, a smaller contribution to sulfate formation by H2O2 oxidation of SO2 when compared to sulfur cycle models that use monthly averaged oxidant distributions as input. In the polluted NH, the differences in calculated sulfate columns are largest in winter and relatively small in summer. Therefore, the coupling between the sulfur cycle and the oxidant chemistry is expected to have a minor impact on the calculation of the indirect and direct radiative forcing by sulfate. An empirical relation between sulfate concentration and cloud drop number concentration, derived from cloud measurements at Grean Dun Fell (UK), is applied to the simulated cloud and sulfate fields to derive distributions of effective could drop radii. Additionally, a relation between wind speed and cloud drop number concentration is applied over marine regions to account for the effect of seasalt aerosol on cloud formation when sulfate concentrations are relatively low. Calculated droplet radii are systematically underestimated by about 10-20% in the NH compared to satellite derived values, but they agree relatively well in the SH.
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