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Cloud Forming Properties of Ambient Aerosol in the Netherlands and Resultant Shortwave Radiative Forcing of Climate"

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Cloud Forming Properties of Ambient Aerosol in the Netherlands and Resultant Shortwave Radiative Forcing of Climate Andrey Khlystov » CLOUD FORMING PROPERTIES OFAMBIEN T AEROSOL IN THE NETHERLANDS ANDRESULTAN T SHORTWAVE RADIATIVE FORCING OF CLIMATE PROEFSCHRIFT ter verkrijging van de graad van doctor op gezag van de rector magnificus van de Landbouwuniversiteit Wageningen prof. dr. C.M. Karssen, in het openbaar te verdedigen opmaanda g 16maar t 1998 des namiddags te vier uur in de Aula Promotor: Dr. J. Slanina hoogleraar in de meetmethoden in atmospherisch onderzoek Co-promotor: Dr. H.M. ten Brink Energieonderzoek Centrum Nederland (ECN) ob 0uce>2dt^ Stellingen behorende bij het proefschrift "CLOUD FORMING PROPERTIES OF AMBIENT AEROSOL IN THE NETHERLANDS AND RESULTANT SHORTWAVE RADIATIVE FORCING OF CLIMATE" A.Khlystov 1. The effective radius of cloud droplets deduced from satellite measurements should not be used to estimate the indirect aerosol forcing unless the liquid water content is simultaneously assessed. 2. A substantial effort has been made to characterize broken clouds. However it is over-optimistic to compare in-cloud measurements and satellite observations ofreflectiv e properties ofsuc h clouds. 3. The continuing use ofinstrumentatio n that has not been calibrated is a major impediment for aerosol research to be considered as a serious branch of science. 4. Contrary to common belief, considerable efforts are required to achieve a reasonable accuracy (of at least 20%) in measurement of aerosol number concentrations. 5. Due to the relatively low size resolution of cascade impactors it is always possible to fit the data to a multimodal distribution. 6. One should not use the observed changes in (global) temperature as proof that present climate models accurately predict the effects ofanthropogeni c pollution on the global climate. 7. The general misconception that nitrate is present only in super-micrometer aerosol particles appears to be very persistent. 8. Though the evaporation coefficient for ammonium nitrate is usually taken to be 0.04 [Harrison et al. (1990) Atmos.Environ., V24A, 1833-1888], increasing evidence suggests that it is close to unity [Bai et al. (1995) Atmos.Environ.,V29, 313-321; Khlystov et al. (1997)J.Aerosol Sei., S59-S60]. 9. Improved tests on the mobility of pollutants in soils indicate that much of the efforts, dedicated to clean up soils, have been at least partial waste of money and labor (Comans and Zevenbergen [1997] ECN rapport nr. ECN-C-97-055, pp.86). 10. Policy makers do not understand that, due to the complexity of the processes in which atmospheric aerosol is involved, measures to combat one environmental effect of aerosols may lead to an enhancement ofothe r adverse effects. 11. Though a reasonable theoretical explanation can now be offered for the discrepancy between measured and predicted deposition velocities of sub-micrometer ambient aerosols (see Chapter 2 of this thesis), there is great reluctance to accept the measured values in transport models. 12. The much increased speed of personal computers has not speeded up the process of reporting scientific data, due to the persistent and malicious development of unnecessarily over-sophisticated software. *Mo«-a>',r*°* We gratefully acknowledge the financial support from the Dutch National Research Program on Global Air Pollution and Climate Change (NOP Project nr. 852066). We are especially thankful for the substantial funding of this research by the department of Economic Affairs of the Netherlands, within Mefis-contract nr.53478. We would also like to thank the European Union for the financial contribution within contract nr. EV-5V-CT92-0171 in the Climatology and Natural Hazards Program of the EC ENVIRONMENT Program 1991-1994. Contents Contents Ill Summary VII Samenvatting XI Chapter 1 Introduction and overview 1 Introduction 1 The Climate system 4 The global energy balance 4 Radiative (climate) forcing 6 The enhanced (anthropogenic) greenhouse effect 7 Aerosol effect on climate 9 Direct forcing 9 The indirect effect 12 Overview ofth e climate forcing 15 Scope ofthi s work 19 Chapter 2 Factors influencing the local direct radiative aerosol forcing 23 Introduction 23 Nitrate 26 Contribution to light-scattering 26 Size distribution 31 III Contents Techniques which avoid artifacts in sampling aerosols. .. 34 Thermodenuders 35 Steam-Jet Aerosol Collector (SJAC) 39 Comparison with filter pack and dénuder measurements 46 Environmental implications i 52 Effects of hygroscopic growth 54 Deliquescence and hysteresis 55 The Köhler equation 57 Equilibrium sizes 61 Kinetics of hygroscopic size changes 64 Characteristic times for hygroscopic growth 66 Effect on the light scattering 68 Influence of the hygroscopic growth on particle deposition velocity 75 Conclusions 80 Chapter 3 The indirect effect 83 Introduction 83 Droplet number and cloud albedo 85 Cloud activation, cloud droplets and cloud condensation nuclei (CCN) 88 Relation between cloud nucleation and size / chemical composition 89 Influence of water soluble gases on cloud activation 91 Cloud condensation nuclei (CCN) and potential cloud nuclei (PCN) 93 Saturation effect 94 Cloud droplets 96 Studies of the influence of anthropogenic aerosols on cloud droplet number concentration 97 Requirements for a cloud chamber study 100 rv Contents Chapter 4 Cloud chamber 103 Introduction 103 Description of the cloud chamber 105 Aerosol losses / artifacts 110 Aerosol generation 112 Aerosol / droplet monitoring and sampling 114 Residence time distribution of air inside the chamber 119 Stabilization of the chamber 121 Stability of the chamber 122 Supersaturation in the chamber 124 Influence of the total number concentration 129 Chapter 5 Activation of ambient aerosol 135 Introduction 135 Data evaluation 138 Aerosol size spectra 140 Chemical composition 150 Droplet number versus aerosol number 155 Effect of insoluble material 165 Droplet number versus aerosol mass 172 Comparison with aircraft studies 175 Conclusions 177 Chapter 6 Estimate of the local aerosol radiative forcing 181 Introduction 181 Local indirect forcing 182 Local direct forcing 185 Conclusions / implications 189 V Contents References 193 Appendix 1 Impactor data AI - 1 Appendix 2 Effect oflaten t heat transfer on radius change rate A2-1 Appendix 3 Weather maps A3 - 1 Acknowledgments XV Curriculum vitae XVII VI Summary This thesis discusses properties of ambient aerosols in the Netherlands which are controlling the magnitude of the local aerosol radiative forcing. Anthropogenic aerosols influence climate by changing the radiative transfer through the atmosphere via two effects, one is direct and a second is indirect. Due to the scattering of solar light on aerosol particles the Earth surface receives less radiation and thus cools, which is called the direct aerosol effect. The indirect effect includes processes by which aerosols influence the radiation balance indirectly - via clouds. The indirect effect includes mechanisms by which anthropogenic aerosol particles enhance the reflectivity and prolong the life time of marine stratoform clouds by increasing their droplet number concentration. Anthropogenic aerosols were demonstrated to have a considerable cooling effect on climate, comparable in magnitude to that of the greenhouse gases, but opposite in sign. However, calculation of the aerosol radiative forcing is much more complex than that for the green house gases. This results in high uncertainties in the estimates of the effects of anthropogenic aerosols on climate. Both types of the aerosol radiative forcing and their magnitude, as estimated by the Intergovernmental Panel on Climate Change (IPCC), are discussed in Chapter 1. The aerosol radiative forcing is a regional phenomenon because of the limited life time of aerosols in the atmosphere. Thus, the aerosol effects should be assessed locally. In Chapter 2 chemical composition of local ambient aerosols in the size range relevant to the radiative forcing is discussed. It is shown that ammonium nitrate, which is omitted by the IPCC, is a major contributor to the direct, and perhaps to the indirect forcing. The contribution of this compound to the local direct forcing is equal to that of anthropogenic sulfate aerosols. Measurements of ammonium nitrate content in submicrometer aerosol VII Summary are subject to artifacts due to the volatility of this compound. Automated techniques which were developed for artifact-free measurements of chemical composition of ambient aerosols are described. Aerosol particles composed of water-soluble salts increase their size at elevated relative humidities. The increase in size enhances light scattering by aerosol particles which in turn enhances the direct radiative forcing. Hygroscopic properties of aerosol particles also play a central role in cloud droplet formation and thus are also relevant for the indirect aerosol effect. Chapter 2 discusses the hygroscopic size changes and characteristic growth times. It is also demonstrated that hygroscopic growth may substantially enhance dry deposition of aerosol particles to forests. According to the IPCC the indirect aerosol effect is the most uncertain factor in the anthropogenic perturbation of the climate, with an uncertainty of the same magnitude as the total radiative effect by the greenhouse gases. One of the major uncertainties in the indirect effect is the unknown relation between the concentration of anthropogenic aerosol particles and the increase in droplet number concentration in clouds. In Europe only few measurements of the relation between the number of particles and the number of cloud droplets
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