Cloud forming properties of ambient aerosol in the Netherlands and resultant shortwave radiative forcing of climate

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 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 have been performed. It should be also noted that those measurements were performed with instruments that did not span the full range of aerosol sizes that is relevant for cloud formation. For this reason, the major part of the thesis was devoted to an experimental assessment of the cloud forming properties of ambient aerosol in the Netherlands. The indirect effect as well as factors controlling cloud droplet formation are discussed in Chapter 3. Chapter 4 describes the large flow-through cloud chamber in which cloud forming properties of ambient aerosol were studied. The advantage of a cloud chamber over in-situ measurements is the use of instrumentation which is too delicate for use in aircraft and the possibility to compare, simultaneously, the aerosol before and after cloud processing. The aerosol was characterized both in terms of its number concentration and size as well as its chemical composition with the emphasis on the amount of soluble material. The unique feature of the cloud chamber is its reproducible stable low supersaturations (around 0.1%) common for marine clouds. Aerosol monitors were used that measure the concentration of particles in the full range of sizes relevant for cloud formation. The unique large size and the high throughput flow of the chamber allowed unperturbed use of conventional cloud instrumentation for sizing and counting of droplets. Also high-flow cascade impactors for chemical analysis of aerosol were used. The cloud chamber was calibrated with artificially generated ammonium sulfate aerosol. This aerosol was selected because in anthropogenically influenced air in Western Europe most particles should consist of this compound. Experiments were done at different total aerosol number concentrations to check its influence on the number of resulting cloud droplets. A sub-linear relation between the aerosol and the droplet number concentrations was observed. The results of the tests with the reference aerosol were used for the interpretation of experiments with ambient aerosols. The cloud activation tests on ambient aerosol are discussed in Chapter 5. The cloud forming efficiency of the aerosols was assessed by comparing the number concentration of droplets formed in the cloud chamber with that found during the tests with the reference ammonium sulfate aerosol at the same number concentration. Similarly to the reference sulfate aerosol, a sub-linear relation between the aerosol and droplet number concentrations was observed. However, the deviation from a linear relation was stronger in the ambient aerosol. This suggested that about one third of the (anthropogenic) particles was water-insoluble. The measurements of the composition supported this hypothesis. This demonstrates the importance of hygroscopicity for cloud formation. It also shows that measurements of the solubility of (individual) particles should be done along with conventional measurements of aerosol size and number. The results of experiments discussed in Chapter 5 were used for an estimate of the local indirect aerosol radiative effect (Chapter 6). The difference between the droplet number concentration in clean marine air and the average droplet number concentration in polluted air was used to estimate the corresponding increase in cloud reflectivity which, in turn, was translated into local indirect aerosol effect. The data on the aerosol light scattering discussed in Chapter 2 are used to estimate the local direct radiative forcing. The estimated total (direct plus indirect) local aerosol radiative forcing is about 4 times higher than the forcing by the anthropogenic greenhouse gases but opposite in sign.