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Abstract. Aerosol–cloud interactions (ACI) constitute the single largest uncertainty in anthropogenic radiative forcing. To reduce the uncertainties and gain more confidence in the simulation of ACI, models need to be evaluated against observations, in particular against measurements of cloud condensation nuclei (CCN). Here we present a data set – ready to be used for model validation – of long-term observations of CCN number concentrations, particle number size distributions and chemical composition from 12 sites on 3 continents. Studied environments include coastal background, rural background, alpine sites, remote forests and an urban surrounding. Expectedly, CCN characteristics are highly variable across site categories. However, they also vary within them, most strongly in the coastal background group, where CCN number concentrations can vary by up to a factor of 30 within one season. In terms of particle activation behaviour, most continental stations exhibit very similar activation ratios (relative to particles > 20 nm) across the range of 0.1 to 1.0 % supersaturation. At the coastal sites the transition from particles being CCN inactive to becoming CCN active occurs over a wider range of the supersaturation spectrum. Several stations show strong seasonal cycles of CCN number concentrations and particle number size distributions, e.g. at Barrow (Arctic haze in spring), at the alpine stations (stronger influence of polluted boundary layer air masses in summer), the rain forest (wet and dry season) or Finokalia (wildfire influence in autumn). The rural background and urban sites exhibit relatively little variability throughout the year, while short-term variability can be high especially at the urban site. The average hygroscopicity parameter, κ, calculated from the chemical composition of submicron particles was highest at the coastal site of Mace Head (0.6) and lowest at the rain forest station ATTO (0.2–0.3). We performed closure studies based on κ–Köhler theory to predict CCN number concentrations. The ratio of predicted to measured CCN concentrations is between 0.87 and 1.4 for five different types of κ. The temporal variability is also well captured, with Pearson correlation coefficients exceeding 0.87. Information on CCN number concentrations at many locations is important to better characterise ACI and their radiative forcing. But long-term comprehensive aerosol particle characterisations are labour intensive and costly. Hence, we recommend operating “migrating-CCNCs” to conduct collocated CCN number concentration and particle number size distribution measurements at individual locations throughout one year at least to derive a seasonally resolved hygroscopicity parameter. This way, CCN number concentrations can only be calculated based on continued particle number size distribution information and greater spatial coverage of long-term measurements can be achieved.

The chemical history of dust particles in the atmosphere is crucial for assessing their impact on both the Earth's climate and ecosystem. So far, a number of studies have shown that, in the vicinity of strong anthropogenic emission sources, Ca-rich dust particles can be converted into aqueous droplets mainly by the reaction with gaseous HNO 3 to form CaðNO 3 Þ 2 . Here we show that other similar processes have the potential to be activated under typical remote marine atmospheric conditions. Based on field measurements at several sites in East Asia and thermodynamic predictions, we examined the possibility for the formation of two highly soluble calcium salts, CaðNO 3 Þ 2 and CaCl 2 , which can deliquesce at low relative humidity. According to the results, the conversion of insoluble CaCO 3 to CaðNO 3 Þ 2 tends to be dominated over urban and industrialized areas of the Asian continent, where the concentrations of HNO 3 exceed those of HCl (½HNO 3 ∕HCl > ∼1). In this regime, CaCl 2 is hardly detected from dust particles. However, the generation of CaCl 2 becomes detectable around the Japan Islands, where the concentrations of HCl are much higher than those of HNO 3 (½HNO 3 ∕ HCl < ∼0.3). We suggest that elevated concentrations of HCl in the remote marine boundary layer are sufficient to modify Ca-rich particles in dust storms and can play a more important role in forming a deliquescent layer on the particle surfaces as they are transported toward remote ocean regions.heterogeneous reactions | marine atmosphere

During June, July and August 2006 five aircraft took part in a campaign over West Africa to observe the aerosol content and chemical composition of the troposphere and lower stratosphere as part of the African Monsoon Multidisciplinary Analysis (AMMA) project. These are the first such measurements in this region during the monsoon period. In addition to providing an overview of the tropospheric composition, this paper provides a description of the measurement strategy (flights performed, instrumental payloads, wing-tip to wing-tip comparisons) and points to some of the important findings discussed in more detail in other papers in this special issue

[1] Aerosol samples were collected in the urban atmosphere of Beijing, China, by deploying a tethered balloon. Coarse particles (d > 1 mm) were individually analyzed using electron microscopes, to investigate the extent of dust modification by acidic gases in the atmosphere. Based on the elemental composition, irregularly shaped mineral dust was separated into carbonate and silicate groups. Both sulfate and nitrate were found to accumulate on carbonate more readily than silicate particles. Interestingly, spherical particles resembling Ca-carbonate in composition were spotted frequently in the samples. These Ca-rich spherical particles were more abundant under humid conditions, suggesting that they are deliquesced carbonate particles that formed in the atmosphere following the uptake of acidic gases. Sulfate and nitrate were more frequently detected in the Ca-rich spherical particles than in carbonate in the original solid form, indicating that the gas uptake efficiency of carbonate is further enhanced after the phase transition.

[1] Continental China has been recognized as one of the most important sources of atmospheric mineral dust particles (called Kosa in Japan, which literally means yellow sand). Many investigators have pointed out the importance of study of the long-range transport of mineral dust particles and their modifications in this process even during the nondust storm periods. Because of these modifications, particles can change their radiative properties and their ability to be a condensation nucleus. Therefore it is important to examine the composition of individual mineral particles in their source region and compare these particles with those after long-range transport. A number of investigations have been carried out on the subject; however, the amount of data is still insufficient. Samples of aerosol particles were collected in Dunhuang, China, in different seasons in 2001 and 2002 during the ACE-Asia campaign. The collected particles were examined using a scanning electron microscope equipped with an energy dispersive X-ray analyzer. The particles in all the samples were mainly mineral particles. Similar types of mineral particles were found in the free troposphere over Japan. A number of differences were found between the particles collected in China and those collected over Japan, and these differences can be explained by chemical modifications that occurred in the particles during their transport from China to Japan. Approximately 40-45% of mineral particles mixed internally with sulphate during their transport in the troposphere. Also, the particles collected over Japan were found to be different from those obtained in ground-based measurements in Nagasaki, Nagoya, and Fukuoka, Japan (reported by other research groups). The portion of mineral particles that mixed internally with sea salt and sulphates was considerably smaller than for the samples obtained in Japan near the ground. It is important to take this fact into account while investigating the impact of mineral particles on the biogeochemical cycle and climate.

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Abstract. In this study, we have investigated the performance of liquid-environment FM-AFM with various cantilevers having different dimensions from theoretical and experimental aspects. The results show that the reduction of the cantilever dimensions provides improvement in the minimum detectable force as long as the tip height is sufficiently long compared with the width of the cantilever. However, we also found two important issues to be overcome to achieve this theoretically-expected performance. The stable photothermal excitation of a small cantilever requires much higher pointing stability of an excitation laser beam than that for a long cantilever. We present a way to satisfy this stringent requirement using a temperature controlled laser diode module and a polarization-maintaining optical fiber. Another issue is associated with the tip. While a small carbon tip formed by electron beam deposition (EBD) is desirable for small cantilevers, we found that an EBD tip is not suitable for atomic-scale applications due to the weak tip-sample interaction. Here we present that the tip-sample interaction can be greatly enhanced by coating the tip with Si. With these improvements, we demonstrate atomic-resolution imaging of mica in liquid using a small cantilever with a megahertz-order resonance frequency. In addition, we experimentally demonstrate the improvement in the minimum detectable force obtained by the small cantilever in the measurements of oscillatory hydration forces.