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Abstract. Physical and chemical properties of boundary layer clouds, together with relevant aerosol properties, were investigated during the first Pallas Cloud Experiment (First Pace) conducted in northern Finland between 20 October and 9 November 2004. Two stations located 6 km apart from each other at different altitudes were employed in measurements. The low-altitude station was always below the cloud layer, whereas the high-altitude station was inside clouds about 75% of the time during the campaign. Direct measurements of cloud droplet populations showed that our earlier approach of determining cloud droplet residual particle size distributions and corresponding activated fractions using continuous aerosol number size distribution measurements at the two stations is valid, as long as the cloud events are carefully screened to exclude precipitating clouds and to make sure the same air mass has been measured at both stations. We observed that a non-negligible fraction of cloud droplets originated from Aitken mode particles even at moderatelypolluted air masses. We found clear evidence on first indirect aerosol effect on clouds but demonstrated also that no simple relation between the cloud droplet number concentration and aerosol particle number concentration exists for this type of clouds. The chemical composition of aerosol particles was dominated by particulate organic matter (POM) and sulphate in continental air masses and POM, sodium and chlorine in marine air masses. The inorganic composition of cloud water behaved similarly to that of the aerosol phase and was not influenced by inorganic trace gases.

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This study demonstrates how positive ion atmospheric pressure photoionization-ion mobility spectrometry-mass spectrometry (APPI-IMS-MS) can be used to produce different ionic forms of an analyte and how these can be separated. When hexane:toluene (9:1) is used as a solvent, 2,6-di-tert-butylpyridine (2,6-DtBPyr) and 2,6-di-tert-4-methylpyridine (2,6-DtB-4-MPyr) efficiently produce radical cations [M] ϩ· and protonated [M ϩ H] ϩ molecules, whereas, when the sample solvent is hexane, protonated molecules are mainly formed. Interestingly, radical cations drift slower in the drift tube than the protonated molecules. It was observed that an oxygen adduct ion, [M ϩ O 2 ] ϩ· , which was clearly seen in the mass spectra for hexane:toluene (9:1) solutions, shares the same mobility with radical cations, [M] ϩ· . Therefore, the observed mobility order is most likely explained by oxygen adduct formation, i.e., the radical cation forming a heavier adduct. For pyridine and 2-tert-butylpyridine, only protonated molecules could be efficiently formed in the conditions used. For 1-and 2-naphthol it was observed that in hexane the protonated molecule typically had a higher intensity than the radical cation, whereas in hexane:toluene (9:1) the radical cation [M] ϩ· typically had a higher intensity than the protonated molecule [M ϩ H] ϩ . Interestingly, the latter drifts slower than the radical cation [M] ϩ· , which is the opposite of the drift pattern seen for 2,6-DtBPyr and 2,6-DtB-4-MPyr. (J Am Soc Mass Spectrom 2010, 21, 1565-1572) © 2010 American Society for Mass Spectrometry I n ion mobility spectrometry (IMS) ions move through an applied electric field in a drift gas flow [1]. Interactions between analyte ions and the drift gas result in a specific drift time for that ion, which is often converted to the reduced mobility (Formula 1) [1,2]. Several general reviews of IMS exist [1,[3][4][5][6][7][8] and the theory behind drift tube ion mobility spectrometry has been presented in detail elsewhere [2,9].where l d ϭ length of the drift region, t d ϭ drift time of ion, V ϭ voltage drop over the drift region, T ϭ temperature, and P ϭ pressure.The most common IMS devices are stand-alone instruments, which are commonly used to screen the environment for significant chemicals such as warfare agents, explosives, and illegal drugs [1, 10 -12]. These devices are often tuned to detect certain chemicals; when more specific information is required, IMS is often combined with other instruments, for example with gas chromatography (GC-IMS) [4,13]. Ion mobility spectrometry-mass spectrometry (IMS-MS) is also becoming more and more popular in various application fields, especially in bioanalysis [5]. Several different types of ion mobility spectrometers have been combined with mass spectrometers, e.g., drift tube IMS-MS [1,5,14], field asymmetric ion mobility spectrometry (FAIMS-MS) [5,[15][16][17], and also an aspiration IMS method has been combined with MS [18]. In bioanalysis, a common application of IMS-MS is structural studies ...