Kulmala, M. & Tammet, H. 2007: Finnish–Estonian air ion and aerosol workshops. Boreal Env. Res. 12: 237–245.

Atmospheric air ions and aerosol particles participate in atmospheric processes and have several important effects on e.g. global climate and human health. When measured, air ions and aerosol particles have been observed to be present always and in every place. In this overview we present a brief summary of the motivation, history and main achievements in air ion research. The papers in this special issue are the result of a set of bi-annual workshops taken place since 2003. The contents of the papers reflect the collaborative research efforts lead by the University of Helsinki and University of Tartu, with an increasing contribution of other partners. The main objective of these workshops has been to provide a discussion forum for the development of air-ion theory and measurement techniques. This special issue presents main achievements in a set of 14 original papers.

Mirme, A., Tamm, E., Mordas, G., Vana, M., Uin, J., Mirme, S., Bernotas, T., Laakso, L., Hirsikko, A. & Kulmala, M. 2007: A wide-range multi-channel Air Ion Spectrometer. Boreal Env. Res. 12: 247–264.

A new multi-channel air ion spectrometer (AIS) is presented. The instrument allows simultaneous measurements of positive and negative ion distributions from 3.2 to 0.0013 cm² V^–1 s^–1 (0.80–40 nm diameter or, using the Tammet correction, from 0.4 to 40 nm). The mobility range is divided into 27 fractions which are measured simultaneously to ensure a high time resolution (down to 10 seconds). The instrument calibration shows a good agreement with the mobility of electrically-classified, mono-dispersed aerosols. The spectrometer overestimates low concentrations of cluster ions. This is caused by a natural production of small ions (< 1 nm, 10 to 100 cm^–3) inside the spectrometer. The instrument specifications, calibration results and measurements performed show a huge application potential of the new spectrometer.

Hirsikko, A., Paatero, J., Hatakka, J. & Kulmala, M. 2007: The ²²²Rn activity concentration, external radiation dose and air ion production rates in a boreal forest in Finland between March 2000 and June 2006. Boreal Env. Res. 12: 265–278.

We measured the ²²²Rn content of the air by continuously collecting particle-bound daughter nuclides ²¹⁴Pb and ²¹⁴Bi onto glass-fibre filters and counting their beta particle emissions with Geiger-Müller tubes. A scintillation gamma spectrometer system measured external radiation, which main components are gamma and cosmic radiation. Our purpose was to detect long-term, seasonal and diurnal variations in ²²²Rn activity concentration and external radiation dose rate in a Finnish boreal forest during the years 2000–2006. The long-term variations in activity concentration and dose rate were small, whereas the annual variations were more pronounced. In late summer and autumn, the diurnal cycle of ²²²Rn activity concentration was strongest, whereas the diurnal cycle of external radiation dose rate was practically non-existing throughout the year. We utilised the ²²²Rn and external radiation measurements also when calculating air ion production rate in the lower boundary layer. Based on our results, the total ion production rate varied in the range 4.2–17.6 ion pairs cm^–3 s^–1. The fraction of ²²²Rn contribution in the ion production varied in the range 0–0.43, with average fraction 0.11 +/- 0.07. These results indicate that ion production was typically dominated by the external radiation on our measurement site.

Laakso, L., Grönholm, T., Kulmala, L., Haapanala, S., Hirsikko, A., Lovejoy, E. R., Kazil, J., Kurtén, T., Boy, M., Nilsson, E. D., Sogachev, A., Riipinen, I., Stratmann, F. & Kulmala, M. 2007: Hot-air balloon as a platform for boundary layer profile measurements during particle formation. Boreal Env. Res. 12: 279–294.

In this study, we used a hot-air balloon as a platform for boundary layer particle and cluster measurements. We did altogether 11 flights during the springs of 2005 and 2006. During the spring of 2006, we observed five new-particle formation days. During all days, new-particle formation took place in the mixed boundary layer. During one of the days, we observed particle formation in the free troposphere, separate from that of the mixed layer. The observations showed that the concentration of freshly-formed 1.5–2 nm negative ions was several times higher than the concentration of positive ions. We also clearly observed that nucleation during one of the days, 13 March 2006, was a combination of neutral and ion-induced nucleation. During some of the days, particle growth stopped at around 3 nm, probably due to lack of condensable organic vapours. Simulations of boundary layer dynamics showed that particles are formed either throughout the mixed layer or in the lower part of it, not at the top of the layer.

Hirsikko, A., Yli-Juuti, T., Nieminen, T., Vartiainen, E., Laakso, L., Hussein, T. & Kulmala, M. 2007: Indoor and outdoor air ions and aerosol particles in the urban atmosphere of Helsinki: characteristics, sources and formation. Boreal Env. Res. 12: 295–310.

We measured air ion size distributions with an air ion spectrometer in the size range of 0.34–40.3 nm both indoors (in July) and outdoors (in August) in Helsinki, Finland in 2004. At the same time we measured particle number concentrations and size distributions with two condensation particle counters (indoors) and differential mobility particle sizer (outdoors). Our main focus was to study new-particle formation in an urban site. We observed new-particle formation indoors almost every day, even many times a day, and four times outdoors. Indoors, the observed growth rates were 2.3–4.9 nm h^–1 for 1.3–3-nm ions, 6.5–8.7 nm h^–1 for 3–7-nm ions and 5.1–8.7 nm h^–1 for 7–20-nm ions. Outdoor ions (3–7 nm) grew at rates as large as 15.4 nm h^–1. Outdoor ion and particle number concentrations were dependent on the wind direction, whereas indoor concentrations were dependent on ventilation conditions. Secondary particle formation and growth affected concentrations both indoors and outdoors. We concluded, based on our measurement results and simulated penetration of outdoor particles through the ventilation system, that we had indoor sources for secondary particles.

Tiitta, P., Miettinen, P., Vaattovaara, P., Laaksonen, A., Joutsensaari, J., Hirsikko, A., Aalto, P. & Kulmala, M. 2007: Road-side measurements of aerosol and ion number size distributions: a comparison with remote site measurements. Boreal Env. Res. 12: 311–321.

We measured the number concentrations and size distributions of aerosol particles and air ions with a differential mobility particle sizer and air ion spectrometer, respectively, during a roadside campaign conducted in Kuopio (Savilahti), Finland, between 16 June and 2 July 2004. The average cluster ion (0.3–1.8 nm) concentrations were quite low (around 320 cm^–3 and 280 cm^–3 for negative and positive ions) during the whole period. For comparison, cluster ion concentrations in a rural SMEAR II station in Hyytiälä, southern Finland, were at the same time almost three times higher. Negative intermediate ions (1.8–7.5 nm) reached maximum concentrations of 620 cm^–3 in Kuopio, while the average concentrations were in the range 60–80 cm^–3 depending slightly on the wind direction. Positive intermediate ion concentrations were lower. We observed higher amounts of the intermediate ions usually during rain but also during non-rain periods indicative of short-term secondary particle formation. Large ion (7.5–40 nm) concentrations (average values of 500–800 cm^–3) were 2–3 times higher in Kuopio than at the SMEAR II station. Straightforward impact of traffic was observed when the wind blew from the road: an increase in the traffic density increased concentrations of large ions.

Komppula, M., Vana, M., Kerminen, V.-M., Lihavainen, H., Viisanen, Y., Hõrrak, U., Komsaare, K., Tamm, E., Hirsikko, A., Laakso, L. & Kulmala, M. 2007: Size distributions of atmospheric ions in the Baltic Sea region. Boreal Env. Res. 12: 323–336.

Number size distributions of air ions and aerosol particles were measured at three sites in the Baltic Sea region in spring 2004. One site was on the island of Utö in the Baltic Sea and the two other sites, Hyytiälä and Tahkuse, had a more continental location not far away from the coast of the Baltic Sea. The concentrations of cluster ions (air ions with a diameter < 1.6 nm) were about three times smaller at the Utö island as compared with those at the Hyytiälä mainland site in Finland, although the particle concentrations (i.e. ion sink) were at about the same level at these two sites. Generally, the Utö island had the lowest air-ion concentration probably due to weaker sources for ions than the other two more continentally-located stations. Intermediate ion concentrations (1.6–7 nm diameter) were generally low, even though they reached several hundreds cm^–3 during nucleation episodes. Charging probabilities of < 8 nm particles were found to be close to the steady state in all the three sites, suggesting that ion-induced nucleation was not playing a major role during this time period.

Lihavainen, H., Komppula, M., Kerminen, V.-M., Järvinen, H., Viisanen, Y., Lehtinen, K., Vana, M. & Kulmala, M. 2007: Size distributions of atmospheric ions inside clouds and in cloud-free air at a remote continental site. Boreal Env. Res. 12: 337–344.

During the late autumn 2004, aerosol and air ion number size distributions inside and outside clouds and cloud droplet number size spectra were measured in Pallas, northern Finland. The concentrations of cluster ions (air ions with a diameter < 1.6 nm) were substantially lower, roughly by an order of magnitude, inside clouds as compared with cloud-free air. The observed concentration levels of cluster ions could be explained by a source rate of a few ion pairs per second. The main sink for cluster ions was the cloud droplet population during the cloudy periods and the ion-ion recombination in cloud-free air. Very few intermediate ions (1.6–7.4 nm diameter) were present during the cloudy periods, indicating that processes capable of generating intermediate ions were rather inactive inside clouds during the measurement campaign.

Venzac, H., Sellegri, K. & Laj, P. 2007: Nucleation events detected at the high altitude site of the Puy de Dôme Research Station, France. Boreal Env. Res. 12: 345–359.

Aerosol and ion number size distributions were measured at the top of the Puy de Dôme (1465 m above the sea level) for a three-month period. The goals were to investigate the vertical extent of nucleation in the atmosphere and the effect of clouds on nucleation. Nucleation and new-particle formation events were classified into four classes: (1) burst of cluster ions, (2) large ion formation starting from 10 nm, (3) burst of cluster ions followed by large ion formation with a gap of intermediate ions, and (4) burst of ions with continuous growth to the sizes > 10 nm. All together these events were observed during nearly half of the analyzed days. Concentrations of cluster ions (< 1.4 nm) varied typically between 100 and 1000 cm^–3. Intermediate ion (1.4–6 nm) concentrations were usually lower than 500 cm^–3 but could exceed 3000 ions cm^–3 during nucleation events. Large concentrations of intermediate ions seem to be appropriate to detect the occurrence of most of nucleation events. In clouds, the aerosol condensation sink and cluster ions concentrations were lower, presumably because of scavenging by cloud droplets, but the intermediate ion concentrations remained unchanged. We observed that large ion formation starting at 10 nm (class 2 events) and continuous growth of ions (class 4 events) occurred preferably under clear-sky conditions, and that all except class 2 events could be observed under cloudy conditions.

Pugatsova, A., Iher, H. & Tamm, E. 2007: Modal structure of the atmospheric aerosol particle size spectrum for nucleation burst days in Estonia. Boreal Env. Res. 12: 361–373.

The modal structure of an atmospheric aerosol size spectrum determines to a significant extent the role of aerosol particles in the formation of weather and climate and their potential danger to living beings. In this paper, the modal structure of the atmospheric aerosol particle size spectra for nucleation burst days was investigated using data acquired at a rural measurement site in Estonia during the year 2005. The measurements were made with the original electrical aerosol spectrometer designed at the Institute of Environmental Physics of the University of Tartu. The performed analysis relied on air mass histories, factor analysis and least-square fitting. The highest particle number concentrations were found in Arctic air masses. The start time of a nucleation burst was found to be dependent on the air mass type: in Arctic air masses nucleation occurred before the noon, whereas in the polar air it occurred in the afternoon.

Vartiainen, E., Kulmala, M., Ehn, M., Hirsikko, A., Junninen, H., Petäjä, T., Sogacheva, L., Kuokka, S., Hillamo, R., Skorokhod, A., Belikov, I., Elansky, N. & Kerminen, V.-M. 2007: Ion and particle number concentrations and size distributions along the Trans-Siberian railroad. Boreal Env. Res. 12: 375–396.

Aerosol concentrations and properties in Russia are not well known. There are only few studies published on aerosols in Russia. However, these aerosols can have a major effect on global climate. We measured aerosol particle and air ion number size distributions together with relevant information on meteorological conditions and atmospheric trace gas concentrations in Russia. Our purpose was to get new insight on number concentrations of aerosol particle and air ions in different parts of Russia, and to examine which sources and sinks affected the observed concentrations. During a two-week TROICA-9 expedition between 4 and 18 October 2005, we travelled on the Trans-Siberian railroad from Moscow to Vladivostok and back, conducting measurements constantly along the route. The lowest aerosol particle number concentrations, around 500 cm^–3, were observed at remote sites and the highest concentrations of around 40000 cm^–3 were observed near large industrial towns. The particle number concentration correlated best with nitrogen oxide and carbon monoxide concentrations. Pollutant levels were at their highest in the vicinity of towns, even though important pollution sources such as wood burning and forest fires also existed in rural areas. Concentrations of positive and negative intermediate and large ions were of the same order of magnitude as has been observed in previous studies made in boreal forests. Concentrations of intermediate ions were often low of the order of a few ions cm^–3, but their concentration increased during nucleation, rain and snowfall events. Concentrations of positive and negative cluster ions were sometimes very high, reaching values of about 5000 cm^–3 in case of negative ions. We also detected exceptionally high ion production rates of up to 30 s^–1 cm^–3 due to 222-radon decay. Concentrations of cluster ions correlated quite well with the ion production rate but less so with the ion sink. Two particle formation events were observed, during which the particle growth rates varied between 2.4 and 11.4 nm h^–1. Smaller particles grew slower than the bigger ones.

Virkkula, A., Hirsikko, A., Vana, M., Aalto, P. P., Hillamo, R. & Kulmala, M. 2007: Charged particle size distributions and analysis of particle formation events at the Finnish Antarctic research station Aboa. Boreal Env. Res. 12: 397–408.

We measured the size distributions of positively and negatively charged particles in the size range 0.34–40 nm at the Finnish Antarctic research station Aboa from 14 December 2004 to 30 January 2005 with an Air Ion Spectrometer (AIS). Our focus was to study secondary particle formation in a clean environment. Therefore, we classified the 48 measurement days either as a particle formation event day, an undefined day or a non-event day. Approximately 23% of the days were particle formation event days, a fraction similar to that observed at a boreal forest site. The median diameter growth rates of negatively charged particles in size classes 1.3–3, 3–7 and 7–20 nm were 1.1, 1.5 and 4.3 nm h^–1, respectively, and somewhat lower for positively charged particles. Furthermore, we observed a clear positive correlation between the wind speed and cluster and intermediate ion concentrations, which suggests that ions were produced by friction processes in fast moving snow and ice crystals during the periods with strong winds. Cluster ions (diameter < 1.6 nm) were present during the whole measurement period. The average (+/- SD) positive and negative cluster ion concentrations were 557 +/- 974 and 587 +/- 543 cm^–3, respectively.

Parts, T.-E., Luts, A., Laakso, L., Hirsikko, A., Grönholm, T. & Kulmala, M. 2007: Chemical composition of waterfall-induced air ions: spectrometry vs. simulations. Boreal Env. Res. 12: 409–420.

Our measurements of ion size distributions near a waterfall provided new evidence for a waterfall-induced modification of air ion sizes. The ion size spectrum near a waterfall permanently differs from that in ordinary tropospheric air. In this paper we investigated the near-waterfall air ions chemical nature in detail. We carried out a simulation series of air small negative ion evolution, proposing that falling water, as a new environmental component, increases the concentration of OH^– cluster ions. The produced OH^– ions were employed as an extra input for our ion evolution model. The presence of additional OH^– ions resulted in a decrease of typically model-provided NO₃^– and/or HSO₄^– cluster ion concentrations and an increase of the abundance of HCO₃^– cluster ions. Near the waterfall the latter ions became dominant in our simulations.

Tammet, H. & Kulmala, M. 2007: Simulating aerosol nucleation bursts in a coniferous forest. Boreal Env. Res. 12: 421–430.

We modified a numerical model of atmospheric aerosol nucleation bursts to include the dry deposition of ions and freshly nucleated particles onto tree needles in a coniferous forest. The dry deposition is estimated using the Churchill-Bernstein approximation, which is adapted from the theory of heat transfer. The model includes an improved submodel of the sink of ions and nucleation mode particles onto large particles of the background aerosol. The user can edit the values of 95 input parameters by altering a control file. The computing time on an ordinary PC is counted in seconds in case of a typical task, which is characterized by several thousands of time steps and several thousands of size sections. The numerical examples show that, during atmospheric aerosol nucleation events, the dry deposition of ions and nanometer scale particles onto the conifer needles has a considerable effect on the respective concentrations.

Kurtén, T., Noppel, M., Vehkamäki, H., Salonen, M. & Kulmala, M. 2007: Quantum chemical studies of hydrate formation of H₂SO₄ and HSO₄^–. Boreal Env. Res. 12: 431–453.

We calculate structures and thermochemical parameters for H₂SO₄•(H₂O)_n, HSO₄^–•(H₂O)_n, H₂SO₄•NH₃•(H₂O)_m and HSO₄^–•NH₃•(H₂O)_m clusters (with n = 0 ... 4 and m = 0 ... 1) using the MP2/aug-cc-pV(D + d)Z quantum chemical method, with higher-order corrections computed at the MP2/aug-cc-pV(T + d)Z and MP4/aug-cc-pV(D + d)Z levels. Equilibrium constants for hydrate formation at different temperatures are computed using the quantum chemical results, and the predicted extent of hydrate formation is compared with experimental results. Hydrate distributions in different RH conditions are derived using the calculated free energies of hydration. The results show that the hydrogensulfate ion is in all conditions much more strongly hydrated than the neutral sulfuric acid molecule. The high-level thermodynamic data calculated for the clusters agree with the experimental data, and the presented hydrate model is expected to perform better than earlier versions based on less reliable quantum chemical data. A comparison to the ammonia-containing clusters indicates that ammonia probably plays at most a minor role in ion-induced nucleation involving the HSO₄^– core ion.