BER 21(3-4), 2016

ISSN 1239-6095 (print), ISSN 1797-2469 (online)
© Boreal Environment Research 2016

Contents of Volume 21 no. 3–4

Kulmala M., Hõrrak U., Manninen H.E., Mirme S., Noppel M., Lehtipalo K., Junninen H., Vehkamäki H., Kerminen V.-M., Noe S.M. & Tammet H. 2016: The legacy of Finnish–Estonian air ion and aerosol workshops. Boreal Env. Res. 21: 181–206.
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Junninen H., Duplissy J., Ehn M., Sipilä M., Kangasluoma J., Franchin A., Petäjä T., Manninen H.E., Kerminen V.-M., Worsnop D. & Kulmala M. 2016: Measuring atmospheric ion bursts and their dynamics using mass spectrometry. Boreal Env. Res. 21: 207–220.
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Palo M., Eller M., Uin J. & Tamm E. 2016: Electric wind in a Differential Mobility Analyzer. Boreal Env. Res. 21: 221–229.
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Wagner R., Manninen H.E., Franchin A., Lehtipalo K., Mirme S., Steiner G., Petäjä T. & Kulmala M. 2016: On the accuracy of ion measurements using a Neutral cluster and Air Ion Spectrometer. Boreal Env. Res. 21: 230–241.
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Buenrostro Mazon S., Kontkanen J., Manninen H.E., Nieminen T., Kerminen V.-K. & Kulmala M. 2016: A long-term comparison of nighttime cluster events and daytime ion formation in a boreal forest. Boreal Env. Res. 21: 242–261.
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Jokinen T., Kausiala O., Garmash O., Peräkylä O., Junninen H., Schobesberger S., Yan C., Sipilä M. & Rissanen M.P. 2016: Production of highly oxidized organic compounds from ozonolysis of β-caryophyllene: laboratory and field measurements. Boreal Env. Res. 21: 262–273.
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Leino K., Nieminen T., Manninen H.E., Petäjä T., Kerminen V.-M. & Kulmala M. 2016: Intermediate ions as a strong indicator for new particle formation bursts in a boreal forest. Boreal Env. Res. 21: 274–286.
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Yli-Juuti T., Tikkanen O.-P., Manninen H.E., Nieminen T. & Kulmala M. 2016: Analysis of sub-3 nm particle growth in connection with sulfuric acid in a boreal forest. Boreal Env. Res. 21: 287–298.
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Chen X., Paatero J., Kerminen V.-M., Riuttanen L., Hatakka J., Hiltunen V., Paasonen P., Hirsikko A., Franchin A., Manninen H.E., Petäjä T., Viisanen Y. & Kulmala M. 2016: Responses of the atmospheric concentration of radon-222 to the vertical mixing and spatial transportation. Boreal Env. Res. 21: 299–318.
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Kulmala M., Luoma K., Virkkula A., Petäjä T., Paasonen P., Kerminen V.-M., Nie W., Qi X., Shen Y., Chi X. & Ding A. 2016: On the mode-segregated aerosol particle number concentration load: contributions of primary and secondary particles in Hyytiälä and Nanjing. Boreal Env. Res. 21: 319–331.
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Noe S.M., Krasnov D., Krasnova A., Cordey H.P.E. & Niinemets Ü. 2016: Seasonal variation and characterisation of reactive trace gas mixing ratios over a hemi-boreal mixed forest site in Estonia. Boreal Env. Res. 21: 332–344.
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Vana M., Komsaare K., Hõrrak U., Mirme S., Nieminen T., Kontkanen J., Manninen H.E., Petäjä T., Noe S.M. & Kulmala M. 2016: Characteristics of new-particle formation at three SMEAR stations. Boreal Env. Res. 21: 345–362.
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Tammet H. 2016: Identification of boundaries between size classes of atmospheric aerosol particles. Boreal Env. Res. 21: 363–371.
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Atmospheric air ions, clusters and aerosol particles participate in a variety of atmospheric processes and considerably affect e.g. global climate and human health. When measured, air ions as well as atmospheric clusters and particles have been observed to be present practically always and everywhere. In this overview, we present a brief summary of the main achievements and legacy of the series of workshops organized mainly by the University of Helsinki and the University of Tartu. The legacy covers the development and standardization of new instruments, such as ion spectrometers, mass spectrometers and aerosol particle counters, as well as work toward theoretical understanding of new-particle formation and evolution of atmospheric clusters. One important legacy is the establishment of the SMEAR-Estonia station at Järvselja.

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Atmospheric ions are produced after a cascade of reactions starting from initial ionization by high energetic radiation. Such ionization bursts generate ions that rapidly react and generate a suite of ion products. Primary ions are in the atmosphere originate from radioactive decay, gamma radiation from the soil or cosmic ray events. In this work, we modified an existing instrumentation and developed a novel setup for detecting ion bursts. The setup consists of a continuous flow ionization chamber coupled to Atmospheric Pressure interface Time-Of-Flight (APi-TOF) mass spectrometer. The APi-TOF sampling rate was set to 100 Hz in order to detect individual ion bursts from ionization events. Besides counting the individual ionization events, the developed setup is able to follow the rapidly changing chemical composition of ions during ion burst cascade. The setup can give us insights into the primary ionization mechanisms and their importance in atmospheric ion and aerosol dynamics.

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Palo M., Eller M., Uin J. & Tamm E. 2016: Electric wind in a Differential Mobility Analyzer. Boreal Env. Res. 21: 221–229.

Electric wind — the movement of gas, induced by ions moving in an electric field — can be a distorting factor in size distribution measurements using Differential Mobility Analyzers (DMAs). The aim of this study was to determine the conditions under which electric wind occurs in the locally-built VLDMA (Very Long Differential Mobility Analyzer) and TSI Long-DMA (3081) and to describe the associated distortion of the measured spectra. Electric wind proved to be promoted by the increase of electric field strength, aerosol layer thickness, particle number concentration and particle size. The measured size spectra revealed three types of distortion: widening of the size distribution, shift of the mode of the distribution to smaller diameters and smoothing out the peaks of the multiply charged particles. Electric wind may therefore be a source of severe distortion of the spectrum when measuring large particles at high concentrations.

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Here, we present a calibration of the Neutral cluster and Air Ion Spectrometer (NAIS, Airel Ltd.) for the size and concentration of ions in the mobility-diameter size-range 0.98–29.1 nm. Previous studies raised accuracy issues in size and concentration determination and highlighted the importance of used data inversion algorithm. Therefore, we investigated the performance of the NAIS by using five inversion methods. The presented results illustrate that the size information given by the NAIS is very accurate, regardless of the version of the data inversion. The number concentrations determined by the NAIS were 15%–30% too low especially at the lower end of the measurement size range (< 5 nm), whereas concentrations at diameters 19.6 nm and larger were overestimated by up to 8%. With the correction presented in this study, the uncertainty of the ion concentration measurement of the NAIS can be reduced to less than 10%, allowing the NAIS to be used in quantitative ion cluster studies and more accurate determination of formation and growth rates.

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New particle formation (NPF) events are typically observed during daytime when photochemical oxidation takes place. However, nighttime nucleation mode particles have been observed across various locations only sporadically. We present 11 years (2003–2013) of air ion number size distribution data from the SMEAR II station in Hyytiälä, Finland, where during a third of the nights a sub-3 nm negative (n = 1324 days) and positive (n = 1174 days) ion events took place. To investigate nocturnal clustering at sizes above the constant small ion pool, we defined cluster events (CE) as a nocturnal event with 2–3 nm ion concentrations reaching ≥ 70 cm^–3 between 18:00 and 24:00 local time. CE (n = 221 days) were characterized by a rapid, 10-fold increase in the median 2–3 nm ion concentration from the start (~10 cm^–3) to the event peak (~100 cm^–3). Furthermore, small and intermediate ions during the CE, NPF events and nonevents were compared: while concentrations of 1.5–2 nm ions were the highest during CE (median 235 cm^–3), as compared with the NPF events (96 cm^–3) or the daytime and nighttime nonevents (~20 cm^–3), 3–7 nm ion concentrations increased notably only during NPF events (median 52 cm^–3). Specifically, ion concentrations during CE decreased for sizes above ~2.4 nm (< 10 cm^–3). In addition, 90% of CE proceeded either a NPF event (55%) or a undefined day (35%), and only 10% of them proceeded a daytime non-event. This study suggests a build-up of 0.9–2.4 nm ion clusters during CE nights (18:00–24:00) that equals or exceeds the ion concentration levels during daytime NPF, but unlike the latter, CE fail to activate and grow clusters > 3 nm in diameter in nighttime Hyytiälä.

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We conducted a laboratory investigation to identify highly-oxidized organic compounds formed in sesquiterpene (C₁₅H₂₄, SQT) ozonolysis. The dominant sesquiterpene previously identified from branch emissions of Scots pine, β-caryophyllene, was used for this study. Using the latest mass spectrometric methods, we identified several highly oxidized organic compounds corresponding to RO₂ radical, closed-shell monomer and dimer species. The most abundant compounds detected were monomers C₁₅H₂₄O_7,9,11 and C₁₅H₂₂O_9,11, and dimers C₂₉H₄₆O_12,14,16 and C₃₀H₄₆O_12,14,16. These oxidized organic compounds have very low saturation vapour pressures, an O-to-C ratio of about 0.3–0.9, and they are all classified as extremely low-volatility products (ELVOC). The molar yield of ELVOC was determined to be 1.7% ± 1.28%. Highly-oxidized organic compounds were also measured at a boreal forest site, and few possible β-caryophyllene oxidation products were identified, but the concentrations were extremely low, reaching a maximum of a few hundred thousand molecules cm^–3 in spring.

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Secondary aerosol formation from gas-phase precursors is a frequent phenomenon occurring in a boreal environment. Traditionally, this process is identified visually from observational data on total and ion number size distributions. Here, we introduce a new, objective classification method for the new particle formation events based on measured intermediate-ion concentrations. The intermediate-ion concentration is a suitable indicator of new particle formation, because it is linked to the atmospheric new particle formation. The concentration of intermediate ions is typically very low (below 5 cm^–3) when there is no new particle formation or precipitation events occurring. In this study, we analysed concentrations of negative intermediate ions at the Station for Measuring Ecosystem Atmosphere Relations (SMEAR II) in Hyytiälä, Finland, during the years 2003–2013. We found that the half-hour median concentration of negative intermediate ions in sizes 2–4 nm was > 20 cm^–3 during 77.5% of event days classified by traditional method. The corresponding value was 92.3% in the case of 2–7 nm negative ions. In addition, the intermediate-ion concentration varied seasonally in a similar manner as the number of event days, peaking in the spring. A typical diurnal variation of the intermediate-ion concentration resembled that of the particle concentration during the event days. We developed here a new method for classifying new particle formation events based on intermediate-ion concentrations. The new method is complementary to the traditional event analysis and it can also be used as an automatic way of determining new particle formation events from large data sets.

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We analyzed nanoparticle growth during new-particle-formation events based on ten years of measurements carried out at a boreal forest site in Hyytiälä, Finland, concentrating on the sub-3 nm particles and the role of sulfuric acid in their growth. Growth rates of 1.5–3 nm diameter particles were determined from ion spectrometer measurements and compared with parameterized sulfuric acid concentration and other atmospheric parameters. The calculated growth rates from sulfuric acid condensation were on average 7.4% of the observed growth rates and the two did not correlate. These suggest that neither sulfuric acid monomer condensation nor coagulation of small sulfuric acid clusters was the primary growth mechanism in these atmospheric conditions. Also no clear sign of organic condensation being the single main growth mechanism was seen. These observations are consistent with the hypothesis that several factors have comparative roles in the sub-3 nm growth.

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Radon-222 (²²²Rn) has traditionally been used as an atmospheric tracer for studying air masses and planetary boundary-layer evolution. However, there are various factors that influence its atmospheric concentration. Therefore, we investigated the variability of the atmospheric radon concentration in response to the vertical air mixing and spatial transport in a boreal forest environment in northern Europe. Long-term ²²²Rn data collected at the SMEAR II station in southern Finland during 2000–2006 were analysed along with meteorological data, mixing layer height retrievals and air-mass back trajectory information. The daily mean atmospheric radon concentration followed a log-normal distribution within the range < 0.1–11 Bq m^–3, with the geometric mean of 2.5 Bq m^–3 and a geometric standard deviation of 1.7 Bq m^–3. In spring, summer, autumn and winter, the daily mean concentrations were 1.7, 2.7, 2.8 and 2.7 Bq m^–3, respectively. The low, spring radon concentration was especially attributed to the joint effect of enhanced vertical mixing due to the increasing solar irradiance and inhibited local emissions due to snow thawing. The lowest atmospheric radon concentration was observed with northwesterly winds and high radon concentrations with southeasterly winds, which were associated with the marine and continental origins of air masses, respectively. The atmospheric radon concentration was in general inversely proportional to the mixing layer height. However, the ambient temperature and small-scale turbulent mixing were observed to disturb this relationship. The evolution of turbulence within the mixing layer was expected to be a key explanation for the delay in the response of the atmospheric radon concentration to the changes in the mixing layer thickness. Radon is a valuable naturally-occurring tracer for studying boundary layer mixing processes and transport patterns, especially when the mixing layer is fully developed. However, complementing information, provided by understanding the variability of the atmospheric radon concentration, is of high necessity to be taken into consideration for realistically interpreting the evolution of air masses or planetary boundary layer.

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Aerosol particle concentrations in the atmosphere are governed by their sources and sinks. Sources include directly-emitted (primary) and secondary aerosol particles formed from gas-phase precursor compounds. The relative importance of primary and secondary aerosol particles varies regionally and with time. In this work, we investigated primary and secondary contributions to mode-segregated particle number concentrations by using black carbon as a tracer for the primary aerosol number concentration. We studied separately nucleation, Aitken and accumulation mode concentrations at a rural boreal forest site (Hyytiälä, Finland) and in a rather polluted megacity environment (Nanjing, China) using observational data from 2011 to 2014. In both places and in all the modes, the majority of particles were estimated to be of secondary origin. Even in Nanjing, only about half of the accumulation mode particles were estimated to be of primary origin. Secondary particles dominated particularly in the nucleation and Aitken modes.

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Tropospheric mixing ratios of reactive trace gases (ozone, sulphur dioxide, nitric oxide and nitrogen dioxide) together with meteorological parameters were measured at SMEAR Estonia (Järvselja, Estonia) from August 2013 until May 2015. Seasonal monthly median mixing ratios of the trace gases were determined. Data of incoming wind directions were used to divide the site into four quadrants to compare the median mixing ratios among quadrants. We calculated 48-hour back-trajectories from January to May 2015 for days with different trace gas mixing ratios to determine the influence of pollutant transport and the impact of local processes on ambient mixing ratios. Overall, long-distance transport contributed to 15% of the total concentrations measured. The data demonstrate that distant air masses carrying higher pollutant concentrations, in particular from eastern directions, significantly contributed to local SO₂ and NO_x mixing ratios. Otherwise, the SMEAR Estonia station is a rural site with pollutant mixing ratios near to the measurement equipment’s detection limit.

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We analyzed the size distributions of atmospheric aerosol particles measured during 2013–2014 at Värriö (SMEAR I) in northern Finland, Hyytiälä (SMEAR II) in southern Finland and Järvselja (SMEAR-Estonia) in Estonia. The stations are located on a transect spanning from north to south over 1000 km and they represent different environments ranging from subarctic to the hemi-boreal. We calculated the characteristics of new-particle-formation events, such as the frequency of events, growth rate of nucleation mode particles, condensation and coagulation sinks, formation rate of 2 nm and 3 nm particles, and source rate of condensable vapors. We observed 59, 185 and 108 new-particle-formation events at Värriö, Hyytiälä and Järvselja, respectively. The frequency of the observed events showed an annual variation with a maximum in spring. The analysis revealed size dependence of growth rate at all locations. We found that the growth rate and source rate of a condensable vapor were the highest in Järvselja and the lowest in Värriö. The condensation sink and particle formation rate were of a similar magnitude at Hyytiälä and Järvselja, but several times smaller at Värriö. Tracking the origin of air masses revealed that the number concentration of nucleation mode particles (3–25 nm) varied from north to south, with the highest concentrations at Järvselja and lowest at Värriö. Trajectory analysis indicated that new-particle-formation events are large-scale phenomena that can take place concurrently at distant stations located even 1000 km apart. We found a total of 26 days with new-particle-formation events occurring simultaneously at all three stations.

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Tammet H. 2016: Identification of boundaries between size classes of atmospheric aerosol particles. Boreal Env. Res. 21: 363–371.

A set of few non-overlapping size classes is a widely used structure for presentation of atmospheric aerosol measurements. The boundaries between size class intervals are usually decided on the basis of visual analysis of distribution diagrams without explicit quantitative argumentation. I propose a simple and transparent method of moving neighbor correlation for choosing the boundaries between the size class intervals. The method is in a straight line based on the general principle of classification: two items picked from the same class should be similar and two items picked from different classes should be as different as possible. A measure of similarity is the correlation coefficient between the values of the distribution function at two diameters. A correlation coefficient does not depend on constant factors and is the same in the case of the particle volume and number distributions. The method is illustrated with examples based on measurements of atmospheric aerosols at Hyytiälä during the years 2008–2010.