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  • Qi, X. M.; Ding, A. J.; Nie, W.; Petäjä, T.; Kerminen, V. -M.; Herrmann, E.; Xie, Y. N.; Zheng, L. F.; Manninen, H.; Aalto, P.; Sun, J. N.; Xu, Z. N.; Chi, X. G.; Huang, X.; Boy, M.; Virkkula, A.; Yang, X. -Q.; Fu, C. B.; Kulmala, M. (2015)
    Aerosol particles play important roles in regional air quality and global climate change. In this study, we analyzed 2 years (2011-2013) of measurements of submicron particles (6-800 nm) at a suburban site in the western Yangtze River Delta (YRD) of eastern China. The number concentrations (NCs) of particles in the nucleation, Aitken and accumulation modes were 5300 +/- 5500, 8000 +/- 4400, 5800 +/- 3200 cm(-3), respectively. The NCs of total particles are comparable to those at urban/suburban sites in other Chinese megacities, such as Beijing, but about 10 times higher than in the remote western China. Long-range and regional transport largely influenced number concentrations and size distributions of submicron particles. The highest and lowest accumulation-mode particle number concentrations were observed in air masses from the YRD and coastal regions, respectively. Continental air masses from inland brought the highest concentrations of nucleation-mode particles. New particle formation (NPF) events, apparent in 44% of the effective measurement days, occurred frequently in all the seasons except winter. The frequency of NPF in spring, summer and autumn is much higher than other measurement sites in China. Sulfuric acid was found to be the main driver of NPF events. The particle formation rate was the highest in spring (3.6 +/- 2.4 cm(-3) s(-1)), whereas the particle growth rate had the highest values in summer (12.8 +/- 4.4 nm h(-1)). The formation rate was typically high in relatively clean air masses, whereas the growth rate tended to be high in the polluted YRD air masses. The frequency of NPF events and the particle growth rates showed a strong year-to-year difference. In the summer of 2013, associated with a multi-week heat wave and strong photochemical processes, NPF events occurred with larger frequency and higher growth rates compared with the same period in 2012. The difference in the location and strength of the subtropical high pressure system, which influences the air mass transport pathways and solar radiation, seems to be the cause for year-to-year differences. This study reports, up to now, the longest continuous measurement records of submicron particles in eastern China and helps to achieve a comprehensive understanding of the main factors controlling the seasonal and year-to-year variation of the aerosol size distribution and NPF in this region.
  • Groess, Johannes; Hamed, Amar; Sonntag, Andre; Spindler, Gerald; Manninen, Hanna Elina; Nieminen, Tuomo; Kulmala, Markku; Horrak, Urmas; Plass-Dülmer, Christian; Wiedensohler, Alfred; Birmili, Wolfram (2018)
    This paper revisits the atmospheric new particle formation (NPF) process in the polluted Central European troposphere, focusing on the connection with gas-phase precursors and meteorological parameters. Observations were made at the research station Melpitz (former East Germany) between 2008 and 2011 involving a neutral cluster and air ion spectrometer (NAIS). Particle formation events were classified by a new automated method based on the convolution integral of particle number concentration in the diameter interval 2-20 nm. To study the relevance of gaseous sulfuric acid as a precursor for nucleation, a proxy was derived on the basis of direct measurements during a 1-month campaign in May 2008. As a major result, the number concentration of freshly produced particles correlated significantly with the concentration of sulfur dioxide as the main precursor of sulfuric acid. The condensation sink, a factor potentially inhibiting NPF events, played a subordinate role only. The same held for experimentally determined ammonia concentrations. The analysis of meteorological parameters confirmed the absolute need for solar radiation to induce NPF events and demonstrated the presence of significant turbu-lence during those events. Due to its tight correlation with solar radiation, however, an independent effect of turbulence for NPF could not be established. Based on the diurnal evolution of aerosol, gas-phase, and meteorological parameters near the ground, we further conclude that the particle formation process is likely to start in elevated parts of the boundary layer rather than near ground level.
  • Vana, Marko; Komsaare, Kaupo; Horrak, Urmas; Mirme, Sander; Nieminen, Tuomo; Kontkanen, Jenni; Manninen, Hanna E.; Petäjä, Tuukka; Noe, Steffen M.; Kulmala, Markku (2016)
    We analyzed the size distributions of atmospheric aerosol particles measured during 2013-2014 at Varrio (SMEAR I) in northern Finland, Hyytiala (SMEAR II) in southern Finland and Jarvselja (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 Varrio, Hyytiala and Jarvselja, 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 Jarvselja and the lowest in Varrio. The condensation sink and particle formation rate were of a similar magnitude at Hyytiala and Jarvselja, but several times smaller at Varrio. 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 Jarvselja and lowest at Varrio. 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.
  • Zaidan, Martha A.; Haapasilta, Ville; Relan, Rishi; Paasonen, Pauli; Kerminen, Veli-Matti; Junninen, Heikki; Kulmala, Markku; Foster, Adam S. (2018)
    Atmospheric new-particle formation (NPF) is a very non-linear process that includes atmospheric chemistry of precursors and clustering physics as well as subsequent growth before NPF can be observed. Thanks to ongoing efforts, now there exists a tremendous amount of atmospheric data, obtained through continuous measurements directly from the atmosphere. This fact makes the analysis by human brains difficult but, on the other hand, enables the usage of modern data science techniques. Here, we calculate and explore the mutual information (MI) between observed NPF events (measured at Hyytiala, Finland) and a wide variety of simultaneously monitored ambient variables: trace gas and aerosol particle concentrations, meteorology, radiation and a few derived quantities. The purpose of the investigations is to identify key factors contributing to the NPF. The applied mutual information method finds that the formation events are strongly linked to sulfuric acid concentration and water content, ultraviolet radiation, condensation sink (CS) and temperature. Previously, these quantities have been well-established to be important players in the phenomenon via dedicated field, laboratory and theoretical research. The novelty of this work is to demonstrate that the same results are now obtained by a data analysis method which operates without supervision and without the need of understanding the physics deeply. This suggests that the method is suitable to be implemented widely in the atmospheric field to discover other interesting phenomena and their relevant variables.
  • Kontkanen, Jenni; Järvinen, Emma; Manninen, Hanna E.; Lehtipalo, Katrianne; Kangasluoma, Juha; Decesari, Stefano; Gobbi, Gian Paolo; Laaksonen, Ari; Petäjä, Tuukka; Kulmala, Markku (2016)
    The concentrations of neutral and charged sub3nm clusters and their connection to new particle formation (NPF) were investigated during the PEGASOS campaign (7 June-9 July 2012) at the San Pietro Capofiume measurement station in the Po Valley, Italy. Continuous high concentrations of sub-3nm clusters were detected during the measurement period, although the condensation sink was relatively high (median value 1.1 x 10(-2) s(-1)). The median cluster concentrations were 2140 and 7980 cm 3 in the size bins of 1.5-1.8 and 1.8-3 nm, and the majority of them were electrically neutral. NPF events were observed during the measurement period frequently, on 86% of the days. The median growth rates of clusters during the events were 4.3, 6.0 and 7.2 nm h(-1) in the size ranges of 1.5-3, 3-7 and 720 nm. The median formation rate of 1.6 nm clusters was high, 45 cm 3 s(-1), and it exceeded the median formation rate of 2 nm clusters by 1 order of magnitude. The ion-induced nucleation fraction was low; the median values were 0.7% at 1.6 nm and 3.0% at 2 nm. On NPF event days the neutral cluster concentration had a maximum around 09: 00 (local winter time), which was absent on a non-event day. The increase in the cluster concentrations in the morning coincided with the increase in the boundary layer height. At the same time radiation, temperature and SO2 concentration increased, and RH and condensation sink decreased. The concentrations of neutral and charged clusters were observed to have a positive correlation with sulfuric acid proxy, indicating the significance of sulfuric acid for the cluster formation in San Pietro Capofiume. The condensation sink had a negative correlation with the concentration of charged clusters but no clear relation to the neutral cluster concentration. This finding, together with back-trajectory analysis, suggests that the precursor vapors of the clusters and background aerosol particles, acting as their sink, have possibly originated from the same sources, including e.g., power plants and industrial areas in the Po Valley.
  • Peräkylä, Otso; Vogt, Matthias; Tikkanen, Olli-Pekka; Laurila, Terhi; Kajos, Maija K.; Rantala, Pekka A.; Patokoski, Johanna; Aalto, Juho; Yli-Juuti, Taina; Ehn, Mikael; Sipila, Mikko; Paasonen, Pauli; Rissanen, Matti; Nieminen, Tuomo; Taipale, Risto; Keronen, Petri; Lappalainen, Hanna K.; Ruuskanen, Taina M.; Rinne, Janne; Kerminen, Veli-Matti; Kulmala, Markku; Back, Jaana; Petaja, Tuukka (2014)
  • Wehner, B.; Werner, F.; Ditas, F.; Shaw, R. A.; Kulmala, M.; Siebert, H. (2015)
    During the CARRIBA (Cloud, Aerosol, Radiation and tuRbulence in the trade wInd regime over BArba-dos) campaign, the interaction between aerosol particles and cloud microphysical properties was investigated in detail, which also includes the influence of clouds on the aerosol formation. During two intensive campaigns in 2010 and 2011, helicopter-borne measurement flights were performed to investigate the thermodynamic, turbulent, microphysical, and radiative properties of trade-wind cumuli over Barbados. During these flights, 91 cases with increased aerosol particle number concentrations near clouds were detected. The majority of these cases are also correlated with enhanced irradiance in the ultraviolet (UV) spectral wavelength range. This enhancement reaches values up to a factor of 3.3 greater compared to background values. Thus, cloud boundaries provide a perfect environment for the production of precursor gases for new particle formation. Another feature of cloud edges is an increased turbulence, which may also enhance nucleation and particle growth. The observed events have a mean length of 100 m, corresponding to a lifetime of less than 300 s. This implies that particles with diameters of at least 7 nm grew several nanometers per minute, which corresponds to the upper end of values in the literature (Kulmala et al., 2004). Such high values cannot be explained by sulfuric acid alone; thus extremely low volatility organic compounds (ELVOCs) are probably involved here.
  • Mogensen, D.; Gierens, R.; Crowley, J. N.; Keronen, P.; Smolander, S.; Sogachev, A.; Noelscher, A. C.; Zhou, L.; Kulmala, Markku; Tang, M. J.; Williams, J.; Boy, M. (2015)
    Using the 1-D atmospheric chemistry transport model SOSAA, we have investigated the atmospheric reactivity of a boreal forest ecosystem during the HUMPPA-COPEC-10 campaign (summer 2010, at SMEAR II in southern Finland). For the very first time, we present vertically resolved model simulations of the NO3 and O-3 reactivity (R) together with the modelled and measured reactivity of OH. We find that OH is the most reactive oxidant (R similar to 3 s(-1)) followed by NO3 (R similar to 0.07 s(-1)) and O-3 (R similar to 2 x 10 5 s(-1)). The missing OH reactivity was found to be large in accordance with measurements (similar to 65 %) as would be expected from the chemical subset described in the model. The accounted OH radical sinks were inorganic compounds (similar to 41 %, mainly due to reaction with CO), emitted monoterpenes (similar to 14 %) and oxidised biogenic volatile organic compounds (similar to 44 %). The missing reactivity is expected to be due to unknown biogenic volatile organic compounds and their photoproducts, indicating that the true main sink of OH is not expected to be inorganic compounds. The NO3 radical was found to react mainly with primary emitted monoterpenes (similar to 60 %) and inorganic compounds (similar to 37 %, including NO2). NO2 is, however, only a temporary sink of NO3 under the conditions of the campaign (with typical temperatures of 20-25 degrees C) and does not affect the NO3 concentration. We discuss the difference between instantaneous and steady-state reactivity and present the first boreal forest steady-state lifetime of NO3 (113 s). O-3 almost exclusively reacts with inorganic compounds (similar to 91 %, mainly NO, but also NO2 during night) and less with primary emitted sesquiterpenes (similar to 6 %) and monoterpenes (similar to 3 %). When considering the concentration of the oxidants investigated, we find that OH is the oxidant that is capable of removing organic compounds at a faster rate during daytime, whereas NO3 can remove organic molecules at a faster rate during night-time. O-3 competes with OH and NO3 during a short period of time in the early morning (around 5 a.m. local time) and in the evening (around 7-8 p.m.). As part of this study, we developed a simple empirical parameterisation for conversion of measured spectral irradiance into actinic flux. Further, the meteorological conditions were evaluated using radiosonde observations and ground-based measurements. The overall vertical structure of the boundary layer is discussed, together with validation of the surface energy balance and turbulent fluxes. The sensible heat and momentum fluxes above the canopy were on average overestimated, while the latent heat flux was un-derestimated.
  • Venter, Marcell; Beukes, Johan Paul; van Zyl, Pieter Gideon; Vakkari, Ville; Virkkula, Aki; Josipovic, Miroslav; Kulmala, Markku; Laakso, Lauri (2020)
    Atmospheric aerosols have a significant effect on earth's radiative budget, particularly on regional scales. This paper presents a similar to 6 year, in situ, ground level aerosol scattering and absorption dataset, measured at a background site strategically positioned to enable differentiation of the effect of anthropogenic, population density and open biomass burning activities on a regional scale. Relatively well-defined seasonal and diurnal patterns were observed for all the aerosol optical properties, i.e. scattering coefficient (sigma(SP)), absorption coefficient (sigma(AP)), single scattering albedo (omega(0)) and Angstrom exponent of scattering (sigma(SP)). These patterns were explained by considering southern African specific sources and metrological conditions. Using a receptor modelling method (auto-generated source maps) it was found that air masses that had higher sigma(SP), sigma(AP) and omega(0), and lower alpha(SP), if compared with the relatively clean background, passed over source regions with significant industrial or other anthropogenic activities, higher population density, re-circulation of polluted air masses and higher open biomass burning frequency. To quantify differences, four source regions were defined, i.e. Karoo, Kalahari, anti-cyclonic recirculation pattern and the industrial hub of South Africa. Air masses that had passed over the Karoo source region represented the cleanest regional background conditions, while air masses that had passed either over the industrial hub and/or the anti-cyclonic recirculation pattern represented the most significant anthropogenically impacted, as indicated by the aerosol optical properties. The omega(0) medians of air masses that had passed over the Karoo (0.80-0.86) were 9, 12 and 7% lower than in air masses that had passed over source regions with the highest omega(0) median, in the warmest/wettest, coldest, and driest, peak open biomass burning periods, respectively.
  • Nieminen, T.; Yli-Juuti, T.; Manninen, H. E.; Petäjä, T.; Kerminen, V. -M.; Kulmala, M. (2015)
    New particle formation (NPF) occurs frequently in the global atmosphere. During recent years, detailed laboratory experiments combined with intensive field observations in different locations have provided insights into the vapours responsible for the initial formation of particles and their subsequent growth. In this regard, the importance of sulfuric acid, stabilizing bases such as ammonia and amines as well as extremely low volatile organics, have been proposed. The instrumentation to observe freshly formed aerosol particles has developed to a stage where the instruments can be implemented as part of airborne platforms, such as aircrafts or a Zeppelin-type airship. Flight measurements are technically more demanding and require a greater detail of planning than field studies at the ground level. The high cost of flight hours, limited time available during a single research flight for the measurements, and different instrument payloads in Zeppelin airship for various flight missions demanded an analysis tool that would forecast whether or not there is a good chance for an NPF event. Here we present a methodology to forecast NPF event probability at the SMEAR II site in Hyytiala, Finland. This methodology was used to optimize flight hours during the PEGASOS (Pan-European Gas Aerosol Climate Interaction Study)-Zeppelin Northern mission in May-June 2013. Based on the existing knowledge, we derived a method for estimating the nucleation probability that utilizes forecast air mass trajectories, weather forecasts, and air quality model predictions. With the forecast tool we were able to predict the occurrence of NPF events for the next day with more than 90% success rate (10 out of 11 NPF event days correctly predicted). To our knowledge, no similar forecasts of NPF occurrence have been developed for other sites. This method of forecasting NPF occurrence could be applied also at other locations, provided that long-term observations of conditions favouring particle formation are available.
  • Carnerero, Cristina; Perez, Noemi; Reche, Cristina; Ealo, Marina; Titos, Gloria; Lee, Hong-Ku; Eun, Hee-Ram; Park, Yong-Hee; Dada, Lubna; Paasonen, Pauli; Kerminen, Veli-Matti; Mantilla, Enrique; Escudero, Miguel; Gomez-Moreno, Francisco J.; Alonso-Blanco, Elisabeth; Coz, Esther; Saiz-Lopez, Alfonso; Temime-Roussel, Brice; Marchand, Nicolas; Beddows, David C. S.; Harrison, Roy M.; Petäjä, Tuukka; Kulmala, Markku; Ahn, Kang-Ho; Alastuey, Andres; Querol, Xavier (2018)
    The vertical profile of new particle formation (NPF) events was studied by comparing the aerosol size number distributions measured aloft and at surface level in a suburban environment in Madrid, Spain, using airborne instruments. The horizontal distribution and regional impact of the NPF events was investigated with data from three urban, urban background, and suburban stations in the Madrid metropolitan area. Intensive regional NPF episodes followed by particle growth were simultaneously recorded at three stations in and around Madrid during a field campaign in July 2016. The urban stations presented larger formation rates compared to the suburban station. Condensation and coagulation sinks followed a similar evolution at all stations, with higher values at urban stations. However, the total number concentration of particles larger than 2.5 nm was lower at the urban station and peaked around noon, when black carbon (BC) levels are at a minimum. The vertical soundings demonstrated that ultrafine particles (UFPs) are formed exclusively inside the mixed layer. As convection becomes more effective and the mixed layer grows, UFPs are detected at higher levels. The morning soundings revealed the presence of a residual layer in the upper levels in which aged particles (nucleated and grown on previous days) prevail. The particles in this layer also grow in size, with growth rates significantly smaller than those inside the mixed layer. Under conditions with strong enough convection, the soundings revealed homogeneous number size distributions and growth rates at all altitudes, which follow the same evolution at the other stations considered in this study. This indicates that UFPs are detected quasi-homogenously in an area spanning at least 17 km horizontally. The NPF events extend over the full vertical extension of the mixed layer, which can reach as high as 3000 m in the area, according to previous studies. On some days a marked decline in particle size (shrinkage) was observed in the afternoon, associated with a change in air masses. Additionally, a few nocturnal nucleation-mode bursts were observed at the urban stations, for which further research is needed to elucidate their origin.