AIR IONS AND CHARGING STATE OF ATMOSPHERIC NANOPARTICLES. AT THE MACE HEAD RESEARCH STATION. M. VANA1,2, H.E. MANNINEN1, ...
AIR IONS AND CHARGING STATE OF ATMOSPHERIC NANOPARTICLES AT THE MACE HEAD RESEARCH STATION M. VANA1,2, H.E. MANNINEN1, T. NIEMINEN1, S. GAGNÉ1, K. LEHTIPALO1, M. SIPILÄ1, D. CEBURNIS3, C.D. O’DOWD3 and M. KULMALA1 1
Department of Physics, University of Helsinki, 00014, Helsinki, Finland 2
Institute of Physics, University of Tartu, 50090, Tartu, Estonia
3
School of Physics and Centre for Climate and Air Pollution Studies, Environmental Change Institute, National University of Ireland Galway, Galway, Ireland
Keywords: AIR IONS, ATMOSPHERIC AEROSOLS, PARTICLE FORMATION, CHARGING STATE. INTRODUCTION The Earth’s atmosphere is a giant aerosol system, as the atmospheric air always contains the suspended particles. New particle formation via gas-to-particle conversion determines the concentration of cloud condensation nuclei, and ultimately, has influence to global climate (Spracklen et al., 2006). The initial nucleation in the atmosphere produces particles with diameters of the order of 1 – 2 nm (Kulmala et al., 2000). Therefore, in order to understand formation mechanisms of atmospheric aerosol particles direct measurements of the sub-3 nm particles are needed. Air ions (charged clusters and aerosol particles) have been measured at different sites around the world (Kulmala and Tammet, 2007). Even very low concentrations of atmospheric ions can initiate the production of a significant number of ion clusters that eventually grow into new particles. Recently, the evidence on the existence of a pool of neutral particles in the sub-3 nm size range has also been reported (Kulmala et al., 2007; Sipilä et al., 2009). Cluster formation mechanisms can vary in different locations. Coastal regions are places where new particle formation takes place frequently, typically during exposure of shore biota during low tide conditions (O’Dowd and Hoffmann, 2005). Therefore, coastal aerosols can significantly contribute to the natural background aerosol population. Recent studies have shown that the bursts of intermediate air ions are common phenomenon at coastal regions (Vana et al., 2008). The aim of this study is to characterize the coastal air ion population, to obtain more information on the behavior of neutral and charged particles with diameter < 3 nm, and to estimate the charging state of nucleation mode particles and the role of charged particles in coastal new particle formation. MATERIALS AND METHODS We measured the air ion mobility distribution and the particle size distribution with an Air Ion Spectrometer (AIS) and a Neutral cluster and Air Ion Spectrometer (NAIS). Both instruments were designed by the University of Tartu, and built by Airel Ltd., Estonia. The NAIS is a further development of the AIS (Mirme et al., 2007) and measures mobility distributions of neutral and charged aerosol particles and clusters in the mobility range from 0.0013 to 3.2 cm2V-1s-1 (the diameter range from 0.8 to 41 nm), whereas the AIS measures only charged particles in the same mobility range. The AIS and NAIS were calibrated during a special calibration and intercomparison workshop (Asmi et al., 2009). As a supporting data sets, we also measured particle size distribution with a Pulse-Height Condensation Particle Counter (PH-CPC) (Sipilä et al., 2009) and a Scanning Mobility Particle Sizer (SMPS) in size ranges of 1.5 – 5 nm and 3 – 1000 nm, respectively.
As a part of the European Union project MAP (Marine Aerosol Production), we deployed an AIS at the Mace Head Atmospheric Research Station (53 19’N, 9 54’W) on the west coast of Ireland from 8 January 2006 to 30 October 2007. As a part of the European Commission 6th Framework program project EUCAARI (European Integrated project on Aerosol Cloud Climate and Air Quality interactions), we deployed a NAIS at the Mace Head Atmospheric Research Station from 13 June 2008 to 7 May 2009. The PH-CPC measured at Mace Head from 13 June to 25 August 2008. The SMPS measures continuously at the research station. The location of the monitoring station provides a good opportunity to study particle formation events at different distances from the tidal source regions. Basic meteorological data, solar radiation, air mass trajectories and modelled data of the tidal height for the measurement location were included in the data analysis. RESULTS AND DISCUSSION A mode of small ions (in this work particles with mobility > 0.42 cm2V-1s-1) in the mobility distribution of air ions always exists in the atmosphere. These particles play a crucial role in the initial steps of nucleation and in the formation of thermodynamically stable clusters. Therefore, it is important to study the character of small ions in the atmosphere. Figure 1 illustrates the time variation of the concentration of small ions in years 2006 – 2009. The data points in the figure are 12-hour medians and horizontal lines present 25%-, 50%- and 75%-percentiles. Figure 1 shows that the median concentration is 360 cm-3 and appears to be the same for the negative and positive small ions. We also studied the dependence of the concentration of small ions on various meteorological parameters. We observed a correlation between the small ion concentrations and both wind speed and direction. The concentration of small ions can be several times higher during low wind speed.
Figure 1. The time variation of the small ion concentration in 2006 – 2009. Data in 2006 – 2007 were measured by the AIS, data in 2008 – 2009 were measured by the NAIS. The horizontal lines present 25%-, 50%- and 75%-percentiles for the concentrations of negative and positive small ions, respectively. The analysis shows that bursts of intermediate ions and neutral clusters are a frequent phenomenon in the marine coastal environment. Nucleation events occurred during most of the measurement days. Particle formation and growth events mostly coincided with the presence of low tide. Measurements by the NAIS and the PH-CPC enable to estimate the concentration of neutral particles with diameter < 3 nm. The concentration of 1.8 - 3 nm particles varied between 103 and 106 cm-3 during nucleation events. Figure 2
shows typical “apple”-type event which coincided with low tide occurrence and the wind direction was from the sector where air was advected over sparsely populated land in the northwest-to-north direction (Vana et al., 2008). We calculated the formation rate values for charged and neutral clusters using calculation scheme described by Kulmala et al., 2007. Formation rates of 2 nm particles (J2) were high during coastal nucleation events. J2 is usually around 10 cm-3s-1, but can be 100 - 1000 cm-3s-1 during “apple” and “hump“-type events, which differ from observations over the boreal forest where values of J2 are typically 1 - 2 cm-3s-1 (Kulmala et al., 2007). For positive and negative ions, values of J2 are about two order of magnitude smaller compared to neutral particles.
Figure 2. The time variations of the particle size distribution measured by the NAIS (particle diameter < 10 nm) and the SMPS (particle diameter > 10 nm), the formation rate of 1.8-nm particles (J2), wind direction, temperature, and tidal amplitude during a coastal nucleation event day. We analysed a charging state of > 1.8 nm particles during different types of new particle formation events. The time variation of the ratio of charged and all particles in the diameter range of 1.8 - 3 nm showed that in some cases negatively charged clusters seem to activate more easily than positively charged and neutral particles. During nucleation events 1.8 - 3 nm particles seem to be highly undercharged. Figure 3 illustrates size distribution of charging probability during a typical coastal new particle formation day. Charging probabilities were calculated as the slope of the linear regression line on the scatterplot of the measured concentrations of total and charged (air ions) particles for the same fraction (diameter
interval). Charging probabilities for nucleation mode particles (with diameter < 20 nm) usually appeared to be smaller than expected by the bipolar equilibrium charge distribution (e.g. Wiedensholer, 1988).
Figure 3. The bipolar equilibrium charge distribution and the charging probabilities as a function of particle diameter during new particle formation event day (17 March 2009) at the Mace Head research station.
CONCLUSIONS Our measurements of the particle size distribution in the diameter range of 0.8 – 41 nm show that nucleation events occurred during most of the measurement days. We can conclude that the NAIS and the PH-CPC can be used for observation of sub-3 nm particles in the atmosphere. The two instruments give better results then they measure high concentrations during nucleation events. The concentration of 1.8 – 3 nm particles varied between 103 to 106 cm-3 during nucleation events. Formation rates of 2 nm particles are high during coastal nucleation events. J2 is usually around 10 cm-3s-1, but can be 100 - 1000 cm-3s-1 during “apple” and “hump“-type events, which differ from observations over the boreal forest where values of J2 are typically 1 – 2 cm-3s-1. Small air ions were characterized. The median concentration during the years 2006 – 2009 was estimated to be 360 cm-3 and appears to be the same for the negative and positive small ions. The concentrations of small ions clearly depend on wind speed and wind direction. The concentration of small ions can be several times higher during low wind speed compared to that of during high wind speed. During nucleation event freshly nucleated particles seem to be highly undercharged compared to the bipolar equilibrium charge distribution. The time variation of the charged fraction of 1.8 – 3 nm particles show that negatively charged particles seem to activate easier than positively charged and neutral particles. Charged particles can sometimes play important role in the beginning of nucleation event.
ACKNOWLEDGEMENTS The financial support by the Academy of Finland Centre of Excellence program (project no 1118615) is gratefully acknowledged. This work was also supported by the European Commission 6th Framework program projects EUCAARI (contract no 036833-2) and MAP (project no 018332), EPA Ireland, the Estonian Science Foundation under grants no. 6988, and by the Estonian Research Council Project SF0180043s08.
REFERENCES Asmi, E., Sipilä, M., Manninen, H. E., Vanhanen, J., Lehtipalo, K., Gagné, S., Neitola, K., Mirme, A., Mirme, S., Tamm, E., Uin, J., Komsaare, K., Attoui, M. and Kulmala, M. (2009). Results of the first air ion spectrometer calibration and intercomparison workshop. Atmos. Chem. Phys., 9, 141154. Kulmala, M., Pirjola, L., Mäkelä, J. M. (2000). Stable sulphate clusters as a source of new atmospheric particles. Nature, 404, 66-69. Kulmala, M., Riipinen, I., Sipilä, M., Manninen, H. E., Petäjä, T., Junninen, H., Dal Maso, M., Mordas, G., Mirme, A., Vana, M., Hirsikko, A., Laakso, L., Harrison, R. M., Hanson, I., Leung, C., Lehtinen, K. E. J. and Kerminen, V.-M. (2007) Towards direct measurement of atmospheric nucleation. Science, 318, 89-92. Kulmala, M. and Tammet, H. (2007). Finnish-Estonian air ion and aerosol workshop. Boreal Env. Res., 12, 237-245. Mirme, A., Tamm, E., Mordas, G., Vana, M., Uin, J., Mirme, S., Bernotas, T., Laakso, L., Hirsikko, A. and Kulmala, M. (2007). A wide-range multi-channel Air Ion Spectrometer. Boreal Env. Res., 12, 247-264. O’Dowd, C. D. and Hoffmann, T. (2005). Coastal new particle formation: a review of the current state-ofthe-art. Environ. Chem., 2, 245-255. Sipilä, M., Lehtipalo, K., Attoui, M., Neitola, K., Petäjä, T., Aalto, P. P., O’Dowd, C. D. and Kulmala, M. (2009). Laboratory verification of PH-CPC’s ability to monitor atmospheric sub-3 nm clusters. Aerosol Sci. Technol., 43, 126-135. Spracklen, D. V., Carslaw, K. S., Kulmala, M., Kerminen, V.-M., Mann, G. W., and Sihto, S.-L. (2006). The contribution of boundary layer nucleation events to total particle concentrations on regional and global scales. Atmos. Chem. Phys., 6, 5631-5648. Vana, M., Ehn, M., Petäjä, T., Vuollekoski, H., Aalto, P., de Leeuw, G., Ceburnis, D., O'Dowd, C. D. and Kulmala, M. (2008). Characteristic features of air ions at Mace Head on the west coast of Ireland. Atmos. Res., 90, 278-286. Wiedensohler, A (1988). An approximation of the bipolar charge distribution for particles in the submicron size range, J. Aerosol Sci., 19, 387-389.
