Peer-reviewed publications


Gierens, R. T., Henriksson, S., Josipovic, M., Vakkari, V., van Zyl, P. G., Beukes, J. P., Wood, C. R. and O'Connor, E. J.: Observing continental boundary-layer structure and evolution over the South African savannah using a ceilometer, Theor. Appl. Climatol., 1–14, doi:10.1007/s00704-018-2484-7, 2018.

Jaars, K., Vestenius, M., van Zyl, P. G., Beukes, J. P., Hellén, H., Vakkari, V., Venter, M., Josipovic, M. and Hakola, H.: Receptor modelling and risk assessment of volatile organic compounds measured at a regional background site in South Africa, Atmos. Environ., 172, 133–148, doi:10.1016/j.atmosenv.2017.10.047, 2018.

Kollonige, D. E., Thompson, A. M., Josipovic, M., Tzortziou, M., Beukes, J. P., Burger, R., Martins, D. K., van Zyl, P. G., Vakkari, V. and Laakso, L.: OMI Satellite and Ground-Based Pandora Observations and Their Application to Surface NO2 Estimations at Terrestrial and Marine Sites, J. Geophys. Res. Atmos., doi:10.1002/2017JD026518, 2018.

Nieminen, T., Kerminen, V.-M., Petäjä, T., Aalto, P. P., Arshinov, M., Asmi, E., Baltensperger, U., Beddows, D. C. S., Beukes, J. P., Collins, D., Ding, A., Harrison, R. M., Henzing, B., Hooda, R., Hu, M., Hõrrak, U., Kivekäs, N., Komsaare, K., Krejci, R., Kristensson, A., Laakso, L., Laaksonen, A., Leaitch, W. R., Lihavainen, H., Mihalopoulos, N., Németh, Z., Nie, W., O'Dowd, C., Salma, I., Sellegri, K., Svenningsson, B., Swietlicki, E., Tunved, P., Ulevicius, V., Vakkari, V., Vana, M., Wiedensohler, A., Wu, Z., Virtanen, A., and Kulmala, M.: Global analysis of continental boundary layer new particle formation based on long-term measurements, Atmos. Chem. Phys., 18, 14737-14756,, 2018.

Vakkari, V., Beukes, J. P., Dal Maso, M., Aurela, M., Josipovic, M. and van Zyl, P. G.: Major secondary aerosol formation in southern African open biomass burning plumes, Nature Geosci., 11, 580¬–583, doi:10.1038/s41561-018-0170-0, 2018.


Chiloane, K. E., Beukes, J. P., van Zyl, P. G., Maritz, P., Vakkari, V., Josipovic, M., Venter, A. D., Jaars, K., Tiitta, P., Kulmala, M., Wiedensohler, A., Liousse, C., Mkhatshwa, G. V., Ramandh, A. and Laakso, L.: Spatial, temporal and source contribution assessments of black carbon over the northern interior of South Africa, Atmos. Chem. Phys., 17(10), 6177–6196, doi:10.5194/acp-17-6177-2017, 2017.

Räsänen, M., Aurela, M., Vakkari, V., Beukes, J. P., Tuovinen, J.-P., Van Zyl, P. G., Josipovic, M., Venter, A. D., Jaars, K., Siebert, S. J., Laurila, T., Rinne, J., and Laakso, L.: Carbon balance of a grazed savanna grassland ecosystem in South Africa, Biogeosciences, 14, 1039-1054, doi:10.5194/bg-14-1039-2017, 2017.

Venter, A. D., van Zyl, P. G., Beukes, J. P., Josipovic, M., Hendriks, J., Vakkari, V., and Laakso, L.: Atmospheric trace metals measured at a regional background site (Welgegund) in South Africa, Atmos. Chem. Phys., 17, 4251-4263, doi:10.5194/acp-17-4251-2017, 2017


Aurela, M., Beukes, J.P., Van Zyl, P.G., Vakkari, V., Teinilä, K., Saarikoski, S., Laakso, L. The composition of ambient and fresh biomass burning aerosols at a savannah site, South Africa, S Afr J Sci., 112, Art. #2015-0223,, 2016

Butt, E. W., Rap, A., Schmidt, A., Scott, C. E., Pringle, K. J., Reddington, C. L., Richards, N. A. D., Woodhouse, M. T., Ramirez-Villegas, J., Yang, H., Vakkari, V., Stone, E. A., Rupakheti, M., S. Praveen, P., G. van Zyl, P., P. Beukes, J., Josipovic, M., Mitchell, E. J. S., Sallu, S. M., Forster, P. M. and Spracklen, D. V.: The impact of residential combustion emissions on atmospheric aerosol, human health, and climate, Atmos. Chem. Phys., 16(2), 873–905, doi:10.5194/acp-16-873-2016, 2016.

Jaars, K., van Zyl, P. G., Beukes, J. P., Hellén, H., Vakkari, V., Josipovic, M., Venter, A. D., Räsänen, M., Knoetze, L., Cilliers, D. P., Siebert, S. J., Kulmala, M., Rinne, J., Guenther, A., Laakso, L., and Hakola, H.: Measurements of biogenic volatile organic compounds at a grazed savannah grassland agricultural landscape in South Africa, Atmos. Chem. Phys., 16, 15665-15688, doi:10.5194/acp-16-15665-2016, 2016.

Mafusire, G., H. J. Annegarn, V. Vakkari, J. P. Beukes, M. Josipovic, P. G. van Zyl, and L. Laakso, Sub-micrometer aerosols and excess CO as tracers for biomass burning air mass transport over southern Africa, J. Geophys. Res. Atmos., 121, doi:10.1002/2015JD023965, 2016.

Venter, A. D., Beukes, J. P., Gideon van Zyl, P., Josipovic, M., Jaars, K. and Vakkari, V.: Regional atmospheric Cr(VI) pollution from the Bushveld Complex, South Africa, Atmos. Pollut. Res., 7(5), 762–767, doi:10.1016/j.apr.2016.03.009, 2016.


Booyens, W., Van Zyl, PG., Beukes, JP., Ruiz-Jimenez, J., Kopperi, M., Riekkola, M-L., Josipovic, M., Venter, A., Jaars, K.,  Laakso, L.,  Vakkari, V., Kulmala, M., Pienaar, J.J. Size-resolved characterisation of organic compounds in atmospheric aerosols collected at Welgegund, South Africa, J. Atmos Chem., DOI 10.1007/s10874-015-9304-6, 2015

Kuik, F., Lauer, A., Beukes, J. P., Van Zyl, P. G., Josipovic, M., Vakkari, V., Laakso, L., and Feig, G. T.: The anthropogenic contribution to atmospheric black carbon concentrations in southern Africa: a WRF-Chem modeling study, Atmos. Chem. Phys., 15, 8809-8830, doi:10.5194/acp-15-8809-2015, 2015

Shrivastava, M., Easter, R., Liu, X., Zelenyuk, A., Singh, B., Zhang, K., Ma, P.-L., Chand, D., Ghan, S., Jimenez, J., Zhang, Q., Fast, J., Rasch, P., and Tiitta, P., Global transformation and fate of SOA: Implications of low-volatility SOA and gas-phase fragmentation reactions, Geophys. Res. Atmos., 120, 4169-4195. doi: 10.1002/2014JD022563, 2015

Vakkari, V., Tiitta, P., Jaars, K., Croteau, P., Beukes, J.P., Josipovic, M., Kerminen, V.-M., Kulmala, M., Venter, A.D., Van Zyl, P.G., Worsnop, D.R., Laakso, L., Reevaluating the contribution of sulfuric acid and the origin of organic compounds in atmospheric nanoparticle growth, Geophys. Res. Lett., 42, doi:10.1002/2015GL066459, 2015.

Venter, A.D., Jaars, K., Booyens, W., Beukes, J.P., Van Zyl, P.G., Josipovic, M., Hendriks, J., Vakkari, V., Hellén, H., Hakola, H., Aaltonen, H., Ruiz-Jimenez, J., Riekkola M-L., and Laakso, L.,  Plume characterisation of a typical South African braai, South African Journal of Chemistry,  68, 181–194, doi: 10.17159/0379-4350/2015/v68a25 , 2015.


Backman, J., Virkkula, A., Vakkari, V., Beukes, J. P., Van Zyl, P. G., Josipovic, M., Piketh, S., Tiitta, P., Chiloane, K., Petäjä, T., Kulmala, M., and Laakso, L.: Differences in aerosol absorption Ångström exponents between correction algorithms for a particle soot absorption photometer measured on the South African Highveld, Atmos. Meas. Tech., 7, 4285-4298, doi:10.5194/amt-7-4285-2014, 2014

Gierens, R., Laakso, L., Mogense, D., Vakkari, V., Beukes, J.P., va Zyl, P.G., Hakola, H., Guenther, A., Pienaar, J.J., Boy, M., Modelling new particle formation events in the South African savannah, South African Journal of Science,110(5/6), Art. #2013-0108,,  2014

Jaars, K., Beukes, J. P., van Zyl, P. G., Venter, A. D., Josipovic, M., Pienaar, J. J., Vakkari, V., Aaltonen, H., Laakso, H., Kulmala, M., Tiitta, P., Guenther, A., Hellén, H., Laakso, L., and Hakola, H.: Ambient aromatic hydrocarbon measurements at Welgegund, South Africa, Atmos. Chem. Phys., 14, 7075-7089, doi:10.5194/acp-14-7075-2014, 2014

Korhonen, K., Giannakaki, E., Mielonen, T., Pfüller, A., Laakso, L., Vakkari, V., Baars, H., Engelmann, R., Beukes, J. P., Van Zyl, P. G., Ramandh, A., Ntsangwane, L., Josipovic, M., Tiitta, P., Fourie, G., Ngwana, I., Chiloane, K., and Komppula, M.: Atmospheric boundary layer top height in South Africa: measurements with lidar and radiosonde compared to three atmospheric models, Atmos. Chem. Phys., 14, 4263-4278, doi:10.5194/acp-14-4263-2014, 2014

Tiitta, P., Vakkari, V., Croteau, P., Beukes, J. P., van Zyl, P. G., Josipovic, M., Venter, A. D., Jaars, K., Pienaar, J. J., Ng, N. L., Canagaratna, M. R., Jayne, J. T., Kerminen, V.-M., Kokkola, H., Kulmala, M., Laaksonen, A., Worsnop, D. R., and Laakso, L.: Chemical composition, main sources and temporal variability of PM1 aerosols in southern African grassland, Atmos. Chem. Phys., 14, 1909-1927, doi:10.5194/acp-14-1909-2014, 2014.

Vakkari, V., Kerminen, V.-M., Beukes, J.P.,  Tiitta, P., van Zyl, P.G., Josipovic, M., Venter, A., Jaars, K., Worsnop, D., Kulmala, M., and Laakso, L., Rapid changes in biomass burning aerosols by atmospheric oxidation, Geophys. Res. Lett., 41, doi:10.1002/2014GL059396, 2014

Van Zyl PG, Beukes JP, Du Toit G, Mabaso D, Hendriks J, Vakkari V., Tiitta, P., Pienaar, J.J., Kulmala, M., Laakso, L.. Assessment of atmospheric trace metals in the western Bushveld Igneous Complex, South Africa. S Afr J Sci. 110(3/4), Art. #2013-0280,, 2014


Hirsikko, A., Vakkari, V., Tiitta, P., Hatakka, J., Kerminen, V.-M., Sundström, A.-M., Beukes, J. P., Manninen, H. E., Kulmala, M., and Laakso, L.: Multiple daytime nucleation events in semi-clean savannah and industrial environments in South Africa: analysis based on observations, Atmos. Chem. Phys., 13, 5523-5532, doi:10.5194/acp-13-5523-2013, 2013

Hyvärinen, A.-P., Vakkari, V., Laakso, L., Hooda, R. K., Sharma, V. P., Panwar, T. S., Beukes, J. P., van Zyl, P. G., Josipovic, M., Garland, R. M., Andreae, M. O., Pöschl, U., and Petzold, A.: Correction for a measurement artifact of the Multi-Angle Absorption Photometer (MAAP) at high black carbon mass concentration levels, Atmos. Meas. Tech., 6, 81-90, doi:10.5194/amt-6-81-2013, 2013

Laakso, L., Merikanto, J., Vakkari, V., Laakso, H., Kulmala, M., Molefe, M., Kgabi, N., Mabaso, D., Carslaw, K. S., Spracklen, D. V., Lee, L. A., Reddington, C. L., and Kerminen, V.-M.: Boundary layer nucleation as a source of new CCN in savannah environment, Atmos. Chem. Phys., 13, 1957-1972, doi:10.5194/acp-13-1957-2013, 2013

Paasonen,P., Asmi, A., Petäjä, T., Kajos, M. K., Äijälä, M., Junninen, H., Holst, T., Abbatt, J.P.D., Arneth, A., Birmili, W., van der Gon, H.D., Hamed, A., Hoffer, A., Laakso, L., Laaksonen, A., Leaitch, W.R., Plass-Dülmer, C. Pryor, S.C., Räisänen, P., Swietlicki, E., Wiedensohler, A., Worsnop, D.R., Kerminen, V.-M. and Kulmala, M., Warming-induced increase in aerosol number concentration likely to moderate climate change, Nature Geoscience, doi:10.1038/ngeo1800, 2013

Petäjä, T., Vakkari, V., Pohja, T., Nieminen, T., Laakso, H., Aalto, P.P., Keronen, P., Siivola, E., Kerminen, V.-M., Kulmala, M., Laakso, L., Transportable Aerosol Characterization Trailer with Trace Gas Chemistry: Design, Instruments and Verification, Aerosol and Air Quality Research, 13: 421–435, 2013

Vakkari, V., Beukes, J. P., Laakso, H., Mabaso, D., Pienaar, J. J., Kulmala, M., and Laakso, L.: Long-term observations of aerosol size distributions in semi-clean and polluted savannah in South Africa, Atmos. Chem. Phys., 13, 1751-1770, doi:10.5194/acp-13-1751-2013, 2013


Hirsikko, A., Vakkari, V., Tiitta, P., Manninen, H. E., Gagné, S., Laakso, H., Kulmala, M., Mirme, A., Mirme, S., Mabaso, D., Beukes, J. P., and Laakso, L., Characterisation of sub-micron particle number concentrations and formation events in the western Bushveld Igneous Complex, South Africa, Atmos. Chem. Phys., 12, 3951-3967, doi:10.5194/acp-12-3951-2012, 2012

Kerminen, V.-M., Paramonov, M., Anttila, T., Riipinen, I., Fountoukis, C., Korhonen, H., Asmi, E., Laakso, L., Lihavainen, H., Swietlicki, E., Svenningsson, B., Asmi, A., Pandis, S. N., Kulmala, M., and Petäjä, T.: Cloud condensation nuclei production associated with atmospheric nucleation: a synthesis based on existing literature and new results, Atmos. Chem. Phys., 12, 12037-12059, doi:10.5194/acp-12-12037-2012, 2012.

Venter, A.D., Vakkari,V., Beukes, J.P., van Zyl, P.G., Laakso, H., Mabaso, D., Tiitta, P., Josipovic, M., Kulmala, M., Pienaar, J.J., Laakso, L., An air quality assessment in the industrialised western Bushveld Igneous Complex, South Africa, South African Journal of Science, 108, No 9/10, 2012


Hirsikko, A., Nieminen, T., Gagné, S., Lehtipalo, K., Manninen, H. E., Ehn, M., Hõrrak, U., Kerminen, V.-M., Laakso, L., McMurry, P. H., Mirme, A., Mirme, S., Petäjä, T., Tammet, H., Vakkari, V., Vana, M., and Kulmala, M: , Atmospheric small ions: A review of observations, Atmos. Chem. Phys., 11, 767-798, doi:10.5194/acp-11-767-2011, 2011

Kulmala, M., Asmi, A., Lappalainen, H. K., Baltensperger, U., Brenguier, J.-L., Facchini, M. C., Hansson, H.-C., Hov, Ø., O'Dowd, C. D., Pöschl, U., Wiedensohler, A., Boers, R., Boucher, O., de Leeuw, G., Denier van der Gon, H. A. C., Feichter, J., Krejci, R., Laj, P., Lihavainen, H., Lohmann, U., McFiggans, G., Mentel, T., Pilinis, C., Riipinen, I., Schulz, M., Stohl, A., Swietlicki, E., Vignati, E., Alves, C., Amann, M., Ammann, M., Arabas, S., Artaxo, P., Baars, H., Beddows, D. C. S., Bergström, R., Beukes, J. P., Bilde, M., Burkhart, J. F., Canonaco, F., Clegg, S. L., Coe, H., Crumeyrolle, S., D'Anna, B., Decesari, S., Gilardoni, S., Fischer, M., Fjaeraa, A. M., Fountoukis, C., George, C., Gomes, L., Halloran, P., Hamburger, T., Harrison, R. M., Herrmann, H., Hoffmann, T., Hoose, C., Hu, M., Hyvärinen, A., Hõrrak, U., Iinuma, Y., Iversen, T., Josipovic, M., Kanakidou, M., Kiendler-Scharr, A., Kirkevåg, A., Kiss, G., Klimont, Z., Kolmonen, P., Komppula, M., Kristjánsson, J.-E., Laakso, L., Laaksonen, A., Labonnote, L., Lanz, V. A., Lehtinen, K. E. J., Rizzo, L. V., Makkonen, R., Manninen, H. E., McMeeking, G., Merikanto, J., Minikin, A., Mirme, S., Morgan, W. T., Nemitz, E., O'Donnell, D., Panwar, T. S., Pawlowska, H., Petzold, A., Pienaar, J. J., Pio, C., Plass-Duelmer, C., Prévôt, A. S. H., Pryor, S., Reddington, C. L., Roberts, G., Rosenfeld, D., Schwarz, J., Seland, Ø., Sellegri, K., Shen, X. J., Shiraiwa, M., Siebert, H., Sierau, B., Simpson, D., Sun, J. Y., Topping, D., Tunved, P., Vaattovaara, P., Vakkari, V., Veefkind, J. P., Visschedijk, A., Vuollekoski, H., Vuolo, R., Wehner, B., Wildt, J., Woodward, S., Worsnop, D. R., van Zadelhoff, G.-J., Zardini, A. A., Zhang, K., van Zyl, P. G., Kerminen, V.-M., S Carslaw, K., and Pandis, S. N.: General overview: European Integrated project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI) – integrating aerosol research from nano to global scales, Atmos. Chem. Phys., 11, 13061-13143, doi:10.5194/acp-11-13061-2011, 2011.

Vakkari, V., Laakso, H., Kulmala, M., Laaksonen, A., Mabaso, D., Molefe, M., Kgabi, N. and Laakso, L., New particle formation events in semi-clean South African savannah, Atmos. Chem. Phys., 11, 3333-3346, 2011.

2010 - 2009

Kerminen, V.-M., Petäjä, T., Manninen, H. E., Paasonen, P., Nieminen, T., Sipilä, M., Junninen, H., Ehn, M., Gagné, S., Laakso, L., Riipinen, I., Vehkamäki, H., Kurten, T., Ortega, I. K., Dal Maso, M., Brus, D., Hyvärinen, A., Lihavainen, H., Leppä, J., Lehtinen, K. E. J., Mirme, A., Mirme, S., Hõrrak, U., Berndt, T., Stratmann, F., Birmili, W., Wiedensohler, A., Metzger, A., Dommen, J., Baltensperger, U., Kiendler-Scharr, A., Mentel, T. F., Wildt, J., Winkler, P. M., Wagner, P. E., Petzold, A., Minikin, A., Plass-Dülmer, C., Pöschl, U., Laaksonen, A., and Kulmala, M.: Atmospheric nucleation: highlights of the EUCAARI project and future directions, Atmos. Chem. Phys., 10, 10829-10848, doi:10.5194/acp-10-10829-2010, 2010

Spracklen, D. V. , K. S. Carslaw, J. Merikanto, G. W. Mann, S. Pickering, J. A. Ogren, E. Andrews, U. Baltensperger, E. Weingartner, M. Boy, M. Kulmala, L. Laakso, H. Lihavainen, N. Kivekäs, N. Mihalopoulos, G. Kouvarakis, S. G. Jennings, W. Birmili, A. Wiedensohler, R. Weller, P. Laj, K. Sellegri, B. Bonn, and R. Krejci, Explaining global surface aerosol number concentrations in terms of primary emissions and particle formation, Atmos. Chem. Phys., 10, 4775-4793, doi:10.5194/acp-10-4775-2010, 2010


Laakso, L.,  H. Laakso, P.P. Aalto, P. Keronen, T. Petäjä, T. Nieminen, T. Pohja, E. Siivola, M. Kulmala, N. Kgabi, D. Phalatse, M. Molefe, D. Mabaso, K. Pienaar, V.-M. Kerminen," Basic characteristics of atmospheric particles, trace gases and meteorology in a relatively clean Southern African Savannah environment", Atmos. Chem. Phys., 8, 4823-4839, doi:10.5194/acp-8-4823-2008, 2008


Laakso, L.,  I.K. Koponen, P. Mönkkönen and M. Kulmala, Veli-Matti Kerminen, Birgit Wehner, Alfred Wiedensohler, Zhijun Wu, Min Hu, "Aerosol particles in the developing world; a comparison between New Delhi in India and Beijing in China", Water, Air and Soil Pollution, pages 5-20, DOI 10.1007/s11270-005-9018-5, 2006


Gierens, R., Understanding the evolution of the boundary layer over the Highveld, South Africa, MSc-thesis, University of Helsinki, 2015

Vakkari, V., On the sources of submicron aerosol particles in savannah: implications for climate and air quality, PhD-thesis, University of Helsinki, 2013

Jaars, K., Temporal assessment of volatile organic compounds at a site with high atmospheric variability in the North-West Province, MSc., 2013

Venter, A.: Air quality assessment of the industrialized western Bushveld Igneous Complex, NWU MSc, 2012

Swartz, J.-S.: Temporal assessment of anion and cation chemical species in atmospheric aerosols collected in the western Bushveld Igneous Complex, NWU Honns. BSc,, 2012

Charne du Preez: "Characterization of ion species and trace metals in aerosol samples collected near Potchefstroom", Honns. BSc, 2010

Wanda Engelbrecht: "Characterization of organic compounds in atmospheric aerosols utilizing 2D gas chromatography and mass spectrometry", MSc, 2009

Ville Vakkari: "Aerosol particles in a semi-clean savannah environment", MSc, 2009

Andrew Venter: "Evaluation of passive samplers for the measurements of inorganic gaseous species", Honns. B.Sc., 2009


Book chapters  (refereed)


Beukes, J.P., Venter, A.D., Josipovic, M., Van Zyl, P.G, Vakkari, V., Jaars, K., Dunn, M. and Laakso,L.
"Chapter 6 – Automated Continuous Air Monitoring" in  "Monitoring of Air Pollutants — Sampling, Sample Preparation and Analytical Techniques", edited by Patricia B.C. Forbes, Comprehensive Analytical Chemistry, Vol. 70, p. 183–208, 2015


Laakso, L., Beukes, J.P., Van Zyl, P.G., Pienaar, J.J., Josipovic, M., Venter, A., Jaars, K., Vakkari, V., Labuschagne, C., Chiloane, K., Tuovinen, J.-P., Chapter 20 - Ozone Concentrations and Their Potential Impacts on Vegetation in Southern Africa, In: R. Matyssek, N. Clarke, P. Cudlin, T.N. Mikkelsen, J.-P. Tuovinen, G. Wieser and E. Paoletti, Editor(s), Developments in Environmental Science, Elsevier, Volume 13, Pages 429-450, ISSN 1474-8177, ISBN 9780080983493, 2013


Laakso, L. and Pienaar, J.J., Chapter "Air pollution and the interactions between atmosphere, biosphere and the anthroposphere" in "Earth Observation and Environmental Change in South Africa", eds. Dr Johan Pauw, J., van Jaarsveld, A., Wessels, K. and Zietsman, L., 8 pages, in print, 2011.


Appearances in media

Beukes, J.P., Vakkari, V., van Zyl, P., Venter, A., Josipovic, M., Jaars, K., Tiitta, P., Siebert, S.,  Pienaar, J.J.,   Kulmala, M., and Laakso, L., "Welgegund - long-term land-atmosphere measurement platform in South Africa", Ileaps Newsletter #12, September 2012.

Laakso, L., Beukes, J.P., Van Zyl, P.G., Pienaar, J.J., Vakkari, V., Kulmala, M. New Southern African Supersite for Atmospheric Observations, MEGAPOLI-NL11-11-06, 2011

Yliopistolehti, "Johannesburgin tuulet", 2011/01

Laakso, L and Suni, T.,  " Suomalaisia aerosolimittauksia maailmalla – miksi ja kenen rahalla? Ilmansuojelulehti 2/2007

Helsingin Sanomat, "Suomalainen vaunu nuuskii savannin ilmaa", HS - Tiede & Luonto - 17.4.2007