ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-14-4263-2014 Atmospheric boundary layer top height in South Africa: measurements with lidar and radiosonde compared to three atmospheric models Korhonen K. 1 2 Giannakaki E. 1 Mielonen T. https://orcid.org/0000-0003-1496-097X 1 Pfüller A. 1 Laakso L. 3 4 Vakkari V. 3 5 Baars H. https://orcid.org/0000-0002-2316-8960 6 Engelmann R. https://orcid.org/0000-0003-4225-9961 6 Beukes J. P. 4 Van Zyl P. G. https://orcid.org/0000-0003-1470-3359 4 Ramandh A. 7 Ntsangwane L. 8 Josipovic M. 4 Tiitta P. https://orcid.org/0000-0003-4694-3648 2 4 9 Fourie G. 7 Ngwana I. 8 Chiloane K. 10 Komppula M. 1 Finnish Meteorological Institute, P.O. Box 1627, 70211, Kuopio, Finland Department of Applied Physics, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland Finnish Meteorological Institute, P.O. Box 503, 00101, Helsinki, Finland Unit for Environmental Sciences and Management, North-West University, Potchefstroom, South Africa Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland Leibniz Institute for Tropospheric Research, Permoserstrasse 15, 04318, Leipzig, Germany Sasol Technology R{&}D (Pty) Ltd., P.O. Box 1, Sasolburg, South Africa South African Weather Service, Pretoria, South Africa Department of Environmental Sciences, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland Eskom Holdings SOC Ltd; Sustainability Division; Research, Testing and Development, South Africa 30 04 2014 14 8 4263 4278 Copyright: © 2014 K. Korhonen et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/14/4263/2014/acp-14-4263-2014.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/14/4263/2014/acp-14-4263-2014.pdf

Atmospheric lidar measurements were carried out at Elandsfontein measurement station, on the eastern Highveld approximately 150 km east of Johannesburg in South Africa throughout 2010. The height of the planetary boundary layer (PBL) top was continuously measured using a Raman lidar, Polly<sup>XT</sup> (<b>PO</b>rtab<b>L</b>e <b>L</b>idar s<b>Y</b>stem e<sup>XT</sup>ended). High atmospheric variability together with a large surface temperature range and significant seasonal changes in precipitation were observed, which had an impact on the vertical mixing of particulate matter, and hence, on the PBL evolution. The results were compared to radiosondes, CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) space-borne lidar measurements and three atmospheric models that followed different approaches to determine the PBL top height. These models included two weather forecast models operated by ECMWF (European Centre for Medium-range Weather Forecasts) and SAWS (South African Weather Service), and one mesoscale prognostic meteorological and air pollution regulatory model TAPM (The Air Pollution Model). The ground-based lidar used in this study was operational for 4935 h during 2010 (49% of the time). The PBL top height was detected 86% of the total measurement time (42% of the total time). Large seasonal and diurnal variations were observed between the different methods utilised. High variation was found when lidar measurements were compared to radiosonde measurements. This could be partially due to the distance between the lidar measurements and the radiosondes, which were 120 km apart. Comparison of lidar measurements to the models indicated that the ECMWF model agreed the best with mean relative difference of 15.4%, while the second best correlation was with the SAWS model with corresponding difference of 20.1%. TAPM was found to have a tendency to underestimate the PBL top height. The wind speeds in the SAWS and TAPM models were strongly underestimated which probably led to underestimation of the vertical wind and turbulence and thus underestimation of the PBL top height. Comparison between ground-based and satellite lidar shows good agreement with a correlation coefficient of 0.88. On average, the daily maximum PBL top height in October (spring) and June (winter) was 2260 m and 1480 m, respectively. To our knowledge, this study is the first long-term study of PBL top heights and PBL growth rates in South Africa.

 References Althausen, D., Engelmann, R., Baars, H., Heese, B., Ansmann, A., Müller, D., and Komppula, M.: Portable Raman Lidar PollyXT for Automated Profiling of Aerosol Backscatter, Extinction, and Depolarization, J. Atmos. Ocean. Technol., 26, 2366–2378, 2009. Amiridis, V., Melas, D., Balis, D. S., Papayannis, A., Founda, D., Katragkou, E., Giannakaki, E., Mamouri, R. E., Gerasopoulos, E., and Zerefos, C.: Aerosol Lidar observations and model calculations of the Planetary Boundary Layer evolution over Greece, during the March 2006 Total Solar Eclipse, Atmos. Chem. Phys., 7, 6181–6189, <a href="http://dx.doi.org/10.5194/acp-7-6181-2007">https://doi.org/10.5194/acp-7-6181-2007</a>, 2007. Ansmann, A., Riebesell, M., Wandinger, U., Weitkamp, C., Voss, E., Lahmann, W., and Michaelis, W.: Combined Raman elastic-backscatter lidar for vertical profiling of moisture, aerosol extinction, backscatter, and lidar ratio, Appl. Phys. B55, 18–28, 1992. Ansmann, A., Baars, H., Tesche, M., Müller, D., Althausen, D., Engelmann, R., Pauliquevis, T., and Artaxo, P.: Dust and smoke trasport from Africa to South America: Lidar profiling over Cape Verde and the Amazon rainforest, Geophys. Res. Lett., 36, L11802, <a href="http://dx.doi.org/10.1029/2009GL037923">https://doi.org/10.1029/2009GL037923</a>, 2009. Ansmann, A., Petzold, A., Kandler, K., Tegen, I., Wendisch, M., Müllet, D., Weinzierl, B., Müller, T., and Heintzenberg, J.: Saharan Mineral Dust Experiments SAMUM-1 and SAMUM-2: what have we learned? Tellus 63B, 403–429, <a href="http://dx.doi.org/10.1111/j.1600-0889.2011.00555.x">https://doi.org/10.1111/j.1600-0889.2011.00555.x</a>, 2011. Baars, H., Ansmann, A., Engelmann, R., and Althausen, D.: Continuous monitoring of the boundary-layer top with lidar, Atmos. Chem. Phys., 8, 7281–7296, <a href="http://dx.doi.org/10.5194/acp-8-7281-2008">https://doi.org/10.5194/acp-8-7281-2008</a>, 2008. Bethlehem, L. and Goldblatt, M.: The Bottom Line, Industry and the Environment in South Africa. University of Cape Town Press, International Development Research Centre, ISBN 0-88936-830-9, 1997. Brooks, I. M.: Finding Boundary Layer Top: Application of a Wavelet Covariance Transform to Lidar Backscatter Profiles, J. Atmos. Ocean. Technol., 20, 1092–1195, 2003. Campbell, J. R., Welton, E., Spinhirne, J. D., Ji, Q., Tsay, S. C., Piketh, S. P., and Barenbrug, M.: Micropulse lidar observations of tropospheric aerosols over northeastern South Africa during the ARREX and SAFARI 2000 dry season experiments, J. Geophys. Res., 108, 8497–8530, <a href="http://dx.doi.org/10.1029/2002JD002563">https://doi.org/10.1029/2002JD002563</a>, 2003. Chen, W., Kuze, H., Uchiyama, A., Suzuki, Y., and Takeuchi, N.: One-year observation of urban mixed layer characteristics at Tsukuba, Japan using a micro pulse lidar, Atmos. Environ., 35, 4273–4280, <a href="http://dx.doi.org/10.1016/S1352-2310(01)00181-9">https://doi.org/10.1016/S1352-2310(01)00181-9</a>, 2001. Cimini, D., De Angelis, F., Dupont, J.-C., Pal, S., and Haeffelin, M.: Mixing layer height retrievals by multichannel microwave radiometer observations, Atmos. Meas. Tech., 6, 2941–2951, <a href="http://dx.doi.org/10.5194/amt-6-2941-2013">https://doi.org/10.5194/amt-6-2941-2013</a>, 2013. ECMWF: IFS Documentation – Cy36r1, part 5: Ensemble prediction system, ECWMF, 2010a. ECMWF: IFS Documentation – Cy36r1, part 4: Physical processes, ECMWF, 2010b. ECMWF: IFS Documentation – Cy36r1, ECMWF, 2010c. Formenti, P., Winkler, H., Fourie, P., Piketh, S., Makgopa, B., Helas, G., and Andreae, M. O.: Aerosol optical depth over a remote semi-arid region of South Africa from spectral measurements of the daytime solar extinction and the nighttime stellar extinction, Atmos. Res., 62, 11–32, <a href="http://dx.doi.org/10.1016/S0169-8095(02)00021-2">https://doi.org/10.1016/S0169-8095(02)00021-2</a>, 2002. Formenti, P., Elbert, W., Maenhaut, W., Haywood, J., Osborne, S., and Andreae, M. O.: Inorganic and carbonaceous aerosols during the Southern African Regional Science Initiative (SAFARI 2000) experiment: Chemical characteristics, physical properties, and emission data for smoke from African biomass burning, J. Geophys. Res., 108, 8488, <a href="http://dx.doi.org/10.1029/2002JD002408">https://doi.org/10.1029/2002JD002408</a>, 2003. Granados-Muñoz, M. J., Navas-Guzmán, F., Bravo-Aranda, J. A., Guerrero-Rascado, J. L., Lyamani, H., Fernandez-Galvez, J., and Alados-Arboledas, L.: Automatic determination of the planetary boundary layer height using lidar: one-year analysis over Southeastern Spain., J. Geophys. Res., 117, D18208, <a href="http://dx.doi.org/10.1029/2012JD017524">https://doi.org/10.1029/2012JD017524</a>, 2012. Groß, S., Gasteiger, J., Freudenthaler, V., Wiegner, M., Geiß, A., Schladitz, A., Toledano, C., Kandler, K., Tesche, M., Ansmann, A. and Wiedensohler, A.: Characterization of the planetary boundary layer during SAMUM-2 by means of lidar measurements. Tellus 63B, 695–705, <a href="http://dx.doi.org/10.1111/j.1600-0889.2011.00557.x">https://doi.org/10.1111/j.1600-0889.2011.00557.x</a>, 2011. Haeffelin, M., Angelini, F., Morille, Y., Martucci, G., Frey, S., Gobbi, G. P., Lolli, S., O'Dowd, C. D., Sauvage, L., Xueref-Rémy, I., Wastine, B., Feist, D. G.: Evaluation of Mixing-Height Retrievals from Automatic Profiling Lidars and Ceilometers in View of Future Integrated Networks in Europe, Bound.-Lay. Meteorol., 143, 49–75, <a href="http://dx.doi.org/10.1007/s10546-011-9643-z">https://doi.org/10.1007/s10546-011-9643-z</a>, 2012. Hennemuth, B. and Lammert, A.: Determination of the atmospheric boundary layer height from radiosonde and lidar backscatter, Bound.-Lay. Meteorol., 120, 181–200, <a href="http://dx.doi.org/10.1007/s10546-005-9035-3">https://doi.org/10.1007/s10546-005-9035-3</a>, 2006. Hurley, P.: TAPM V4. Part 1: Technical Description. CSIRO Marine and Atmospheric Research Paper No. 25, October 2008. ISBN: 978-1-921424-71-7, 2008. Hurley, P. J., Physick, W. L., and Luhar, A. K.: TAPM: a practical approach to prognostic meteorological and air pollution modelling, Environ. Model. Softw., 20, 737–752, 2005a. Hurley, P., Physick, W., Luhar, A., and Edwards, M.: The Air Pollution Model (TAPM) Version 3. Part 2: Summary of some verification studies, CSIRO, Atmos. Res., 72, 20–36, 2005b. Hurley, P., Edwards, M., and Luhar, A.: TAPM V4. Part 2: Summary of Some Verification Studies. CSIRO Marine and Atmospheric Research Paper No. 26, October 2008, ISBN: 978-1-921424-72-4, 2008. Hänel, A., Baars, H., Althausen, D., Ansmann, A., Engelmann, R., and Sun, J. Y.: One-year aerosol profiling with EUCAARI Raman lidar at Shangdianzi GAW station: Beijing plume and seasonal variations, J. Geophys. Res., 117, D13201, <a href="http://dx.doi.org/10.1029/2012JD017577">https://doi.org/10.1029/2012JD017577</a>, 2012. Johansson, C. and Bergström, H.: An auxiliary tool to determine the height of the boundary layer, Bound.-Lay. Meteorol., 115, 423–432, <a href="http://dx.doi.org/10.1007/s10546-004-1424-5">https://doi.org/10.1007/s10546-004-1424-5</a>, 2005. Jordan, N. S., Hoff, R. M., and Bacmeister, J. T.: Validation of Goddard Earth Observing System-version 5 MERRA planetary boundary layer heights using CALIPSO, J. Geophys. Res., 115, D24218, <a href="http://dx.doi.org/10.1029/2009JD013777">https://doi.org/10.1029/2009JD013777</a>, 2010. 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, <a href="http://dx.doi.org/10.5194/acp-11-13061-2011">https://doi.org/10.5194/acp-11-13061-2011</a>, 2011. Laakso, L., Vakkari, V., Virkkula, A., Laakso, H., Backman, J., Kulmala, M., Beukes, J. P., van Zyl, P. G., Tiitta, P., Josipovic, M., Pienaar, J. J., Chiloane, K., Gilardoni, S., Vignati, E., Wiedensohler, A., Tuch, T., Birmili, W., Piketh, S., Collett, K., Fourie, G. D., Komppula, M., Lihavainen, H., de Leeuw, G., and Kerminen, V.-M.: South African EUCAARI measurements: seasonal variation of trace gases and aerosol optical properties, Atmos. Chem. Phys., 12, 1847–1864, <a href="http://dx.doi.org/10.5194/acp-12-1847-2012">https://doi.org/10.5194/acp-12-1847-2012</a>, 2012. Lai, L. and Cheng, W. L.: Air quality influenced by urban heat island coupled with synoptic weather patterns, Sci. Total Environ., 4, 2724–2733, 2009. Landman, S., Engelbrecht, F. A., Engelbrecht, C. J., Dyson, L. L., and Landman, W.: A short-range prediction system based on a multi-model approach, Water SA, 38, ISSN 0378-4738, 2012. Liu, L.: Improving GCM Aerosol Climatology using satellite and ground based measurements, paper presented at 15th ARM Science Team Meeting, Atmos. Radiat. Meas. (ARM) Program, Daytona Beach, Florida, 14–18 March, 2005. Lourens, A. S. M., Beukes, J. P., van Zyl, P. G., Fourie, G. D., Burger, J. W., Pienaar, J. J., Read, C. E. and Jordaan, J. H. L., Spatial and Temporal assessment of Gaseous Pollutants in the Mpumalanga Highveld of South Africa, South Afr. J. Sci., 107, 8 pp. <a href="http://dx.doi.org/10.4102/sajs.v107i1/2.269">https://doi.org/10.4102/sajs.v107i1/2.269</a>, 2011. Lourens, S. M., Butler, T. M., Beukes, J. P., van Zyl, P. G., Beirle, S., Wagner, T., Heue, K.-P., Pienaar, J. J., Fourie, G. D., and Lawrence, M. G.: Re-evaluating the NO<sub>2</sub> hotspot over the South African Highveld, South Afr. J. Sci., 108, 6 pp.. <a href="http://dx.doi.org/10.4102/sajs.v108i11/12.1146">https://doi.org/10.4102/sajs.v108i11/12.1146</a>, 2012. Luhar, A. K. and Hurley P. J.: Application of a prognostic model TAPM to sea-breeze flows, surface concentrations, and fumigating plumes, Environ. Model. Softw., 19, 591–601, 2004. Matthias, V., Balis, D., Bösenberg, J., Eixmann, R., Iarlori, M., Komguem, L., Mattis, I., Papayannis, A., Pappalardo, G., Perrone, M. R., and Wang, X.: Vertical aerosol distribution over Europr: Statistical analysis of Raman lidar data from 10 European Aerosol Research Lidar Network (EARLINET) stations, J. Geophys. Res., 109, D18201, <a href="http://dx.doi.org/10.1029/2004JD004638">https://doi.org/10.1029/2004JD004638</a>, 2004. Menut, L., Flamant, C., Pelon, J., and Flamant, P. H.: Urban boundary-layer height determination from lidar measurements over the Paris area, Appl. Opt., 38, 6, 945–954, 1999. Piketh, S. J., Annegarn, H. J., and Tyson, P. D.: Lower tropospheric aerosol loadings over South Africa: The relative contribution of aeolian dust, industrial emissions, and biomass burning, J. Geophys. Res., 104, 1597–1607, <a href="http://dx.doi.org/10.1029/1998JD100014">https://doi.org/10.1029/1998JD100014</a>, 1999. Piketh, S. J., Swap, R. J., Maenhaut, W., Annegarn, H. J., and Formenti, P.: Chemical evidence of long-range atmospheric transport over southern Africa, J. Geophys. Res., 107, 4817, <a href="http://dx.doi.org/10.1029/2002JD002056">https://doi.org/10.1029/2002JD002056</a>, 2002. Queface, A. J., Piketh, S. J., Eck, T. F., Tsay, S. C., and Mavume, A. F.: Climatology of aerosol optcal properties in Southern Africa, Atmos. Environ., 45, 2910–2921, 2011. Raghunandan, A., Scott, G., Zunckel, M., and Carter, W.: TAPM verification in South Africa: modelling surface meteorology at Alexander Bay and Richards Bay, Report done on behalf CSIR Natural Resources and the Environment, Congella, 2008. Seibert, P., Beyrich, F., Gryning, S.-E., Joffre, S., Rasmussen, A., and Tecier, P.: Review and intercomparison of operational methods for the determination of the mixing height, Atmos. Environ., 34, 1001–1027, 2000. Seidel, D. J., Zhang, Y., Beljaars, A., Golaz, J.-C., Jacobson, A. R., and Medeiros, B.: Climatology of the planetary boundary layer over the continental United States and Europe, J. Geophys. Res., 117, D17106, <a href="http://dx.doi.org/10.1029/2012JD018143">https://doi.org/10.1029/2012JD018143</a>, 2012. Stull, R. B.: An introduction to boundary layer meteorology, Kluwer Academic Publishers, ISBN: 90-277-2768-6, 1988. Stull, R. B.: Meteorology for Scientists and Engineers, Second Edition, Brooks&Cole, ISBN: 0-534-37214-7, 2000. Swap, R. J., Annegarn, H. J., Suttles, J. T., King, M. D., Platnick, S., Privette, J. L., and Scholes, R. J.: Africa burning: A thematic analysis of the Southern African Regional Science Initiative (SAFARI 2000), J. Geophys. Res., 108, 8465, <a href="http://dx.doi.org/10.1029/2003JD003747">https://doi.org/10.1029/2003JD003747</a>, 2003. Troen, I. B. and Mahrt, L.: A simple model of the Atmospheric Boundary Layer; sensitivity to surface evaporation. Bound.-Lay. Meteorol., 37, 129–148, 1986. Tsaknakis, G., Papayannis, A., Kokkalis, P., Amiridis, V., Kambezidis, H. D., Mamouri, R. E., Georgoussis, G., and Avdikos, G.: Inter-comparison of lidar and ceilometer retrievals for aerosol and Planetary Boundary Layer profiling over Athens, Greece. Atmos. Meas. Tech., 4, 1261–1273, <a href="http://dx.doi.org/10.5194/amt-4-1261-2011">https://doi.org/10.5194/amt-4-1261-2011</a>, 2011. van Pul, W. A. J., Holtslag, A. A. M., and Swart, D. P. J.: A comparison of atmospheric boundary layer heights inferred routinely from lidar and radiosondes at noon, Bound.-Layer Meteorol., 68, 173–191, 1994. Vaughan, M. A., Winker, D. M., and Powell, K. A.: CALIOP Algorithm Theoretical Basis Document Part 2: Feature Detection and Layer Properties Algorithms, available at: <a href="http://www-calipso.larc.nasa.gov/resources/pdfs/PC-SCI-202_Part2_rev1x01.pdf">http://www-calipso.larc.nasa.gov/resources/pdfs/PC-SCI-202_Part2_rev1x01.pdf</a> (last access: 28 June 2013), 2005. 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., and Laakso, L.: An air quality assessment in the industrialised western Bushveld Igneous Complex, South Africa, S. Afr. J. Sci., 108, 84–93, <a href="http://dx.doi.org/10.4102/sajs.v108i9/10.1059">https://doi.org/10.4102/sajs.v108i9/10.1059</a>, 2012. Winker, D., Hostetler, C., and Hunt, W.: CALIOP: The CALIPSO Lidar, Proc. 22nd International Laser Radar Conference (ESASP 561), Matera, Italy, 941–944, Vol. 6409 (SPIE, Bellingham, WA 2006), 640902, 2004. Winker, D., Vaughan, M., and Hunt, W., The CALIPSO mission and initial results from CALIOP, Proc. SPIE, 6409, <a href="http://dx.doi.org/10.1117/12.698003">https://doi.org/10.1117/12.698003</a>, 2006. Winker, D. M., Hunt, W. H., and McGill, M. J.: Initial performance assessment of CALIOP, Geophys. Res. Lett., 34, L19803, <a href="http://dx.doi.org/10.1029/2007GL030135">https://doi.org/10.1029/2007GL030135</a>, 2007. Zawar-Reza, P. and Sturman, A.: Application of airshed modelling to the implementation of the New Zealand National Environmental Standards for air quality, Atmos. Environ., 42, 8785–8794, 2008. Ångström, A.: On the atmospheric transmission of Sun radiation and on dust in the air, Geograf. Ann., 11, 156–166, 1929.