RESEARCH ARTICLES

 

Physical, chemical and optical properties of aerosol particles collected over Cape Town during winter haze episodes

 

 

Patience Gwaze^{I, II}; Günter Helas^I; Harold J. Annegarn^II; Joachim Huth^III; Stuart J. Piketh^IV

^IDepartment of Biogeochemistry, Max Planck Institute for Chemistry, Mainz, Germany
^IIDepartment of Geography, Environmental Management and Energy Studies, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa
^IIIDepartment of Particle Chemistry, Max Planck Institute for Chemistry, Mainz, Germany
^IVClimatology Research Group, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa

 

 

---

ABSTRACT

Airborne measurements were conducted in the winter months of July and August 2003 over the metropolitan area of Cape Town to characterize physical, chemical and optical properties of aerosol particles during intense brown haze episodes. Particles were collected on highly temporally and spatially resolved samples and investigated using a high-resolution scanning electron microscope (SEM). From morphology and elemental composition, particles were categorized in terms of seven groups: aggregated soot particles, mineral dust, sulphates (SO²₄ ), sea-salt, tar balls/fly ash, rod-shaped particles associated with soot agglomerates, and those that could not be attributed to any of these groups. Refractive indices of aerosols were derived from chemical distributions obtained from SEM analysis and combined with in situ measurements of number-size distributions to determine optical properties of dry particles in the size range 0.1-3.5 pm. Particles exhibited marked spatial and temporal variability in chemical composition. They were externally mixed with highly absorbing soot particles. From number concentrations, light extinction and absorption coefficient ranges were σ_ep= 19-755 Mm^-1 and σ_ap= 7-103 Mm^-1, respectively (at wavelength λ = 550 nm). Single scattering albedo, ω₀, varied from 0.61 to 0.87 with a mean value of 0.72 ± 0.08; this value was much lower than generally reported in the literature, a result that was attributed to high concentrations of highly absorbing soot (fractional number concentrations of up to 46% were observed in the SEM). The haze could be attributed to extinction of light by fine aerosols composed mainly of anthropogenic particles. High extinction coefficients and low single scattering albedo computed here demonstrate quantitatively the contributions of particulate matter to visibility reduction and the brown haze phenomenon in Cape Town.

---

 

 

“Full text available only in PDF format”

 

 

References

1. Zunckel M., John J. and Naidoo M. (2006). The National Air Quality Management Programme (NAQMP), Output c.1., Air Quality Information Review. A Report for Directorate Environmental Information and Reporting, Environmental Quality and Protection Chief Directorate: Air Quality Management & Climate Change. Environmentek, CSIR, Pretoria.         [ Links ]

2. Statistics South Africa (2001). South African Census 2001. Online: http://www.statssa.gov.za/census01/htm]/default.asp        [ Links ]

3. Pineda C.A. and de Villiers M.G. (1995). Air pollution study in the Cape Town area by proton-induced X-ray emission spectroscopy. S. Afr. J. Chem. 48, 90-93.         [ Links ]

4. Wicking-Baird M.C., de Villiers M.G. and Dutkiewicz R.K. (1997). Cape Town Brown Haze Study, Report No. Gen. 182. Energy Research Institute, University of Cape Town.         [ Links ]

5. Jury M., Tegen A., Ngeleza E. and Dutoit M. (1990). Winter air pollution episodes over Cape Town. Bound. Lay. Meteorol. 53, 1-20.         [ Links ]

6. Piketh S.J., Otter L.B., Burger R.P, Walton N., van Nierop M.A., Bigala T, Chiloane K.E. and Gwaze P (2004). Cape Town Brown Haze II Report. Climatology Research Group, University of the Witwatersrand, Johannesburg.         [ Links ]

7. City of Cape Town (2002). State of Environment Report Year 5, 2002, Air Quality Monitoring Network. Online: http://www.capetown.gov.za/airqual/        [ Links ]

8. Nel A. (2005). Air pollution-related illness: effects of particles. Science 308, 804-806.         [ Links ]

9. Eck TF., Holben B.N., Dubovik O., Smirnov A., Goloub P, Chen H.B., Chatenet B., Gomes L., Zhang X.Y., Tsay S-C., Ji Q., Giles D. and Slutsker I. (2005). Columnar aerosol optical properties at AERONET sites in central eastern Asia and aerosol transport to the tropical mid-Pacific. J. Geophys. Res. 110, D06202, doi:10.1029/2004JD005274.         [ Links ]

10. Gwaze P. (2006). Physical and chemical properties of aerosol particles in the troposphere: an approach from microscopy methods. Ph.D. thesis, University of the Witwatersrand, Johannesburg.         [ Links ]

11. Walton N.M. (2005). Characterisation of Cape Town Brown Haze. M.Sc. thesis, University of the Witwatersrand, Johannesburg.         [ Links ]

12. Chiloane K.E. (2005). Volatile organic compounds (VOCs) analysis from Cape Town Brown Haze II Study. M.Sc. thesis, University of the Witwatersrand, Johannesburg.         [ Links ]

13. Ebert M., Weinbruch S., Rausch A., Gorzawski G., Hoffmann P., Wex H. and Helas G. (2002). Complex refractive index of aerosols during LACE 98 as derived from the analysis of individual particles. J. Geophys. Res. 107, 8121, doi:10.1029/2000JD000195.         [ Links ]

14. Annegarn H.J., Flanz M., Kenntner T., Kneen M.A., Helas G. and Piketh S.J. (1996). Airborne streaker sampling for PIXE analysis. Nucl. Instr. Meth. B 109/110, 548-550.         [ Links ]

15. Liu Y. and Daum PH. (2000). The effect of refractive index on size distributions and light scattering coefficients derived from optical particle counters. J. Aerosol. Sci. 31, 945-957.         [ Links ]

16. Pósfai M., Gelencsér A., Simonics R., Arató K., Li J., Hobbs P.V and Buseck PR. (2004). Atmospheric tar balls: particles from biomass and biofuel burning. J. Geophys. Res. 109, D06213, doi:10.1029/2003JD004169.         [ Links ]

17. Mallet M., Roger J.C., Despiau S., Putaud J.P. and Dubovik O. (2004). A study of the mixing state of black carbon in urban zone. J. Geophys. Res. 109, D04202, doi:10.1029/2003JD003940.         [ Links ]

18. Hasegawa S. and Ohta S. (2002). Some measurements of the mixing state of soot-containing particles at urban and non-urban sites. Atmos. Environ. 36, 3899-3908.         [ Links ]

19. Neusüß C., Wex H., Birmili W., Wiedensohler A., Koziar C., Busch B., Brüggemann E., Gnauk T., Ebert M. and Covert D.S. (2002). Characterization and parameterization of atmospheric particle number-, mass-, and chemical-size distributions in central Europe during LACE 98 and MINT. J. Geophys. Res. 107, 8127, doi:10.1029/2001JD000514.         [ Links ]

20. Ebert M., Weinbruch S., Hoffmann P. and Ortner H.M. (2004). The chemical composition and complex refractive index of rural and urban influenced aerosols determined by individual particle analysis. Atmos. Environ. 38, 6531-6545.         [ Links ]

21. Li J., Pósfai M., Hobbs P.V and Buseck PR. (2003). Individual aerosol particles from biomass burning in southern Africa: 2. Composition and aging of inorganic particles. J. Geophys. Res. 108, 8484, doi:10.1029/2002JD002310.         [ Links ]

22. Baumbach G. (1996). Air Quality Control. Springer-Verlag, Berlin, Heidelberg.         [ Links ]

23. Buseck P.R. and Pósfai M. (1999). Airborne minerals and related aerosol particles: effects on climate and the environment. Proc. Natl Acad. Sci. U.S.A. 96, 3372-3379.         [ Links ]

24. Ikegami M., Okada K., Zaizen Y., Tsutsumi Y., Makino Y., Jensen J.B. and Gras J.L. (2004). The composition of aerosol particles in the middle troposphere over the western Pacific Ocean: aircraft observations from Australia to Japan, January 1994. Atmos. Environ. 38, 5945-5956.         [ Links ]

25. Hara K., Yamagata S., Yamanouchi T., Sato K., Herber A., Iwasaka Y., Nagatani M. and Nakata H. (2003). Mixing states of individual aerosol particles in spring Arctic troposphere during ASTAR 2000 campaign. J. Geophys. Res. 108, 4209, doi:10.1029/2002JD002513.         [ Links ]

26. Seinfeld J.H. and Pandis S.N. (1998). Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. Wiley-Interscience, New York.         [ Links ]

27. Norman A-L., Belzer W. and Barrie L. (2004). Insights into the biogenic contribution to total sulphate in aerosol and precipitation in the Fraser Valley afforded by isotopes of sulphur and oxygen. J. Geophys. Res. 109, D05311, doi:10.1029/2002JD003072.         [ Links ]

28. South African Weather Service (2003). Daily Weather Bulletin August 2003.         [ Links ]

29. Baumgardner D., Raga G.B., Kok G., Ogren J., Rosas I., Báez A. and Novakov T (2000). On the evolution of aerosol properties at a mountain site above Mexico City. J. Geophys. Res. 105, 22243-22253.         [ Links ]

30. Horvath H. (1993). Atmospheric light absorption - a review. Atmos. Environ. A 27, 293-317.         [ Links ]

31. d'Almeida G., Koepke P and Shettle E. (1991). Atmospheric Aerosols. Global Climatology and Radiative Characteristics. A. Deepak, Hampton.         [ Links ]

32. Tegen I., Lacis A.A. and Fung I. (1996). The influence on climate forcing of mineral aerosols from disturbed soils. Nature 380, 419-422.         [ Links ]

33. Horvath H. (1998). Influence of atmospheric aerosols upon the global radiation balance. In Atmospheric Particles, eds R.M. Harrison and R.E. van Grieken, pp. 543-596. Wiley-Interscience, New York.         [ Links ]

34. Formenti P., Boucher O., Reiner T., Sprung D., Andreae M.O., Wendisch M., Wex H., Kindred D., Tzortziou M., Vasaras A. and Zerefos C. (2002). STAAARTE-MED 1998 summer airborne measurements over the Aegean Sea 2. Aerosol scattering and absorption, and radiative calculations. J. Geophys. Res. 107, 4451, doi:10.1029/2001JD001536.         [ Links ]

35. Wiscombe W.J. (1980). Improved Mie scattering algorithms. Appl. Opt. 19, 1505-1509.         [ Links ]

36. Mallet M., Roger J.C., Despiau S., Dubovik O. and Putaud J.P. (2003). Microphysical and optical properties of aerosol particles in urban zone during ESCOMPTE. Atmos. Res. 69, 73-97.         [ Links ]

37. Carrico C.M., Bergin M.H., Xu J., Baumann K. and Maring H. (2003). Urban aerosol radiative properties: measurements during the 1999 Atlanta Supersite Experiment. J. Geophys. Res. 108, 8422, doi:10.1029/2001JD001222.         [ Links ]

38. Mallet M., Van Dingenen R., Roger J.C., Despiau S. and Cachier H. (2005). In situ airborne measurements of aerosol optical properties during photochemical pollution events. J. Geophys. Res. 110, D03205, doi:10.1029/2004JD005139.         [ Links ]

39. Malm WC., Sisler J.F., Huffman D., Eldred R.A. and Cahill TA. (1994). Spatial and seasonal trends in particle concentration and optical extinction in the United Sates. J. Geophys. Res. 99, 1347-1370.         [ Links ]

40. Bergin M.H., Cass G.R., Xu J., Fang C., Zeng L.M., Yu T., Salmon L.G., Kiang C.S., Tang X.Y., Zhang Y.H. and Chameides W.L. (2001). Aerosol radiative, physical, and chemical properties in Beijing during June 1999. J. Geophys. Res. 106, 17969-17980.         [ Links ]

41. Reid J.S., Eck T.F., Christopher S.A., Hobbs P.V. and Holben B. (1999). Use of the Angstrom exponent to estimate variability of optical and physical properties of aging smoke particles in Brazil. J. Geophys. Res. 104, 27473-27489.         [ Links ]

42. Tang I N. (1996). Chemical and size effects of hygroscopic aerosols on light scattering coefficients. J. Geophys. Res. 101, 19245-19250.         [ Links ]

43. Weingartner E., Burtscher H. and Baltensperger U. (1997).Hygroscopic properties of carbon and diesel soot particles. Atmos. Environ. 31, 2311-2327.         [ Links ]

44. Bundke U., Hänel G., Horvath H., Kaller W., Seidl S., Wex H., Wiedensohler A., Wiegner M. and Freudenthaler V. (2002). Aerosol optical properties during the Lindenberg Aerosol Characterization Experiment (LACE 98). J. Geophys. Res. 107, 8123, doi:10.1029/2000JD000188.         [ Links ]

45. Reus de M., Formenti P, Ström J., Krejci R., Müller D., Andreae M.O. and Lelieveld J. (2002). Airborne observations of dry particle absorption and scattering properties over the northern Indian Ocean. J. Geophys. Res. 107, 8002, doi:10.1029/2002JD002304.         [ Links ]

46. Russell P.B., Redemann J., Schmid B., Bergstrom R.W., Livingston J.M., McIntosh D.M., Ramirez S.A., Hartley S., Hobbs PV., Quinn P.K., Carrico C.M., Rood M.J., Öström E., Noone K.J., Hoyningen-Huene von W. and Remer L. (2002). Comparison of aerosol single scattering albedos derived by diverse techniques in two North Atlantic experiments. J. Atmos. Sci. 59, 609-619.         [ Links ]

47. Kittelson D.B. (1998). Engines and nanoparticles: a review. J. Aerosol Sci. 29, 575-588.         [ Links ]

 

 

Received 8 September 2006.
Accepted 8 December 2006.

 

 

* Author for correspondence. E-mail: pgwaze2@yahoo.com