Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 3 1 2003 10.5194/acpd-3-615-2003 http://www.atmos-chem-phys-discuss.net/3/615/2003/ http://www.atmos-chem-phys-discuss.net/3/615/2003/acpd-3-615-2003.html http://www.atmos-chem-phys-discuss.net/3/615/2003/acpd-3-615-2003.pdf 615 653 2003-02-10 Haze in the Klang Valley of Malaysia M. D. Keywood G. P. Ayers J. L. Gras R. Boers C. P. Leong CSIRO Atmospheric Research, Australia Atmospheric Research Division, KNMI, The Netherlands Malaysian Meteorological Service, Malaysia now at: California Institute of Technology, USA Continuous measurements of dry aerosol light scattering (Bsp) were made at two sites in the Klang Valley of Malaysia between December 1998 and December 2000. In addition 24-h PM2.5 samples were collected on a one-day-in-six cycle and the chemical composition of the aerosol was determined. Periods of excessive haze were defined as 24-h average Bsp values greater than 150 Mm^-1 and these occurred on a number of occasions, between May and September 1999, during May 2000, and between July and September 2000. The evidence for smoke being a significant contributor to aerosol during periods of excessive haze is discussed and includes features of the aerosol chemistry, the diurnal cycle of Bsp, and the coincidence of forest fires on Sumatra during the southwest (SW) monsoon period, as well as transport modelling for one week of the southwest Monsoon of 2000. The study highlights that whilst transboundary smoke is a major contributor to poor visibility in the Klang Valley, smoke from fires on Peninsular Malaysia is also a contributor, and at all times, the domestic source of secondary particle production is present.