Abstract. In the urban area of Guangzhou, observations on aerosol light extinction effect were conducted at a monitoring site of the South China Institute of Environmental Sciences (SCIES) during April 2009, July 2009, October 2009 and January 2010. The main goal of these observations is to recognise the impact of relative humidity (RH) and particles number distribution on aerosol light extinction. PM_2.5 was sampled by Model PQ200 air sampler; ions and OC/EC in PM_2.5 were identified by the Dionex ion chromatography and the DRI model 2001 carbon analyser, respectively; particles number size distribution was measured by TSI 3321 APS, while total light scattering coefficient was measured by TSI 3563 Nephelometer. Chemical composition of PM_2.5 was reconstructed by the model ISORROPIA II. As a result, possible major components in PM_2.5 were (NH₄)₂SO₄, Na₂SO₄, K₂SO₄, NH₄NO₃, HNO₃, water, POM and EC. Regarding ambient RH, mass concentration of PM_2.5 ranged from 26.1 to 279.1 μg m⁻³ and had an average of 94.8, 44.6, 95.4 and 130.8 μg m⁻³ in April, July, October and January, respectively. With regard to the total mass of PM_2.5, inorganic species, water, POM, EC and the Residual accounted for 34–47%, 19–31%, 14–20%, 6–8% and 8–17%, respectively. Under the assumption of "internal mixture", optical properties of PM_0.5–20 were estimated following the Mie Model. Optical refractive index, hygroscopic growth factor and the dry aerosol density required by the Mie Model were determined with an understanding of chemical composition of PM_2.5. With these three parameters and the validated particles number size distribution of PM_0.5–20, the temporal variation trend of optical property of PM_0.5–20 was estimated with good accuracy. The highest average of b_{ep,pm0.5–20} was 300 Mm⁻¹ in April while the lowest one was 78.6 Mm⁻¹ in July. Regarding size distribution of b_{ep,pm0.5–20}, peak value was almost located in the diameter range between 0.5 and 1.0 μm. Furthermore, hygroscopic growth of optical properties of PM_0.5–20 largely depended on RH. As RH increased, b_{ep,pm0.5–20} grew and favoured a more rapid growth when aerosol had a high content of inorganic water-soluble salts. Averagely, f_{bep,pm0.5–20} enlarged 1.76 times when RH increased from 20% to 90%. With regard to the temporal variation of ambient RH, f_{bep,pm0.5–20} was 1.29, 1.23, 1.14 and 1.26 on average in April, July, October and January, respectively.