Impact of PM_2.5 chemical compositions on aerosol light scattering in Guangzhou - the largest megacity in South China

Daily PM_2.5 samples were collected in Guangzhou - the largest megacity in South China, for a period of one month in each season during 2009-2010. Mass concentrations of water-soluble inorganic ions, organic carbon (OC) and elemental carbon (EC) in PM_2.5 were determined, and aerosol scattering coefficient (b_sp) was synchronously measured. The daily PM_2.5 mass concentrations ranged from 21.0 to 213.6μgm^-3 with an annual average of 76.8±41.5μgm^-3. The highest seasonal average PM_2.5 was observed in winter (103.3±50.1μgm^-3) and the lowest in summer (38.6±15.7μgm^-3). Annual average PM_2.5 mass scattering efficiency (MSE) was 3.5±0.9m²g^-1, with obvious seasonal variations in sequence of autumn (4.5±0.2m²g^-1)>winter (3.9±0.5m²g^-1)>spring (3.0±0.4m²g^-1)>summer (2.3±0.3m²g^-1).To determine the relationship between b_sp and the chemical components of PM_2.5, b_sp was reconstructed in each season using the original IMPROVE formula with a modification of including sea salt aerosols. The estimated b_sp using this method was 22±28% smaller on annual average compared to the measurements. Multiple linear regression of measured b_sp against (NH₄)₂SO₄, NH₄NO₃, OM (Organic Mass), SS (Sea Salt), FS (Fine Soil), and CM (Coarse Mass) were also performed in all the four seasons. The estimated b_sp from using the regression equation was 4±12% larger than the measured values. On average, (NH₄)₂SO₄, NH₄NO₃, OM, SS, FS and CM accounted for 50±11%, 18±10%, 19±5%, 5±4%, 3±2% and 5±6%, respectively, of the estimated b_sp.