Impact of size distributions of major chemical components in fine particles on light extinction in urban Guangzhou

To evaluate the impact of fine particulate matter (PM_2.5) size distribution on aerosol chemical and optical properties, dominant chemical components including water-soluble inorganic ions (WSII), organic carbon (OC) and elemental carbon (EC) in PM₁ and PM_2.5, aerosol scattering coefficient (b_sp), and aerosol absorption coefficient (b_ap) were collected synchronously at an urban site in Guangzhou, south China during a typical summer month in 2009 and a winter month in 2010. PM₁ (sizes smaller than 1 μm) constituted 77% and 63% of PM_2.5 in summer and winter, respectively. From the reconstructed mass concentrations, the sum of SO₄ ^2 −, NO₃ ⁻ and NH₄ ⁺ (SNA) distributed more in PM₁ than in PM_1–2.5 (PM_2.5 minus PM₁) in summer and the opposite was found in winter, while carbonaceous aerosols distributed more in PM₁ in both summer and winter. With the aggravation of PM_2.5 pollution, the mass fraction of PM₁/PM_2.5 increased for (NH₄)₂SO₄ (AS), NH₄NO₃ (AN) and EC but decreased for organic matter (OM) in summer, and the opposite was found in winter. B_sp of PM₁ and PM_1–2.5 was estimated from the mass extinction efficiencies (MSEs) of the dominant chemical components, which showed good correlations (R² = 0.99) with measured ones and those estimated using the IMPROVE formula. The fractional contributions of dominant chemical components to extinction coefficient (b_ext) were consistent with their respective mass size distributions, indicating the importance of chemically-resolved aerosol size distributions on aerosol optical properties and haze formation.