Impacts of aerosol compositions on visibility impairment in Xi'an, China

Daily particle light scattering coefficient, PM _2.5 mass and chemical composition were measured in Xi'an from February to December 2009. Visibility was strongly affected by anthropogenic air pollution sources, resulting in an average visual range (VR) of 6.4 ± 4.5 km. The threshold PM _2.5 mass concentration, corresponding to VR <10 km, was ∼88 μg m ^-3. The revised IMPROVE equation was applied to estimate chemical extinction (b _ext), which on average was ∼15% lower than measured b _ext. PM _2.5 ammonium sulfate was the largest contributor, accounting for ∼40% of b _ext, followed by organic matter (∼24%), ammonium nitrate (∼23%), and elemental carbon (∼9%), with minor contributions from soil dust (∼3%), and NO ₂ (∼1%). High secondary inorganic aerosol contributions (i.e., SO ₄ ^2- and NO ₃ ^-) were the main contributors for VR <5 km. A Positive Matrix Factorization (PMF) solution to the Chemical Mass Balance (CMB) receptor model showed that coal combustion was the dominant factor, accounting for ∼52% of the dry particle light scattering coefficient, followed by the engine exhaust factor (∼31%). Other factors included biomass burning (∼12%) and fugitive dust (∼5%).