Evaluation of the IMPROVE formulas based on Mie model in the calculation of particle scattering coefficient in an urban atmosphere

To assess the uncertainties in particle scattering coefficient (b(sp)) estimated from using the original and revised IMPROVE formulas, particle mass size distribution, bulk PM2.5 and PM10 and their major chemical compositions, and b(sp) under dry condition were synchronously measured in urban Guangzhou during 2015-2016. The estimated b(sp), mass concentrations (C-m), and mass scattering efficiencies (MSEs) of the dominant chemical species in the fine, condensation and droplet modes were compared between those calculated using the original and revised IMPROVE formulas and the Mie model. On annual average, the reconstructed b(sp) using the original IMPROVE formula, the revised IMPROVE formula, and the Mie model explained 81%, 91%, and 98% of the measured b(sp) at 550 nm, respectively. The better performance of the Mie model than the original and revised IMPROVE formula in the reconstructed b(sp) was mainly due to the inclusion of the unidentified chemical species which explained 14% of the measured b(sp) at 550 nm. The estimated percentage contributions of the dominant chemical species to the measured b(sp) differed by < 3% in most cases, except for (NH4)(2)SO4 between the Mie model and the revised IMPROVE formula (6%). By including the unidentified chemical species in the revised IMPROVE formula can best reconstructed b(sp) to within 5% of the measured value.