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To explore the controlling factors for mass absorption efficiency (MAE) of elemental carbon (EC) in fine particles (PM 2.5), major chemical compositions in size-segregated aerosol samples and bulk PM 2.5 samples and light absorption coefficient (b ap) of PM 2.5 samples under dry condition were measured during four seasons in an urban environment in Guangzhou of south China. On seasonal average, absorption Ångström exponent (AAE) in the wavelength range of 370–880 nm measured by a transmissometer or in the range of 370–950 nm measured by an aethalometer ranged from 0.95 to 1.02 and from 1.01 to 1.14, respectively. The estimated EC MAE at 550 nm in PM 2.5 were 10.1 ± 1.0, 8.9 ± 0.7, 9.1 ± 1.0 and 9.1 ± 0.7 m2 g−1 in spring, summer, autumn and winter, respectively, which were evidently higher than the value of 7.5 m2 g−1 for "pure EC". More than 61% of EC mass in PM 2.1 was distributed in the droplet mode (0.43–2.1 μm), which should be related to hygroscopic growth of aged EC particle as well as external mixing with coating materials. No significant correlations were found between EC MAE and mass fraction of EC or mass ratio of (SO 4 2-+NO 3 −+OC)/EC in the droplet mode. The deliquescence of aerosol was primarily determined by NH 4 NO 3 , and the high EC MAE was mainly related to high nitrate mass concentration and positive ΔRH (ambient relative humidity minus deliquescence relative humidity of NH 4 NO 3). [Display omitted] • Elemental carbon was mainly originated from vehicle emissions in urban Guangzhou. • EC was mainly distributed in the droplet mode largely due to external mixing. • The estimated EC MAE at 550 nm was evidently higher than that for pure EC. • Enhanced EC MAE was mainly caused by aerosol deliquescence by nitrate. [ABSTRACT FROM AUTHOR]

To assess the uncertainties in particle scattering coefficient (b sp) estimated from using the original and revised IMPROVE formulas, particle mass size distribution, bulk PM 2.5 and PM 10 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 (NH 4) 2 SO 4 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. Image 1 • b sp was reconstructed using three models with similar input data. • Model-estimated b sp were biased low by 2–19% on average than measurements. • Mie model performed better than the original and revised IMPROVE formulas. [ABSTRACT FROM AUTHOR]