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This study investigates seasonal variations of mass absorption efficiency of elemental carbon (MAE EC) and possible influencing factors in urban Guangzhou of South China. Mass concentrations of elemental carbon (EC) and organic carbon (OC) in PM 2.5 and aerosol absorption coefficient (b ap) at multi-wavelengths were simultaneously measured in four seasons of 2018–2019 at hourly resolution by a semi-continuous carbon analyzer and an aethalometer. Seasonal average mass concentrations of EC were in the range of 1.36–1.70 µgC/m3 with a lower value in summer than in the other seasons, while those of OC were in the range of 4.70–6.49 µgC/m3 with the lowest value in summer and the highest in autumn. Vehicle exhaust from local traffic was identified to be the predominant source of carbonaceous aerosols. The average aerosol absorption Ångström exponents (AAE) were lower than 1.2 in four seasons, indicating EC and b ap were closely related with vehicle exhaust. Seasonal MAE EC at 550 nm was 11.0, 8.5, 10.4 and 11.3 m2/g in spring, summer, autumn, and winter, respectively. High MAE EC was related with the high mass ratio of non-carbonaceous aerosols to EC and high ambient relative humidity. [ABSTRACT FROM AUTHOR]

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]