Impact of deliquescence of aerosol on mass absorption efficiency of elemental carbon in fine particles in urban Guangzhou in south China

To explore the controlling factors for mass absorption efficiency (MAE) of elemental carbon (EC) in fine particles (PM2.5), major chemical compositions in size-segregated aerosol samples and bulk PM2.5 samples and light absorption coefficient (bap) of PM2.5 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 PM2.5 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 PM2.1 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 (SO42-+NO3−+OC)/EC in the droplet mode. The deliquescence of aerosol was primarily determined by NH4NO3, and the high EC MAE was mainly related to high nitrate mass concentration and positive ΔRH (ambient relative humidity minus deliquescence relative humidity of NH4NO3). [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.