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Measurements of ageing and hygroscopicity variation of black carbon\ud(BC) particles in Beijing were conducted using a 1.2 m3\udquasi-atmospheric aerosol evolution study (QUALITY) chamber, which consisted\udof a bottom flow chamber through which ambient air was pulled continuously\udand an upper reaction chamber where ageing of BC particles occurred. Within\udthe reaction chamber, transmission of the solar ultraviolet irradiation was\udapproximately 50–60 %, wall loss of primary gaseous pollutants was\udnegligible, and BC\udexhibited a half-lifetime of about 3–7 h. Typically, equilibrium for the\udprimary gases, temperature and relative humidity between the reaction\udchamber and ambient air was established within 1 h. Rapid growth of BC\udparticles occurred, with an average total growth of 77 ± 33 nm and\udaverage growth rate of 26 ± 11 nm h−1. Secondary organic aerosols\ud(SOA) accounted for more than 90 % of the coating mass. The O ∕ C ratio of\udSOA was 0.5, lower than the ambient level. The hygroscopic growth factor of\udBC particles decreased slightly with an initial thin coating layer because\udof BC reconstruction, but subsequently increased to 1.06–1.08 upon further\udageing. The κ (kappa) values for BC particles and coating materials\udwere calculated as 0.035 and 0.040 at the subsaturation and supersaturation\udconditions, respectively, indicating low hygroscopicity of coated SOA on BC\udparticles. Hence, our results indicate that initial photochemical ageing of\udBC particles leads to considerable modifications to morphology and optical properties but does not appreciably alter the particle hygroscopicity in\udBeijing

Ageing and hygroscopicity variation of black carbon particles in Beijing measured by a quasi-atmospheric aerosol evolution study (QUALITY) chamber