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

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