Soot aging time scales in polluted regions during day and night

[摘要] The aging of soot is one of the key uncertainties in the estimation of both the direct andindirect climate effect. While freshly emitted soot is initially hydrophobic and externallymixed, it can be transferred into an internal mixture by coagulation, condensation orphotochemical processes. These aging processes affect the hygroscopic qualities and hencethe growth behaviour, the optical properties and eventually the lifetime of the soot particles.However, due to computational limits the aging of soot in global climate models is often onlyparameterised by an estimated turnover rate resulting in a lifetime of soot of several days.Hence, the aging process of soot is one of the key uncertainties governing the burden andeffect of black carbon. In this study, we discuss the time scale on which diesel soot istransferred from an external to an internal mixture based on the results of our simulations witha comprehensive mesoscale model. For daytime conditions during summer condensation ofsulphuric acid is dominant and the aging process occurs on a time scale of τ =8h close to thesources and τ =2h above the source region. During winter comparable time scales are foundbut ammonium nitrate becomes more important. During night time condensation is noteffective. Then coagulation is the most important aging process and our results show timescales between 10h and 40h.

[学科分类] 大气科学