Abstract
Soot particles are an important component of atmospheric aerosol and their interactionwith water is important for their climate effects. The hygroscopicity of fresh andphotochemically aged soot and secondary organic aerosol (SOA) from diesel passenger caremissions was studied under atmospherically relevant conditions in a smog chamber at sub-and supersaturation of water vapor. Fresh soot particles show no significant hygroscopicgrowth nor cloud condensation nucleus (CCN) activity. Ageing by condensation of SOAformed by photooxidation of the volatile organic carbon (VOC) emission leads to increasedwater uptake and CCN activity as well as to a compaction of the initially non-sphericalsoot particles when exposed to high relative humidity (RH). It is important toconsider the latter effect for the interpretation of mobility based measurements. Thevehicle with oxidation catalyst (EURO3) emits much fewer VOCs than the vehiclewithout after-treatment (EURO2). Consequently, more SOA is formed for the latter,resulting in more pronounced effects on particle hygroscopicity and CCN activity.Nevertheless, the aged soot particles did not reach the hygroscopicity of pureSOA particles formed from diesel VOC emissions, which are similarly hygroscopic(0.06 < κH − TDMA < 0.12 and0.09 < κCCN < 0.14) as SOA from other precursor gases investigated in previous studies.
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