Characterization of size-resolved effective density of atmospheric particles in an urban atmosphere in Southern China

Effective density (ρeff) is one of the most important physical properties of atmospheric particles,providing important references in exploring the emissions and aging processes of fresh particles.In this study,a combined system of differential mobility analyzer,centrifugal particle mass analyzer,and condensation particle counter was used to periodically measure the ρeffof atmospheric particles in Shenzhen from Oct.2021 to Jan.2022.Results showed that the ρeffof particles with various size presented a bimodal distribution,which could be divided into main density (ρm,main peak,corresponding to relatively dense particles after aging) and sub density (ρs,sub peak,corresponding to fresh particles).The occurrence frequencies of ρsof particles with diameters of 50 and 80 nm were less than 20%,but were as high as about 40% of that with diameters from 120 to 350 nm.The ρmshowed increasing trend with the size of particles,whileρsdecreased as the increasing of the size of particles.The ρeffon pollution day varied significantly with chemical compositions.The increasing of the proportion of sulfate could promote the increasing of ρeff,while black carbon and organic matter caused opposite effects,which may be related to various factors,including the difference of the material density and morphology of various chemical components.The ρeffof 50,80 and 120 nm particles decreased considerably during the new particle formation event,indicating that organic condensation was an important contributor to new particle growth.