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. Effective density is one of the most important physicalproperties of atmospheric particles. It is closely linked to particlechemical composition and morphology and could provide special informationon particle emissions and aging processes. In this study, size-resolvedparticle effective density was measured with a combined differential mobility analyzer–centrifugal particle mass analyzer–condensation particle counter (DMA–CPMA–CPC) systemin autumn 2019 as part of the Multiphase chemistry experiment in Fogs andAerosols in the North China Plain (McFAN). With a newly developed flexibleGaussian fit algorithm, frequent (77 %–87 %) bimodal distribution of particle effective density is identified, with a low-density mode (namedsub-density mode) accounting for 22 %–27 % of the total number of observedparticles. The prevalence of the sub-density mode is closely related tofresh black carbon (BC) emissions. The geometric mean for the main densitymode (ρ‾eff,main) increases from1.18 ± 0.10 g cm−3 (50 nm) to 1.37 ± 0.12 g cm−3 (300 nm) due to a largerfraction of high-density components and a more significant restructuringeffect at large particle sizes but decreases from 0.89 ± 0.08 g cm−3 (50 nm) to 0.62 ± 0.12 g cm−3 (300 nm) for the sub-density mode (ρ‾eff,sub), which could be mainly ascribed to the agglomerate effect of BC. ρ‾eff,main and ρ‾eff,sub show similar diurnal cycles with peaksin the early afternoon, mainly attributed to the increasing mass fraction of high material density components associated with secondary aerosolproduction, especially of secondary inorganic aerosol (SIA). To investigatethe impact of chemical composition, bulk particle effective density wascalculated based on measured chemical composition (ρeff,ACSM) and compared to the average effective density at 300 nm (ρ‾eff,tot,300nm). The best agreement between the two densities is achieved when assuming a BC effective density of 0.60 g cm−3. The particle effective density is highly dependent on SIA and BCmass fractions. The influence of BC on the effective density is evenstronger than SIA, implying the importance and necessity of including BC inthe estimate of effective density for ambient particles.

Bimodal distribution of size-resolved particle effective density: results from a short campaign in a rural environment over the North China Plain