Abstract
Abstract. Simultaneous measurements of particle number size distribution, particle hygroscopic properties, and size-resolved chemical composition were made during the summer of 2014 in Beijing, China. During the measurement period, the mean hygroscopicity parameters (κs) of 50, 100, 150, 200, and 250 nm particles were respectively 0.16 ± 0.07, 0.19 ± 0.06, 0.22 ± 0.06, 0.26 ± 0.07, and 0.28 ± 0.10, showing an increasing trend with increasing particle size. Such size dependency of particle hygroscopicity was similar to that of the inorganic mass fraction in PM1. The hydrophilic mode (hygroscopic growth factor, HGF > 1.2) was more prominent in growth factor probability density distributions and its dominance of hydrophilic mode became more pronounced with increasing particle size. When PM2.5 mass concentration was greater than 50 μg m−3, the fractions of the hydrophilic mode for 150, 250, and 350 nm particles increased towards 1 as PM2.5 mass concentration increased. This indicates that aged particles dominated during severe pollution periods in the atmosphere of Beijing. Particle hygroscopic growth can be well predicted using high-time-resolution size-resolved chemical composition derived from aerosol mass spectrometer (AMS) measurements using the Zdanovskii–Stokes–Robinson (ZSR) mixing rule. The organic hygroscopicity parameter (κorg) showed a positive correlation with the oxygen to carbon ratio. During the new particle formation event associated with strongly active photochemistry, the hygroscopic growth factor or κ of newly formed particles is greater than for particles with the same sizes not during new particle formation (NPF) periods. A quick transformation from external mixture to internal mixture for pre-existing particles (for example, 250 nm particles) was observed. Such transformations may modify the state of the mixture of pre-existing particles and thus modify properties such as the light absorption coefficient and cloud condensation nuclei activation.
Highlights
Particle hygroscopicity is one of the important parameters controlling direct and indirect climate effects of atmospheric particles (McFiggans et al, 2006; Haywood and Boucher, 2000)
Our work provided a general overview of particle hygroscopic behavior as well as a chemical closure study on the particle hygroscopicity using aerosol mass spectrometer (AMS)-based chemical particle composition, emphasizing the organic mass fraction
The Hygroscopicity Tandem Differential Mobility Analyzer (H-TDMA) consists of three main parts: (1) a differential mobility analyzer (DMA1) that selects quasi-monodisperse particles, and a condensation particle counter (CPC1) that measures the particle number concentration, leaving the DMA1 at the selected particle size; (2) an aerosol humidifier conditioning the particles selected by DMA1 to a defined relative humidity (RH); (3) the second DMA (DMA2) coupled with another condensation particle counter (CPC2) to measure the number size distributions of the humidified particles
Summary
Particle hygroscopicity is one of the important parameters controlling direct and indirect climate effects of atmospheric particles (McFiggans et al, 2006; Haywood and Boucher, 2000). Some studies have been performed to investigate the relationship between particle hygroscopicity and chemical composition in both field measurements and laboratory experiments (Massoli et al, 2010; Wong et al, 2011; Lambe et al, 2011; Rickards et al, 2013; Moore et al, 2012a, b; Suda et al, 2014; Paramonov et al, 2013; Levin et al, 2012) These works specially focused on parametrizing the empirical correlations between the atomic oxygen : carbon (O : C) ratio and organic hygroscopicity parameter (κ) derived from either hygroscopic growth factor (e.g., Wu et al, 2013; Rickards et al, 2013) or cloud condensation nuclei (CCN) activity (e.g., Mei et al, 2013; Wong et al, 2011; Lambe et al, 2011; Chang et al, 2010). The evolution of particle hygroscopicity during the new particle formation event was investigated to understand the effects of strong photochemistry-driven atmospheric oxidation processes on particle hygroscopicity and the mixing state
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