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. Hygroscopic and volatile properties of atmospheric aerosolparticles with dry diameters of (20), 50, 75, 110 and 145 nm were determinedin situ by using a volatility–hygroscopicity tandem differential mobility analyser (VH-TDMA)system with a relative humidity of 90 % anddenuding temperature of 270  ∘ C in central Budapest during 2months in winter 2014–2015. The probability density function of thehygroscopic growth factor (HGF) showed a distinct bimodal distribution. Oneof the modes was characterised by an overall mean HGF of approximately 1.07(this corresponds to a hygroscopicity parameter  κ of 0.033)independently of the particle size and was assigned to nearly hydrophobic (NH)particles. Its mean particle number fraction was large, and itdecreased monotonically from 69 to 41 % with particle diameter. The othermode showed a mean HGF increasing slightly from 1.31 to 1.38 ( κ  valuesfrom 0.186 to 0.196) with particle diameter, and it was attributed toless hygroscopic (LH) particles. The mode with more hygroscopic particleswas not identified. The probability density function of the volatility GF (VGF)also exhibited a distinct bimodal distribution with an overall meanVGF of approximately 0.96 independently of the particle size, and withanother mean VGF increasing from 0.49 to 0.55 with particle diameter. Thetwo modes were associated with less volatile (LV) and volatile (V) particles.The mean particle number fraction for the LV mode decreased from 34 to21 % with particle diameter. The bimodal distributions indicated that theurban atmospheric aerosol contained an external mixture of particles with adiverse chemical composition. Particles corresponding to the NH and LV modeswere assigned mainly to freshly emitted combustion particles, morespecifically to vehicle emissions consisting of large mass fractions of sootlikely coated with or containing some water-insoluble organic compounds suchas non-hygroscopic hydrocarbon-like organics. The hygroscopic particles wereordinarily volatile. They could be composed of moderately transformed agedcombustion particles consisting of partly oxygenated organics, inorganicsalts and soot. The larger particles contained internally mixed non-volatilechemical species as a refractory residual in 20–25 % of the aerosol material (by volume).

Wintertime hygroscopicity and volatility of ambient urban aerosol particles