Winter time hygroscopicity and volatility of ambient urban aerosol particles
Authors: Enroth Joonas , Mikkilä Jyri , Németh Zoltán , Kulmala Markku , Salma Imre 
Published: November 2017

Abstract. Hygroscopic and volatile properties of atmospheric aerosol particles with dry diameters of (20), 50, 75, 110 and 145 nm were determined in situ by using a VH-TDMA system with a relative humidity of 90 % and denuding temperature of 270 °C in central Budapest during two months in winter 2014–2015. The probability density function of the hygroscopic growth factor (GF) showed a distinct bimodal distribution. One of the modes was characterized by an overall mean GF of approximately 1.07 (the corresponding 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 it was decreasing monotonically from 71 % to 41 % with particle diameter. The other mode showed a mean GF increasing slightly from 1.31 to 1.38 (κ values from 0.186 to 0.196) with particle diameter, and it was attributed to less hygroscopic (LH) particles. These hygroscopicity values are low in general. The mode with more hygroscopic particles was not identified. The probability density function of the volatility GF also exhibited a distinct bimodal distribution with an overall mean volatility GF of approximately 0.96 independently of the particle size, and with another mean GF increasing from 0.49 to 0.55 with particle diameter. The two modes were associated with less volatile (LV) and volatile (V) particles. The mean particle number fraction for the LV mode was decreasing from 34 % to 21 % with particle diameter. The bimodal distributions in the GF spectrum indicated that the urban atmospheric aerosol contained an external mixture of particles with a diverse chemical composition. The mean diurnal variability of the particle number fractions for the NH and LV modes, and of the volatility GF for the LV mode followed the diurnal pattern of the vehicular road traffic, while the mean diurnal variability of the hygroscopicity parameter for the NH, and of the particle number fractions for the V mode on workdays were inversely linked to the road traffic. The particles corresponding to the NH and LV modes were assigned mainly to freshly emitted combustion particles, more specifically to vehicle emissions consisting of large mass fractions of soot likely coated with or containing some water-insoluble organic compounds such as non-hygroscopic hydrocarbon-like organics. The particles related to the LH and V modes could be composed of moderately transformed aged combustion particles consisting of partly oxygenated organics, inorganic salts and soot. Both regional background sources and urban (local) emissions contribute to these particles. Dependency of the volatility GF and the volume fraction remaining after the thermal treatment on the mean hygroscopic GF suggested that the hygroscopic compounds were ordinarily volatile, and that the larger particles contained internally mixed non-volatile chemical species as refractory residuals in 20–25 % of the aerosol material (by volume), which could be core-like soot or organic polymers.