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Hygroscopic properties of aerosol particles at high relative humidity and their diurnal variations in the North China Plain

by P F Liu, C S Zhao, T Goebel, E Hallbauer, A Nowak, L Ran, W Y Xu, Z Z Deng, N Ma, K Mildenberger, S Henning, F Stratmann, A Wiedensohler show all authors


The hygroscopic properties of submicron aerosol particles were\ndetermined at a suburban site (Wuqing) in the North China Plain among a\ncluster of cities during the period 17 July to 12 August, 2009. A High\nHumidity Tandem Differential Mobility Analyser (HH-TDMA) instrument was\napplied to measure the hygroscopic growth factor (GF) at 90%, 95% and\n98.5% relative humidity (RH) for particles with dry diameters between\n50 and 250 nm. The probability distribution of GF (GF-PDF) averaged over\nthe period shows a distinct bimodal pattern, namely, a dominant\nmore-hygroscopic (MH) group and a smaller nearly-hydrophobic (NH) group.\nThe MH group particles were highly hygroscopic, and their GF was\nrelatively constant during the period with average values of 1.54 +/-\n0.02, 1.81 +/- 0.04 and 2.45 +/- 0.07 at 90%, 95% and 98.5% RH (D(0)\n= 100 nm), respectively. The NH group particles grew very slightly when\nexposed to high RH, with GF values of 1.08 +/- 0.02, 1.13 +/- 0.06 and\n1.24 +/- 0.13 respectively at 90%, 95% and 98.5% RH (D(0) = 100 nm).\nThe hygroscopic growth behaviours at different RHs were well represented\nby a single-parameter Kohler model. Thus, the calculation of GF as a\nfunction of RH and dry diameter could be facilitated by an empirical\nparameterization of K as function of dry diameter. A strong diurnal\npattern in number fraction of different hygroscopic groups was observed.\nThe average number fraction of NH particles during the day was about\n8%, while during the nighttime fractions up to 20% were reached.\nCorrespondingly, the state of mixing in terms of water uptake varied\nsignificantly during a day. Simulations using a particle-resolved\naerosol box model (PartMC-MOSAIC) suggest that the diurnal variations of\naerosol hygroscopicity and mixing state were mainly caused by the\nevolution of the atmospheric mixing layer. The shallow nocturnal\nboundary layer during the night facilitated the accumulation of freshly\nemitted carbonaceous particles (mainly hydrophobic) near the surface\nwhile in the morning turbulence entrained the more aged and more\nhygroscopic particles from aloft and diluted the NH particles near the\nsurface resulting in a decrease in the fraction of NH particles.

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