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

The hygroscopic properties of submicron aerosol particles were determined at a suburban site (Wuqing) in the North China Plain among a cluster of cities during the period 17 July to 12 August, 2009. A High Humidity Tandem Differential Mobility Analyser (HH-TDMA) instrument was applied to measure the hygroscopic growth factor (GF) at 90%, 95% and 98.5% relative humidity (RH) for particles with dry diameters between 50 and 250 nm. The probability distribution of GF (GF-PDF) averaged over the period shows a distinct bimodal pattern, namely, a dominant more-hygroscopic (MH) group and a smaller nearly-hydrophobic (NH) group. The MH group particles were highly hygroscopic, and their GF was relatively constant during the period with average values of 1.54 +/- 0.02, 1.81 +/- 0.04 and 2.45 +/- 0.07 at 90%, 95% and 98.5% RH (D-0 = 100 nm), respectively. The NH group particles grew very slightly when exposed to high RH, with GF values of 1.08 +/- 0.02, 1.13 +/- 0.06 and 1.24 +/- 0.13 respectively at 90%, 95% and 98.5% RH (D-0 = 100 nm). The hygroscopic growth behaviours at different RHs were well represented by a single-parameter Kohler model. Thus, the calculation of GF as a function of RH and dry diameter could be facilitated by an empirical parameterization of K as function of dry diameter. A strong diurnal pattern in number fraction of different hygroscopic groups was observed. The average number fraction of NH particles during the day was about 8%, while during the nighttime fractions up to 20% were reached. Correspondingly, the state of mixing in terms of water uptake varied significantly during a day. Simulations using a particle-resolved aerosol box model (PartMC-MOSAIC) suggest that the diurnal variations of aerosol hygroscopicity and mixing state were mainly caused by the evolution of the atmospheric mixing layer. The shallow nocturnal boundary layer during the night facilitated the accumulation of freshly emitted carbonaceous particles (mainly hydrophobic) near the surface while in the morning turbulence entrained the more aged and more hygroscopic particles from aloft and diluted the NH particles near the surface resulting in a decrease in the fraction of NH particles.