Research on the hygroscopic properties of aerosols by measurement and modeling during CAREBeijing-2006

To further understand and improve air quality for the 2008 Beijing Olympic Games, the Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing‐2006) were carried out in an urban and a suburban area from 10 August to 12 September 2006. As a part of an intensive series of measurements, the optical and physical properties of the aerosol were monitored together with identification of the chemical species involved. A method to calculate the hygroscopic factor for aerosol scattering f(RH), defined as the ratio of the aerosol scattering coefficient at given relative humidity (RH) to that at 35% RH, is proposed on the basis of the optical parameters. Over the course of the study f(80%) = 1.63 ± 0.19. The observation that the molar ratio of NH4+ to (2*SO42− plus NO3−) was very close to 1 implies that the chemical form of the sulfate aerosol may be ammonium sulfate (NH4)2SO4 and that nitrate possibly existed as NH4NO3. On the basis of the measurements of size‐resolved chemistry, RH, and published functional relationships between the chemical composition and water uptake, the aerosol scattering coefficients could be calculated by the Mie theory for the major particle species (ammonium sulfate, ammonium nitrate, sodium chloride, particulate organic matter, elemental carbon, and residual material). This retrieval method synthesizes the high temporal resolution of mass concentration measurements and low temporal resolution size distribution for water soluble ionic components and carbonaceous aerosols. A local closure experiment is obtained by comparing the measured f(RH) with model calculations using aerosol chemical composition and chemical thermodynamics. Results from the closure study show that the measured and the predicted values of f(RH) agree within measurement uncertainties.