Semi‐volatile aerosols in Beijing (R.P. China): Characterization and influence on various PM 2.5 measurements

During summer, the Beijing urban area meets contrasting meteorological conditions including warm and humid monsoon winds from the southeast bringing high levels of pollution, with PM 2.5 mass concentrations often exceeding 100 μ g/m 3 . The specific weather conditions and aerosol chemical composition observed at that time offer a unique opportunity to address the question of the contribution of semi‐volatile material (SVM) in the continuous PM records available for the south East Asian urban regions. Different PM 2.5 measurements were evaluated during a 3‐week field campaign performed in Beijing downtown during the summer 2004, and consisting of a Rupprecht & Patashnik (R&P TEOM) (heating air sample at 50°C), R&P TEOM‐FDMS (enabling SVM measurement), optical GRIMM counter, and filter weighing. A good agreement was found between the different TEOM measurements during the campaign with the exception of the periods of high Relative Humidity (RH), which exhibited SVM levels (derived from PM 2.5 loss in the TEOM heated at 50°C) as high as 140 μ g/m 3 . Continuous artefact‐free PM 2.5 nitrate measurements were performed simultaneously and showed a close relationship with the TEOM‐derived SVM, accounting for half of this SVM. To better document the role of RH on SVM, estimates of liquid water content (LWC) in aerosols were derived from light scattering coefficient and integrated aerosol volume measurements performed in the field at different RH. LWC is shown to be related in a quantitative way to the levels of SVM and nitrate in aerosols, and thus is believed to play a major role in the gas‐particle partitioning of semi‐volatile species in Beijing aerosols.