Online hygroscopicity and chemical measurement of urban aerosol in Shanghai, China

Submicron aerosol hygroscopicity and composition were simultaneously measured with a Hygroscopicity-Tandem Differential Mobility Analyzer (HTDMA) in-line with an Aerosol Time-of-Flight mass spectrometer (ATOFMS) in wintertime of 2009 in Shanghai. 250 nm dry diameter aerosol particles were humidified (85% RH) and selected based on their hygroscopicity. Two Growth Factor (GF) modes were observed persistently: a weak nearly-hydrophobic (NH) mode with GF at around 1.05 and a strong hydrophilic mode with GF around 1.46. Aerosol particles at different GFs were chemically analyzed by ATOFMS to link the aerosol hygroscopicity and chemistry. Particles were grouped into five major classes: inorganic dust/ash, biomass burning particles (BB), elemental carbon and organic carbon mixed particles (ECOC), Amine rich organic carbon particles (OC-Amine), and high mass organic carbon particles (HMOC). Different particle types were found enriched in the two GF modes. ECOC and OC-Amine particles internally mixed with secondary inorganic species were found mostly in the hydrophilic mode. Pure EC particles and a small group of clay particles among the dust/ash type with strong signals of aluminum and silicon oxides appeared in the NH mode. HMOC particles were exclusively found in the NH mode. Chemical signature intensities were analyzed to examine the particle mixing states and their impact on the diversity of hygroscopicity for each particle type. BB particles in the NH mode had stronger organic carbon signals, while those in hydrophilic mode had stronger potassium salt signals. In general, an elevated OC mass fraction increased particle's hydrophobicity. Amine and secondary inorganic species such as nitrate strongly increased particle's hygroscopicity.