Regional differences of chemical composition and optical properties of aerosols in the Tibetan Plateau

Tibetan Plateau (TP) is an important area that influences regional meteorology, climate and environment through its dynamic and thermal effects. Ambient aerosol over the TP has a significant impact on the atmospheric radiative forcing. In this study, PM_2.5 filter samples (atmospheric particulate matter with aerodynamic diameter less than 2.5 µm) were collected from two high elevation sites located in the southern and northern areas of TP: the Qomolangma Station (QOMS) and Waliguan Baseline Observatory (WLG), respectively. Collected samples were analyzed to examine the regional differences in aerosol properties and relate them to potential climate effects in the TP area. The aerosol mass concentrations inferred from measured chemical components were higher at WLG (11.2 µg m^-3) compared to QOMS (6.8 µg m^-3). The chemical composition shows higher contribution of organic aerosol at QOMS than that of WLG. The optical properties of water-soluble organic carbon (WSOC) from the QOMS samples show higher light absorption than those collected at WLG. The light absorption of WSOC at QOMS indicates significant pH dependence with enhanced light absorption at higher pH values, while the light absorption of WSOC from WLG samples show very weak pH dependence. Photolysis experiments of WSOC fractions were conducted at 365 nm for 4 h and showed significant photochemical bleaching at the first few minutes and then remained constant. The molecular-level chemical composition investigated using high resolution mass spectrometry (HRMS) assisted with an electrospray ionization (ESI) shows significant differences in WSOC composition representative of two sampling sites. Relatively high molecular weight molecules were prominent in the WLG samples, while smaller molecules were more abundant in the QOMS samples. For QOMS samples, CHO and CHON compounds are the major chemical species detected in the negative (-) ESI mode, while CHO and CHOS compounds are the most abundant chemical species detected by the same method in samples collected at WLG. The reported differences in aerosol composition are discussed in a context of aerosol sources and chemical processes typical for the southern and northern TP regions.