Abstract
Abstract. Aerosol filter samples were collected at a high-elevation mountain observatory (4180 m a.s.l.) in the northeastern part of the Qinghai–Xizang (Tibet) Plateau (QXP) during summer 2012 using a low-volume sampler and a micro-orifice uniform deposit impactor (MOUDI). These samples were analyzed for water-soluble inorganic ions (WSIs), organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and total organic nitrogen (TON) to elucidate the size-resolved chemical composition of free tropospheric aerosols in the QXP region. The average mass concentration of the sum of the analyzed species in PM2.5 (particle matter) (WSIs + OC + EC + TON) was 3.74 μg sm−3, 36% of which was sulfate, 18% OC, 17 % nitrate, 10% ammonium, 6.6% calcium, 6.4% TON, 2.6% EC, 1.5 % sodium, 0.9% chloride, 0.5% magnesium, and 0.3% potassium. The size distributions of sulfate and ammonium peaked in the accumulation mode (0.32–0.56 μm), whereas the size distributions of both nitrate and calcium peaked in the range of 1.8–3.2 μm, suggesting the formation of nitrate on mineral dust. OC, EC and TON were also predominantly found in the accumulation mode. The bulk chemical composition and the average oxidation degree of water-soluble organic matter (WSOM) were assessed using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). WSOM was found to be highly oxidized in all PM2.5 samples with an average oxygen-to-carbon atomic ratio (O / C) of 1.16 and an organic mass-to-organic carbon ratio (OM / OC) of 2.75. The highly oxidized WSOM was likely related to active cloud processing during upslope air mass transport coupled with strongly oxidizing environments caused by snow/ice photochemistry. High average ratios of OC / EC (7.6) and WSOC / OC (0.79) suggested that organic aerosols were primarily made of secondary species. Secondary organic aerosol (SOA) was estimated on average accounting for 80% (62–96%) of the PM2.5, indicating that SOA is an important component of free tropospheric aerosols over the northern QXP.
Highlights
The Qinghai–Xizang (Tibet) Plateau (QXP), often called “the third pole” (Yao et al, 2012), is one of the most remote and isolated regions in the world
The primary Organic aerosol (OA) (POA) concentration was estimated based on primary organic carbon (POC), which was subtracted from the total OM calculated as the product of the measured organic carbon (OC), and the OM / OCOM determined in Eq (3) to determine the secondary OA (SOA) concentration
An intensive study was conducted at a high elevation remote site (QSS) in the northern part of the QXP to characterize the chemical compositions of PM2.5 during the summer of 2012 – a period that was influenced by a strong exchange of air masses between boundary layer and free troposphere
Summary
The Qinghai–Xizang (Tibet) Plateau (QXP), often called “the third pole” (Yao et al, 2012), is one of the most remote and isolated regions in the world. The high altitude of this region has long been recognized as ideal for studying the long-range transported air pollutants Measurements from this area have been rare, usually due to the harsh natural conditions and logistic difficulties. Previous studies in the QXP region have focused on the chemical properties of aerosols and their source signatures due to the important roles of aerosols on climate forcing. Analysis of the chemical compositions of snow pit samples collected from a glacier in the central Himalayas indicated that biomass burning particles were significantly enhanced in snow during the winter– spring periods, due to transport of polluted air masses from northwest India and Nepal (Xu et al, 2013b). Organic aerosol (OA) is usually dominated by secondary species in remote regions (Zhang et al, 2007) because of atmospheric aging processes during long-range transport. The size distributions of the chemical species were assessed to understand the sources and chemical processes of aerosol
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