Chemical composition and size distribution of summertime PM<sub>2.5</sub> at a high altitude remote location in the northeast of the Qinghai–Xizang (Tibet) Plateau: insights into aerosol sources and processing in free troposphere
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 wate...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-05-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/15/5069/2015/acp-15-5069-2015.pdf |
| Summary: | 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 PM<sub>2.5</sub> (particle matter) (WSIs + OC
+ EC + TON) was 3.74 μg sm<sup>−3</sup>, 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
PM<sub>2.5</sub> 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 PM<sub>2.5</sub>, indicating that SOA is an
important component of free tropospheric aerosols over the northern QXP. |
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| ISSN: | 1680-7316 1680-7324 |