Wintertime organic and inorganic aerosols in Lanzhou, China: sources, processes, and comparison with the results during summer
Lanzhou, which is located in a steep alpine valley in western China, is one of the most polluted cities in China during the wintertime. In this study, an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), a seven-wavelength aethalometer, and a scanning mobility parti...
| Main Authors: | , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-12-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/14937/2016/acp-16-14937-2016.pdf |
| Summary: | Lanzhou, which is located in a steep alpine valley in western China, is one
of the most polluted cities in China during the wintertime. In this study,
an Aerodyne high-resolution time-of-flight aerosol mass spectrometer
(HR-ToF-AMS), a seven-wavelength aethalometer, and a scanning mobility
particle sizer (SMPS) were deployed during 10 January to 4 February 2014 to
study the mass concentrations, chemical processes, and sources of
submicrometer particulate matter (PM<sub>1</sub>). The average PM<sub>1</sub>
concentration during this study was 57.3 µg m<sup>−3</sup> (ranging from
2.1 to 229.7 µg m<sup>−3</sup> for hourly averages), with organic aerosol
(OA) accounting for 51.2 %, followed by nitrate (16.5 %), sulfate
(12.5 %), ammonium (10.3 %), black carbon (BC, 6.4 %), and chloride
(3.0 %). The mass concentration of PM<sub>1</sub> during winter was more than
twice the average value observed at the same site in summer 2012
(24.5 µg m<sup>−3</sup>), but the mass fraction of OA was similar in the two
seasons. Nitrate contributed a significantly higher fraction to the PM<sub>1</sub>
mass in winter than summer (16.5 % vs. 10 %), largely due to more
favored partitioning to the particle phase at low air temperature. The mass
fractions of both OA and nitrate increased by ∼ 5 % (47
to 52 for OA and 13 to 18 % for nitrate) with the increase of the
total PM<sub>1</sub> mass loading, while the average sulfate fraction decreased
by 6 % (17 to 11 %), indicating the importance of OA and nitrate for
the heavy air pollution events in Lanzhou. The size distributions of OA,
nitrate, sulfate, ammonium, and chloride all peaked at ∼ 500 nm, with OA being
slightly broader, suggesting that aerosol particles were
internally mixed during winter, likely due to frequently calm and stagnant
air conditions during wintertime in Lanzhou (average wind speed: 0.82 m s<sup>−1</sup>).<br><br>The average mass spectrum of OA showed a medium oxidation degree (average
O ∕ C ratio of 0.28), which was lower than that during summer 2012
(O ∕ C = 0.33). This is consistent with weaker photochemical processing during
winter. Positive matrix factorization (PMF) with the multi-linear engine
(ME-2) solver identified six OA sources, i.e., a hydrocarbon-like OA (HOA),
a biomass burning OA (BBOA), a cooking-emitted OA (COA), a coal combustion
OA (CCOA), and two oxygenated OA (OOA) factors. One of the OOAs was
less oxidized (LO-OOA), and the other one more oxidized (MO-OOA). LO-OOA
was the most abundant OA component (22.3 % of OA mass), followed by CCOA
(22.0 %), COA (20.2 %), MO-OOA (14.9 %), BBOA (10.8 %), and HOA
(9.8 %). The mass fraction of primary OA
( = HOA + BBOA + COA + CCOA) increased during high PM pollution periods, indicating that local
primary emissions were a main reason for the formation of air pollution
events in Lanzhou during winter. Radiocarbon (<sup>14</sup>C) measurement was
conducted on four PM<sub>2.5</sub> filter samples from this study, which allowed
for a quantitative source apportionment of organic carbon (OC). The
non-fossil sources on average accounted for 55 ± 3 % of OC, which
could be mainly from biomass burning and cooking activities, suggesting the
importance of non-fossil sources for the PM pollution in Lanzhou. Together
with the PMF results, we also found that a large fraction (66 ± 10 %) of the secondary OC was from non-fossil OC. |
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| ISSN: | 1680-7316 1680-7324 |