Seasonal variation of fine- and coarse-mode nitrates and related aerosols over East Asia: synergetic observations and chemical transport model analysis
We analyzed long-term fine- and coarse-mode synergetic observations of nitrate and related aerosols (SO<sub>4</sub><sup>2−</sup>, NO<sub>3</sub><sup>−</sup>, NH<sub>4</sub><sup>+</sup>, Na<sup>+</sup>, Ca<sup>...
| Main Authors: | , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-11-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/17/14181/2017/acp-17-14181-2017.pdf |
| Summary: | We analyzed long-term fine- and coarse-mode synergetic observations of
nitrate and related aerosols (SO<sub>4</sub><sup>2−</sup>, NO<sub>3</sub><sup>−</sup>,
NH<sub>4</sub><sup>+</sup>, Na<sup>+</sup>, Ca<sup>2+</sup>) at Fukuoka
(33.52° N, 130.47° E) from August 2014 to October 2015. A
Goddard Earth Observing System chemical transport model (GEOS-Chem) including
dust and sea salt acid uptake processes was used to assess the observed
seasonal variation and the impact of long-range transport (LRT) from the
Asian continent. For fine aerosols (fSO<sub>4</sub><sup>2−</sup>, fNO<sub>3</sub><sup>−</sup>,
and fNH<sub>4</sub><sup>+</sup>), numerical results explained the seasonal changes,
and a sensitivity analysis excluding Japanese domestic emissions clarified
the LRT fraction at Fukuoka (85 % for fSO<sub>4</sub><sup>2−</sup>, 47 % for
fNO<sub>3</sub><sup>−</sup>, 73 % for fNH<sub>4</sub><sup>+</sup>). Observational data
confirmed that coarse NO<sub>3</sub><sup>−</sup> (cNO<sub>3</sub><sup>−</sup>) made up the
largest proportion (i.e., 40–55 %) of the total nitrate (defined as the
sum of fNO<sub>3</sub><sup>−</sup>, cNO<sub>3</sub><sup>−</sup>, and HNO<sub>3</sub>) during the
winter, while HNO<sub>3</sub> gas constituted approximately 40 % of the
total nitrate in summer and fNO<sub>3</sub><sup>−</sup> peaked during the winter.
Large-scale dust–nitrate (mainly
cNO<sub>3</sub><sup>−</sup>) outflow from China to Fukuoka was confirmed during all
dust events that occurred between January and June. The modeled
cNO<sub>3</sub><sup>−</sup> was in good agreement with observations between July and
November (mainly coming from sea salt NO<sub>3</sub><sup>−</sup>). During the winter,
however, the model underestimated cNO<sub>3</sub><sup>−</sup> levels compared to the
observed levels. The reason for this underestimation was examined
statistically using multiple regression analysis (MRA). We used
cNa<sup>+</sup>, nss-cCa<sup>2+</sup>, and cNH<sub>4</sub><sup>+</sup> as independent
variables to describe the observed cNO<sub>3</sub><sup>−</sup> levels; these variables
were considered representative of sea salt cNO<sub>3</sub><sup>−</sup>, dust
cNO<sub>3</sub><sup>−</sup>, and cNO<sub>3</sub><sup>−</sup> accompanied by
cNH<sub>4</sub><sup>+</sup>), respectively. The MRA results explained the observed
seasonal changes in dust cNO<sub>3</sub><sup>−</sup> and indicated that the
dust–acid uptake scheme
reproduced the observed dust–nitrate levels even in winter. The annual
average contributions of each component were 43 % (sea salt
cNO<sub>3</sub><sup>−</sup>), 19 % (dust cNO<sub>3</sub><sup>−</sup>), and 38 %
(cNH<sub>4</sub><sup>+</sup> term). The MRA dust–cNO<sub>3</sub><sup>−</sup> component had a
high value during the dust season, and the sea salt component made a large
contribution throughout the year. During the winter, cNH<sub>4</sub><sup>+</sup> term
made a large contribution. The model did not include aerosol microphysical
processes (such as condensation and coagulation between the fine
anthropogenic aerosols NO<sub>3</sub><sup>−</sup> and SO<sub>4</sub><sup>2−</sup> and coarse
particles), and our results suggest that inclusion of aerosol microphysical
processes is critical when studying observed cNO<sub>3</sub><sup>−</sup> formation,
especially in winter. |
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| ISSN: | 1680-7316 1680-7324 |