Classification of summertime synoptic patterns in Beijing and their associations with boundary layer structure affecting aerosol pollution
Meteorological conditions within the planetary boundary layer (PBL) are closely governed by large-scale synoptic patterns and play important roles in air quality by directly and indirectly affecting the emission, transport, formation, and deposition of air pollutants. Partly due to the lack of long-...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/17/3097/2017/acp-17-3097-2017.pdf |
| Summary: | Meteorological conditions within the planetary boundary layer
(PBL) are closely governed by large-scale synoptic patterns and play
important roles in air quality by directly and indirectly affecting the
emission, transport, formation, and deposition of air pollutants. Partly due
to the lack of long-term fine-resolution observations of the PBL, the
relationships between synoptic patterns, PBL structure, and aerosol pollution
in Beijing have not been well understood. This study applied the obliquely
rotated principal component analysis in T-mode to classify the summertime
synoptic conditions over Beijing using the National Centers for Environmental
Prediction reanalysis from 2011 to 2014, and investigated their relationships
with PBL structure and aerosol pollution by combining numerical simulations,
measurements of surface meteorological variables, fine-resolution soundings,
the concentration of particles with diameters less than or equal to
2.5 µm, total cloud cover (CLD), and reanalysis data. Among the
seven identified synoptic patterns, three types accounted for 67 % of the
total number of cases studied and were associated with heavy aerosol
pollution events. These particular synoptic patterns were characterized by
high-pressure systems located to the east or southeast of Beijing at the
925 hPa level, which blocked the air flow seaward, and southerly PBL winds
that brought in polluted air from the southern industrial zone. The
horizontal transport of pollutants induced by the synoptic forcings may be
the most important factor affecting the air quality of Beijing in summer. In
the vertical dimension, these three synoptic patterns featured a relatively
low boundary layer height (BLH) in the afternoon, accompanied by high CLD and
southerly cold advection from the seas within the PBL. The high CLD reduced
the solar radiation reaching the surface, and suppressed the thermal
turbulence, leading to lower BLH. Besides, the numerical sensitive
experiments show that cold advection induced by the large-scale synoptic
forcing may have cooled the PBL, leading to an increase in near-surface
stability and a decrease in the BLH in the afternoon. Moreover, when warm
advection appeared simultaneously above the top level of the PBL, the thermal
inversion layer capping the PBL may have been strengthened, resulting in the
further suppression of PBL and thus the deterioration of aerosol pollution
levels. This study has important implications for understanding the crucial
roles that meteorological factors (at both synoptic and local scales) play in
modulating and forecasting aerosol pollution in Beijing and its surrounding
area. |
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| ISSN: | 1680-7316 1680-7324 |