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Research progress: Dirtier Air from a Weaker Monsoon

Aerosols are major air pollutants that affect human health and climate change. The increase of the aerosol level in China has been commonly attributed to the fast rise in pollutant emissions from the rapid economic development in the region. However, a research team in the Institute of Atmospheric Physics, led by Professors Hong Liao and Jianping Li, has found that the observed decadal-scale weakening of the East Asian summer monsoon also contributed to the increases of aerosol concentrations in China.

The research team showed by using a chemical transport model driven by the assimilated meteorological fields that the simulated aerosol concentrations have strong negative correlations with the strength of the East Asian Summer monsoon. Accounting for sulfate, nitrate, ammonium, black carbon, and organic carbon aerosols, the summer surface-layer PM2.5 concentration averaged over eastern China (110–125ºE, 20–45ºN) can be 17.7% higher in the weakest monsoon years than in the strongest monsoon years. The weakening of the East Asian Summer monsoon increases aerosol concentrations mainly by the changes in atmospheric circulation (the convergence of air pollutants) in eastern China.

This work was published recently in Geophysical Research Letters (Zhu, J., H. Liao, and J. Li, Increases in aerosol concentrations over eastern China due to the decadal-scale weakening of the East Asian summer monsoon, Geophys. Res. Lett., 39, L09809, doi:10.1029/ 2012GL051428, 2012). The new approach and results of this study were immediately reported by the "News and Views" of Nature Geoscience (Vol. 5, p.449-450, 2012),which commented that “the evolution of the East Asian summer monsoon may just be part of the atmospheric response to global warming, if so, Zhu and Colleagues have presented a compelling case for the effects of climate change on air quality”.

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The normalized time series of EASMI (bars, the left y-axis) and the simulated JJA surface-layer PM2.5 concentrations (blue line, right y-axis, mg m-3) averaged over eastern China (110°–125°E, 20°–45°N) for years of 1986–2006. The EASMI-GEOS for years 1986–2006 (red bars) are calculated with the GEOS-4 assimilated meteorological data, while the EASMI-NCEP for years 1948–2010 (green bars) are calculated using the NCEP/NCAR reanalysis data. The thick dash line is the 9-year Gaussian-type filtered value of EASMI-NCEP, which represents the decadal variation of EASM. This figure shows a statistically significant negative correlation between the strength of East Asian summer monsoon and aerosol concentrations over eastern China.


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