The pollution of PM2.5 is a serious environmental problem in Beijing. The annual average concentration of PM2.5 in 2001 from seasonal monitor results was more than six times that of the US national ambient air quality standards proposed by US EPA. The major contributors to mass of PM2.5 were organics, crustal elements and sulfate. The chemical composition of PM2.5 varied largely with season, but was similar at different monitor stations in the same season. The fine particles (PM2.5) cause atmospheric visibility deterioration through light extinction. The mass concentrations of PM2.5 were anti-correlated to the visibility, the best fits between atmospheric visibility and the mass concentrations of PM2.5 varied throughout the year: following a power law in spring, exponential in summer, logarithmic in autumn, and power or exponential in winter. As in each season the meteorological parameters such as air temperature and relative humidity change from day to day, the reason for the above correlations between PM2.5 and visibility obtained at different seasons probably come from the differences in chemical compositions of PM2.5.

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A number of previous studies have indicated that boreal spring Pacific Meridional Mode (SPMM) can exert notable impacts on the occurrence of El Niño-Southern Oscillation (ENSO) event during the subsequent winter. Here, we reveal that the impact of the SPMM on the following winter ENSO is asymmetric. During positive SPMM (+SPMM) years, strong westerly wind anomalies are induced over the tropical western Pacific via wind-evaporation-SST (WES) feedback. These westerly wind anomalies lead to pronounced sea surface temperature warming in the tropical central and eastern Pacific, which further develop to an El Niño-like pattern via positive atmosphere-ocean interaction.However, easterly wind anomalies over the tropical western Pacific during negative SPMM (−SPMM) years are much weaker. The weak easterly wind anomalies cannot lead to a clear La Niña-like pattern in the following winter.We suggest that the asymmetric strength of the westerly and easterly wind anomalies over the tropical western Pacific is due to the asymmetric intensity of the WES feedback over the tropical north Pacific, with stronger intensity in the +SPMM years. The asymmetric trade wind charging process also partly contributes to the asymmetric SPMM-ENSO relation. Atmospheric general circulation model simulations confirm that the intensity of the +SPMM-generated westerly wind anomalies over the tropical western Pacific is stronger than that of the easterly wind anomalies related to the −SPMM. The present study indicates that it is necessary to consider the phases of the SPMM when predicting winter ENSO events based on the preceding SPMM.

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