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Effect of climate change on winter haze pollution in Beijing: uncertain and likely small

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This paper is available in a repository.

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Several recent studies have suggested that 21 st -century climate change will significantly worsen the meteorological conditions leading to very high concentrations of fine particulate matter (PM 2.5 ) in Beijing in winter (Beijing haze). We find that 81 % of the variance in observed monthly PM 2.5 during 2010–2017 winters can be explained by a single meteorological mode, the first principal component (PC1) of the 850 hPa meridional wind velocity (V850) and relative humidity (RH). V850 and RH drive stagnation and chemical production of PM 2.5 , respectively, and thus have a clear causal link to Beijing haze. PC1 explains more of the variance in PM 2.5 than either V850 or RH. Using additional meteorological variables does not explain more of the variance in PM 2.5 . Therefore PC1 can serve as a proxy for Beijing haze in the interpretation of long-term climate records and in future climate projections. Previous studies suggested that shrinking Arctic sea ice would worsen winter haze conditions in eastern China, but we show with the PC1 proxy that Beijing haze is correlated with a dipole structure in the Arctic sea ice rather than with the total amount of sea ice. Beijing haze is also correlated with dipole patterns in Pacific sea surface temperatures (SSTs). We find that these dipole patterns of Arctic sea ice and Pacific SSTs shift and change sign on interdecadal scales, so that they cannot be used reliably as future predictors for the haze. Future 21 st -century trends of the PC1 haze proxy computed from the CMIP5 ensemble of climate models are overall very small and variable in sign. We thus find no evidence for a significant effect of climate change on Beijing haze.

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