Variability of winter haze over the Beijing-Tianjin-Hebei region tied to wind speed in the lower troposphere and particulate sources

This study analyzes the variability of winter haze days and visibility in the Beijing-Tianjin-Hebei (BTH) region, in relation to the regional average wind speed changes in the lower troposphere and emissions for 1961-2014. Winter mean surface meteorological data, NCEP/NCAR atmospheric reanalysis data, and fossil fuel emission data are used. The results reveal a significant increase in the haze days of +0.8 days decade(-1) (p < .01), and a subsequent decline in visibility of-1.56 km decade-1 (p < .01). Most interestingly, an accelerated increase in the number of haze days was observed for the last 11-year period (+8.3 days decade(-1), p <.01) of the study period (2004-2014). Regression analysis indicates that the increase of haze occurrence and decrease in visibility are partly due to the significant (p <.01) declining trend of the mean wind speed in the lower troposphere (-0.19 m s(-1) decade(-1) at 10 m, (-0.23 m s(-1) decade(-1) at 925 hPa), and -0.21 m s(-1) decade(-1) at 850 hPa), and partly due to the declining (dust storm frequency as a proxy, -0.41 days dec(-1)) surrounding particulate sources and increasing fossil fuel emissions (total carbon emission as a proxy, +48,206.8 thousand metric tons dec(-1)). Specifically, wind speed changes in the lower troposphere explain 41.3% of the interannual varation of the winter haze days and 71.2% of the visibility variance. These are extended to 51.7% and 81.6% respectively when combined with information about the natural (dust storm frequency) and anthropogenic (fossil fuel emissions) particulate sources. Therefore, the analyses show that wind speed changes in the lower troposphere, together with the varied natural and anthropogenic sources of particulates, play a key role in modulating winter haze and visibility conditions in the BTH area.