北京大气能见度和消光特性变化规律及影响因素

利用长时间序列的大气能见度与湿度等气象资料以及近年来大气污染物的监测数据,探讨了北京大气能见度及消光特性的变化规律及影响因素.结果显示:近50年来北京大气消光作用存在降-升-降的变化过程,1954~1967年以下降为主,20世纪60年代中期至70年代明显上升,此后特别是20世纪90年代以来北京大气消光作用基本呈缓慢下降趋势,能见度变化过程与此相反.从区域分布看北京大气消光作用北部及西部山区低于平原区,平原区存在由北向南逐渐升高的分布规律,即北部平原区低于中部市区,中部市区低于南部平原区.近10年来北京大气颗粒物消光作用区域差异逐渐减小,这与大气污染区域分布变化趋势基本一致.北京大气消光作用20世纪80年代之前冬高夏低,之后转为冬低夏高,对应于大气污染由煤烟型向综合型的转变.大气消光作用平均日变化呈双峰双谷型,09:00和21:00形成双峰,06:00和16:00处于双谷,但月际差异明显.大气消光作用受颗粒物浓度与相对湿度影响显著.高消光作用通常与高相对湿度和高颗粒物浓度有关;低消光作用出现在湿度和颗粒物质量浓度同时较小情况.相对湿度低于70%时,大气颗粒物消光作用会随着PM2.5浓度的升高明显增强,消光作用与PM2.5浓度存在线性关系;当相对湿度大于70%时,消光作用对PM2.5浓度变化的响应并不明显.

The variation characteristics and influence factors of atmospheric visibility and extinction effect in Beijing

To well understand the long-term variation characteristics and influence factors of atmospheric visibility and extinction characteristics, data of extinction effect, visibility, relative humidity, and PM2.5 concentrations, had been analyzed in Beijing. It was found that the atmospheric extinction effect in Beijing showed a slow decreasing trend during 1954 to 1967, an increasing trend from the mid- 1960s to 1970s, and then especially since the 1990s a slow downward trend. The variation of visibility was opposite to that of extinction effect. Atmospheric extinction effect over the mountainous regions was lower than that over the plain. Within the plain area, the extinction coefficient in the south was higher than that in the north, but the gap was reduced in the last 10years. Summer was the season with the lowest visibility and winter with the highest in recent years. But it was the opposite before the 1980s. It might result from the change in pollution emission and aerosol chemical components. The visibility reached its peaks at 09:00and 21:00LT, and the valley value of visibility appeared at 06:00and 16:00LT. Atmospheric extinction was affected by both aerosol concentration and relative humidity. The strong atmospheric extinction was generally associated with high RH and high PM concentration, while small extinction effect occurred when RH and PM concentration were low. There was a linear relation between extinction coefficient and PM2.5 mass concentration when RH was lower than 70%. When RH was higher than 70%, the response of extinction coefficient to PM2.5 mass concentration was not observable.