延庆地区山谷风对PM_(2.5)浓度的影响

Effect of mountain-valley wind on PM_(2.5) concentrations in Yanqing Area

基于2015~2019年北京生态环境监测和气象数据,分析了延庆地区山谷风对PM_(2.5)浓度的影响,揭示了含山谷风环流污染过程(事件1)与未有山谷风污染过程(事件2)初始阶段的异同及其气象影响机制.结果表明,延庆持续性污染过程集中在9月~次年3月,共计63次,其中27次(43%)伴随1d或多天的山谷风日,39d山谷风中有32d(82%)出现在污染过程的初始阶段,18%出现在峰值阶段;36次过程未出现山谷风日.山谷风日逐时PM_(2.5)浓度大于非山谷风日4.5~15.4μg/m^(3),全日差值最大时段为谷风阶段(15:00~19:00)均大于13μg/m^(3),山谷风日存在SSE-ESE风频中心0.59%,15:00~16:00风速3.3m/s左右,非山谷风日风频中心在WSW-SW和SE-ESE,最大值为0.41%,风速较山谷风日小.事件1和2初始阶段PM_(2.5)浓度变化关键期为15:00~19:00,事件1风向E-SSE风速2~4m/s,PM_(2.5)增长速率大于事件2,与露点变化趋势基本一致,23:00事件1PM_(2.5)浓度显著高于事件220μg/m^(3)左右,污染过程发展初期出现的山谷风环流谷风阶段的偏东南风形成气溶胶和绝对水汽的区域传输,对PM_(2.5)浓度的升高有正贡献.平原空气污染过程(延庆未出现)特殊污染型占比20%,该类污染型白天风频中心分布分散,NNW-WNW、SW-SSW和ENE-NNE均有0.7%左右的风频中心,未出现S-ESE的风频.

Based on the environmental monitoring data and meteorological data of Beijing from 2015 to 2019,this study analyzed the impact of mountain-valley wind on PM_(2.5) concentrations in Yanqing and revealed the separateness,the similarities and the differences between(the event 1)initial stage of air pollution event with mountain-valley wind and(the event 2)non-mountain-valley wind and its meteorological mechanism.The result showed that persistent pollution concentrated in September to March of the next year with a total of 63 times,of which 27 times(43%)were accompanied by one or more mountain-valley days and 82% mountain-valley days appeared in the initial stage of the pollution event and 18% appeared in the peak stage.The hourly PM_(2.5) concentrations in mountain-valley days(the event 1)were higher than those in non-mountain-valley days(the event 2)by 4.5~15.4μg/m^(3).The maximum diurnal difference was in the stage of valley wind,which was greater than 13μg/m^(3) from 15:00 to 19:00.The wind frequency center of SSE-ESE was 0.59% with the wind speed about 3.3m/s from 15:00 to 16:00.The wind frequency centers of non-mountain-valley wind days were WSW-SW and SE-ESE with the maximum value of 0.41% and the wind speed was smaller than that of mountain-valley days.In the initial stage of both events 1 and 2,the critical period of PM_(2.5) concentration change was from 15:00 to 19:00.The wind direction of event 1 was E-SSE and the wind speed was 2~4m/s.In the event 1,the growth rate of PM_(2.5) was greater than that of event 2,which was basically consistent with the trend of dew point temperature change.The PM_(2.5) concentration of event 1was significantly higher than that of event 2.The meteorological mechanism can be so summarized:the mountain-valley wind circulation appears in the initial stage of the pollution event,then in the later stage,the southeast wind in the valley from the regional transport of aerosol and water vapor,which have a positive contribution to the increase of PM_(2.5) concentration.This special type of air pollution events accounted for 20% in the Beijing plain(outside of Yanqing)aera and the wind orientation centers were scattered in the daytime with NNW-WNW,SW-SSW and ENE-NNE about 0.7% and S-ESE orientation.