夏季局地环流对北京下风向地区O3输送的影响

于2007年6~9月,北京地区环流背景属于局地“山谷风”环流期间,在沿主导风向路径分布的4个代表性测点开展了地面O3浓度观测,并采用Models-3/CMAQ模式对典型个例进行模拟,以研究城市对下风向地区O3输送的影响.结果表明,城市下风向测点上甸子本底站O3平均浓度、超标率最高,上风向测点超标率最低,O3小时极大值出现在城区站,为271.56×10-9;不同测点O3浓度日变化规律一致,但浓度日峰值出现时间呈现自城市至下风向地区逐渐滞后的规律;各测点主导风向下O3浓度分布具有沿输送路径逐渐升高的趋势,且两主导风向下的浓度差值也具有沿西南输送路径逐渐升高的趋势;本底地区O3的浓度受来自清洁地区和城市污染地区不同气流影响,观测资料估算得出城市对区域背景地区的输送贡献为36.7×10-9;模拟结果验证了北京地区城市对下风向地区O3的输送影响.

Influence of summer local circulation on the transportation of ozone from urban to the downwind area in Beijing

Surface ozone concentration was measured at four stations in Beijing area, which are located along the route of prevalent wind, from June to September 2007. During the experiment period, the local circulation - mountain and valley breeze was prevalent in Beijing. The community multiscale air quality model (CMAQ) was applied simulating a typical case to validate the influence of transportation of ozone from urban to rural area. Both the maximum average O_3 concentration and the highest frequency of 1h O_3 mixing ratio exceeding 102×10~9 was observed at the urban-downwind, regional background station, whereas the lowest frequency appeared at the urban-upwind station. The peak hourly ozone concentration of up to 271.56×10~9 was captured at the downtown station. The patterns of ozone diurnal variations were similar at the different stations. The peak time of hourly ozone concentration lagged along the city-to-downwind route. O_3 concentration observed under the prevalent wind conditions increased along the route of wind direction. The difference in O3 concentration between the two prevalent wind directions (SW and NE) also increased along the route of SW wind direction. O_3 concentration at the regional background station was affected by different air masses from the clean and polluted areas. The contribution of the city-to-downwind transport to regional background O_3 was estimated to be 36.7×10~(-9). Simulated results validated the influence of O_3 transportation from urban to the downwind areas.