输送对区域本底站痕量气体浓度的影响

利用HYSPLIT4轨迹模式计算了临安、上甸子和龙凤山区域大气本底站2005年7月-2007年6月的气块后向轨迹, 并通过聚类分析取得了各站点平均轨迹的空间分布和季节变化等结果。将各站点同期观测到的O₃, SO₂, NO_x和CO浓度通过滑动平均法去除季节变化, 取得其短期变化信号, 并将轨迹与这种短期变化信号对应, 比较真实地反映了排放源空间分布和轨迹移动特性对污染物浓度的影响。研究结果表明:移动高度高、速度快的气块不利于污染物积累, 对站点污染物起清除作用。但个别来自高空的轨迹会将富含O₃的气块输送到地面, 从而使站点O₃浓度升高。各站均有大约50%~60%的轨迹输送污染气块, 导致站点气体浓度升高。临安站的污染主要来自S-SW和N-SE扇区; 上甸子站的污染最主要来自SE-SW扇区, 其次来自N-SE和W-N扇区; 龙凤山的污染主要由来自S-WSW扇区。

Impacts of Air Parcel Transport on the Concentrations of Trace Gases at Regional Background Stations

To investigate the impacts of air parcel transport on the concentrations of trace gases (SO_2,NO_x,CO and O_3) at regional background stations,backward air parcel trajectories from July 2005 to June 2007 are calculated for Lin'an,Shangdianzi,and Longfengshan stations using the HYSPLIT4 model and the NCEP reanalysis meteorological data.Four trajectories are calculated for each day,with starting time at 02:00,08:00,14:00 and 20:00 Beijing Time and a travelling time of 72 hours.Individual trajectories are clustered to obtain the mean trajectories and monthly statistics are made for different clusters of trajectories.Statistics of trace gas concentrations observed at the stations during the same period are conducted,corresponding to different clusters of trajectories.However,these statistics do not facilitate interpreting relative influences of different trajectories on the pollutants concentrations at the stations,because both trajectories and pollutants concentrations vary seasonally.Therefore,seasonal trends in the time series of the O_3,SO_2,NO,and CO concentrations are filtered by 30-day smooth-averaging to obtain the short-term variations of these gases,which is nearly independent of seasonal variations.The impacts of air parcel transport over different areas on the pollutants concentrations are studied by combining the trajectories and the corresponding short-term variation data of the gases.The results show that air parcels originating from higher altitudes and transported at higher speeds can effectively remove gaseous pollutants from surface air over the stations.However,the surface O_3 level may occasionally be enhanced by downward transport of O_3 rich air from near the top of boundary layer.About 50%-60% of the backward trajectories for the stations are related to the transport of polluted air parcels,leading to increases in the concentrations of the gaseous pollutants.The polluted air parcels are mainly from the S-SW and N-SE sectors for Lin'an,from the SE-SW sector for Shangdianzi,and from the S-WSW sector for Longfengshan.On the average,air parcels from the S-SW sector can enhance the concentrations of SO_2,CO and O_3 at Lin'an by 1.2×10~(-9),57.5×10~(-9) and 4.6× 10~(-9),respectively,while those from the E-SE sector can abate concentrations of SO_2,CO and O_3 by 0.7× 10~(-9),5.0× 10~(-9) and 9.5 × 10~(-9),respectively.For Shangdianzi,air parcels from the SE-SW sector can increase the concentrations of SO_2,NO_x,CO and O_3 by 3.6× 10~(-9),2.8×10~(-9),406×10~(-9) and 1.8×10~(-9),respectively,while those from the high altitudes in the NW sector can reduce concentrations of SO_2,NO_x and CO by 8.4×10~(-9),10×10~(-9) and 599×10~(-9),respectively.For Longfengshan,air parcels from the SW sector can increase the concentrations of SO_2,NO_x,CO and O_3 by 1.3×10~(-9),0.4×10~(-9),53×10~(-9) and 1.4×10~(-9) respectively,while those from the other sectors decrease the concentrations of SO_2,NO_x and O_3 by about 0.3 × 10~(-9) - 5.9 × 10~(-9),and that of CO by about 24×10~(-9)-100×10~(-9).