西安郊区泾河秋、冬季大气黑碳变化及粒径特征

Analysis of Variation and Particle Size Characteristics of Black Carbon at Jinghe,Suburban of Xi’an in Autumn and Winter

利用西安郊区泾河大气成分站ρ(BC)(BC为黑碳)、粒子数浓度资料和观象台自动站、探空站数据,对西安郊区秋、冬季ρ(BC)变化特征、粒径分布特征和变化原因进行了分析.结果表明:①泾河郊区ρ(BC)比城区的低.城区ρ(BC)日变化呈三峰特征,峰值分别出现在凌晨02:00、清晨07:00和傍晚20:00,分别为17.0、12.5和21.5μgm3.泾河郊区ρ(BC)日变化呈双峰特征,峰值出现在上午09:00和晚上23:00,数值分别为5.5和6.1μgm3;谷值出现在清晨06:00和下午16:00,数值分别为4.9和2.6μgm3.②ρ(BC)与不同粒径颗粒物数浓度关系不同,其中与粒径<4.0μm粒子的数浓度成正比,与粒径≥4.0μm粒子(如沙尘)的数浓度成反比;波长指数分析表明,沙尘可致ρ(BC)实测值偏大.③ρ(BC)与近地面逆温强度有密切相关性,R(相关系数)达到0.412 5(P<0.000 1);风速小于1.5 ms时BC易堆积,大于1.5 ms时则相反;连续降水对BC清除效果显著.

Black carbon （BC） is one of the most important research fields in aerosol climate effect studies due to its strong absorption effect. Compared to early international efforts, systematic BC observations were started late since 1990s＇ in several cities of China. The variation and particle size characteristics and their causing factors of BC concentration were analyzed based on data collected at Jinghe station, suburban of Xi＇ an, during autumn and winter （October 1st, 2008-February 28, 2009 ）. The station is suitable for BC characteristics study since it is far from downtown area without main BC source nearby. The observational data included BC and particle concentration data collected by atmospheric elements station, meteorological variables collected by automatic meteorological station and sounding station. Results showed that： 1 ） suburban has lower BC concentration than urban area; Contrast to tri-peak diurnal cycle at urban area, suburban showed double-peak characteristic with maximum values of 5.5 p,g/m3 （09：00 o＇ clock） and 6. 1 p^g/m3 （ 23：00 o＇ clock）. The two corresponding trough values were 4.9 p^g/m3 （06：00 o＇ clock） and 2. 6 p,g/m3 （ 16：00 o＇ clock） , respectively; 2 ） Relationship between BC concentration and particle size varied with particle sizes. The BC concentration was directly proportional to ultrafine particles and inversely proportional to big particles （ e. g. dust）. Dust affects measurement of BC concentration; 3 ） BC concentration was significantly related with temperature inversion near the ground （ correlation coefficient）. Wind speed had different effects in the processes of transport and diffusion of BC. Continuous precipitation had remarkable removal effects to BC. The increasing BC concentration during autumn could be contributed by the burning of crop stalks during the harvest seasons. In December and January, residential heating through burning coal and firewood by simple equipment caused relative high BC concentration. The reasons for lowest BC concentration in February were particles removal by precipitation （rain and snow） and dispersal by strong wind. Diurnal cycle of BC concentration reflected local BC emission variation and was affected by planetary boundary layer height. These results are useful to further studies of atmospheric environmental pollution and aerosol climate effect for other inland cities in the northwest of China.