不同季节气象条件对北京城区高黑碳浓度变化的影响

Impact of meteorological conditions on high black carbon concentrations in urban area of Beijing in different seasons

利用2013年至2015年北京城区黑碳气溶胶(下文统称为"BC")和PM2.5观测资料,结合地面气象观测资料、ECMWF边界层高度再分析资料和FNL/NCEP不同高度风速再分析资料,讨论了BC质量浓度及其在PM_(2.5)质量浓度中所占比例(下文统称"黑碳占比")的季节、月、日变化特征,并通过计算北京城区BC浓度与不同高度风速的相关矢量,分析了气象条件和外来输送对北京城区BC浓度变化的影响.结果发现:研究时段内北京城区BC浓度平均值为(4.77±4.49)μg·m^(-3);黑碳占比为8.23%±5.47%.BC浓度和黑碳占比在春、夏季低,秋、冬季高,其日变化特征在4个季节均为"白天低夜间高"的单峰型特征.随着PM_(2.5)浓度的升高,BC浓度增大,黑碳占比减小.当北京地区风向为东北、东北偏东、东南和西南偏西(主风向)时,BC浓度与风速和边界层高度均呈反向变化,即随风速和边界层高度的增大而减小.另外不同季节BC浓度随风速变化的临界值及其变化速率不同.冬季高BC浓度时段,北京城区BC浓度在低层大气的关键影响区分别位于河北南部与山东交界地区以及河北西北部与山西内蒙交界地区;高空关键影响区主要位于北京以西的河北西部、山西北部和内蒙古地区.

Based on the black carbon （BC） and PM2.5 concentration data, ground-based meteorological observation data, ECMWF boundary layer height reanalysis data and FNL/NCEP wind speed reanalysis data in urban area of Beijing from 2013 to 2015, seasonal, monthly, and diurnal changes of BC mass concentrations and the ratio of BC in PM2.5 were characterized, the correlation vector of BC concentration in the urban area with wind at various heights was studied, and the influences of meteorological conditions and regional transport on the change of BC concentrations in Beijing were analyzed.It was found that the average BC concentration was （4.77±4.49） μg·m-3, and the ratio of BC in PM2.5 was 8.23%±5.47%.BC concentrations and the ratio of BC in PM2.5 in autumn and winter were higher than those in the spring and summer.In all seasons, BC concentration and the ratio of BC in PM2.5 presented the diurnal change pattern with low in daytime and high at night.When the PM2.5 concentrations increased, BC concentrations increased while the ratio of BC in PM2.5 decreased.Under the prevailing NE, ENE, SE and WSW winds, BC concentrations decreased with the increases of wind speed and boundary layer height.In different seasons, variation of BC concentration with wind speed shows different threshold values and change rates.During high BC periods in winter, the key areas influencing BC concentrations in the lower atmosphere over the urban Beijing were located in the bordering regions between southern Hebei and northern Shandong and between northwestern Hebei, northern Shanxi and Inner Mongolia, while the key areas influencing BC concentration in the upper atmosphere in the urban Beijing were situated in western Hebei, northern Shanxi and Inner Mongolia.