华北地区一次强灰霾污染的天气学效应

Synoptic Effect of a Heavy Haze Episode over North China

利用在线耦合的大气化学模式WRF-Chem V3.6（Weather Research Forecasting Model with Chemistry Version 3.6）及环境、气象观测数据,在完成大气化学方案优选的基础上,研究了华北地区一次重霾污染过程（2013年2月15-17日）对气象条件的反馈作用。重点关注一次颗粒物、无机气态成分和挥发性有机污染物的人为排放对PM2.5（空气动力学当量直径小于等于2.5μm的颗粒物,即细颗粒物）生成的贡献,探讨了由此引发的气象条件的变化。模拟结果显示,上述3种人为源的综合排放对华北地区PM2.5浓度的平均贡献率为91.27%,其中对北京、秦皇岛和沧州的贡献率分别达96.9%、95.9%和97.2%。这使区域地面太阳向下短波辐射降低近15.99%,区域平均地面辐射强迫达-26.51 W m^-2,由此导致地面温度下降0.14°C（3.68%）,逆温增强,垂直温度梯度（？T/？z）升高0.026 K km^-1,边界层高度降低18.92 m（8.77%）,平均风速减少约0.014 m s^-1（0.35%）,相对湿度绝对值升高0.51%,地面平均气压降低0.86 Pa。对于15-17日污染过程,人为源综合排放的气溶胶对短波辐射的影响在天气过程中占主导地位,对边界层高度的影响较大,但不起主导作用,对温度、风速、相对湿度、气压的作用则远小于天气系统本身。挥发性有机污染物（Volatile Organic Compounds,VOCs）作为二次有机气溶胶（Secondary Organic Aerosol,SOA）的前体物,其人为排放对SOA浓度的贡献率约为99.6%。同时,VOCs通过调整大气反应活性促进无机气态成分向无机盐转化,它对硫酸盐和硝酸盐浓度的贡献达50%以上。然而,VOCs对整个PM2.5浓度的贡献不及各种源综合贡献的1/4。人为排放的VOCs对气象场的反馈与综合排放的作用基本一致,但对地面气压的影响VOCs排放时以热力因子为主,而人为源综合排放时以动力因子为主。上述结果暗示,灰霾污染过程所引发的气象条件向不利于污染物扩散方向改变,这可能促进污染物的局地累积、增强污染程度并延长区域内重污染的持续时间。因此,在探讨区域性灰霾污染成因时,灰霾自身通过辐射强迫作用对大气的调节是不可忽视的重要影响因素。

Version 3.6 of the online-coupled meteorology-chemistry model WRF-Chem （Weather Research Forecasting Model with Chemistry） was applied to simulate the feedback effect of a heavy haze episode （15-17 February 2013） on the synoptic conditions, based on optimization of the model scheme with observed data. We focus on the contribution of anthropogenic emissions, including primary particles, inorganic gas compositions, and volatile organic compounds （VOCs）, to the formation of fine particulate matter （PM2.5）, and the feedback effect on meteorological variables. The model results show that the contribution rate of anthropogenic emissions to the regional average PM2.5 concentration is 91.27% over North China and 96.9%, 95.9%, and 97.2% in Beijing, Qinhuangdao, and Cangzhou, respectively, leading to a 15.99% decrease in regional downward shortwave radiation, with regional radiative forcing at the surface estimated to be -26.51 W m^-2. Consequently, the 2-m temperature decreases by 0.14 ℃ （3.68%）, and the inversion intensifies so that the vertical temperature gradient （？T/？z） increases by 0.026 K km^-1, which leads to a boundary layer height reduction of 18.92 m （8.77%）. The 10-m wind speed decreases by 0.014 m s^-1 （0.35%）, surface relative humidity increases by 0.51%, and surface pressure reduces by 0.86 Pa. For the haze episode during 15-17 February, the feedback effect of anthropogenic emissions on downward shortwave radiation plays a leading role in the weather process, which has a great influence--but not a leading role--on boundary layer height, and has little influence on temperature, wind speed, relative humidity, and surface pressure. As the precursors of secondary organic aerosol （SOA）, volatile organic compounds （VOCs） contribute 99.6% to the concentration of SOA; moreover, VOCs transform inorganic gas into inorganic salt by adjusting the reactivity of the atmosphere, and they contribute greater than 50% of sulfates and nitrates, Even so, the contribution of VOCs to the PM2.5 concentration is less than 1/4 of the contribution of all anthropogenic emissions. The feedback effect of anthropogenic VOCs is about the same as anthropogenic emissions, but the effect of VOCs on surface pressure is led by thermal factors and all the anthropogenic emissions are led by dynamic factors. These results imply that the meteorological feedback effect induced by haze pollution was not conducive to the dispersal of air pollutants but contributed to local accumulation, which resulted in a more prolonged and serious pollution episode than might otherwise have been the case. Hence, the self-adjustment of air pollutants is an ineligible factor in analyzing the formation mechanism of regional haze.