Observational Analysis of the Influence of Aerosol Radiation Effect on Planetary Boundary Layer Structure and Entrainment Characteristics
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摘要: 2017年12月22日至2018年1月18日利用无人机携带温、湿和颗粒物浓度探测仪对南京地区灰霾污染条件下大气边界层垂直结构开展加密观测。通过比较不同灰霾污染条件下温度、湿度和PM2.5（直径小于2.5微米的颗粒物）浓度的垂直结构差异，结合地面热通量、2米空气温度、相对湿度、风速、风向及主要大气污染物（如臭氧和PM2.5）浓度，定量评估了气溶胶辐射效应对边界层和夹卷过程的影响。分析表明，灰霾或气溶胶削弱到达地表太阳辐射，减小地表感热通量，延迟边界层发展，增加近地层大气稳定度，降低边界层高度，并加重灰霾污染。灰霾污染物在混合层顶处累积，导致PM2.5浓度最大变化出现在边界层顶部而不是近地层。气溶胶辐射效应对夹卷特征及其特征参数有重要影响。灰霾浓度升高时，夹卷区厚度增加；无量纲化夹卷速度随对流理查逊数的变化不再符合负1次方幂函数关系，与大涡模拟结果一致。本研究进一步指出，为提高重霾污染条件下天气和空气质量数值预报水平，必须考虑气溶胶辐射效应对边界层和夹卷参数化的影响。Abstract: An unmanned aerial vehicle (UAV) equipped with portable instruments was used to measure the vertical profiles of temperature, specific humidity, and particulate matter under different haze-polluted weather conditions in Nanjing from December 22, 2017 to January 18, 2018. The study aimed to assess the impact of the aerosol radiative effect on the atmospheric boundary layer (ABL) and the entrainment zone structures as well as their evolution on days with heavy haze pollution and other days with good air quality. The vertical profiles of the potential temperature, water vapor, and PM2.5 concentrations were obtained through detailed analyses of the UAV-measured vertical profiles and surface observations, including the surface heat flux, 2-m air temperature, specific humidity, wind speed, wind direction, and major air pollutants concentration (e.g., O3 and PM2.5) under different air pollution conditions. The results indicate that aerosols reduce the amount of surface-reaching solar radiation and the surface sensible heat flux, postpone the development of the ABL, enhance the atmospheric stability near the surface, decrease the ABL height, and exacerbate air pollution. The maximum concentrations of PM2.5 and the largest increase rate were observed at the top of the ABL rather than near the surface. Furthermore, the aerosol radiative effect was found to have an important impact on entrainment and its characteristic parameters. The depth of the entrainment zone increased with increasing surface PM2.5 concentrations. The entrainment rate normalized with convective velocity did not follow a negative?1 power function with the convective Richardson number under heavy haze or PM2.5 pollution conditions, which is consistent with the findings of large-eddy simulation studies. These results indicate that the aerosol radiative effect must be included in ABL and entrainment parameterization schemes to improve numerical predictions of weather and air quality under heavy pollution conditions.
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图 1 PDR-1500无人机（PDR）与热电β射线颗粒物监测仪FH62C14（β-ray）观测的PM2.5浓度在不同相对湿度（RH）条件下比较（虚线为1∶1线）
Figure 1. A comparison of PM2.5 concentrations measured by PDR-1500 unmanned aerial vehicle (PDR) with a thermo beta ray particle monitor FH62C14 (β-ray) under different relative humidity (RH) conditions (the dotted line is the 1∶1 line)
图 2 2017年12月23日至2018年1月18日落桥试验站（32°30'N，118°37'E）观测的2 m温度、相对湿度（RH）、风速（WS）、风向（WD）及南京环监站（全市站点平均）臭氧、PM2.5浓度（C）时间序列
Figure 2. Time series of surface temperature, relative humidity (RH), wind speed (WS), wind direction (WD), O3, and PM2.5 concentration (C) observed at the Luoqiao Test Station (32°30'N, 118°37'E) and Nanjing environmental monitoring station (average of all sites in Nanjing) from December 23, 2017 to January 18, 2018
图 3 重霾日（2017年12月23日；左列）和干净日（2018年1月12日；右列）不同时次PM2.5浓度、位温和比湿垂直廓线比较
Figure 3. Comparison of the vertical profiles of PM2.5 concentration, potential temperature, and specific humidity on December 23, 2017 (Heavy haze day, left column) and January 12, 2018 (clear day, right column). BT means Beijing time
图 4 2017年12月23日与2018年1月12日（a）气温、（b）感热通量（Hs）的日变化
Figure 4. (a) Diurnal variation of 2-m air temperature and (b) surface sensible heat fluxes (Hs) on December 23, 2017 and January 12, 2018
图 5 地表感热通量（Hs）随地面观测PM2.5浓度变化关系。
Figure 5. The relationship between surface sensible heat fluxes (Hs) and PM2.5 concentrations
图 6 夹卷厚度随近地层PM2.5浓度的变化
Figure 6. Change in entrainment thickness with PM2.5 concentrations near surface
图 7 对数坐标系下无量纲化夹卷速度与对流理查逊数对应关系。填色代表PM2.5浓度，单位：μg m−3，实线为干净天时两者函数关系（Deardorff et al., 1980），即
${w_{\rm{e}}}/{w_*} = 0.25Ri_*^{ - 1}$ Figure 7. Correspondence between dimensionless entrainment rate and convective Richardson number in the logarithmic coordinate system. Colour filling represents PM2.5 concentration, units: μg m−3; the solid line is the function relationship on clean days (Deardorff et al., 1980),
${w_{\rm{e}}}/{w_*} = 0.25Ri_*^{ - 1}$ 表 1 大气边界层垂直探测所用仪器的主要技术指标
Table 1. Summary of key technical indicators of the instruments used for vertical detection of the atmospheric boundary layer
仪器名称 型号 探测要素 测量范围 探测精度 误差范围 小流量便携式气溶胶颗粒物检测仪 Thermo PDR-1500 PM2.5浓度 0～400 mg m−3 0.01 μg m−3 ±5%读数 温湿度传感器 sht75 温度 相对湿度 ‒40～123.8°C 0～100% 0.01°C 0.05% ±0.3°C ±1.8% 数字压力传感器 bmp085 大气压强 300～1100 hPa 0.03 hPa ±0.5 hPa 
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表 2 2017年12月23日与2018年1月12日气温（T）、地表PM2.5浓度（C）、地表感热通量（Hs）、边界层高度（PBLH）、夹卷厚度（△h）、摩擦速度（u*）、夹卷速度（we）、对流速度尺度（w*）的比较（表中所给值均为一天中最大值）
Table 2. A comparison of measured temperature (T), surface PM2.5 concentrations (C), surface sensible heat flux (Hs), planetary boundary layer height (PBLH), entrainment thickness (△h), friction velocity (u*), the winding speed (we), the convection speed scale (w*) between December 23, 2017 and January 12, 2018 (the values represent the maximum ones on each day)
T/°C C(PM2.5)/μg m−3 Hs/W m−2 PBLH/m △h/m u*/m s−1 we/m s−1 w*/m s−1 2017年12月23日 14.2 163.61 48.50 490 249 0.13 0.015 0.67 2018年1月12日 1.7 38.65 150.84 811 150 0.24 0.031 1.33
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