Correction of Lidar Ratio and Its Vertical Distribution Characteristics Using Aircraft Observations
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摘要: 雷达比是激光雷达反演气溶胶光学特性的重要参数和影响因素。利用北京地区2016年一次清洁过程（12月10日）和两次污染过程（11月15～18日和12月16～19日）的微脉冲激光雷达、机载浊度计和黑碳仪以及多种地基观测设备，综合研究基于飞机观测订正雷达比的方法及其分布特征。清洁过程地面PM2.5浓度低于40 μg m−3；污染严重时期的PM2.5均高于150 μg m−3且能见度低于5 km，污染过程1存在高空传输的特征。研究结果表明相较于采用单一的柱平均雷达比，利用本文方法获得的雷达比垂直廓线反演得到的气溶胶消光系数和光学厚度更接近原位跟踪观测，精度均有提升。基于此方法获得的雷达比在污染发展不同时期垂直分布差异较大，主要分布在19～76 sr之间，清洁时期雷达比较小且垂直分布差异不大。污染过程1雷达比随高度波动增加至边界层顶（19～45 sr）；污染过程2严重期边界层内雷达比随高度由70 sr降低到20 sr；边界层以上均呈现小幅波动变化。边界层内雷达比垂直分布与气溶胶来源特别是高空气溶胶传输有密切联系，混有沙尘的区域传输显著提升了所在高度的雷达比值。边界层以上雷达比受少量大粒子或者强吸收性的气溶胶粒子的影响波动变化。边界层内消光系数增大时雷达比呈增加趋势；当相对湿度高于40%，边界层内雷达比随相对湿度增加而增大。Abstract: The lidar ratio (LR) is an important parameter and impact factor for the retrieval of aerosol optical properties using micro-pulse lidar. We analyze the vertical LR retrieved by the method of correcting micro-pulse lidar with aircraft observation using multiple-observation equipment during one clean case (December 10, 2016) and two pollution cases (November 15–18, December 16–19, 2016) in Beijing. PM2.5 concentration was lower than 40 μg m−3,while PM2.5 concentration was higher than 150 μg m−3, and visibility was lower than 5 km during the severe-pollution period. The first pollution case was characterized via high-altitude transportation. The vertical extinction coefficient profiles and aerosol optical depth (AOD) obtained by the iterative algorithm using the LR values were closer to the in-situ observation than those obtained by the single column average value. LR obtained from this method primarily varied between 19 and 76 sr in different periods. The results showed a small LR value and insignificant differences in vertical distribution during the cleaning period, and LR increased in height from 19 to 45 sr in the boundary layer during the first pollution case. During the second pollution case, LR presented little variation during the pollution-development period and later decreased in height from 70 to 20 sr at the boundary layer under the severe-pollution period, while there were slight fluctuations above the boundary layer. The vertical distribution of LR along the boundary layer is related to the source of aerosols, especially the regional transportation in the high layer, and regional dust transportation may significantly increase the LR. LR fluctuates due to the influence of large particles or strong absorbing particles above the boundary layer. The results showed that LR increased with the increase in the extinction coefficient at the boundary layer as well as relative humidity (RH) when RH was higher than 40%.
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Key words:
- Lidar ratio /
- Vertical distribution /
- Extinction coefficient /
- Lidar /
- Aircraft observation
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图 1 2016年（a–d）11月15～18日和（e−f）12月16～19日（a，e）能见度（黑色实线）、相对湿度（蓝色实线）、（b，f）地面PM2.5浓度、（c，g）地面直接辐射和（d，h）激光雷达标准后向散射信号垂直剖面随时间分布
Figure 1. Temporal variations in (a, e) visibility (black solid line), relative humidity (blue solid line), (b, f) PM2.5 mass concentration, (c, g) solar radiation, and (d, h) normalized relative backscatter (NRB) during (a–d) November 15–18, 2016 and (e−f) December 16–19, 2016
图 2 2016年（a，b）11月15日12:00（北京时，下同）和（c，d）12月10日14:00不同雷达比反演的气溶胶消光系数廓线和光学厚度（左）及其相对于飞机实测的相对偏差（右）。图2a和2c中灰色实心圆点线为飞机观测的消光系数，灰色阴影为飞机观测的±10%，红色空心圆圈线为采用飞机订正激光雷达的雷达比计算的消光系数，其他颜色为不同雷达比反演的消光系数
Figure 2. Profiles of extinction coefficient observed by the aircraft and retrieved by different lidar ratios (LRs) from lidar, aerosol optical depth (AOD), and relative bias for (a, b) at 1200 BJT on November 15, 2016 and (c, d) at 1400 BJT on December 10, 2016. The gray line denotes the extinction coefficient from aircraft measurement and gray shadow for ±10% of the extinction coefficient. The red line denotes LRs retrieved by aircraft and lidar, and the blue, orange, and green lines respectively denote LRs for 50, 30, and 20 sr in Fig. 2a and 2c. The two insets in Fig. 2a and c denote AOD from different LRs, and the measurement is the same as above. The relative bias between lidar and aircraft is shown in Fig. 2b and 2d
图 3 不同边界层高度激光雷达光学厚度与AERONET光学厚度
Figure 3. Comparison between lidar and AERONET AOD with a color gradient representing different levels of planetary boundary layer (PBL) height
图 4 2016年（a）11月15～18日和（b）12月16～19日污染过程雷达比垂直分布
Figure 4. Vertical distribution of LRs from (a) November 15 to 18, 2016 and from (b) December 16 to 19, 2016
5 2016年11月15～18日12:00（左）和12月16～19日14:00（右）HYSPLIT （Hybrid Single-Particle Lagrangian Integrated Trajectory model）48小时后向轨迹
5. Backward trajectories of 48 h at 1200 BJT from November 15 to 18 in 2016 (left) and at 1400 BJT from December 16 to 19 in 2016 (right) based on Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT)
图 6 两次污染过程（2016年11月15～18日和12月16～19日）和清洁过程（12月10日）期间雷达比垂直分布的分类统计特征
Figure 6. Classification characteristics for the vertical distribution of LRs during November 15–18, 2016, December 10, 2016 and December 16–19, 2016
图 7 2016年11月15～18日和12月16～19日两次污染过程边界层内外雷达比随消光系数和相对湿度的变化
Figure 7. Variation of LRs with extinction coefficient and RH in PBL and above PBL during November 15–18, 2016 and December 16–19, 2016
表 1 2016年11月15～18日、12月10日和12月16～19日飞机探测时间、地面污染物浓度和气象要素特征Tabble1 Flight schedule and corresponding PM2.5 mass concentration, direct radiation, visibility, surface relative humidity, and planetary boundary layer height
during November 15–18, 2016, December 10, 2016 and December 16–19, 2016 探测时间 地面PM2.5浓度/μg m−3 气象因素 日期 飞行时间 直接辐射/W m−2 能见度/m 相对湿度 边界层高度/m 11月15日 12:00 36 426 30629 19% 1800 14:00 44 337 25722 22% 1700 11月16日 12:00 84 359 10408 40% 1000 14:00 79 273 10365 28% 900 11月17日 12:00 75 219 10033 53% 1200 14:00 84 186 9210 57% 1200 11月18日 12:00 187 98 3197 70% 1100 12月10日 14:00 37 32286 19% 1800 12月16日 14:00 126 302 16276 23% 750 12月17日 14:00 226 315 3422 39% 700 12月18日 14:00 151 224 3026 29% 350 12月19日 14:00 138 242 3548 28% 350 
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