Measurement report: Vehicle-based and In Situ Multi- 1 lidar Observational Study ofn the Effect of Meteorological 2 Elements on the Three-dimensional Distribution of 3 Particles in the Western Guangdong–Hong Kong–Macao 4 Greater Bay Area

: The distribution of meteorological elements has always been an important factor in 13 determining the horizontal and vertical distribution of particles in the atmosphere. To study the effect 14 of meteorological elements on the three-dimensional distribution structure of particles, mobile vehicle 15 lidar observations, and in situfixed-location observations were were collectedpresented in the western 16 Guangdong–Hong Kong–Macao Greater Bay Area of China during September and October ofin 2019 17 and 2020. Vertical aerosol extinction coefficient, depolarization ratio, and wind and temperature 18 profiles were measured by using a micro pulse lidar, a Raman scattering lidar, and a Doppler wind 19 profile lidar installed on a mobile monitoring vehicle. The mechanism of how wind and temperature in 20 the boundary layer affects the horizontal and vertical distribution of particles was analyszed. The 21 results showsed that particles were mostly distributed in downstream areas on days with moderate wind 22 speed in the boundary layer, while whereas they presented were distributed homogeneously on days 23 with weaker wind. There are three typical types of vertical distribution of particles in the western Guangdong–Hong Kong–Macao Greater Bay Area (GBA): surface single layer, elevated single layer, 25 and double layer. Analysis of wind profiles and Hybrid Single-Particle Lagrangian Integrated 26 Trajectory Model ( HYSPLIT) backward trajectory revealsed different sources of particles for the three 27 types. Particles concentratinged near the temperature inversion and multiple inversions could cause 28 more than one peak in the extinction coefficient profile. There wereare two mechanisms that 29 affectinged the distribution of particulate matter in the upper and lower boundary layers. Based on this 30 observational study, a general model of meteorological elements affecting the vertical distribution of 31 urban particulate matter was is madeproposed. Results show that the difference between lidar and

human lives and activities. Therefore, it is essential to study their distribution characteristics.

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The distribution of particles is influenced not only by changes in source emissions but also by changes 55 in meteorological factors, such as temperature and wind. It has previously been observed that a low

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The concentration of particulate matter also shows characteristics of wind-dependent spatial 65 distributions in which pollutant transport within the GBA city cluster is significant (Xie et al., 2019).

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Hence, the issue of how meteorological factors affecting the distribution of particles has received 67 considerable critical attention.

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Lidar is an active remote sensing device. It emits a laser light beam and receives a backscatter signal, 70 which can be further used to retrieve the vertical distribution of particle optical properties, as well as 71 wind, and temperature. It has been wildly widely applied in the fields of meteorology and 72 environmental science. In most of the researchstudies, it was is used as a ground-based or satellite-

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In recent years, vehicle-based lidar observation has been gradually developed and become a powerful

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The vehicle-based observation lasted for seven continuous days, which started on 29 August 29th and 97 ended on 4 September 4th, 2020. During most of the mobile observations, the relative humidity of 98 Zhuhai, the closest city to the sea, was below 60 %. Therefore, the influence of hygroscopic growth on         150 151 where ( ) is the power received at altitude ,; and denote the particle extinction and 154 molecular extinction, respectively and. = 50 Sr is the particle extinction-to-backscatter ratio, 6 which is the default value given by the manufacturer. This value is consistent with prior work in the 156 GBA area (Li et al., 2020). = 8π/3 is the molecular extinction-to-backscatter ratio, and. is the 157 calibration height of the micro pulse lidar, which is variable, ranging from 10-15 km, and depending on 158 the signal intensity.

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The micro pulse lidar (MPL) system uses the scattering of polarized light to distinguish between 161 spherical and non-spherical particles to ascertain the particle species (

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During 0300 LT and 0400 LT, there is a significant increase in the maximum height of the particle 241 layer. After 0430 LT, the maximum height of the particle layer dropped, and the near-ground extinction 242 coefficient fell below 0.3. Figure 45

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It is worth noting that the wind at an altitude of 540 m at night gradually shifted to southerly wind,

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Arrows represents the wind direction.

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The backward trajectories analysis of the same period ( Figure 67) shows that on a large scale, the

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The vertical distribution of particulate matter was closely related to the horizontal wind speed at 348 various heights (Figure 1213). It can be seen that the wind speed of more than 1000 m increased 349 significantly with the altitude, reaching more than 6 m/s. By 2300 LT, the wind speed below 500 m was 350 approximately 4 m/s, obviously higher than the wind speed between 500-1000 m, and there were 351 significant differences in the wind direction. After 2300 LT, the wind speed near the ground decreased,

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The vertical observations of the temperature (Fig. 1415.) showed that on the night of 15 September

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As shown in Figure 1617, the surface single layer occurred when calmlight horizontal wind dominated 408 near the ground, which was not conducive to removing particles from local emissions. An elevated 409 single layer was caused by upward airflow near the ground and vertical wind shear at a higher altitude.

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In this kind of wind structure, particle layer formation was dominated by upward convection and 411 regional transport. A double layer existed because a layer with stronger horizontal wind existed 412 between two layers with weaker wind, which facilitated the transport of particles from local emission 413 and horizontal transport to downstream areas and resulted in a cleaner layer inside the polluted air 414 mass.

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Another key factor that influenced the vertical distribution of particles was temperature inversion,

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The horizontal distribution of particles in the GBA was closely related to wind speed and wind

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The vertical distribution of particles in the GBA was classified into three typical types according to the

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The statistics of average extinction coefficients at different altitudes and horizontal wind speeds