## 3 Results and Discussion
### Diurnal Variation Characteristics
Figure 4: Box plots of maximum detection ranges for DBS mode (**a,b**) and PPI mode (**c,d**), along with the diurnal variation in average temperature and wind speed (**e,f**) for January (**a,c,e**) and April (**b,d,f**) 2024. The orange arrows indicate wind direction. The center, upper, and lower ends of the box plots represent the mean, upper quartile and lower quartile, respectively. In contrast, the PPI mode detection range is inverse to the DBS mode. The average detection range remains stable at around 9.7 km in the early morning and through the morning. As the DBS mode detection range increases and the conditions for vertical aerosol diffusion improve, the PPI mode detection range decreases significantly. Around 2:00 p.m., when the temperature reaches its peak of 18 \({}^{\circ}\)C, the average detection range in PPI mode reaches its daily minimum of about 6.5 km. As wind speed continues to increase, which aids in the uplift of more particles, the PPI mode detection range begins to rise gradually. Therefore, the diurnal variation in vertical diffusion conditions in the river valley in April has a significant impact on the PPI mode detection range. From the scatter plots of hourly detection ranges for DBS mode and PPI mode shown in Figure 5, it can be observed that there is a moderate correlation between the two detection ranges. This is because when the detection range of the DBS mode reaches its maximum range, the detection range of the PPI mode has not yet reached its upper limit, which limits the correlation. However, this does not affect the consistent trend in their variations. Studies have shown that the annual variation in boundary layer height over the Qinghai-Tibet Plateau is insignificant in the morning. In contrast, the afternoon shows a large variation, with the most pronounced seasonal change occurring in the spring. The diurnal amplitude of the boundary layer height is more prominent in the spring and summer and smaller in the autumn and winter [29], which agrees with the characteristics observed by the DWL in this study. The performance of the PPI and DBS modes under dynamic meteorological conditions (Figure 6) is further analyzed. Notably, the maximum altitude of the wind profile product in DBS mode is approximately 4.6 km. From 00:00 to 02:00, the detection range in DBS mode shows a downward trend. Below 3 km, horizontal winds and surface winds are relatively weak (Figure 6a), while strong descending airflow gradually develops (Figure 6b). As a result, particles are carried downward with the subsiding air, causing the strong-echo region to shift downward. At the same time, echoes below 1 km are weak (Figure 6c), suggesting that the concentration of particulate matter may be low, leading to a decrease in the detection range of the PPI mode (Figure 6d). Between 02:00 and 06:00, surface wind speeds increase to 5 m/s, and the airflow below 2 km is predominantly upward. Echoes below 1 km are strong, allowing the PPI mode to reach its maximum detection range. The strong-echo region between 2 and 3 km shows an upward trend, consistent with the mixed rising and descending airflows observed in Figure 6b). This indicates that particles are dispersed to higher altitudes due to vertical solid mixing, resulting in a higher detection range in DBS mode. From 07:00 to 13:00, the upward airflow below 2 km is gradually replaced by descending airflow, which  
Figure 5: Scatter plots of hourly detection ranges for PPI mode and DBS mode during different time intervals in April 2024: 00:00–05:00 (**a**), 06:00–11:00 (**b**), 12:00–17:00 (**c**), and 18:00–24:00 (**d**). The linear regression equations and correlation coefficients are indicated. develops further upwards, accelerating particle sedimentation. The significant reduction in echo intensity below 2 km reflects a continuous decrease in particle concentration, leading to a noticeable reduction in backscattering intensity. Consequently, the PPI detection range drops to its daily minimum of approximately 4 km at 13:00.