Long-term measurements of planetary boundary layer height and interactions with PM2.5 in Shanghai, China

Abstract

The planetary boundary layer (PBL) height plays important roles in modulating the local air pollution near surface, such as PM2.5 (particles with diameter ≤ 2.5 μm) concentrations. However, it is very difficult to continuously measure a long-term PBL height. This study is the first time to analyze the long-term variability of PBL height (from 2010 to 2015) by using the continuous measurements from a micro-pulse Lidar (MPL) in Shanghai, China. The aim of this paper is to investigate the long-term variation of PBL height and its relationship with meteorological factors and PM2.5 concentration in the mega city of Shanghai, China. The results showed that there was no significant long-term trend of PBL height in Shanghai during the 6-year period with a mean value of 400 m, indicating the changes of air pollution during the same period were likely driven by other factors, e.g. wind speed or emissions. The PBL height was positively correlated with solar radiation and wind speed, while negatively correlated with vertical temperature gradient and PM2.5 concentration. By comparison, the PBL height was more sensitive to solar radiation and vertical temperature gradient, indicating that the variation of PBL height was mostly driving by thermal effect. The diurnal variation of PBL height had strong impact on the diurnal cycle of PM2.5 concentration. For example, the PM2.5 diurnal variation presented different patterns between summer and winter. In winter the PM2.5 diurnal cycle exhibited two-peaks pattern, appeared in the morning at 8:00 BJT and at night around 20:00 BJT respectively. Both peaks were well corresponding to the low PBL conditions (200–300 m). While in summer, the PM2.5 peaked at noon under high PBL condition (1000 m). This noon peak was likely due to the elevation of secondary aerosol formation, which offsets the diffusion effect on PM2.5 resulted from PBL developments. There was a strong interaction among the solar radiation, PBL, and PM2.5 concentration. The higher PM2.5 caused the reduction of solar radiation to inhibit PBL, further depressed the aerosols in a shallow layer to yield higher PM2.5 concentration. As a result, PBL and PM2.5 concentrations presented non-linearly anti-correlations. During the lower range of PBL (less than 400 m), the PM2.5 variability was very sensitive to the changes in the PBL. On the other hand PM2.5 presented substantial effects on PBL evolutions. The daytime PBL developed more fully under low PM2.5 conditions compared with that under high PM2.5 levels. The PBL height decreased about 100 m when PM2.5 concentration increased about 30–50 μg/m3 under the condition that daily PM2.5 concentration was greater than 70 μg/m3. The statistical analysis showed that the mean ratios of PM2.5/PBL during 6-year period were 0.37, 0.11, 0.30, and 0.37 (μg/m4) in spring, summer, autumn, and winter respectively, suggesting that the PM2.5 levels were more sensitive to PBL in spring and winter.