Methods to Retrieve PBLH from Ground-Based and Space-Borne Lidars and Application to Air Pollution Studies

Planetary boundary layer height (PBLH) is a key factor in the vertical mixing and dilution of near-surface pollutants, and is closely associated with air pollution episodes. Ground-based lidars measure the vertical distribution of aerosol with fine vertical resolution, and space-borne lidars provide spatially extensive vertical coverage, offering a potential for tracking PBLH. In our study, we explore and develop different techniques for retrieving PBLH from space-borne lidar (CALIPSO) and ground-based lidar, and validate them against those obtained from radiosonde data, which we consider ground truth. For CALIPSO, two techniques are examined: MSD (maximum standard deviation) and WCT (wavelet covariance transform), and their hybrid, which shows better performance. The uncertainties and their causes are investigated; the worst performance occurs at low aerosol loading and for elevated aerosol layers (Su et al., 2017a). For ground-based lidar, the performance is assessed under different thermodynamic stability conditions using 8-years of micropulse lidar and radiosonde measurements at the ARM Southern Great Plains site. A new method is proposed to improve the accuracy of PBLH estimation, especially its diurnal variation, by fusing lidar data with radiosonde data and imposing temporal continuity conditions.

By utilizing CALIPSO and ground-based lidars, we investigate the relationship between PBLH and surface PM (particulate matter) over China, its spatial pattern and influencing factors (Su et al., 2018). A generally negative correlation is observed between PM and the PBLH, as expected, albeit varying greatly in magnitude and significance with location and season. Nonlinear responses of PM to PBLH evolution are found, especially over the North China Plain (NCP). Strongest PBLH-PM interaction is found when the PBLH is shallow and PM concentration is high, which typically corresponds to wintertime cases. Correlations are much weaker over highland than plains regions, which may be associated with lighter pollution loading and a significant contribution from mountain breezes. The influence of larger-scale horizontal transport on surface PM is considered as well, manifested as a negative correlation between surface PM and wind speed over the whole nation. Moreover, inter-comparison between PBLH retrieved by different methods and data sources suggests that the method chosen for determining PBLH is critical when deriving PM from satellite (Su et al., 2017b). The improved method for retrieving PBLH from space-borne and ground-based lidars would be quite useful for calculating PM from satellites, especially where radiosonde data are available to initialize the diurnal time-series. In addition, the effects of topography and horizontal transport need to be thoroughly taken account when deriving PM from remote sensing observations.

References

Su, T., Z. Li, and R. Kahn, 2018, Relationships between the planetary boundary layer height and surface pollutants derived from lidar observations over China, Atmos. Chem. Phy. under revision.

Su, T., et al, 2017a. An intercomparison of long-term planetary boundary layer heights retrieved from CALIPSO, ground-based lidar, and radiosonde measurements over Hong Kong. J. Geophys. Res. Atmos. 122(7), 3929- 3943.

Su, T., et al, 2017b. An intercomparison of AOD-converted PM2.5 concentrations using different approaches for estimating aerosol vertical distribution. Atmos. Environ.