Evolution of planetary boundary layer under different weather conditions, and its impact on aerosol concentrations

Abstract

A field experiment was conducted in Tianjin, China from September 9–30, 2010, focused on the evolution of Planetary Boundary Layer (PBL) and its impact on surface air pollutants. The experiment used three remote sensing instruments, wind profile radar (WPR), microwave radiometer (MWR) and micro-pulse lidar (MPL), to detect the vertical profiles of winds, temperature, and aerosol backscattering coefficient and to measure the vertical profiles of surface pollutants (aerosol, CO, SO2, NOx), and also collected sonic anemometers data from a 255-m meteorological tower. Based on these measurements, the evolution of the PBL was estimated. The averaged PBL height was about 1000–1300m during noon/afternoon-time, and 200–300m during night-time. The PBL height and the aerosol concentrations were anti-correlated during clear and haze conditions. The averaged maximum PBL heights were 1.08 and 1.70km while the averaged aerosol concentrations were 52 and 17μg/m3 under haze and clear sky conditions, respectively. The influence of aerosols and clouds on solar radiation was observed based on sonic anemometers data collected from the 255-m meteorological tower. The heat flux was found significantly decreased by haze (heavy pollution) or cloud, which tended to depress the development of PBL, while the repressed structure of PBL further weakened the diffusion of pollutants, leading to heavy pollution. This possible positive feedback cycle (more aerosols→lower PBL height→more aerosols) would induce an acceleration process for heavy ground pollution in megacities.