Impact of Nocturnal Planetary Boundary Layer on Air Pollutants: Measurements from a 250 meter tower over Tianjin, China
【摘要】：It is well known that nocturnal planetary boundary layer (NPBL) has important effects on urban air pollutants However, the direct measurements of the interactions between NPBL and urban air pollutants are difficult, because such measurements require continuous vertical profiles of air pollutants and meteorological parameters. This paper provides a unique data, which temperature, nocturnal planetary boundary layer (NPBL), NOx and O3 concentrations are measured at a 250 meter meteorological tower in the city of Tianjin, China (a much polluted city located in central-eastern China). The results are analyzed to study the trend of NPBL and the impacts of NPBL on air pollutants in the city. The results show that the NPBL height ranges from 100 to 150 meters. The measurement of 10 year trend of the NPBL suggests that the averaged NPBL height is increased by about 20% between 1995 and 2006. The results also show that the NPBL height has important effects on air pollutants. This study suggests that NOx and O3 concentrations are strongly anti-correlated inside of the NPBL. During nighttime, NOx is directly emitted from the surface and is limited to inside of the NPBL (40 meters), resulting in high NOx concentrations near the surface. The high NOx concentrations depress O3, producing low O3 concentrations near the surface. The measurements of vertical gradient of O3 show that about 30-50 ppbv of O3 concentrations are chemically destroyed due to the surface emission of NOx during nighttime, suggesting that the NPBL plays important roles in regulating the diurnal cycle of O3 at the surface.