Impact of nocturnal planetary boundary layer on urban air pollutants: Measurements from a 250-m tower over Tianjin, China

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 the NPBL height and urban air pollutants are normally difficult, because such measurements require continuous vertical profiles of air pollutants and meteorological parameters. This paper provides an unique data, which temperature, NPBL, NOx and O3 concentrations are measured at a 250-m 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 measured NPBL height ranges from 100 m to 150 m. The measurement of 10-year trend of the NPBL height suggests that the averaged NPBL height increases 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 height. During nighttime, NOx is directly emitted from the surface and is limited to inside of NPBL (40 m), 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 NPBL plays important roles in regulating the diurnal cycle of O3 at the surface.