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, NO(x) and O(3) 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 100m to 150m. 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 NO(x) and O(3) concentrations are strongly anti-correlated inside of the NPBL height. During nighttime, NO(x) is directly emitted from the surface and is limited to inside of NPBL (40m), resulting in high NO(x) concentrations near the surface. The high NO(x) concentrations depress O(3), producing low O(3) concentrations near the surface. The measurements of vertical gradient of O(3) show that about 30-50ppbv of O(3) concentrations are chemically destroyed due to the surface emission of NO(x) during nighttime, suggesting that NPBL plays important roles in regulating the diurnal cycle of O(3) at the surface.