Mixing layer height estimated from AMDAR and its relationship with PMs and meteorological parameters in two cities in North China during 2014-2017

The mixing layer height (MLH) plays an important role in atmospheric pollution by influencing the vertical diffusion of pollutants. The Aircraft Meteorological Data Relay (AMDAR) from the airports in Beijing (BJ) and Shijiazhuang (SJZ) during 2014–2017 was used to investigate temporal characteristics of the daily maximum MLH (MLHmax) and their relationships with surface meteorological parameters and particulate matters (PM1, PM2.5, and PM10).The seasonal mean of MLHmax in BJ was higher than that of SJZ, and shows the seasonal variation trend of summer> spring >autumn > winter in both cities. The MLHmax was found to have a positive correlation with daily average wind speed at 2 m (WS) in all seasons except summer and a significant negative correlation with daily average relative humidity at 2 m (RH) in four seasons. These implied a high risk of air pollution during low-MLHmax period. Subsequently, a higher negative relationship between MLHmax and PMs was obtained in both cities in autumn and winter based on the polynomial regression method, with a coefficient of determination (R2) of 0.37–0.71 in autumn and 0.52–0.88 in winter. In BJ, the sequence of R2 was PM1 >PM2.5 >PM10, yet a total opposite trend was observed in SJZ. It illustrated that the greatest contribution to local emissions in BJ and SJZ is PM1 and PM10 respectively, which means that the impact of MLHmax to local emissions and the secondary formed pollutants is stronger than that of long range transport. The ammoniate (NH4+), nitrate (NO3−) and sulfate (SO42−) also performed a better correlation with MLHmax than elemental carbon (EC) and organic carbon (OC) in BJ; while in SJZ, a better correlation between MLHmax and EC is easy to be found, while NO3− and SO42− performed a worse correlation with it. Therefore, PM pollution was characterized by secondary formation in BJ but by primary emissions in SJZ.