Characteristics of Pollutants and Boundary Layer Structure during Two Haze Events in Summer and Autumn 2014 in Shenyang, Northeast China

The characteristics of pollutants and the boundary-layer structure during two haze events in the summer and autumn of 2014 in Shenyang, Northeast China, were comparatively analyzed by using measurements of the mass concentrations of PM10, PM2.5, O-3, NO2, SO2, and CO; vertical profiles of meteorological parameters from a 100-m high tower; and radiosonde data. The results showed that PM concentrations increased rapidly during the two haze events, resulting in visibility that decreased to 1400 and 405 m, respectively. The weak haze event on 16 June was characterized by high O-3 but low NO2 mainly due to the photochemical reaction, while all the pollutants increased during the severe haze event on 31 October, which was affected by pollutant emissions and meteorological conditions. The PM2.5 concentration had a good correlation with friction velocity (u*) but did not have an obvious relationship with (w'theta') over bar, which means that the haze events were largely affected by the dynamic effect of turbulence and less so by its thermal effect. According to the radiosonde data, a single inversion layer with an inversion intensity of 1.6 degrees C/100 m existed during the weak haze event, whereas double inversions and even more occurred during the severe haze event, with the inversion intensity larger than 2-4 degrees C/100 m. Such stable atmospheric conditions favored the accumulation of pollutants. Backward trajectory analyses showed that the weak haze event was probably caused by pollutant transport from North China, whereas the severe haze event was generated mostly by local pollutants.