Rapid formation and evolution of an extreme haze episode in Northern China during winter 2015

Abstract

We investigate the rapid formation and evolutionary mechanisms of an extremely severe and persistent haze episode that occurred in northern China during winter 2015 using comprehensive ground and vertical measurements, along with receptor and dispersion model analysis. Our results indicate that the life cycle of a severe winter haze episode typically consists of four stages: (1) rapid formation initiated by sudden changes in meteorological parameters and synchronous increases in most aerosol species, (2) persistent evolution with relatively constant variations in secondary inorganic aerosols and secondary organic aerosols, (3) further evolution associated with fog processing and significantly enhanced sulfate levels, and (4) clearing due to dry, cold north-northwesterly winds. Aerosol composition showed substantial changes during the formation and evolution of the haze episode but was generally dominated by regional secondary aerosols (53-67%). Our results demonstrate the important role of regional transport, largely from the southwest but also from the east, and of coal combustion emissions for winter haze formation in Beijing. Also, we observed an important downward mixing pathway during the severe haze in 2015 that can lead to rapid increases in certain aerosol species.

Figure 1

Evolution of (a,b) wind direction (WD), (c,d) wind speed (WS), (e) relative humidity (RH), temperature (T) and liquid water content (LWC), and (f) PM_2.5 mass concentration and particle extinction coefficient (b_ext) from November 25 to December 2, 2015. Six episodes (Ep1–Ep6) are marked for further discussion. Four initial stages in the formation of the episodes (F1–F4) and four events with evident downward mixing (D1–D4) are also marked.

Figure 2. Evolution of aerosol chemical composition during the severe haze episode in 2015.

Pie charts show the average chemical composition for each episode and the description of each episode is also included in the figure.

Figure 3

Average vertical profiles of meteorological parameters during four haze episodes in 2015 (a–c) and 2014 (d–f). The temperature of Ep2 in 2015 was offset by +3 °C for clarity.

Figure 4

(a–f) Footprint regions of air mass origin for air arriving at 50 m altitude for the six episodes (Ep1–Ep6) marked in Figs 1 and 2. The legend indicates the concentrations of tracer particles. (g,h) Simulated average spatial distributions of PM_2.5 (μg m⁻³) and surface wind fields (m s⁻¹) for the six episodes (Ep1–Ep6). The cities marked in (e,k) are Beijing (BJ), Tianjin (TJ), Langfang (LF), Baoding (BD), Shijiazhuang (SJZ), and Hengshui (HS). The maps were drawn by IGOR Pro (version 6.3.7.2, WaveMetrics, Inc., Oregon USA), http://www.wavemetrics.com/.

Figure 5

Scatter plots of (a) BC vs. CO, (b) NO_x vs. CO, (c) POA vs. BC, (d) NO₂ vs. NO_x, (e) NO₂ vs. NO₃, and (f) SOA vs. NO₃ for the episodes marked in Fig. 2 in 2015 (blue) and Fig. 3 in 2014 (red). Error bars show one standard deviation. Note that Ep4 was excluded from the 2015 linear fit in (b,d) due to strong local source influences.