Reflections on the threshold for PM_2.5 explosive growth in the cumulative stage of winter heavy aerosol pollution episodes (HPEs) in Beijing

Figures & data

Fig. 1. Temporal variations in PM_2.5 mass concentration, surface radiation, and surface and vertical distributions of meteorological factors from 25 December 2017 to 1 January 2018 (a) PM_2.5 mass concentration (gray or colored line: Beijing; light gray line: Baoding; gray dot: Shijiazhuang); (b) winds (vectors) and wind velocity (shadings; units: m/s); (c) temperature (shadings; units:°C); (d) RH (shadings; units: %); (e) direct (of the vertical surface to the direction of solar radiation) radiant exposure and global radiant exposure; (f, g, h, i) surface wind distribution in Beijing (the shading represents topography). (Green boxes or lines: the transport stage; blue boxes: relative strong southerly winds; red boxes or lines: the cumulative stage; white or gray boxes: the meteorological characteristics of the cumulative stage).

Table 1. Characteristics of HPEs with the CS in winter (Jan., Feb, and Dec) from 2013 to 2017.

Year	No.	HPEs with the CS	Duration (d)	PM2.5 peak value (μg m^-3)	Start time of CSs	Initial PM2.5 concentration (μg m^-3)
2013	1	9–14 Jan.	6	790	6:00, 12 Jan.	126
	2	17–19 Jan.	3	535	9:00, 18 Jan.	100.5
	3	11–14 Feb.	4	329	21:00, 12 Feb.	159.25
	4	20–22 Feb.	3	256	6:00, 21 Feb.	123
	5	23–28 Feb.	6	497	10:00, 23 Feb.	43.29
					0:00, 26 Feb.	122.12
	6	5–8 Dec.	4	417	6:00, 6 Dec.	151.67
2014	7	1–3 Jan.	3	182	10:00, 2 Jan.	70.86
	8	9–12 Jan.	4	239	11:00, 11 Jan.	68.25
	9	28–30 Jan.	3	197	7:00, 29 Jan.	69.5
	10	19–27 Feb.	8	513	14:00, 24 Feb.	227.83
	11	8–10 Dec.	3	308	20:00, 8 Dec.	97.71
2015	12	2–5 Jan.	4	283	11:00, 3 Jan.	124.25
	13	6–9 Jan.	4	288	12:00, 8 Jan.	100.5
	14	11–16 Jan.	6	462	8:00, 15 Jan.	226.88
	15	22–26 Jan.	5	261	5:00, 25 Jan.	71
	16	12–16 Feb.	4	349	5:00, 14 Feb.	189.62
	17	26 Nov.–1 Dec.	6	641	15:00, 29 Nov.	183.75
	18	16–26 Dec.	11	622	10:00, 22 Dec.	157.5
					19:00, 24 Dec.	141.62
	19	27–30 Dec.	4	495	09:00, 29 Dec.	223.75
2016	20	31 Dec. 2015–4 Jan.	5	477	09:00, 1 Jan.	107
	21	1–5 Dec.	5	475	3:00, 3 Dec.	159.62
	22	16–22 Dec.	7	466	8:00, 19 Dec.	183.5
	23	29 Dec.–1 Jan. 2017	4	551	9:00, 31 Dec.	214.75
2017	24	2 Jan.–8 Jan.	7	579	12:00, 3 Jan.	247.25
	25	23–26 Jan.	4	318	13:00, 25 Jan.	150
	26	11–16 Feb	6	272	9:00, 14 Feb.	149.88
	27	30 Nov.–3 Dec.	4	222	6:00, 2 Dec.	29
	28	26–30 Dec.	5	223	7:00, 28 Dec.	30

Fig. 2. Mean profiles of temperature before the CS, during the earlier period in the CS, and during the later period in the CS respectively in all the 30 CSs.

Fig. 3. Threshold values for PM_2.5 explosive growth in all 30 CSs.

Fig. 4. Threshold values for PM_2.5 explosive growth during the CSs in lighter HPEs (left) and heavier HPEs (right) respectively.