Impacts of additional HONO sources on O₃ and PM_2.5 chemical coupling and control strategies in the Beijing–Tianjin–Hebei region of China

Figures & data

Fig. 1 Modelling domains and the monitoring sites over the BTH region.

Table 1. Design of WRF–Chem simulations to evaluate the effects of the additional HONO sources

Case ID	HONO sources included in the WRF–Chem simulations
Case R	Reference case with HONO gas-phase production from OH and NO
Case Emis	Case R+HONO emissions
Case NO2 ^*	Case R+NO2 ^* chemistry
Case Het	Case R+NO2 heterogeneous reaction on aerosol surfaces
Case E	Enhanced case with all the three additional HONO sources

Fig. 2 Comparison of simulated 24-h average (a) HONO, (b) NO₂, (c) and (d) concentrations with observations at Peking University during August 2007 (Ianniello et al., 2011; Spataro et al., 2013). Case R is a reference case; Case E includes the HONO emissions, NO₂ ^* chemistry and NO₂ heterogeneous reaction on aerosol surfaces.

Table 2. Statistics for comparison of the observed (Obs.) and simulated (Sim.) hourly data during August 13–20, 2007

	T	RH	WS	WD	HONO	NO2	O3	PM2.5
Units	°C	%	m s^−1	deg	ppb	ppb	ppb	µg m^−3
Number of data	184	184	184	184	117	1304	1194	1340
Median (Obs.)	28.00	68.00	1.40	135.0	0.89	17.02	37.23	94.78
Median (Sim.)	29.15	41.11	2.11	190.7	1.49	20.79	32.77	143.86
Mean (Obs.)	28.05	66.45	1.46	136.35	1.27	19.29	44.61	100.11
Mean (Sim.)	28.92	43.29	2.34	185.74	1.69	24.62	38.62	161.18
MB	0.87	−23.15	0.87	49.40	0.42	5.33	−5.99	61.07
NMB^a	3	−34	52	36	33	28	−13	61
RMSE	1.85	24.78	1.60	133.56	1.05	19.01	18.59	96.84
R	0.94	0.88	0.47	0.42	0.71	0.52	0.87	0.51
FAC2^b	1.0	0.98	0.55	0.58	0.60	0.50	0.62	0.64

Observations of HONO are from the Beijing Meteorological Tower and the meteorological parameters are from the Olympic Village near to the tower site (see Section 3). NO₂, O₃ and PM_2.5 measurements are from the seven urban sites over the BTH region (Fig. 1). Model results are for Case E.

^aThe unit of NMB is in %.

^bFraction of simulations within a factor of two of observations.

Fig. 3 Observed and simulated (a) daytime (D, 7:00–19:00) and night-time (N, 20:00–6:00) mean concentrations of HONO at the Beijing Meteorological Tower site and (b–h) daytime mean concentrations of O₃ and PM_2.5 at the seven urban sites over the BTH region during 13–20 August, 2007. The unit used is µg m⁻³ (at 1.01325×10⁵ Pa and 25°C, 1 µg m⁻³ is ~0.52 ppb for HONO and ~0.51 ppb for O₃).

Fig. 4 Impacts of the additional HONO sources on the chemical coupling between ozone and particulate matter. HONO sources increase OH concentrations and subsequently promote O₃ and PM production. HNO₃ produced from the NO₂ heterogeneous reaction on aerosol surfaces enhances nitrate concentrations in presence of NH₃.

Fig. 5 Enhancements of daytime mean concentrations of O₃ and PM_2.5 due to (a) the NO₂ ^* chemistry (b) the NO₂ heterogeneous reaction on aerosols and (c) all the three additional HONO sources at the seven urban sites demonstrated in Fig. 3 during 13–20 August, 2007.

Fig. 6 Values of (a) the correlation coefficient and (b) the linear-fitting slope for O₃ and PM_2.5 daytime enhancements due to the three additional HONO sources during August 2007; monthly mean daytime (c) O₃ enhancements due to the three additional HONO sources and (d) H₂O₂/HNO₃ ratio over the BTH region.

Fig. 7 Percentage increases of monthly-mean daytime (a) , (b) and (c) in PM_2.5 and (d) PM_2.5, (e) OH, (f) HNO₃, (g) H₂O₂ and (h) H₂SO₄ concentrations over the BTH region during August 2007 due to the three additional HONO sources.

Fig. 8 Impacts of the additional HONO sources on O_{3_8h;max} concentrations resulting from the control of NO_x, VOC and NH₃ emissions. (a) O_{3_8h;max} concentrations for Case R in Beijing on 17 August 2007; (b) O_{3_8h;max} concentrations for Case E minus those for Case R; (c) ratio of Relative Reduction Factors (RRF) for Case E divided by RRF for Case R.

Fig. 9 Twenty four-hour average PM_2.5 concentrations for Case R (a–c) and for Case E (d–f) in Beijing on 17 August 2007.

Fig. 10 The number of concentration reductions (C, with unit of %) of (a) PM_2.5 in Case R, (b) PM_2.5 in Case E, (c) O₃ in Case R, and (d) O₃ in Case E in a certain reduction range among the sensitivity simulations using the 64 emission scenarios (see Section 2.2).

Fig. 11 Comparison of NO_x, VOC and NH₃ emission reductions between Case R (bars with solid baselines) and Case E (bars with dotted baselines) to gain the same range of PM_2.5 or O₃ concentration reductions (C, in unit of %) of (a) −25%<C<−20%; (b) −20%<C<−15%; (c) −15%<C< −10%; (d) −10%<C<−5%; and (e) −5%<C<0. The frequency for each scaling factor is computed by T _i/T, where T is the total times for PM_2.5 or O₃ concentration reductions falling in a certain reduction range; T _i is the occurrence times within T for a specific scaling factor (1, 0.75, 0.5 or 0.25 shown in Section 2.2).

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