Production of NO3 and N2O5
The first step in the nighttime nitrate formation via N2O5
chemistry is the production of NO3 and N2O5. To gain
insight into the key factors affecting the NO3/N2O5
chemistry, the variation of N2O5 and the production rate of
NO3 were examined in relation to some relevant gases and meteorological
parameters on the selected five nights. Figure 5 shows the data for the night from 4 to
5 January as an example. Some common features were identified for all five nights. In
general, low wind speed (<2.0 m s-1) at night facilitated the
accumulation of air pollutants, and high RH was favorable for N2O5
heterogeneous uptake. In addition, high aerosol surface area density provided
interfaces for N2O5 heterogeneous reactions.
In the first couple of hours after sunset (Fig. 5, red rectangle),
N2O5 exhibited a peak and then quickly dropped to hundreds of pptv,
while nitrate and ClNO2 concurrently increased, which was indicative of
the local production and loss of N2O5. NO was below the detection
limit during this period. The production rates of NO3
(PNO3=kNO2+O3NO2O3)
were the fastest just after sunset and decreased gradually due to reduced
O3 levels. There was a period later in the night (22:00 to 01:00)
when fresh emissions of NO were observed, and the production of NO3
was suppressed due to the titration of O3 by NO. In the later
nighttime, NO was below the detection limit (Fig. 5, blue rectangle). During
this period, NO3 and N2O5 were produced at moderate
rates, and the very low N2O5 concentrations (below the detection
limit) suggested a fast loss of N2O5 probably leading to the local
production of ClNO2 and nitrate, which was not revealed in the observed
variations of ClNO2 and nitrate. The concentrations of ClNO2 and
nitrate during this period fluctuated due to the change in the air masses
indicated by the change in SO2 concentrations and wind speeds.
The N2O5 uptake coefficient and ClNO2 yield
The N2O5 uptake coefficient and ClNO2 yield, in
combination with the reactivity of NO3 with NO and VOCs, determines the loss
pathways of NO3 and N2O5. To derive the uptake
coefficient of N2O5, a method suggested by McLaren et al. (2010)
was applied which treated NO3 and N2O5 as a whole
([NO3]+[N2O5]) without assuming that the chemical
system was in a steady state. This approach considers that the change of
NO3 and N2O5 concentrations is mainly due to
NO3/N2O5 chemistry; thus, it requires that
the air mass has relatively stable chemical conditions and is not subject to
fresh NO emissions. It also requires that ClNO2 is produced from
the N2O5 chemistry and has an increasing trend to derive the
yield of ClNO2. This method is applicable for the early nighttime
(Fig. 5, red rectangle; Sect. 3.2.1) for these five nights.
The variation rate of [NO3]+[N2O5] can be calculated
by deducting the production rate of [NO3]+[N2O5] with
its loss rate as follows:
d([N2O5]+[NO3])dt=PNO3-LN2O5+NO3
Average N2O5 concentration values, N2O5
uptake coefficients, ClNO2 yields, and other related parameters and
maximum ClNO2 concentration values in the early nighttime for the five
selected nights.
Date/time
N2O5
Max ClNO2
NO2
O3
RH
Sa
PNO3
kNO3′
LN2O5
kNO3′/(Keq[NO2])
kN2O5′
γN2O5
ϕClNO2
(pptv)
(pptv)
(ppbv)
(ppbv)
(%)
(µm2 cm-3)
(ppbv h-1)
(10-3 s-1)
(ppbv h-1)
(10-5 s-1)
(10-3 s-1)
3 January 17:40–19:00
200
1029
20
78
59
2170
4.3
0.516
4.3
3.03
8.81
0.066
0.18
4 January 17:00–22:00
700
4608
24
61
82
6452
3.3
1.54
3.2
6.07
4.16
0.009
0.32
5 January 17:00–22:00
338
4828
18
73
81
8399
3.4
0.790
3.3
4.06
9.00
0.015
0.29
6 January 17:00–22:40
326
2908
13
82
77
5092
2.8
0.677
2.6
4.95
3.78
0.013
0.20
9 January 19:00–00:20
121
2553
19
41
85
5173
1.9
0.516
1.9
1.40
4.28
0.015
0.28
The loss of [NO3]+[N2O5] is via the reaction
of NO3 with VOCs and N2O5 heterogeneous reactions, which can both
be expressed as pseudo-first-order losses:
LN2O5+NO3=LNO3+LN2O5=k′NO3NO3+k′N2O5N2O5,
where kNO3′ and kN2O5′ represent the total first-order rate constants
for NO3 and N2O5, respectively. The loss rate of
N2O5 can then be obtained from Eq. (3):
LN2O5=kN2O5′N2O5=kNO2+O3NO2O3-dN2O5dt-d[NO3]dt-kNO3′NO3
Because NO3 was not measured, it was calculated by assuming an
equilibrium of NO2–NO3–N2O5 as shown in Eq. (4).
High levels of NO would break this equilibrium. Thus, the periods with
detected NO were excluded. d[NO3]/dt and
d[N2O5]/dt were
calculated as the rate of change of NO3 and N2O5, respectively, at a
time resolution of 10 min. kNO3′ was derived using the measured
concentrations of NMHCs as in Eq. (5) by interpolating the data of the NMHCs to a 10 min time
resolution. The NO3 reactivity with VOCs (kNO3′) in
the early nighttime ranged from 0.516 to 1.54×10-3 s-1 (Table 2), which was
higher than those derived at Mt. TMS in winter 2013
(0.17 to 1.1×10-3 s-1) (Brown et al., 2016), but
lower than those in the North China Plain during the summertime (2 to
57×10-3 s-1) (Tham et al., 2016; H. Wang et al.,
2017, 2018b; Z. Wang et al., 2017). NMHCs were not measured from 9 to 10 January 2017.
We used the average kNO3′ in the early nighttime from 3 to 4 January as
a replacement because these two periods had similar pollution levels for most
pollutants. For the later nighttime (Fig. 5, blue rectangle), low levels of
N2O5 and moderate levels of PNO3 made Eq. (3)
inapplicable even though NO was not detected.
NO3=N2O5NO2⋅Keqk′NO3=∑ki[VOCi]
Finally, the uptake coefficient of N2O5 was derived using Eq. (6)
for every 10 min and averaged for each selected period.
In Eq. (6), CN2O5
is the mean molecular speed of N2O5, and Sa is the aerosol
surface area density. The yield of ClNO2 was derived via
dividing the integrated production of ClNO2 ([ClNO2]max)
by the integrated loss of N2O5 since sunset as Eq. (7).
k′N2O5=LN2O5[N2O5]=14CN2O5SaγN2O5ϕ=[ClNO2]max∫LN2O5dt
The relative importance of the NO3 reactions with VOCs and
N2O5 heterogeneous reactions can be examined by comparing the
values of the loss coefficient of the NO3 reactions
k′NO3NO2⋅Keq and N2O5 heterogeneous
reactions (kN2O5′) (Tham et al., 2016). Based on the
calculations, the values of k′NO3NO2⋅Keq were 1.40×10-5 to 6.07×10-5 s-1 (see Table 2), while those of
kN2O5′ were 3.78×10-3 to 9.00×10-3 s-1, which was 2 orders of magnitude higher than
k′NO3NO2⋅Keq, suggesting that N2O5 heterogeneous
reactions were the dominant loss pathway for both NO3 and
N2O5.
Average values of the N2O5 loss rate and related parameters
for selected periods in the later nighttime.
Date/time
NO2
O3
PNO3
k'NO3
LN2O5
(ppbv)
(ppbv)
(ppbv h-1)
(10-3 s-1)
(ppbv h-1)
3–4 January 21:00–05:00
20.8
20.7
1.00
0.684
1.00
5 January 01:30–06:50
22.4
19.5
0.96
1.45
0.96
5–6 January 23:40–01:10
21.1
25.5
1.26
1.13
1.26
6–7 January 23:00–06:00
22.1
14.4
0.82
0.709
0.82
10 January 01:50–03:30
24.8
15.6
0.90
–
0.90
The average γN2O5 and ϕClNO2 derived for
the early night in the five cases are listed in Table 2. The data show that
the uptake coefficient ranged from 0.009 to 0.066, which was comparable with
previous values derived at Mt. Tai Mo Shan (TMS) in Hong Kong (0.004 to
0.022) (Brown et al., 2016) and in the North China Plain (0.006 to 0.102)
(Tham et al., 2016, 2018; H. Wang et al., 2017, 2018b; X. Wang
et al., 2017; Z. Wang et al., 2017; Zhou et al., 2018). It is interesting to
see a much higher γN2O5 value (0.066) on 3 January than those on
other four nights (0.009–0.015), which resulted from higher PNO3 but
much lower Sa and relatively low N2O5 concentrations on
3 January. We examined known factors affecting the loss of NO3 and
N2O5 such as the concentrations of NO, NMHCs, and aerosol
compositions, but found no obvious difference between 3 January and the other
nights. The yield in this study varied from 0.18 to 0.32, which was similar
to most studies in China (Tham et al., 2016, 2018; Z. Wang et
al., 2017; Yun et al., 2018; Zhou et al., 2018).
The uncertainty of the above γN2O5 was estimated to be ±45 % due to the measurement uncertainty of N2O5 (±25 %), NO2 (±20 %), O3 (±5 %), and Sa
(±30 %). The uncertainty of ϕClNO2 was mainly caused by the
uncertainty of NO2 (±20 %), O3 (±5 %), and
ClNO2 (±25 %) and was estimated to be ±30 %. The
correlation between γN2O5, ϕClNO2, and the concentrations of
aerosol compositions (see Table S2) or RH was investigated, and the results
(not shown here) did not indicate any significant dependence of
γN2O5 or ϕClNO2 on these parameters.
Comparison between the measured NO3- increase and the
NO3- formation potential in the early nighttime (periods in
Table 2: 3 January 17:40–19:00, 4 January 17:00–22:00, 5 January
17:00–22:00, 6 January 17:00–22:40, and 9 January 19:00–00:20) and in the
later nighttime (periods in Table 3: 3–4 January 21:00–05:00, 5 January
01:30–06:50, 5–6 January 23:40–01:10, 6–7 January 23:00–06:00, and
10 January 01:50–03:30).