Atmospheric 1 17 O ( NO − 3 ) reveals nocturnal chemistry dominates nitrate production in Beijing haze

The rapid mass increase of atmospheric nitrate is a critical driving force for the occurrence of fine-particle pollution (referred to as haze hereafter) in Beijing. However, the exact mechanisms for this rapid increase of nitrate mass have not been well constrained from field observations. Here we present the first observations of the oxygen-17 excess of atmospheric nitrate (1O(NO3 )) collected in Beijing haze to reveal the relative importance of different nitrate formation pathways, and we also present the simultaneously observed δN(NO3 ). During our sampling period, 12 h averaged mass concentrations of PM2.5 varied from 16 to 323 μg m−3 with a mean of (141±88(1SD)) μg m−3, with nitrate ranging from 0.3 to 106.7 μg m−3. The observed1O(NO3 ) ranged from 27.5 ‰ to 33.9 ‰ with a mean of (30.6±1.8) ‰, while δN(NO3 ) ranged from −2.5 ‰ to 19.2 ‰ with a mean of (7.4± 6.8) ‰. 1O(NO3 )-constrained calculations suggest nocturnal pathways (N2O5+H2O/Cl and NO3+HC) dominated nitrate production during polluted days (PM2.5 ≥ 75 μg m−3), with a mean possible fraction of 56–97 %. Our results illustrate the potentiality of 117O in tracing nitrate formation pathways; future modeling work with the constraint of isotope data reported here may further improve our understanding of the nitrogen cycle during haze.


Introduction
Severe and frequent haze pollution has become a crucial threat for the air quality in the megacity of Beijing and the North China Plain in recent years.The high concentrations of PM 2.5 (particulate matter with an aerodynamic diameter equal or less than 2.5 µm) during severe haze, of which the hourly average can reach 1000 µg m −3 (B.Zheng et al., 2015), are harmful to public health by contributing to cardiovascular morbidity and mortality (Cheng et al., 2013;Brook et al., 2010).Nitrate is an important component of PM 2.5 , accounting for 1-45 % of PM 2.5 mass in Beijing and the North China Plain (Wen et al., 2015;B. Zheng et al., 2015;G. Zheng et al., 2015).The main formation pathways of atmospheric nitrate, defined herein as gas-phase HNO 3 plus particulate NO − 3 , in the urban area are summarized in Fig. 1, which includes (i) NO 2 oxidation by OH radicals in the gas phase, (ii) heterogeneous uptake of NO 2 on wet aerosols, (iii) NO 3 radicals reacting with hydrocarbon (HC), and (iv) heterogeneous uptake of N 2 O 5 on wet aerosols and chlorine-containing aerosols.Since OH radicals are mainly present in the daytime, while NO 3 radicals and N 2 O 5 are mainly present in the nocturnal atmosphere (Brown and Stutz, 2012), NO 2 + OH is usually referred to as the daytime nitrate formation pathway, while N 2 O 5 + H 2 O/Cl − and NO 3 + HC are referred to as nocturnal formation pathways (Vicars et al., 2013;Sofen et al., 2014).During haze 3 ) traces nitrate production in haze Figure 1.Simplified schematic of the main nitrate formation pathways in urban air; "het."refers to heterogeneous reactions on aerosols.
in Beijing, the mixing ratio of daytime OH is modeled to be low (G.Zheng et al., 2015;Rao et al., 2016), while a relatively high mixing ratio of nocturnal N 2 O 5 is observed in several studies (Wang et al., 2017a, b;Li et al., 2018); therefore, nocturnal pathways are suggested to be most responsible for the high concentrations of atmospheric nitrate during haze (Su et al., 2017;Pathak et al., 2009Pathak et al., , 2011)).In addition, the high PM 2.5 concentration and relative humidity during haze in Beijing favor heterogeneous reactions, which renders NO 2 + H 2 O a potentially significant pathway for nitrate production (J.Wang et al., 2017;Tong et al., 2015;B. Zheng et al., 2015).
Nitrogen isotopic composition of nitrate (δ 15 N(NO − 3 ), wherein δ 15 N = (R sample /R reference − 1), with R representing isotope ratios of 15 N/ 14 N in the sample and the reference atmospheric N 2 ) is useful in tracing the source of its precursor NO X (Xiao et al., 2015;Beyn et al., 2014;Fang et al., 2011;Hastings et al., 2013).Anthropogenic sources of NO X such as coal combustion are generally enriched in δ 15 N, while natural NO X sources such as soil emissions or lighting typically have a negative or zero δ 15 N signature (Hoering, 1957; Yu and Elliott, 2017; Felix et al., 2012).Therefore highly positive values of observed δ 15 N(NO − 3 ) can be considered as an indicator of anthropogenic combustion (Elliott et al., 2009;Fang et al., 2011), although this judgment may be influenced by isotopic exchange between NO and NO 2 (Freyer et al., 1993;Walters et al., 2016), isotopic fractionations associated with nitrate formation pathways, and isotopic effects occurring during transport, such as deposition of NO − 3 and HNO 3 partitioning between the gas and particle phase (Freyer, 1991;Geng et al., 2014).The oxygen-17 excess ( 17 O) of nitrate, defined as 17 O = δ 17 O − 0.52δ 18 O, wherein δ X O = (R sample /R reference − 1), with R representing isotope ratios of X O/ 16 O in the sample and the reference Vienna Standard Mean Ocean Water and X = 17 or 18, is particularly useful in reflecting nitrate formation pathways (Michalski et al., 2003).Atmospheric nitrate from nocturnal reaction pathways has higher 17 O than that from daytime OH oxidation at a given 17 O(NO 2 ) (Table 1).And once formed, atmospheric 17 O(NO − 3 ) cannot be altered by mass-dependent processes such as deposition during trans-port (Brenninkmeijer et al., 2003).Previous studies have shown the utility of atmospheric 17 O(NO − 3 ) in quantifying the relative importance of various nitrate formation pathways (Alexander et al., 2009;Michalski et al., 2003;Patris et al., 2007;Savarino et al., 2013;Vicars et al., 2013).For example, the 17 O(NO − 3 )-constrained box modeling work of Michalski et al. (2003) suggests that more than 90 % of atmospheric nitrate is from nocturnal N 2 O 5 +H 2 O pathways in winter in La Jolla, California, which is reflected by the highest 17 O(NO − 3 ) values being observed in winter.In another study, Alexander et al. (2009)  2 Materials and methods

Sampling and atmospheric observations
PM 2.5 filter samples were collected at a flow rate of 1.05 m 3 min −1 using a high volume air sampler (model TH-1000C II, Tianhong Instruments Co., Ltd, China).The quartz microfiber filter (Whatman Inc., UK) is pre-combusted at 450 • C for 4 h before sampling.Our sampling period lasted from October 2014 to January 2015, with the collection interval being 12 h (08:00-20:00 LT or 20:00-08:00 LT) for each sample.Blank control samples were also collected.The blank was sampled identically to the real sample except that the collection interval was 1 min.Due to the fact that gaseous HNO 3 is likely to adsorb onto particulate matter already trapped by the filter material (Vicars et al., 2013), the nitrate species collected is likely to include both particulate nitrate and gaseous HNO 3 , which is referred to as atmospheric nitrate in previous studies (Vicars et al., 2013;Morin et al., 2009;Michalski et al., 2003) and in this study.The sampling site is at the campus of University of the Chinese Academy of Sciences (40.41 • N, 116.68 • E; ∼ 20 m high) in suburban Beijing, about 60 km northeast of downtown Beijing (Fig. 2), which is a supersite set up by HOPE-J 3 A (Haze Observation Project Especially for Jing-Jin-Ji Area), with various observations being reported (Zhang et al., 2017;Xu et al., 2016;Chen et al., 2015;Tong et al., 2015;He et al., 2018) ) is equal to 0 ‰ in the troposphere (Morin et al., 2011); in contrast, observations suggest 17 O(HO 2 ) = 1-2 ‰ (Savarino and Thiemens, 1999).However, the difference in calculated 17 O(NO − 3 ) between assuming 17 O(HO 2 ) = 0 ‰ and 17 O(HO 2 ) = 2 ‰ is negligible in this study (< 0.1 ‰).And the assumption that 17 O(HO 2 ) = 0 ‰ simplifies calculations and is also consistent with previous studies (Michalski et al., 2003;Alexander et al., 2009;Morin et al., 2008;Kunasek et al., 2008;Sofen et al., 2014).α is the proportion of O 3 oxidation in NO 2 production rate, calculated by Eq. ( 3).b Previous studies suggest that in R7 one oxygen atom of NO − 3 is from H 2 O and the other two are from NO 2 (Li et al., 2010;Cheung et al., 2000;Goodman et al., 1999)

Measurements of ions and isotopic ratios
Ion concentrations of NO − 3 and Cl − were measured in Anhui Province Key Laboratory of Polar Environment and Global Change in the University of Science and Technology of China.A detailed description of this method can be found in the literature (Ye et al., 2015).Briefly, ions in the PM 2.5 filter sample were extracted with Millipore water (≥ 18 M ) and insoluble substances in the extract were filtered.Then the ion concentrations were analyzed using an ion chromatograph system (model Dionex ICS-2100, Thermo Fisher Scientific Inc., USA).The measured ion concentrations of blank samples were subtracted when determining the ion concentrations of real samples.Typical analytical precision by our method is better than 10 % relative standard deviation (RSD) (Chen et al., 2016).
δ 15 N(NO − 3 ) and 17 O(NO − 3 ) were measured with a bacterial denitrifier method (Kaiser et al., 2007) in IsoLab at the University of Washington, USA.Briefly, ions in the filter sample were extracted with Millipore water (≥ 18 M ) and the insoluble substances were filtered.NO − 3 in each sample was converted to N 2 O by the denitrifying bacteria, Pseudomonas aureofaciens.Then N 2 and O 2 , which were decomposed from N 2 O in a gold tube at 800 • C, were separated using a gas chromatograph.The isotopic ratios of each gas were then measured by a Finnigan Delta-Plus Advantage isotope ratio mass spectrometer.Masses of 28 and 29 from N 2 were measured to determine δ 15 N. Masses of 32, 33, and 34 from O 2 were measured to determine δ 17 O and δ 18 O, and 17 O was then calculated.We use international nitrate reference materials, USGS34, USGS35, and IAEANO 3 , for data calibration.The uncertainty (1σ ) of δ 15 N and 17 O measurements in our method is 0.4 ‰ and 0.2 ‰, respectively, based on replicate analysis of the international reference materials.All the samples including blank samples were measured in triplicate to quantify the uncertainty in each sample.The blank was subtracted for each sample by using an isotopic mass balance on the basis of isotopic ratios and concentrations of the blank.To minimize the blank effect, samples with blank concentrations being > 10 % of their concentrations were not analyzed for isotopic ratios.This ruled out 3 of the total 34 samples, all of which are from non-polluted days (NPD, PM 2.5 < 75 µg m −3 ).In total, isotopic composi- 3 ) traces nitrate production in haze tions of 7 samples from NPD and 24 samples from polluted days (PD, PM 2.5 ≥ 75 µg m −3 ) are reported here.

Estimate of different nitrate formation pathways based on 17 O(NO −
3 ) The observed 17 O(NO − 3 ) is determined by the relative importance of different nitrate formation pathways and the relative importance of O 3 oxidation in NO X cycling as shown in Eq. ( 1): where 17 where α is the proportion of O 3 oxidation in NO 2 production rate, calculated by Eq. ( 3): In Eq. (3), k R1 , k R2a , and k R2b are, respectively, the reaction rate constants listed in Table 2. To evaluate α, we estimated HO 2 mixing ratios on the basis of empirical formulas between HO 2 and O 3 mixing ratios derived from observations in winter (Kanaya et al., 2007); i.e., [HO 2 ]/(pmol mol −1 ) = exp(5.7747 ) − 1.6363) at night.Then the RO 2 mixing ratio was calculated as 70 % of HO 2 mixing ratios based on previous studies (Liu et al., 2012;Elshorbany et al., 2012;Mihelcic et al., 2003).As the NO mixing ratio was not observed in our study, we estimated NO mixing ratios following the empirical formulas between NO X and CO mixing ratios derived from observations in winter Beijing (Lin et al., 2011); i.e., By using Eq. ( 2), the relative importance of nocturnal formation pathways (f R8 +f R9 +f R10 ) can be written as Eq. ( 4): Equation ( 4) suggests that the relative importance of nocturnal pathways is solely a function of the assumption of f R9 at given 17 O(NO − 3 ) and α.Sincef R9 , f R8 + f R10 , and f R8 + f R9 + f R10 should be in the range of 0-1 all the time, f R9 is further limited to meet Eq. ( 5): We estimated the relative importance of nocturnal pathways (f R8 + f R9 + f R10 ) by using concentration-weighted 17 O(NO − 3 ) observations and production-rate-weighted α from PD of each haze event rather than each sample due to the lifetime of atmospheric nitrate is typically on the order of day (Liang et al., 1998), larger than our sampling collection interval.

Simulation of surface N 2 O 5 and NO 3 radicals
To see whether the relative importance of nocturnal pathways constrained by 17 O(NO − 3 ) can be reproduced by models, we use the standard Master Chemical Mechanism (MCM, version 3.3; http://mcm.leeds.ac.uk/, last access: 3 September 2018) to simulate the mixing ratios of surface N 2 O 5 and NO 3 radicals during our sampling period.The input for this modeling work includes (i) 1 h averaged mixing ratios of observed surface CO, NO 2 , SO 2 , and O 3 and estimated NO (see Sect. 2.3), (ii) observed RH and T , and (iii) the mixing ratios of organic compounds from the literature (Table S1) (Wang et al., 2001;Wu et al., 2016;Rao et al., 2016).

Relationships between 17 O(NO −
3 ) and other data 3 ) shows a positive correlation with PM 2.5 concentration in Fig. 4b and NOR in Fig. 4c when NO − 3 < 50 µg m −3 (r = 0.71 and r = 0.80, p < 0.01, respectively).Figure 4d shows that 17 O(NO − 3 ) is negatively correlated with visibility in general (r = −0.66,p < 0.01).The significant decrease of visibility will largely reduce surface radiation and thereby OH mixing ratios (G.Zheng et al., 2015), which is unfavorable for nitrate production via the NO 2 + OH pathway.Since the NO 2 + OH pathway produces low 17 O(NO − 3 ) (Table 1), the decreased importance of the NO 2 + OH pathway will conversely increase 17 O(NO − 3 ).While the rise of RH accompanying the large increase of PM 2.5 favors nitrate production via the heterogeneous uptake of gases, e.g., N 2 O 5 (G.Zheng et al., 2015;B. Zheng et al., 2015), and the heterogeneous uptake of N 2 O 5 produces relative high 17 O(NO − 3 ) (Table 1), the enhanced heterogeneous uptake of N 2 O 5 will increase 17 O(NO − 3 ) too.Therefore, the decrease of the importance of NO 2 + OH and the increase of the importance of the heterogeneous uptake of N 2 O 5 should be responsible for the positive correlation between 17 O(NO − 3 ) and NO − 3 concentrations.In addition, for samples with NO − 3 > 50 µg m −3 , visibility was always low with narrow variations (2.3 ± 1.0 km), and RH was always high with a narrow range   (67 ± 7 %), which may be the reason for the relatively high 17 O(NO − 3 ) being observed (31.2±1.7 ‰). Figure 4f shows that 17 O(NO − 3 ) is not correlated with δ 15 N(NO − 3 ).

Estimate of nocturnal formation pathways
Before estimating the relative importance of different nitrate formation pathways, we estimate the proportion of O 3 oxidation in the NO 2 production rate, α.The possible α range can be calculated based on observed 17 O(NO − 3 ).It can be obtained from Table 1 3 ) < (25α + 14) ‰, so the lower limit of possible α is ( 17 O(NO − 3 ) −14 ‰) /25 ‰.And since 17 O(NO − 3 ) ≥ 27.5 ‰ in our observation, the higher limit of α is always 1 for all the samples.Figure 5 presents the possible range of calculated α based on 17 O(NO − 3 ).The calculated lower limit of α ranged from 0.56 to 0.81 with a mean of 0.68 ± 0.07, which directly suggests that O 3 oxidation played a dominated role in NO X cycling during Beijing haze.To estimate the specific α value, chemical kinetics in Table 2 and Eq.(3) were used.Specific α is estimated to range from 0.86 to 0.97 with a mean of (0.94 ± 0.03), which is in the possible range of α value calculated directly based on 17 O(NO − 3 ) (Fig. 5) and close to the range of 0.85-1 determined in other midlatitude areas (Michalski et al., 2003;Patris et al., 2007).
Figure 6a shows the estimated relative importance of nocturnal formation pathways (N 2   PD of case f R9 assumption (%)   3 ).Possible fractional contributions of nocturnal formation pathways range from 49 to 97 %, 58 to 100 %, 60 to 100 %, 45-90 % and 70-100 % on PD of Case I to V, respectively, with a mean of 56-97 %.This directly implies that nocturnal chemistry dominates atmospheric nitrate production in Beijing haze.This finding is consistent with the suggested importance of the heterogeneous uptake of N 2 O 5 during Beijing haze by previous studies (Su et al., 2017;Wang et al., 2017b).The other pathways (NO 2 + OH and NO 2 + H 2 O) account for the remaining fraction with a mean possible range of 3-44 %.Since NO 2 + OH and NO 2 + H 2 O produce the same 17 O(NO − 3 ) signature in our assumptions (Table 1), we cannot distinguish their fractional contributions purely from the observed 17 O(NO − 3 ) in the present study.However, the overall positive correlation between 17 O(NO − 3 ) and RH (r = 0.55, p < 0.01; Fig. 4e) suggests that the heterogeneous uptake of NO 2 should be less important than the heterogeneous uptake of N 2 O 5 ; otherwise, a negative relationship between 17 O(NO − 3 ) and RH is expected.Our calculations also suggest that the sum of possible fractional contributions of N 2 O 5 + Cl − and NO 3 + HC is in the range of 0-49 %, 17-58 %, 20-60 %, 0-45 %, and 41-70 % on PD of Case I to V, respectively, with a mean of 16-56 % (Table 4), which emphasizes that N 2 O 5 + Cl − and NO 3 + HC played a unignorable role in nitrate production during Beijing haze.Due to the fact that N 2 O 5 + Cl − and NO 3 + HC produce the same 17 O(NO − 3 ) in our assumptions (Table 1), we cannot distinguish their fractional contributions purely from the observed 17 O(NO − 3 ) in this study either.However, NO 3 + HC should be minor for nitrate production.For example, the 3-D modeling work of Alexander et al. (2009) suggests that the NO 3 + HC pathway only accounts for 4 % of global tropospheric nitrate production annually on average, and Michalski et al. (2003) found that the NO 3 + HC pathway contributes 1-10 % to nitrate production on the basis of an annual observation at La Jolla, California, with low values in winter.Therefore, in addition to NO 3 + HC, N 2 O 5 + Cl − is likely to also contribute to nitrate production during haze in Beijing.In support of this, the concentrations of Cl − are as high as (5.5 ± 4.1) µg m −3 during PD of all the cases in our observation and the mixing ratios of ClNO 2 , an indicator of the N 2 O 5 + Cl − pathway, reached up to 2.9 nmol mol −1 during a summer observation in suburban Beijing (Wang et al., 2018b) and reached up to 5.0 nmol mol −1 in another modeling work in summer rural Beijing (Wang et al., 2017c Figure 6b presents the simulated mixing ratios of surface N 2 O 5 and NO 3 radicals during our observational period by using the box model MCM.The 12 h averaged mixing ratios of simulated N 2 O 5 ranged from 3 to 649 pmol mol −1 , while simulated NO 3 radicals ranged from 0 to 27 pmol mol −1 .In comparison, previous observations in Beijing suggest that 5 s averaged N 2 O 5 can be as high as 1.3 nmol mol −1 and 30 min averaged NO 3 radicals can be as high as 38 pmol mol −1 , with large day-to-day variability (Wang et al., 2015(Wang et al., , 2017b)).During Case I and II in October, simulated N 2 O 5 and NO 3 radicals present similar trends with the observed NO − 3 and remain relatively high during PD (346 ± 128 and 9 ± 7 pmol mol −1 , respectively, Fig. 6b), which supports the dominant role of nocturnal formation pathways suggested by 17 O(NO − 3 ).However, during Case III-V in the residential heating season, the simulated surface mixing ratios of N 2 O 5 and NO 3 radicals remain relatively low during PD (63 ± 80 and < 1 pmol mol −1 , respectively, Fig. 6b), which seems to be inconsistent with 17 O(NO − 3 ) observations.We note that a recent study suggests that the heterogeneous uptake of N 2 O 5 is negligible at the surface but larger at higher altitudes (e.g., > 150 m) during winter haze in Beijing (Wang et al., 2018a).So during PD of Case III-V in our observational period, large nitrate production via heterogeneous uptake of N 2 O 5 may occur aloft rather than at the surface, which leads to the dominant role of nocturnal formation pathways as suggested by 17 O(NO − 3 ).

Conclusions
We report the first observation of isotopic composition ( 17 O and δ 15 N) of atmospheric nitrate in Beijing haze.The observed 17 O(NO − 3 ) ranged from 27.5 ‰ to 33.9 ‰ with a mean of (30.6 ± 1.8) ‰. δ 15 N(NO − 3 ) ranged largely from −2.5 ‰ to 19.2 ‰ with a mean of (7.4±6.8)‰.When NO − 3 is < 50 µg m −3 , a positive correlation was observed between 17 O(NO − 3 ) and NO − 3 concentration (r = 0.81, p < 0.01).This is likely to result from the variation of relative importance of different nitrate formation pathways.Calculations with the constraint of 17 O(NO − 3 ) suggest that nocturnal pathways (N 2 O 5 + H 2 O/Cl − and NO 3 + HC) dominated nitrate production during polluted days (PM 2.5 ≥ 75 µg m −3 ), with a mean possible contribution of 56-97 %. 17 O(NO − 3 ) also indicates that O 3 dominated NO oxidation during Beijing haze.
Data availability.All data needed to draw the conclusions are present in the main text and/or the Supplement.For additional data, please contact the corresponding author (zqxie@ustc.edu.cn).
Author contributions.ZX conceived this study.PH conducted isotope measurements.PH, XC, SF, HZ, and HK performed the field experiments and ion measurements.PH, ZX, and XY interpreted the data.CL contributed to the field observation support.PH wrote the manuscript with input from ZX.All authors were involved in the discussion and revision of the manuscript.
Competing interests.The authors declare that they have no conflict of interest.Special issue statement.This article is part of the special issue "Multiphase chemistry of secondary aerosol formation under severe haze".It is not associated with a conference.
use observed 17 O(NO − 3 ) to constrain a 3-D model and found that the daytime NO 2 +OH pathway dominates global tropospheric nitrate production, with an annual mean contribution of 76 %.Until now, however, field observations of atmospheric 17 O(NO − 3 ) have not been conducted in north China to constrain the relative importance of different nitrate formation pathways during haze.In this work, we present the first observations of atmospheric 17 O(NO − 3 ) during Beijing haze from October 2014 to January 2015, and use this observation to examine the importance of nocturnal formation pathways.We also present the signature of simultaneously observed δ 15 N(NO − 3 ).

Figure 2 .
Figure 2. A brief map of the sampling site in Beijing.The map scale of base map is 1 : 1 250 000.Huairou station is set up by the Beijing Municipal Environmental Monitoring Center, where hourly PM 2.5 , SO 2 , CO, NO 2 , and O 3 were observed.

Figure 3 .
Figure 3. General characteristics of haze events in Beijing (October 2014-January 2015).(a) Time series of PM 2.5 and NO − 3 concentrations.(b) Time series of nitrogen oxidation ratio (NOR, which is equal to the NO − 3 molar concentration divided by the sum of NO − 3 and NO 2 molar concentration) and Cl − concentrations.(c) Time series of 17 O(NO − 3 ) and visibility.(d) Time series of δ 15 N(NO − 3 ) and relative humidity (RH).The error bars in (c, d) are ±1σ of replicate measurements (n = 3) of each sample.The khaki shaded area indicates polluted days (PD, PM 2.5 ≥ 75 µg m −3 ).

Figure 4 .
Figure 4. Relationships between 17 O(NO − 3 ) and other parameters.The relationship between 17 O(NO − 3 ) and NO − 3 concentrations (a), PM 2.5 concentrations (b), nitrogen oxidation ratio (NOR, c), visibility (d), relative humidity (RH, e), and δ 15 N(NO − 3 ) (f).The dark red dots are samples with NO − 3 < 50 µg m −3 and the orange dots are samples with NO − 3 > 50 µg m −3 .The black dashed lines are linear least-squares fitting lines for all samples, the dark red solid lines are linear least-squares fitting lines for samples with NO − 3 < 50 µg m −3 , and the orange solid lines are linear least-squares fitting lines for samples with NO − 3 > 50 µg m −3 .The error bars are ±1σ of replicate measurements of each sample.

Figure 5 .
Figure 5. Estimate of the proportion of O 3 oxidation in NO X cycling, α.The gray column represents the possible α range determined by 17 O(NO − 3 ).The blue dot represents a specific α value calculated by Eq. (3).

Figure 6 .
Figure 6.Estimate of the nocturnal formation pathways.The estimated relative importance of nocturnal formation pathways (f R8 + f R9 + f R10 ) during PD of each case on the basis of observed 17 O(NO − 3 ) (see Sect. 2.3, a) and the simulated mixing ratios of N 2 O 5 and NO 3 radicals by MCM (b).R8, R9, and R10 in (a) represent NO 3 + HC, N 2 O 5 + H 2 O, and N 2 O 5 + Cl − pathways, respectively.

Table 2 .
Reaction expressions for different NO 2 production pathways.

Table 4 .
The possible range of fractional contribution of different nitrate formation pathways during PD of each case estimated on the basis of observed 17 O(NO − 3 ) a .