NO3 and N2O5 chemistry at a suburban site during the EXPLORE-YRD campaign in 2018

Atmospheric Environment xxx (xxxx) xxx

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Atmospheric Environment journal homepage: http://www.elsevier.com/locate/atmosenv

NO3 and N2O5 chemistry at a suburban site during the EXPLORE-YRD campaign in 2018 Haichao Wang a, 1, Xiaorui Chen a, 1, Keding Lu a, b, *, Renzhi Hu c, **, Zhiyan Li c, Hongli Wang d, Xuefei Ma a, Xinping Yang a, Shiyi Chen a, Huabin Dong a, Ying Liu a, Xin Fang a, Limin Zeng a, Min Hu a, b, Yuanhang Zhang a, b, e a

State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Nanjing University of Information Science & Technology, Nanjing, 210044, China c Key Lab. of Environmental Optics and Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, 230031, China d State Environmental Protection Key Laboratory of Formation and Prevention of the Urban Air Complex, Shanghai Academy of Environmental Sciences, Shanghai, 200223, China e CAS Center for Excellence in Regional Atmospheric Environment, Chinese Academy of Science, Xiamen, 361021, China b

H I G H L I G H T S

� Low N2O5 was observed with a nocturnal NO3 production rate of 1.01 � 0.47 ppbv h 1. � Rapid losses of NO3 and N2O5 attributed to high monoterpenes and fast N2O5 uptake. � NO3 and N2O5 chemistry was responsible for more than half of daily NOx removal. A R T I C L E I N F O

A B S T R A C T

Keywords: Nitrate radical Dinitrogen pentoxide Monoterpenes Nighttime chemistry NOx removal

During the EXPLORE-YRD campaign (EXPeriment on the eLucidation of the atmospheric Oxidation capacity and aerosol foRmation, and their Effects in Yangtze River Delta) in May–June 2018, we measured N2O5, NO2, O3 and relevant parameters at a regional site in Taizhou, Jiangsu Province. The nocturnal average NO3 production rate was 1.01 � 0.47 ppbv h 1, but the mixing ratio of N2O5 was low, with a maximum of 220 pptv in 1 min, sug­ gesting rapid loss of NO3 and N2O5. The nocturnal steady-state lifetime of N2O5 was 43 � 52 s on average, which may be attributed to the elevated monoterpene and fast N2O5 uptake. VOCs (mainly monoterpenes) dominated daily NO3 loss with the percentage of 36.4% and N2O5 uptake accounted for 14.4%, when taking NO þ NO3 and NO3 photolysis into consideration. We demonstrated that the nonnegligible daytime NO3 oxidation of mono­ terpene in YRD region, which contributes to the daytime formation of organic nitrate and secondary organic aerosol. The daily average NOx consumption rate via rapid NO3 reaction reached 0.63 ppbv h 1, corresponding to 57.3% NOx loss in comparison with the OH oxidation pathway at this site, highlighting the key role of NO3 and N2O5 in NOx removal and subsequent photochemistry in the YRD region.

1. Introduction

degradation of volatile organic compounds (VOCs) and air quality at regional and global scales (Wayne et al., 1991). NO3 is predominantly formed by the reaction of NO2 with O3 (R1). N2O5 is formed by the re­ action of NO3 with NO2 (R2), with which it is in thermal equilibrium

The chemistry of reactive nitrogen species, including nitrate radical (NO3) and dinitrogen pentoxide (N2O5), affect the lifetime of NOx, the

* Corresponding author. State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China. ** Corresponding author. E-mail addresses: [email protected] (K. Lu), [email protected] (R. Hu). 1 The first two authors contributed equally to this work and should be considered as the co-first authors. https://doi.org/10.1016/j.atmosenv.2019.117180 Received 25 August 2019; Received in revised form 11 November 2019; Accepted 22 November 2019 Available online 25 November 2019 1352-2310/© 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Haichao Wang, Atmospheric Environment, https://doi.org/10.1016/j.atmosenv.2019.117180

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Fig. 1. Map of the field measurement site (red diamond) in Taizhou, Jiangsu Province, which is located approximately 200 km northwest of Shanghai; the inset plot is the wind rose for the sampling site during the campaign. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

(R2-R3). The production rate of NO3 (P(NO3)) is calculated from the NO2 and O3 concentrations and a reaction rate constant (kNO2þO3) (Eq. (1)). During the daytime, the rapid photolysis of NO3 in sunlight and its reaction with NO lead to a short NO3 lifetime (several seconds), so NO3 and N2O5 are usually below the instrument detection limit. In the dark, without sunlight to regenerate NO via basic photochemical cycling and in the absence of emissions sources, NO3 and N2O5 can accumulate in the atmosphere. NO2 þ O3 → NO3 þ O2

(R1)

NO2 þ NO3 þ M → N2O5 þ M

(R2)

N2O5 þ M → NO2 þ NO3 þ M

(R3)

P(NO3) ¼ kNO2þO3 � [NO2] � [O3]

N2O5 þ (H2O or Cl ) → (2 - φ) NO3 þ φClNO2 kN2O5 ¼ 0:25⋅c⋅γðN2 O5 Þ⋅Sa

(R4) (Eq. 2)

Here, c is the mean molecular speed of N2O5, Sa is the aerosol surface area density and γ(N2O5) is the N2O5 uptake coefficient. φ is the ClNO2 yield. N2O5 heterogeneous hydrolysis is one of the major uncertainties in NO3 budget studies since the N2O5 uptake coefficient is highly variable and difficult to quantify and parameterize (Chang et al., 2011). High mixing ratios of NO3 associated with high NO3 reactivity have been observed in Chinese megacities, such as Beijing and Shanghai (Wang et al., 2013, 2015); N2O5 has been measured widely in North China and Hong Kong in recent years (Tham et al., 2016; Wang et al., 2018b; Wang et al., 2017f; Xia et al., 2019; Yan et al., 2019; Yun et al., 2018b). They found high N2O5 uptake coefficients which even up to 0.1 during summer (Tham et al., 2016; Wang et al., 2018b; Xia et al., 2019), but the key mechanism regulating the uptake processes is still unclear. Observations and model simulations have revealed that N2O5 hetero­ geneous uptake is an important pathway of pNO-3 formation (Su et al., 2017; Sun et al., 2018; Wang et al., 2017b, 2017c, 2019; Wen et al., 2015; Zheng et al., 2015) and considerably contributes to NOx removal in China (Wang et al., 2017c; Yun et al., 2018a). Large amounts of NOx have been emitted for the past several decades in China, but comprehensive field studies of the chemical processes of reactive nitrogen oxides such as NO3 and N2O5 remain sparse. The urban agglomerations in the Yangtze River Delta (YRD) are among the largest urban agglomerations worldwide and are key areas for air pollution control in China. As one of the fastest developing areas in China, the YRD is now suffering severe air pollution with high concentrations of both ozone and fine particles, so-called photochemical smog. To better un­ derstand the air pollution mechanism in the YRD region, the EXPLOREYRD campaign was carried out in the summer of 2018 in Taizhou, Jiangsu Province. The site is located downwind of Shanghai at a distance of 200 km. In this study, we report the measurement of N2O5 and rele­ vant species during the campaign. The roles of NO3 and N2O5 in nighttime chemistry and their effect on NOx fate at this site will be characterized.

(1)

NO3 is an important oxidant in the atmosphere and is regarded as the nighttime analogue of hydroxyl radical (OH) for certain VOCs (Wayne et al., 1991), but the loss mechanism of NO3 and N2O5 strongly depends on the air mass in different geophysical regions; for example, the reac­ tion of NO3 with VOCs is significant downwind of industrial cities, and both the lifetime and concentration of NO3 are limited (Brown et al., 2011; Crowley et al., 2011; Geyer et al., 2003; Stutz et al., 2010). Tens of kilometers downwind of urban areas or above cities, where NO is titrated by O3 or free of ground-level NO emissions, NO3 can reach up to several hundred pptv, and N2O5 can reach up to a few ppbv (Asaf et al., 2009; Brown et al., 2016). The reaction of NO3 and VOCs accounts for organic nitrates (ONs) formation with a considerable mass yield (Ng et al., 2017). The oxidation of NO3 with isoprene has a secondary organic aerosol (SOA) mass yield of 23.8% (Ng et al., 2008), and the yield of NO3 with limonene can reach 174% at ambient temperatures (Boyd et al., 2017). As ONs are important precursors of SOA, the re­ actions of NO3 with BVOCs are critical in terms of both regional and global air quality (Ayres et al., 2015; Fry et al., 2009, 2014; Kiend­ ler-Scharr et al., 2016; Ng et al., 2017; Pye et al., 2010; Rollins et al., 2009). The heterogeneous hydrolysis of N2O5 produces soluble nitrate (HNO3 or NO3 ) and releases nitryl chloride (ClNO2) in chloridecontaining aerosols (R4) (Finlaysonpitts et al., 1989). The rate coeffi­ cient of N2O5 heterogeneous uptake is determined by Eq. (2). 2

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19:00 CNST.

Table 1 The observed gas and particle parameters during the campaign. Parameters

Limit of detection

Methods

Accuracy

N2O5

2.7 pptv (1σ, 1 min)

CEAS

�19%

N2O5

3.1 pptv (1σ, 2.5 s)

CRDS

�15%

OH

0.32 � 10 cm (1σ, 32 s)

LIF

�11%

NO

60 pptv (2σ, 1 min)

NO2

0.3 ppbv (2σ, 1 min)

PCc þ CL

�20%

0.5 ppbv (2σ, 1 min)

UV photometry

�5%

PNSD PM2.5

14 nm–700 nm (4 min) 0.1 m-3μg (1 min)

SMPS TEOMd

�20% �5%

20-300 pptv (60 min) 20 pptv (10 s) 5 � 10 5 s 1 (1 min) 0.06 ppbv (30 min) 0.05 μg m 3 (30 min)

GC-MS PTR-MS SRe GAC-ICf GAC-ICf

�15% �15% �10% �20% �20%

O3

VOCs Monoterpene Photolysis frequency HNO3, NO3, HCl, 2NHþ 4 , NO3 , Cl , SO4

a b c d e f

6

3

a

CLb

2.2. Instrumentation A comprehensive suite of trace gas compounds and aerosol proper­ ties was measured in the field study, and the related instruments are given in Table 1. N2O5 was measured by a cavity enhanced absorption spectrometer, CEAS (Wang et al., 2017a). Ambient N2O5 is thermally decomposed to NO3 in a perfluoroalkoxy alkane (PFA) tube heated to 120 � C and is then detected within a PFA-coated resonator cavity, which is heated to 80 � C to prevent the formation of N2O5 by reversible reac­ tion. Ambient gas is sampled with a 2.5 m sampling line with a flow rate of 2.0 L min 1. NO is injected for 30 s to destroy NO3 from N2O5 thermal decomposition every 5 min during the cycle. The corresponding mea­ surements are then used as a dynamic background, and the NO3 is titrated by NO. The ambient H2O is almost constant in both the back­ ground and sampling spectra in a cycle due to the short time scale. A Teflon polytetrafluoroethylene (PTFE) filter is used in the front of the sampling line to remove ambient aerosol particles. The filter is replaced with a fresh filter every 2 h to avoid a decrease in N2O5 transmission efficiency due to the increased loss of N2O5 from the accumulated aerosols on the filter. The loss of N2O5 in the sampling line and filter were considered in data correction. The limit of detection (LOD) was estimated to be 2.7 pptv (1σ) with an uncertainty of 19%. N2O5 was also measured by cavity ring down spectroscopy (CRDS) (Li et al., 2018a, 2018b). Fig. S1 shows the scatter plot of the data from CEAS and CRDS during the campaign. The N2O5 measured by CRDS is slightly higher than that measured by CEAS, which may be caused by the uncertainties of N2O5 transmission efficiency in the sampling line in each instrument. Overall, a reasonable agreement between the two instruments was achieved, with a correlation coefficient (R2) of 0.79, a slope of 1.24 and an intercept of 2.1 pptv. In the following analysis, the N2O5 data measured by CEAS will be used. NOx and O3 were measured by commercial instruments (Thermo Electron models 42i and 49i, respectively). The PM2.5 mass concentra­ tion was measured using a standard tapered element oscillating micro­ balance (TEOM, 1400A analyzer). OH radical was measured by laserinduced fluorescence, LIF (Tan et al., 2017, 2018). VOCs were measured using an automated gas chromatograph equipped with a mass

�20%

Laser-induced fluorescence. Chemiluminescence. Photolytic converter. Tapered element oscillating microbalance. Spectroradiometer. Gas and aerosol collector combined with ion chromatography.

2. Method 2.1. Sampling site The measurement site is in the north of Taizhou (32.56� , 119.99� ). The site is surrounded by flat fishponds and farmland. The closest road is more than 0.2 km away, and there are no major industrial surroundings. As shown in Fig. 1, the wind was mainly from the east and southeast during the measurement period, suggesting that the site is a receptor site in the YRD pollution region. The air masses in this site feature strong influences of both urban and biogenic emissions and occasionally biomass burning. Instruments were set up in five containers, with the inlets on the tops of the containers, and placed approximately 5 m above the ground. The data presented here were collected from May 18 to June 12, 2018. Time is given as Chinese National Standard Time (CNST ¼ UTC þ 8 h). During the campaign, sunrise was at 05:00 and sunset was at

Fig. 2. Time series of N2O5 measured by CEAS and CRDS, temperature, RH, NO2, O3, NO and PM2.5. The pink line in the O3 panel denotes the Chinese national air quality standard for O3 (ca. 93 ppbv). The dark gray and yellow strips on the top panel label air masses in the P1 and P2 periods, respec­ tively. The light gray shadowed areas in the panels represent the nighttime period. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

3

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Fig. 3. Mean diurnal profiles of NO2, O3, P(NO3) and N2O5 in this campaign; the error bar represents the standard deviation in each hour of day.

spectrometer or flame ionization detector (GC-MS). Monoterpenes were measured by proton transfer reaction mass spectrometry (PTR-MS) (de Gouw and Warneke, 2007). The photolysis frequencies were calculated from the spectral actinic photon flux density measured by a spectror­ adiometer (Bohn et al., 2008). The particle number and size distribution (PNSD) was measured by a scanning mobility particle sizer (SMPS, TSI 3936) and an aerosol particle sizer (APS, TSI 3321). SMPS was used to measure particles in the range between 14 nm and 523 nm in diameter, and APS was used to measure particles with a diameter range from 597.6 nm to 10.0 μm. The aerosol surface area density (Sa) was calculated based on the dry-state particle number and geometric diameter in each size bin (14 nm - 2.5μm), which was corrected to the wet particle-state Sa with a hygroscopic growth factor (Liu et al., 2013). The uncertainty of the wet Sa was estimated to be ~30%. Meteorological data, including the relative humidity (RH), temperature, pressure, wind speed, and wind direction, were also available. A gas and aerosol collector combined with ion chromatog­ raphy (GAC) instrument was used to measure the water-soluble gases and ionic constituents in PM2.5 (Dong et al., 2012).

Fig. 4. A case of observed and calculated N2O5, P(NO3) and NO values during the daytime on June 5th, 2018. The red dots represent the observed N2O5 values in 5 s intervals, and the black line represents the data averaged in 5 min intervals. The purple line is the calculated N2O5 concentration obtained from Eq. (3). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

in Fig. 3. NO2 and O3 are highly anti-correlated, leading to the double peak of P(NO3). The first and second peaks of P(NO3) occurred at 09:00–10:00 and 18:00–19:00, respectively, with values of 1.39 ppbv h 1 and 1.60 ppbv h 1. The daily average of P(NO3) was calculated to be 0.96 � 0.38 ppbv h 1. The average daytime and night­ time P(NO3) values were 0.90 � 0.47 ppbv h 1 and 1.01 � 0.47 ppbv h 1, respectively. The nocturnal P(NO3) was lower than values previously reported in the YRD (2–3 ppbv h 1), except for an observation at a suburban site in Shaoxing (Chen et al., 2019), and slightly lower than that observed in North China Plain (1–2 ppbv h 1) (Tham et al., 2016; Wang et al., 2017d, 2018b). The P(NO3) in the nocturnal residual layer on a Seoul tower from MAPS 2015 showed a comparable median value of 1.3 ppbv h 1 (Brown et al., 2017). The comparison implies that large P(NO3) values in urban agglomerations in Asia are ubiquitous, which implies a significant NO3 oxidizing capacity in these megacities. The chemical conditions in this study are similar to those in Beijing in summer, except for the low RH in Beijing (Wang et al., 2018b). The nocturnal average N2O5 of 8 pptv during the campaign, which is an order of magnitude lower than that observed in Beijing (Wang et al., 2017d, 2018b), suggests faster loss processes of NO3 and N2O5 in the YRD than in Beijing. In this study, the N2O5 concentration increased very quickly after sunset, peaked near 20:00, and then decreased rapidly until dropping below the limit of detection in the beginning of the sec­ ond half of night (Fig. 3). The diurnal pattern of N2O5 in the YRD is quite similar to that observed in urban Beijing (Wang et al., 2017d). After 20:00, the levels of P(NO3) and N2O5 decreased simultaneously, indi­ cating that a lack of NO3 formation is one of the reasons for the low N2O5 concentration.

3. Results 3.1. Overview of measurements During the measurement period, the temperature was high, with an average of 25 � 15 � C. RH was highly varied from 25% to 100%, with an average of 55 � 15%. The nocturnal RH was usually greater than 70%. Fig. 2 shows the time series of N2O5 and the relevant parameters. Depending on the metrological condition, we divided the air masses into two patterns. The P1 period included cloudy or rainy days that featured low O3, NO2 and PM2.5 mass concentrations. During the P1 period, covering eight days in total (05/18-05/22, 05/25-05/26, and 06/09), the average concentration of O3 was below 40 ppbv with little variation, and the wind was predominantly from the east. The mixing ratio of N2O5 remained almost completely below the instrument detection limit. The P2 pattern covered days excluding the P1 period and consisted of 18 days in total. In the P2 period (sunny days), the mixing ratios of NO2, O3, PM2.5 and N2O5 were elevated. Five days in the P2 period suffered O3 pollution, and the maximum 8 h O3 concentration was higher than the Chinese national air quality standard for O3 (93 ppbv). The maximum O3 concentration reached 150 ppbv, and the maximum N2O5 concentration was 220 pptv on a high-O3 day. During the measurement period, the mass concentration of PM2.5 was below 100 μg m 3, except in several biomass burning episodes observed during the second half of the night in 05/24, 05/29, 05/30, 06/03, and 06/05. The mean diurnal variations in N2O5, NO2, O3 and P(NO3) are shown

3.2. Daytime N2O5 Daytime NO3 and N2O5 chemistry is generally regarded as less important due to rapid NO3 photolysis and the titration reaction initi­ ated by NO. However, in this campaign, the daytime production rate of NO3 was large because of the elevated O3 and NO2 concentrations. Although the loss of NO3 is fast and the lifetime of NO3 and N2O5 is short during the daytime, the mixing ratio of NO3 and N2O5 may be nonzero. 4

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4. Discussion 4.1. Steady-state lifetimes of NO3 and N2O5 The steady-state lifetime is widely used to assess the fate of NO3 and N2O5 by assuming the production and loss of NO3 and N2O5 are in balance after sunset and can be expressed as Eqs. (4) and (5) (Allan et al., 1999; Brown et al., 2003; Platt et al., 1980).

τss ðN2 O5 Þ ¼

τss ðNO3 Þ ¼

Here, the mixing ratio of NO3 and N2O5 can be calculated by the steady state method, as shown in Eqs. (2) and (3) (Osthoff et al., 2006). kNO2þO3 ⋅½NO2 �⋅½O3 � kNO3þNO ⋅½NO� þ jðNO3 Þ

½N2 O5 �ðdayÞ ¼ Keq ⋅½NO2 �⋅½NO3 �ðdayÞ

1 ½NO3 � ¼ kss ðNO3 Þ kNO2þO3 ⋅½NO2 �⋅½O3 �

(4) (5)

Here, τss ðNO3 Þand τss ðN2 O5 Þdenote the steady-state lifetime of NO3 and N2O5, respectively; kss(NO3) and kss(N2O5) denote the loss rate of N2O5 and NO3 corresponding to the steady-state lifetime, respectively. Here, NO3 is calculated based on thermal equilibrium. The nocturnal N2O5 lifetime ranged from 1 s to 40 min with an average of 43 � 52 s (a me­ dian value of 29 s), and the average nocturnal NO3 lifetime was 12 � 26 s with a median of 5 s. The steady-state lifetime of N2O5 in this study is much shorter than that in Wangdu (77 s, average) and Beijing (270 s, average) (Tham et al., 2016; Wang et al., 2018a), suggesting high NO3–N2O5 reactivity and rapid loss of NO3 and N2O5 at this site. To understand the key factors regulating the N2O5 lifetime, a series of functional dependence analyses were conducted with the relevant pa­ rameters. Fig. 5(a) shows that the N2O5 lifetime had a clear negative dependence on Sa, indicating that N2O5 uptake is an important part of NO3 and N2O5 losses. As the aerosol liquid water content (ALWC) and Sa are enlarged due to aerosol hygroscopicity under high RH conditions, both of these parameters could promote N2O5 uptake (Chang et al., 2011). Here, we examined the relationship between the N2O5 lifetime and the parameters RH and ALWC. The calculation method of ALWC is detailed in the Supporting Information (Text S1). As shown in Fig. S2, the N2O5 lifetime also exhibited a clear negative dependence on RH and ALWC. The trend of RH was also similar to that observed at a moun­ taintop site with high RH in Hong Kong and a tower site in Seoul (Brown et al., 2016, 2017). The increased N2O5 uptake rate caused by RH (ALWC) effectively reduced the N2O5 lifetime, suggesting that the het­ erogeneous N2O5 process is an important NO3 and N2O5 loss pathway in the YRD. By concurrently observing the mixing ratio of total monoterpenes at this site, we found that total monoterpenes were abundant, with an average of 0.30 � 0.22 ppbv at a 1 min time resolution. Fig. 5(b) shows that the N2O5 steady-state lifetime depends negatively on the total monoterpene content, especially when the monoterpene concentration is larger than 0.4 ppbv, corresponding to a lifetime of N2O5 below 1 min. The trend of N2O5 lifetime with respect to monoterpenes provides field evidence that monoterpenes are a large sink of NO3 and N2O5 in the YRD region during summer.

Fig. 5. Box and whisker plots of the functional dependence of the nocturnal N2O5 steady state lifetime on the (a) aerosol surface area density (Sa) and (b) monoterpenes concentration. The intervals of Sa and monoterpenes concen­ tration are set to 1000 μm2 cm 3 and 0.2 ppbv, respectively. The pink dots represent the scattered data points. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

½NO3 �ðdayÞ ¼

1 ½N2 O5 � ¼ kss ðN2 O5 Þ kNO2þO3 ⋅½NO2 �⋅½O3 �

(2) (3)

Here, kNO3þNO is the rate constant for the reaction of NO3 with NO, and j (NO3) is referred to as the NO3 photolysis rate constant; the loss rate of NO3 does not include VOCs, RO2 and HO2. Fig. 4 shows an example of an air mass with daytime N2O5 values on June 5, 2018. P(NO3) increased rapidly after sunrise, with a maximum of 4.2 ppbv h 1 occurring at 09:20, which was attributed to the rapid in­ crease in O3 due to the enhanced photochemistry. During the morning period, NO dropped below 0.5 ppbv at 11:00. The observed N2O5 was near the limit of detection of 2 pptv at 06:00. With increasing solar ra­ diation, the mixing ratio of N2O5 did not decrease, but increased continuously to a dozen pptv at 09:00. The elevated N2O5 level was sustained until 12:00 and then slowly decreased. The observed daytime N2O5 trend seems to be a time lag of the P(NO3) trend and is generally consistent with the calculated N2O5 when taking the measurement un­ certainty into consideration. In essence, the nonnegligible daytime N2O5 observed in this campaign is the same as the values observed in the late afternoon in previous studies, which were attributed to substantial NO3 formation due to high concentrations of NO2 and O3 (Brown et al., 2005, 2016, 2017; Geyer et al., 2003; Osthoff et al., 2006). The elevated daytime N2O5 implied that the corresponding daytime NO3 concentra­ tion may also be nonnegligible, demonstrating the importance of NO3–N2O5 in daytime NOx regulation and atmospheric oxidation in the YRD region.

4.2. Reactivity of NO3 and N2O5 To understand the details of the loss rates of NO3 and N2O5, the summed NO3 reactivity (kNO3) is calculated according to Eq. (6). kNO3 ¼ kNO3þNO ⋅ ½NO� þ jðNO3 Þ þ kNO3þVOCs ⋅ ½VOCs� þ kN2O5 ​ ⋅ Keq ⋅½NO2 �

(6)

Here, kNO3þVOCs is the reaction rate constant of NO3 with VOCs. kN2O5 ​ represents the pseudo-first-order loss rate of N2O5 heterogeneous hydrolysis. Keq is the temperature-dependent equilibrium constant be­ tween NO2, NO3 and N2O5. The N2O5 uptake coefficient can be deter­ mined by the steady-state method (Brown et al., 2009), but the NO3 loss via VOCs was significant during this campaign. The highly varied NO3 loss rate due to VOCs leads to the steady-state method being almost 5

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the parameterized N2O5 uptake coefficient. Especially during the second half of the night, the kNO3 from N2O5 uptake increased fast due to the increasing RH. During the daytime, VOCs accounted for 9.6% of the NO3 reactivity when taking the reaction of NO into account, which was even comparable with NO3 photolysis (15.5%). An intercomparison of the nocturnal steady state kNO3 (Eq. (5)) with the calculated summed NO3 reactivity on the nighttime of June 2–3 is shown in Fig. 8. Here, we did not include the reaction of NO in the calculation of summed NO3 reactivity because of the nonzero NO near the limit of instrument detection affected the calculations. The trends of the two calculations agree well in general, but the summed kNO3 is consistently lower than the steady-state kNO3, which was a universal phenomenon during this campaign (Fig. S5). The discrepancy seems small under high N2O5 concentrations but large under low N2O5 con­ ditions, especially during the second half of the night. The discrepancy may be attributed to the following: (1) other NO3 scavengers, such as intermediate species (e.g., RO2), are not considered in the calculation; thus, NO3 reactivity or other processes, such as NO3 uptake, may be missing, although NO3 uptake is not regarded as an important NO3 loss pathway; (2) the parameterized N2O5 uptake coefficient is smaller than the real value; (3) the summed monoterpenes include some specific monoterpenes, such as limonene, that are more reactive than α-pinene, which causes a higher bias of the summed lifetime.

Fig. 6. Fitting of the N2O5 lifetime on the night of 23–24 May.

infeasible. In this study, we obtained only one valid fitting result. As shown in Fig. 6, the fitted N2O5 uptake coefficient was 0.041 for the second half of the night, with a kNO3 of 0.10 s 1. Due to the lack of sufficient data points for the derived N2O5 uptake coefficient, we used a parameterized N2O5 uptake coefficient according to (Evans and Jacob, 2005) in the following calculation, which considered the effect of enhanced RH and temperature on N2O5 uptake. The parameterized average N2O5 uptake coefficient was 0.028 � 0.012, and the time series is shown in Fig. S3. Both the derived and predicted uptake coefficients are comparable with those observed in summer in China (Tham et al., 2018; Wang et al., 2017d, 2018b; Wang et al., 2017e; Xia et al., 2019; Yan et al., 2019; Yun et al., 2018b). In the kNO3 calculation, we neglected the contribution of RO2 to kNO3 due to a lack of knowledge of the re­ action rate constant of NO3 with RO2 radicals. The direct measurement of individual monoterpenes are not valid, so we presumed the rate constant for the reaction of total monoterpenes with NO3 to be the same as that for α-pinene, which is reasonable because α-pinene dominates biogenic emissions and has a moderate monoterpene-NO3 rate constant (Brown et al., 2017). Fig. 7 shows the time series of summed kNO3 and its constituents from May 27 to June 10, 2018. The detailed distribution for each term during the daytime, nighttime and 24 h periods is listed in Table S1. The daily average NO3 reactivity was 0.56 s 1. In a 24 h period, NO contributes to 62.5% kNO3, followed by N2O5 uptake, NO3 photolysis and VOCs, with percentages of 15.4%, 11.2%, and 10.8%, respectively. Monoterpenes clearly dominated the kNO3 from VOCs, with an average of 0.0559 s 1; other VOCs only contributed to kNO3 with an average of 0.0044 s 1. The following top three contributors were styrene, isoprene and cis-2-butene (Fig. S4). During the nighttime, kNO3 was dominated by N2O5 uptake and VOCs. The increased RH during nighttime largely enhanced both Sa and

4.3. NO3 loss and NOx loss Fig. 9(a) shows the daily distribution of the loss rate of NO3, which is quite different from the distribution of kNO3. Although NO is the

Fig. 8. Intercomparison of NO3 reactivity calculated by the steady-state method (red dot) with summed* (black dot), that calculated from NO3þVOCs and N2O5 uptake, but excluding NO3þNO, owing to the nonzero N2O5. The light blue line is the concentration of N2O5. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7. Time series of the distribution of NO3 reactivity (kNO3) via the reaction of NO, VOCs, N2O5 uptake and photolysis from May 27 to June 10; total kNO3 values higher than 1.0 s 1 are only shown as 1.0 s 1 here. 6

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Fig. 9. (a) Mean diurnal variation in the NO3 loss rate via reaction with NO and VOCs, N2O5 uptake and NO3 photolysis. (b) The NOx loss rate via the reaction of OH with NO2, the reaction of NO3 with VOCs and N2O5 uptake; here, the ClNO2 yield is set to 0.

dominant factor of kNO3, owing to the titration effect of NO to O3, the production of NO3 has a close negative relationship with NO (Fig. S6), suggesting that under high NO conditions, the relatively low NO3 loss rate initiated by NO is not limited by the NO concentration, but limited by the production of NO3. During the daytime, although NO3 photolysis was fast, the NO3 loss due to VOCs and N2O5 uptake make up a pro­ portion of 26.6%. This result is significant and consistent with the observation of elevated N2O5 mentioned before, implying that NO3 chemistry plays a role during the daytime. At night, the loss of NO3 was dominated by VOCs, followed by N2O5 uptake. The maximum NO3 loss rate via VOCs (monoterpenes) occurs at 19:00–20:00, reaches up to approximately 1.4 ppbv h 1, and then de­ creases continuously throughout the night. Overall, nocturnal NO3 loss via reaction with monoterpenes and N2O5 uptake is 58.9% and 31.9%, respectively, and the result that VOCs dominate nighttime NO3 loss is similar to that observed in Wangdu (Tham et al., 2016). Even when the data were averaged over a 24-h period, VOCs were responsible for 36.4% of the NO3 loss (monoterpene percentage 33.9%) when taking the reaction of NO3 with NO into account, suggesting the importance of the NO3 oxidation of monoterpenes in the YRD, as well as the role of NO3 chemistry in the formation of ONs and SOA. The role of NO3 and N2O5 in NOx removal was compared with the gas-phase reaction of OH þ NO2, which is regarded as an important daytime sink of NOx. As shown in Fig. 9(b), the NOx loss by the oxidation of OH is fast, with an average rate of 0.63 ppbv h 1 during the daytime. The NOx loss rate peaked at 09:00–10:00 and decreased sharply in the afternoon, with an average daily loss of NOx via OH þ NO2 of 0.47 ppbv h 1. Assuming that N2O5 heterogeneous uptake only produces nitrate without the formation of ClNO2, namely, the ClNO2 yield is 0, the average NOx loss by NO3 chemistry is 0.63 ppbv h 1 in a 24-h period. NO3 chemistry accounted for more than half of the NOx loss (57.3%) at this site, which is consistent with previous studies in NCP, U.S. and South China (Stutz et al., 2010; Tsai et al., 2014; Wang et al., 2017d; Yun et al., 2018a, 2018b); as a result, NO3–N2O5 chemistry is significant in regional NOx removal in the YRD.

than 60 s on average, which was attributed to VOCs, especially mono­ terpenes. Although the aerosol mass loading is not serious compared with that during wintertime, high RH may aggravate N2O5 heteroge­ neous hydrolysis. The average daily NOx loss via NO3 chemistry was 0.63 ppbv h 1, which was dominant over the NOx loss attributed to the OH oxidation pathway at the surface level in the YRD region. Without a field quantification of the time series of N2O5 uptake co­ efficient during the campaign to further constrain the N2O5 uptake processes, the above analysis based on the parameterized N2O5 uptake coefficient may have some uncertainties. Here, we aimed not to quantify each term of kNO3, NO3 and NO2 loss with high accuracy, but to highlight the importance of NO3 and N2O5 chemistry in eastern China. More work is needed to provide insight into the NO3 oxidation of monoterpenes and explicitly diagnose the N2O5 uptake processes in similar regions with high NOx, high RH and strong biogenic VOC emissions in future studies. Author contribution Haichao Wang: Conceptualization, Methodology, Software, WritingReviewing and Editing Xiaorui Chen: Investigation, Data Curation, Writing- Reviewing and Editing Keding Lu: Conceptualization, Super­ vision, Writing- Reviewing and Editing, Project administration Renzhi Hu: Conceptualization, Supervision, Writing- Reviewing and Editing Zhiyan Li: Data Curation Hongli Wang: Data Curation Xuefei Ma: Data Curation Xinping Yang: Data Curation Shiyi Chen: Data Curation Huabin Dong: Data Curation Ying Liu: Data Curation Xin Fang: Data Curation Limin Zeng: Data Curation Min Hu: Data Curation, Project administra­ tion Yuanhang Zhang: Data Curation, Project administration. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements

5. Conclusion

The work was supported by the National Natural Science Foundation of China (Grant Nos. 91544225, 91844301, 41907185, 91644107, and 61575206) and Projects funded by the China Postdoctoral Science Foundation (Grant Nos. 2018M641095 and 2019T120023).

These measurements during EXPLORE-YRD 2018 provide new measurements of N2O5 and relevant data in the YRD, China. The average NO3 production rate was 1.01 � 0.47 ppbv h 1 during the nighttime, and the production rate was comparable with the data from other sites in the YRD and NCP regions. However, the mixing ratio of N2O5 was low, with maximum and nocturnal averages of 220 pptv and 8 pptv, respectively, which is 5-10-fold smaller than that reported in summer in Beijing, suggesting a rapid nocturnal loss in the YRD. The daytime N2O5 mixing ratios were more than 10 pptv and were associated with a P (NO3) of more than 4.0 ppbv h 1, implying a nonnegligible role for NO3 and N2O5 during the daytime. The nocturnal N2O5 lifetime was shorter

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.atmosenv.2019.117180.

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