# Evidence for Strong HONO Emission from Fertilized Agricultural Fields and its Remarkable Impact on Regional O 3 Pollution in the Summer North China Plain

Chaoyang Xue, Can Ye, Chenglong Zhang, Valéry Catoire, Pengfei Liu, Rongrong Gu, Jingwei Zhang, Zhuobiao Ma, Xiaoxi Zhao, Wenqian Zhang, et al.

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Chaoyang Xue, Can Ye, Chenglong Zhang, Valéry Catoire, Pengfei Liu, et al.. Evidence for Strong HONO Emission from Fertilized Agricultural Fields and its Remarkable Impact on Regional O 3 Pollution in the Summer North China Plain. ACS Earth and Space Chemistry, 2021, 5 (2), pp.340-347. 10.1021/acsearthspacechem.0c00314 . hal-03173926

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Evidence for strong HONO emission from fertilized agricultural fields and its
remarkable impact on regional O₃ pollution in the summer North China Plain

Chaoyang Xue<fnref n="1" />, <fnref n="2" />, #, Can Ye<fnref n="1" />, #, a, Chenglong Zhang<fnref n="1" />, <fnref n="3" />, Valéry Catoire<fnref n="2" />, Pengfei Liu<fnref n="1" />, <fnref n="3" />, Rongrong Gu<fnref n="7" />, Jingwei Zhang<fnref n="6" />, Zhuobiao Ma<fnref n="1" />, Xiaoxi Zhao<fnref n="1" />, Wenqian Zhang<fnref n="5" />, Yangang Ren<fnref n="4" />, Gisèle Krysztofiak<fnref n="2" />, Shengrui Tong<fnref n="5" />, Likun Xue<fnref n="7" />, Junling An<fnref n="6" />, Maofa Ge<fnref n="5" />, Abdelwahid Mellouki<fnref n="4" />, <fnref n="7" />, Yujing Mu<fnref n="1" />, <fnref n="3" />*

<footnote n="1">¹ Research Centre for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China</footnote>

<footnote n="2">² Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPC2E), CNRS–Université Orléans–CNES, 45071 Orléans Cedex 2, France</footnote>

<footnote n="3">³ Centre for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China</footnote>

<footnote n="4">⁴ Institut de Combustion Aérothermique, Réactivité et Environnement, Centre National de la Recherche Scientifique (ICARE-CNRS), Observatoire des Sciences de l'Univers en région Centre, CS 50060, 45071 cedex02, Orléans, France</footnote>

<footnote n="5">⁵ Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China</footnote>

<footnote n="6">⁶ State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, Beijing 100029, China</footnote>

<footnote n="7">⁷ Environmental Research Institute, Shandong University, Qingdao, Shandong 266237, China</footnote>

<footnote>a now at: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China</footnote>
Abstract

Summertime HONO concentrations were synchronously measured at two (an agricultural and a non-agricultural) sites in the North China Plain (NCP). Daytime HONO (1.4±0.6 ppbv) and HONO/NO₂ ((12±8)%) over the agricultural field after fertilization were found to be remarkably higher than those before fertilization, implying strong HONO emission from the fertilized fields. Synchronous enhancements of HONO and O₃ after fertilization at both sites suggested that the emitted HONO accelerated the local and the regional O₃ pollution. HONO budget analysis further revealed that its emission was significantly enhanced after fertilization. Soil HONO emission flux and its uncertainty were estimated and discussed. The estimated emission flux exhibited a distinct diurnal variation with a noontime maximum. Net OH production rate from HONO photolysis greatly exceeded that from O₃ photolysis over the agricultural field, and their maximum ratio of 4.7 was obtained after fertilization. We provide field evidence that fertilized fields in the NCP act as a strong HONO source, which accelerates daytime photochemistry, leading to an increase of regional photo-oxidants such as O₃. Considering the severe O₃ pollution in the summer NCP and that the large area of the agricultural field is regularly treated with high fertilization amount in this region, HONO emission should be taken into account in the regional air quality deterioration.

**Keywords:** HONO, nitrogen fertilization, agricultural fields, emission flux, O₃
# 1. Introduction

It is generally accepted that the photolysis of nitrous acid (HONO) initiates photochemistry in early morning¹ and makes a significant contribution to hydroxyl radicals (OH) during the whole day²⁻⁴. The sources of HONO, however, are still not well understood. Besides the known sources such as direct emission from vehicle exhaust⁵,⁶, the gas-phase reaction of NO+OH, (photo-enhanced) NO₂ heterogeneous reactions on surfaces⁷,⁸, and other sources⁹,¹⁰, laboratory studies recently found that soil could be a potential HONO source through abiotic or biotic formation¹¹⁻¹⁶ with emission fluxes up to 20-3000 ng-N m⁻² s⁻¹ and it was influenced by various factors such as soil nutrient, pH, and bacteria type (Table S1). However, such high HONO emission fluxes obtained in the laboratory studies were not supported by field flux measurements; that is, the flux obtained in the field measurements (Table S2) was generally much less than 60 ng-N m⁻² s⁻¹, most of which, however, was related to other HONO formation paths rather than emission from the soil. For example, Zhou et al.¹⁷ found upward HONO fluxes <2.7 ng-N m⁻² s⁻¹ at a forest site, which was attributed to the photolysis of leaf surface nitrate. Laufs et al.¹⁸ measured soil HONO emission fluxes <2.3 ng-N m⁻² s⁻¹ at an agricultural site, and the photosensitized NO₂ heterogeneous reaction on the ground surface was suggested to be responsible for the flux. The large difference between HONO emission fluxes measured in the laboratory and over fields may be due to different soil characteristics and types/amounts of the applied nitrogen fertilizer, which needs further research work.

In the North China Plain (NCP), a large area of cultivated land (>0.2×10⁶ km²) is regularly treated with intensive chemical nitrogen fertilizers¹⁹ and could constitute a potential HONO source. However, there were only a few atmospheric HONO measurements²⁰ over the agricultural fields in the rural NCP by now to recognize the possible HONO emission from fertilized fields and its impact. Liu et al.²⁰ have measured HONO at an agricultural site in the NCP and performed model tests on various HONO formation paths. They found that strong unknown HONO sources might be related to the fertilized fields, but did not provide sufficient evidence or specifically estimate soil HONO emissions. Besides, our previous studies²¹,²², in which a twin open-top dynamic chamber (OTC) method was developed, provided the first direct soil HONO emission flux measurement and found that soil HONO emission could be enhanced by chemical nitrogen fertilizer. The impact on atmospheric HONO levels was yet not quantified. Herein, atmospheric HONO and related parameters were simultaneously measured at an agricultural site and a non-
agricultural site in June 2017. Based on the measurement, we then 1) provided evidence of HONO emission from the fertilized soil through the atmospheric HONO budget analysis at the agricultural site during different fertilization periods, 2) provided evidence of the regional impact of fertilization on air quality in the NCP through the comparison between the two sites, and 3) quantified the contribution of the enhanced HONO to the atmospheric oxidizing capacity during different periods.

## 2. Experimental

### 2.1 Sites and instrumentation

The Station of Rural Environmental Chinese Academy of Sciences (SRE-CAS), located in Dongbaituo village (38°42′N, 115°15′E), Hebei province-China, was used as the agricultural site (Figure S1). It is surrounded by agricultural fields with winter-wheat and summer-maize rotation.²² During the campaign, one intensive fertilization event for planting summer maize, which happens every year, was performed (see details in Section 2.2). The non-agricultural site (37°46′N, 118°59′E), Yellow River Delta Ecology Research Station of Coastal Wetland (YelRD) in the NCP, is surrounded by wide-open lands with no significant anthropogenic emissions within 10 km (Figure S1). Even the non-agricultural site is ~15 km west of the sea, air mass during the measurement periods was mainly originated from the inner NCP, and no fertilization was conducted within 20 km around this site, leading to an excellent background site to verify the regional impact of agricultural fertilization.²³ At both sites, HONO was continuously measured online by the LOPAP technique (LOng Path Absorption Photometer, Model-03, QUMA GmbH, Germany)²⁴. Other instruments used at the two stations are shown in Text S1 of the Supporting Information (SI).

### 2.2 The fertilization event

In the NCP region, fertilization is regularly conducted for planting summer-maize in June every year, and it happened during the present campaign. Regarding the large area of agricultural fields in the NCP, the regional fertilization event may last 1-2 weeks. In the present campaign, few fertilization processes for the fields within 3 km around our station started 1-2 days after we started the measurements on 7th June 2017; as a consequence, we recorded limited data without the influence of fertilization, but it may still be affected by the regional fertilization, making it hard to get observations completely without fertilization impact. Note that we have performed a week
measurement at the same site from 1st to 6th August 2016, which was more than 6 weeks after this year's fertilization (A6FP), and the impact of fertilization on soil HONO emission was expected to be less effective. HONO levels in this period were similar to the first day of the present campaign. HONO levels showed a significant increase after fertilization compared to the first day. Therefore, we consider the observation in the first day (with the smallest fertilization impact) as the representative for the rural background HONO levels during the non-fertilization period (NFP, see details in Text S2).

Most fertilization within 3 km around our station was conducted between 12th and 15th June 2017, and fields surrounding the station were fertilized on 13th June with an ammonium-based fertilization amount of ~180 kg-N ha⁻¹. We then classified the observations before 13th June as the pre-intensive fertilization period (PFP) and after 13th June as the intensive fertilization period (IFP). For comparison between the two sites, data at YelRD station was similarly classified into three periods (NFP: 1st-7th June 2017, PFP: 8th-13th June 2017, IFP: 14th-22nd June 2017).

## 3. Results

Figure S2 shows the time series of hourly average HONO, NO₂, O₃, H₂O₂, and solar irradiance (Ra) measured at the agricultural site during the four periods. During NFP, the averaged daytime (6:00-18:00) mixing ratios of HONO, NO₂, and O₃ were 0.36±0.26, 13.3±7.6, and 50±25 ppbv, respectively. Both daytime HONO and O₃ clearly showed significant increases during PFP (0.72±0.32 and 53±30 ppbv) and IFP (1.36±0.60 and 70±32 ppbv) in comparison with NFP (Table S3). Daytime H₂O₂ showed similar levels during NFP (0.66 ppbv) and PFP (0.60 ppbv), and a significant increase during IFP (0.95 ppbv). In contrast, there were no evident variations of NO₂ (an essential precursor for both HONO and O₃) and solar irradiance during the three periods (Table S3, Figure S2, and Figure S3).

Figure 1A exhibits the diurnal variations of HONO during the three periods (NFP, PFP, and IFP) at the agricultural site. During NFP, HONO showed a typical diurnal variation with high values during nighttime and noontime minimum. After fertilization, HONO mixing ratios increased significantly during PFP and IFP compared to NFP, and an unexpectedly high peak of 1.70 ppbv occurred at noontime (13:00) during IFP. In particular, the noontime HONO concentration during IFP was more than 2 times higher than that during PFP (0.71 ppbv) and more than 11 times greater than that during NFP (0.15 ppbv).
Figure 1B and 1C illustrate the diurnal variations of O₃ and H₂O₂ at the agricultural site. Both O₃ and H₂O₂ exhibited similar diurnal variations with peaks in the early afternoon and minima at night. However, O₃ and H₂O₂ in the morning (6:00-13:00) during IFP increased much faster than those during NFP or PFP, leading to higher noontime peaks of O₃ (103 ppbv) and H₂O₂ (1.98 ppbv). Besides, the occurrence time of peak O₃ during IFP (13:00) shifted >3 hours earlier than that during the other two periods (NFP: 16:00, PFP: 17:00). It approached the occurrence time (13:00) of the high daytime HONO peak when OH production from HONO photolysis is the largest in a day, indicating the possible influence of elevated HONO on O₃ formation after the intensive fertilization.

<figure><img src="image_1.png" /></figure>

Figure 1 Diurnal variations of HONO (A), O₃ (B), and H₂O₂ (C) during the three periods at the agricultural site, HONO (D) and O₃ (E) during the three periods (non-fertilization period: NFP in blue, pre-intensive fertilization period: PFP in grey, intensive fertilization period: IFP in red) at the non-agricultural site, and the daytime enhancement factor of HONO or O₃ during PFP and IFP compared to NFP (F). The error bars represent the standard deviations (±1σ).

Figure 1D and 1E show diurnal variations of HONO and O₃ at the non-agricultural site, respectively. HONO mixing ratios were generally lower than 1 ppbv, similar to those during NFP at the agricultural site. Evident enhancement of HONO with a small noontime peak was also observed during IFP compared to NFP or PFP at the non-agricultural site. Similar to the
agricultural site, O₃ at the non-agricultural site showed a small increase during PFP but an evident
increase during IFP with respect to NFP.

The enhancement factors (EF, defined as average daytime mixing ratio of one specific pollutant during PFP or IFP divided by that during NFP) for HONO and O₃ at the two sites are shown in Figure 1F. EF values for HONO or O₃ at the agricultural site were much larger than those at the non-agricultural site, and all the EFs were larger than 1, implying that both HONO and O₃ at the two sites were enhanced during PFP and IFP. Due to the significant daytime increase of HONO and O₃, the EFs for HONO and O₃ during IFP at the agricultural site reached 3.8 and 2.0, respectively. Note that EF for HONO at noontime was even larger. For example, noontime average HONO during IFP was more than one order of magnitude higher than that during NFP (EF>10).

# 4. Discussion

## 4.1 HONO budget

At noontime in summer, the photolytic loss of HONO is usually very rapid, e.g., the photolytic lifetime of HONO (1/J(HONO)) was <15 min at noontime during this campaign, and thus, its source strength is expected to be nearly equal to its sink strength.²⁵,²⁶ HONO formation from homogeneous reaction NO+OH (P<sub>NO+OH</sub>) and photo-enhanced NO₂ uptake on the ground surface (P<sub>het</sub>) could be calculated. Then, HONO formation from the unknown sources (P<sub>unknown</sub>) was calculated in Text S4.

Figure S4 shows the average daytime HONO formation rate from each pathway during the three periods at the agricultural site. HONO formation during NFP was dominated by P<sub>NO+OH</sub> and P<sub>het</sub> with a sum contribution of 97%, leading to a small contribution (3%) from P<sub>unknown</sub>. However, P<sub>unknown</sub> remarkably exceeded P<sub>NO+OH</sub> and P<sub>het</sub> during PFP and IFP with contributions of 63% and 81%, respectively. It is evident that HONO formation from unknown sources significantly increased after fertilization (during PFP and IFP).

The key question is which source made a dominant contribution to P<sub>unknown</sub> during PFP and IFP. A recent study²⁰ revealed that vehicle emission, NO<sub>2</sub> reaction on the aerosol surface, NO<sub>2</sub> reaction on soot surface, acid displacement, photolysis of nitrophenol, and biomass burning had negligible impacts on the daytime HONO formation at this site. And Liu et al. proposed that soil emission and total nitrate (TNO<sub>3</sub>=HNO<sub>3</sub>+pNO<sub>3</sub>) photolysis might be the potential HONO sources that account for unexpectedly high daytime HONO levels. Assuming a high TNO<sub>3</sub> concentration of 12
ppbv observed at this site in summer 2014<sup>20</sup>, a high photolysis frequency of 1×10<sup>-5</sup> s<sup>-1</sup> (~2 orders of magnitude higher than HNO<sub>3</sub>(g) photolysis<sup>27-29</sup>, and a maximum HONO yield of 100%, HONO production rate from TNO<sub>3</sub> photolysis was about 0.4 ppbv h<sup>-1</sup>, which is far to explain P<sub>unknown</sub> during PFP (1.6±1.0 ppbv h<sup>-1</sup>) and IFP (4.3±2.0 ppbv h<sup>-1</sup>). Therefore, the other source, soil emission from the fertilized field, was suspected to be responsible for the extremely high P<sub>unknown</sub> (also discussed in the following section).

## 4.2 Evidence for soil HONO emission: HONO/NO<sub>2</sub>

Firstly, as discussed before, P<sub>unknown</sub> and its contribution to daytime HONO formation showed a significant increase after fertilization (Figure 2B and Figure S4), implying the potential HONO enhancement caused by fertilization.

Secondly, photo-enhanced heterogeneous reactions of NO<sub>2</sub> on ground surfaces have been considered as important daytime HONO sources<sup>7,8,18</sup>, which could be verified by the P<sub>het</sub> obtained by the present study during non-fertilization periods in summer (Figure S4) as well as by our previous investigation in winter<sup>30</sup>. Thus, the HONO/NO<sub>2</sub> ratio was used to evaluate the role of NO<sub>2</sub>-relevant reactions in HONO formation. As shown in Figure 2A and Table S3, the average HONO/NO<sub>2</sub> during A6FP and NFP was (2.1±1.2)% and (2.6±1.0)%, respectively, comparable to previous observations (1%-5%) without significant impact from soil emission<sup>3,18,31-33</sup>, indicating the important role of NO<sub>2</sub> reactions in HONO formation before fertilization and 6 weeks after fertilization. However, the significantly higher HONO/NO<sub>2</sub> ratios during PFP (mean: (5.4±3.7)%, maximum: 22%) and IFP (mean: (11.8±8.0)%, maximum: 38%) could not be explained by NO<sub>2</sub>-relevant reactions with respect to the comparable NO<sub>2</sub> mixing ratios during the three periods (Figure S3 and Table S3). Note that the HONO/NO<sub>2</sub> used here are expected even larger if NO<sub>2</sub> is furtherly corrected by HNO<sub>3</sub>, etc. (See Text S1). In particular, the maximum HONO/NO<sub>2</sub> ratio after fertilization reached 38% (Table S3), which dramatically exceeds the values mainly dominated by NO<sub>2</sub>-relevant reactions.<sup>3,18,31,32</sup>

Taken together, the remarkable increase of atmospheric HONO levels and the extremely high HONO/NO<sub>2</sub> ratios, as well as P<sub>unknown</sub> during IFP, were reasonably attributed to HONO emission from the fertilized field, which is in agreement with the results of our previous field flux studies at this site, in which we found strong HONO emissions (especially around noontime) from the fertilized soil in summer 2016<sup>21,22</sup>.
<figure><img src="image_2.png" /><figcaption>**Figure 2 (A) & (B):** HONO/NO₂ or P<sub>unknown</sub> during NFP, PFP, and IFP at the two stations; (C): The inferred HONO emission flux based on MLH\*0.35 during PFP and IFP. (D) & (E): Correlations of E with solar radiation or soil temperature.</figcaption></figure>

**4.3 Soil HONO emission flux estimation and discussion on the mechanism**

The increase of P<sub>unknown</sub> during PFP and IFP in comparison with NFP was mainly attributed to the HONO emission from the fertilized field. Thus, the soil HONO emission flux (E, in ng-N m<sup>-2</sup> s<sup>-1</sup>) was estimated based on the enhanced P<sub>unknown</sub> and the mixing layer height (MLH), as shown in Text S5. However, vertical distribution of HONO is expected when a ground-based source is dominated, leading to an overestimated emission flux when using the ground measurement (3.4 m above the ground) as the average HONO within the calculated MLH. The maximum value and the variation trend of MLH can be scaled by the turbulence and the boundary layer height (BLH), respectively (see Text S5 and Figure S5), but this results in an overestimated MLH because HONO decreases to 1/e (37%) within its lifetime. Hence, we furtherly analyzed the HONO budget during NFP when daytime HONO formation was dominated by photo-enhanced NO₂ uptake on the ground surface. A more reasonable MLH could be achieved when HONO formation could be nearly fully explained by NO+OH and the photo-enhanced NO₂ uptake on the ground surface. We found that when MLH\*0.35 (maximum: 70 m at noontime) was used in P(HONO)<sub>het</sub> calculation, almost all the HONO formation (97%) could be explained by NO+OH and the photo-enhanced NO₂ uptake on the ground surface, revealing that MLH\*0.35 is reasonable to represent the mixing layer height when ground sources dominate HONO formation. Because both soil emission and the
photo-enhanced NO₂ reaction happen on the ground surface, MLH*0.35 is also valid for soil HONO emission flux calculation.

Figure 2C shows the inferred E during PFP and IFP. Both the emission fluxes during PFP and IFP exhibit a distinct diurnal variation, with a noontime peak. Maximum E during IFP is 84 ng-N m⁻² s⁻¹, which is comparable to those from our field flux measurement from ammonium-fertilized soils²², but higher than those measured in other places with upward fluxes less than (generally much less than) 60 ng-N m⁻² s⁻¹ (Table S2). The inferred emission flux is also lower than most of the high values obtained from the laboratory studies (Table S1). Nevertheless, it should be noted that the evidence of soil HONO emission enhanced by the agricultural nitrogen fertilization is consistent with our previous direct flux measurements. Still, the estimation of soil HONO emission flux is of some uncertainties; for example, the uncertainty caused by the estimated MLH as discussed before. Therefore, further direct flux measurement is necessary to quantify the emission flux and its regional impact accurately.

In the NCP, agricultural fields were treated with long-term and high amounts of ammonium- or urea-based chemical nitrogen fertilizers, which has been expected to accelerate nitrification significantly.¹³,³⁴⁻³⁷ Nitrite (NO₂⁻) is the key intermediate product of ammonium nitrification and could lead to HONO formation through coupling with a hydrogen ion. Although the agricultural soils in the NCP are slightly alkaline (the measured pH is ~8), which is not in favor of HONO formation based on the acid-based equilibrium¹¹, the micro-environment, for example, at the nitrifying bacteria level, is expected to be different from the bulk condition because of the release of hydrogen ions from the nitrification³⁷,³⁸ as well as the surface water loss¹¹,¹³, which may influence surface/microscale pH.¹²,³⁹ It should also be noted that the strong HONO emission from the fertilized agricultural field during IFP occurred under the condition with the measured soil water content greater than 80% of the water holding capacity (WHC) due to the flooding irrigation after fertilization, and it is not consistent with laboratory results that high emission occurred at 10%-30% WHC (obtained from dry air flushed flow tube experiments)¹³,⁴⁰. HONO is soluble easily under high soil water content (e.g., 80% WHC for bulk soil within 5 cm depth). The water content of the soil surface is, however, expected to be lower than the bulk one because of water evaporation that is in favor of HONO emission, indicating the key role of microscale parameters (e.g., on the soil surface) in soil HONO emission.¹²,³⁹ Besides, HONO might be emitted by bacteria under high soil water content, i.e., HONO could be emitted under (75%-140%)WHC reported by
Wu et al.<sup>16</sup> We then support that $NO_2^-$ is produced by soil bacteria under a proper water content and the surface process governs the transmission of $NO_2^-$ (ag) to gas-phase HONO. Further studies focused on the link of changes in the microenvironment with reactive nitrogen emission are still of significant importance.

Besides, the correlation of E with soil temperature (R<sup>2</sup>=0.74) is much higher than that with solar radiation (R<sup>2</sup>=0.17) (Figure 2D and 2F), suggesting the critical role of soil temperature in soil HONO emission, e.g., soil temperature could largely influence the biogenic process (e.g., nitrification<sup>13,40,41</sup>) and enhance surface water evaporation.<sup>11–13,39</sup>

# 5 Atmospheric implications on the atmospheric oxidizing capacity and $O_3$ pollution in the NCP

The remarkable increase of atmospheric HONO due to soil emission could accelerate OH formation through its photolysis (see detailed OH production rate calculation in Text S6). Figure 3 shows the OH production rates from the photolysis of HONO ($P(OH)_{HONO}$) and $O_3$ ($P(OH)_{O_3}$). $P(OH)_{HONO}$ quickly increased from early morning to around noontime, whereas $P(OH)_{O_3}$ started to rise 2 hours later. Daytime $P(OH)_{O_3}$ was generally lower than $P(OH)_{HONO}$ during NFP and much lower during PFP and IFP, implying the predominant role of HONO in OH production. Compared to NFP (or PFP), daytime average $P(OH)_{HONO}$ increased from 0.8±0.5 (or 1.5±1.2) to 3.0±2.1 ppbv h<sup>-1</sup>, while $P(OH)_{O_3}$ kept at a low level (mean <0.7 ppbv h<sup>-1</sup>). The largest $P(OH)_{HONO}$ reached 10.7 ppbv h<sup>-1</sup> at 13:00 on 18th June when HONO reached as high as 3.1 ppbv under high solar irradiance expressed as $J(NO_2)=6.2×10^{-3}$ s<sup>-1</sup>. To the best of our knowledge, such a high OH production rate from HONO photolysis, is the largest one found in rural sites, including (fertilized) agricultural sites<sup>18,20,25,30</sup>, forest sites<sup>2,17</sup>, grass sites<sup>42–44</sup>, or other rural sites<sup>23,28</sup>, suggesting the crucial role of soil HONO emission in the atmospheric oxidizing capacity in this region.
<figure><img src="image_3.png" /><figcaption>Figure 3 The average diurnal variations of the net OH production rates from photolysis of HONO (A) and O₃ (B) during the three periods. The shadowed areas represent the standard deviations (±1σ). The average ratio of P(OH)ₐ₅₅ to P(OH)O₃ was also shown as a column plot (B).</figcaption></figure>

The evident rises of O₃ and H₂O₂ (Figure 1B and 1C) after fertilization could be attributed to the remarkable increase of P(OH)ₐ₅₅ that dominates the enhanced OH production, which could consequently promote the production of O₃ and H₂O₂ through reactions (1)-(5):

$$OH + VOCs/CO \xrightarrow{O_2} HO_2/RO_2 \qquad (1)$$

$$HO_2/RO_2 + NO \rightarrow NO_2 + OH/RO \qquad (2)$$

$$NO_2 + h\nu \rightarrow NO + O(^3P) \qquad (3)$$

$$O(^3P) + O_2 + M \rightarrow O_3 + M (M = N_2/O_2) \qquad (4)$$

$$HO_2 + HO_2 + (M) \rightarrow H_2O_2 + (M) \qquad (5)$$

It is worth noting that both the largest enhancements of O₃ (Figure 1B) and P(OH)ₐ₅₅ (Figure 3A) from PFP to IFP occurred at the same time of 13:00, in concert with the maximum emission flux (Figure 2C). Meanwhile, a recent WRF-Chem model study⁴⁵ and our box model study³⁰ also showed that additional HONO sources significantly impacted HOₓ and O₃ concentrations, consistent with our conclusion.

Additionally, the emitted HONO may also influence regional HONO and O₃ levels, e.g., the significant increase of HONO, O₃, and Punknown values during IFP at the non-agricultural site (Figure 1Figure 2) resulted from the air mass from the inner NCP during IFP (Figures S6 and S7). Because the HONO level at the non-agricultural site is much lower than that at the agricultural site,
the impact on the non-agricultural site could be very significant even if only a small part of HONO
emitted by the agricultural field reached the non-agricultural site. On the one hand, enhanced
regional OH levels caused by the elevated HONO could accelerate NO+OH reaction; on the other
hand, high wind speeds (e.g., 9 m s⁻¹ from the inner NCP) observed at the non-agricultural site
may also enable a rapid air mass transport from the agricultural fields to the non-agricultural site.
As is expected, high HONO, O₃, and HONO/NO₂ levels were also frequently observed in the
airmass originated from the inner NCP (Figure S7), demonstrating the potential regional impact of
fertilization on the air quality of the whole NCP.

Given that the maize planting with intensive fertilization (96-482 kg-N ha⁻¹) is regularly conducted every June in the NCP¹⁹, soil HONO emission is suspected to occur in most agricultural fields (>0.2×10⁶ km²) because the soil characters and meteorological conditions are quite similar in this region⁴¹. Thus, the significant elevation of atmospheric oxidants (i.e., O₃ and H₂O₂) could also be expected in the whole NCP region after fertilization for planting summer maize, and it can be furtherly confirmed by the synchronous enhancements of O₃ concentrations during IFP at the agricultural site and the non-agricultural site. Therefore, the impact of HONO emission from the agricultural fields on regional atmospheric oxidants is necessary to be quantitatively evaluated by further long-term field measurements along with direct OH measurement and model simulations, and it is, as well, essential for disclosing the reasons for the severe O₃ pollution in the summer NCP and similar regions.⁴⁶,⁴⁷

## Acknowledgments

We are grateful to Liwei Guan for his contribution to the field measurements. We thank Yunqiao Zhou for his help with map plotting. **Funding:** This work was supported by the National Natural Science Foundation of China (No. 91544211, 41727805, 41931287, 41975164). **Author contribution:** Y.M. led the study. C.X., C.Z., P.L., C.Y., Z.M., W.Z., S.T., and X.Z. performed measurements at SRE-CAS station. R.G. and L.X. provided and analyzed field data at YelRD station. C.Y., J.Z., Y.R., M.G., and J.A. made discussions on data analysis and writing process. C.X. and Y.M. wrote this manuscript with all the authors' contributions. V.C., G.K., L.X., and A.M. provided scientific advice on the study and revised this manuscript. # C.X. and C.Y. contributed equally. **Competing interests:** The authors declare no competing financial interest.
**Data availability:** Observation data used in this study can be requested from the corresponding author.

**Supporting Information Available**

A description of instruments used in this campaign (Text S1), HONO levels during the non-fertilization period (Text S2), J values calculation method (Text S3), P<sub>unknown</sub> calculation (Text S4), HONO emission flux calculation (Text S5), net OH production rate calculation (Text S6), 7 additional figures, and 3 additional tables.

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