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ACS Earth and Space Chemistry 1

Supporting Information for 2

Evidence for strong HONO emission from fertilized agricultural fields and its remarkable 3

impact on regional O3 pollution in the summer North China Plain 4

Chaoyang Xue1, 2, #, Can Ye1, #, a, Chenglong Zhang1, 3, Valéry Catoire2, Pengfei Liu1, 3, Rongrong 5

Gu7, Jingwei Zhang6, Zhuobiao Ma1, Xiaoxi Zhao1, Wenqian Zhang5, Yangang Ren4, Gisèle 6

Krysztofiak2, Shengrui Tong5, Likun Xue7, Junling An6, Maofa Ge5, Abdelwahid Mellouki4, 7, 7

Yujing Mu1, 3* 8

1 Research Centre for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China 9

2 Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (LPC2E), CNRS–Université 10

Orléans–CNES, 45071 Orléans Cedex 2, France 11

3 Centre for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese 12

Academy of Sciences, Xiamen 361021, China 13

4 Institut de Combustion Aérothermique, Réactivité et Environnement, Centre National de la Recherche 14

Scientifique (ICARE-CNRS), Observatoire des Sciences de l’Univers en région Centre, CS 50060, 45071 15

cedex02, Orléans, France 16

5 Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China 17

6 State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), 18

Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, Beijing 100029, China 19

7 Environmental Research Institute, Shandong University, Qingdao, Shandong 266237, China 20

a now at: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of 21

Environmental Sciences and Engineering, Peking University, Beijing 100871, China 22

Correspondence to: yjmu@rcees.ac.cn 23

Contents of this file (18 pages) 24

Texts S1 to S6 25

Figures S1 to S7 26

Tables S1 to S3 27

References 28

29

30

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Text S1. 31

Instruments used in this campaign 32

At both sites, HONO was continuously measured online by Long Path Absorption Photometer 33

(LOPAP-03, QUMA GmbH, Germany). At the agricultural site, the external sampling unit for 34

ambient measurement was mounted at a height of 3.4 m above the ground. NO and NO2 were 35

measured by a chemiluminescence technique analyzer (Thermo Fisher Model 42i, USA). The 36

instrument can directly quantify NO by the reaction 𝑁𝑂 + 𝑂3 → 𝑁𝑂2∗ → 𝑁𝑂2 + ℎ𝜐 . O3 was 37

measured by a UV photometric analyzer (Thermo Scientific Model 49i, USA). H2O2 was measured 38

by a wet liquid chemistry fluorescence detector (Aero-Laser AL2021, Germany).1 Besides, the 39

meteorological parameters (air temperature, relative humidity, solar radiation, wind speed, and 40

wind direction) and soil temperature were measured by a meteorological station. Soil water content 41

was measured once every two days through the mass loss of soil samples (5 cm depth) by heating 42

for 24 hours at 105 °C.2 Soil pH was measured once every two days based on the method of ISO 43

10390:2005-2012. 44

Since NO2 was quantified indirectly through conversion to NO by a molybdenum converter with 45

possible interference from other NOy species like HONO, HNO3, and PAN, then NO2 46

concentrations used here were corrected by subtracting HONO from the measured NO2. HNO3 and 47

PAN were unfortunately not available in this campaign, which leads to an overestimation of NO2. 48

While, the overestimated NO2 may not have a significant impact on the HONO budget and the 49

following soil HONO emission flux estimation during the fertilization period, which is because 50

NO2 heterogeneous reactions were not the dominated HONO source after fertilization. For example, 51

during the intensive fertilization period (Figure S4), NO2 heterogeneous reactions only contribute 52

6% of the daytime HONO formation, much lower than the other sources (85%). Therefore, the 53

overestimation of NO2, even by a factor of 2, is still far to significantly affect the HONO budget 54

and the soil HONO emission flux estimation. Meanwhile, if HNO3 and PAN are furtherly 55

subtracted from the measured NO2, 1) HONO/NO2 will be larger so that NO2 reactions are farther 56

to explain the observed HONO/NO2, and 2) the unknown HONO strength and the estimated soil 57

HONO emission flux will be even larger, suggesting an even larger impact of fertilization on 58

daytime atmospheric oxidizing capacity. 59

At the non-agricultural site, NO, NO2, O3, and J(NO2) were continuously measured by a 60

chemiluminescence method (T-API Model T200U, USA), a Cavity Attenuated Phase Shift detector 61

(T-API Model T500U, USA), a standard UV photometric detector (TEI Model 49C, USA), and a 62

spectroradiometer (Metcon GmbH, Germany), respectively. Details about other instruments could 63

be found in Gu et al.3 64

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Text S2. 65

HONO levels during NFP and A6FP at the agricultural site 66

HONO levels during the non-fertilization period (NFP) at the agricultural site was needed as a 67

background level for estimation on HONO emission flux during PFP and IFP. As fertilization for 68

summer maize is a regionwide event in the NCP and may last 1-2 weeks if all the agricultural 69

fertilization is finished, we got limited observations completely without fertilization impact during 70

the present campaign. While we had measurements from 1st to 6th in August 2016 (six weeks after 71

fertilization in this year, A6FP), and we found the HONO levels were very similar to those on the 72

first day (no fertilization within 3 km around our station) of measurement in 2017. Therefore, we 73

used data on the first day to represent the levels during NFP. All these daytime HONO and the 74

calculated Punknown during NFP or A6FP, were much lower than those during IFP, suggesting that 75

the background HONO level has a small impact on estimating soil HONO emission flux. HONO 76

variations during NFP and A6FP showed a typical diurnal variation, indicating one fertilization 77

event's impact may last less than 6 weeks. 78

Text S3. 79

Photolysis frequency (J) values 80

During the campaign, solar irradiance (Ra, W m-2) was continuously measured, while J values were 81

not measured. Then we conducted a field measurement of solar irradiance and J(NO2) (10-3 s-1) by 82

a spectroradiometer (Metcon, GmbH, Germany) in June 2020 at the same place, in which the high 83

quadratic correlation of Ra and J(NO2) (J(NO2)=-9.96×10-6×Ra2+1.62×10-2×Ra, R2=0.95) is used 84

to estimate J(NO2) in June 2017. Then J(HONO) and J(O(1D)) were then calculated by the TUV 85

model (https://www2.acom.ucar.edu/) and scaled by the estimated J(NO2), as shown in the 86

following equation. 87

𝑱(𝑿) = 𝑱(𝑿)𝒎𝒐𝒅𝒆𝒍 ∗ 𝑱(𝑵𝑶𝟐)𝒆𝒔𝒕𝒊𝒎𝒂𝒕𝒆𝒅/𝑱(𝑵𝑶𝟐)𝑻𝑼𝑽 (S1) 88

where X represents HONO or O(1D). 89

Text S4. 90

Punknown calculation 91

Except for homogeneous reaction NO+OH (PNO+OH) and photo-enhanced NO2 uptake on the ground 92

surface (Phet), unknown HONO strength could be calculated by the following equation: 93

𝑷𝒖𝒏𝒌𝒏𝒐𝒘𝒏 = 𝑳𝒑𝒉𝒐 + 𝑳𝑯𝑶𝑵𝑶+𝑶𝑯 − 𝑷𝑵𝑶+𝑶𝑯 − 𝑷𝒉𝒆𝒕 94

= [𝑯𝑶𝑵𝑶] ∗ 𝑱(𝑯𝑶𝑵𝑶) + 𝒌𝟏 ∗ [𝑯𝑶𝑵𝑶] ∗ [𝑶𝑯] − 𝒌𝟐 ∗ [𝑵𝑶] ∗ [𝑶𝑯] −𝒗(𝑵𝑶𝟐)×[𝑵𝑶𝟐]

𝟒×𝑴𝑳𝑯× 𝜸𝑵𝑶𝟐

×𝑱(𝑵𝑶𝟐)

𝟓×𝟏𝟎−𝟑 𝒔−𝟏 (S2) 95

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where Lpho is the photolysis rate of HONO, LHONO+OH is the loss rate of HONO due to reaction with 96

OH, (NO2) is the molecular speed of NO2 (m s-1), NO2 (1.5×10-5) is the uptake coefficient of NO2 97

on the ground surface4,5, MLH is the mixing layer height (see Text S5 for detailed calculation), and 98

J(NO2) is the photolysis frequency of NO2. J(NO2)/(5×10-3 s-1) plays as the photo-enhancement 99

factor of the NO2 heterogeneous reaction. J(HONO) represents the photolysis frequency of HONO, 100

which was derived from solar irradiance measurement and the TUV model (see details in Text S2). 101

Reaction rate constants k1 and k2 were taken from IUPAC (http://iupac.pole-ether.fr/). OH 102

concentrations were estimated based on the correlation of OH with J(O(1D)) from a previous field 103

campaign at the same site in the same month, in which high correlation (R2=0.86) of OH with 104

J(O(1D)) was observed.6 105

Text S5. 106

Calculation of HONO emission flux 107

Within the mixing layer, soil HONO emission causes the increase of Punknown, and the increment of 108

Punknown is the upper limit of soil HONO emission flux (E, ng m-2 s-1) through the following equation. 109

𝑬 =(𝑷𝒖𝒏𝒌𝒏𝒐𝒘𝒏−𝑷𝑰−𝑷𝒖𝒏𝒌𝒏𝒐𝒘𝒏−𝑵)×𝑴𝑵×𝑴𝑳𝑯

𝑽𝒎×𝟑𝟔𝟎𝟎× 𝟏𝟎𝟗 (S3) 110

where Punknown-PI represents Punknown (m3 m-3 h-1) during PFP or IFP, Punknown-N, MN, and Vm represent 111

the average Punknown during NFP, nitrogen molar mass (14 g mol-1), and gas molar volume (0.0245 112

m-3 mol-1, at 101 kPa and 298K), respectively. 3600 and 109 have units of s h-1 and ng g-1, 113

respectively. 114

Regarding MLH, vertical mixing is determined by turbulence, and the vertical travel distance ∆z 115

(therefore, it is the upper limit of MLH) over time τ can be calculated by the following equation. 116

∆𝒛 = √𝟐 × 𝑲𝒛 × 𝝉 (S4) 117

where Kz and τ represent the turbulent diffusion coefficient and the photolytic lifetime of HONO, 118

respectively. For a typical Kz of 3×105 cm2 s-1, the calculated ∆z is 200 m at 13:00 when the 119

photolytic lifetime of HONO is shortest (667 s, J(HONO)=1.5×10-3 s-1) in the daytime. 120

Since MLH is expected to have the same variation as BLH, then the diurnal variation of MLH is 121

calculated based on the diurnal BLH obtained from ECMWF (https://www.ecmwf.int/) scaled by 122

the ratio of ∆z-to-BLH at 13:00. Then the BLH obtained from ECMWF, and the calculated MLH 123

are shown in Figure S5. Note that ∆z is the upper limit of MLH as gradient distribution is expected 124

to be significant when the ground-based source is dominated. Therefore, the flux inferred based on 125

the present MLH represents the upper limit of the soil HONO emission flux. But the evidence of 126

the main conclusion that the fertilized agricultural field is a strong HONO source is still sufficient. 127

S5 /S18

Text S6. 128

Net OH production 129

1. Net OH production rate from the photolysis of HONO (P(OH)HONO) 130

𝑷(𝑶𝑯)𝑯𝑶𝑵𝑶 = [𝑯𝑶𝑵𝑶] × 𝑱(𝑯𝑶𝑵𝑶) − 𝒌𝑵𝑶+𝑶𝑯 × [𝑵𝑶] × [𝑶𝑯] − 𝒌𝑯𝑶𝑵𝑶+𝑶𝑯 × [𝑯𝑶𝑵𝑶] ×131

[𝑶𝑯] (S5) 132

2. OH production rate from the photolysis O3 (P(OH)O3) 133

𝑶𝟑 + 𝒉𝝊 → 𝑶(𝟏𝑫), 𝑱(𝑶(𝟏𝑫)) (S6) 134

𝑶(𝟏𝑫) + 𝑴 → 𝑶 + 𝑴 (𝑴 = 𝑵𝟐), 𝒌𝒔𝟕 (S7) 135

𝑶(𝟏𝑫) + 𝑴 → 𝑶 + 𝑴 (𝑴 = 𝑶𝟐), 𝒌𝒔𝟖 (S8) 136

𝑶(𝟏𝑫) + 𝑯𝟐𝑶 → 𝑶𝑯 + 𝑶𝑯, 𝒌𝒔𝟗 (S9) 137

𝑷(𝑶𝑯)𝑶𝟑= [𝑶𝟑] × 𝑱(𝑶𝟑) × ∅ × 𝟐 (S10) 138

The reaction rate constants kNO+OH, kHONO+OH, ks7, ks8, and ks9 were taken from IUPAC 139

(http://iupac.pole-ether.fr/). H2O concentration could be calculated by the measured relative 140

humidity (RH), temperature, and pressure. During this campaign, the daytime average branching 141

ratio of O(1D) with H2O (∅) was 12.3%. 142

S6 /S18

Figures 143

144

Figure S1. Locations of the agricultural site (star A, 3842N, 11515E) and the non-145

agricultural site (star B, 37°46N, 118°59E) in the map colored by land type distribution 146

(from Peng et al.7). 147

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148

Figure S2. Time series of hourly HONO, NO2, O3, H2O2, and solar irradiance (Ra) during the 149

three periods at the agricultural site. A6FP: six weeks after the fertilization (1st-6th August 150

2016); NFP: the non-fertilization period (7th June 2017); PFP: the pre-fertilization period (8th-151

13th June 2017); IFP: the intensive fertilization period (14th-21st June 2017). 152

153

154

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155

156

Figure S3. Diurnal variations of NO and NO2 mixing ratios during the three periods at the 157

agricultural site. NFP: the non-fertilization period; PFP: the pre-fertilization period; IFP: 158

the intensive fertilization period. 159

160

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161

Figure S4. HONO formation rates and contribution from each source during the three 162

periods at the agricultural site. NFP: the non-fertilization period; PFP: the pre-fertilization 163

period; IFP: the intensive fertilization period. 164

165

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166

Figure S5. The BLH obtained from ECMWF and the calculated MLH 167

168

169

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170

171

Figure S6. Backward trajectories (12 hours) for YelRD station at 12:00 in each day during 172

IFP (obtained from NOAA HYSPLIT MODEL with GDAS meteorological data, 173

https://www.ready.noaa.gov/HYSPLIT.php). 174

175

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176

177

Figure S7. Pollution rose plot for the observations at the non-agricultural sites during IFP. 178

179

180

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Tables 181

182

Table S1. Laboratory HONO emission flux (in ng-N m-2 s-1) measurement and the key factor 183

in the literature. 184

Soil information Maximum flux Key factors Reference

1-3000a pH, nitrite 8

pH>7 5-258 AOB 2

Agricultural land and other substrates Surface acidity 9

Agricultural land, Parkland 8-19 Nitrification 10

Biocrusts 5-173 Nitrogen cycling by biocrusts 11

High water content 5-190 Nitrate reduction 12

Pure cultures of AOB, NOB, and AOA 0-800 Biogenic hydroxylamine 13

Soil surface and biocrusts ~150 Microscale pH 14

Bare soil and biocrusts 27-175 Soil cover type, nutrient contents 15

a: the calculated maximum emission 185

AOB: ammonia-oxidizing bacteria 186

Biocrusts: biological soil crusts 187

188

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Table S2. Field HONO emission flux (in ng-N m-2 s-1) measurement and the main 189

precursor/source in the world. 190

Date Location Land type Method Range Mean Precursor Reference

Sept, 1989 Halvergate, UK Grassland AG -12-25 3.2 NO2 16

Feb-May, 1990 Essex U, UK Grassland AG -24-33 -6.6 NO2 16

May-June, 1998 Milan, Italy Grassland AG -20-8 -0.2 NO2 17

July-Aug, 2008 Michigan, USA Forestland REA -24-61 0.4* nitrate 18

July, 2009 Blodgett, USA Forestland REA -24-33 -0.1* 19

May-June, 2010 Bakersfield, USA Agricultural

land REA -14-24 0.7* NO2

19

June-July, 2011 Bavaria, Germany Clearing land AG 0.1-1.0 0.6 NO2 20

Sept, 2012 Bavaria, Germany Forestland AG -0.2-0 -0.1* 20

Aug, 2009 Grignon, France Agricultural

land AG 0.1-2.3 1.2* NO2

21

Apr, 2010 Grignon, France Agricultural

land AG -0.1-0.2 0.1* NO2

21

Aug, 2011 Grignon, France Agricultural

land AG 0.1-2.3 1.2* NO2

21

Aug, 2015 Wangdu, China Agricultural

land OTC 0.6-3.2 1.9 NO2

22

Aug, 2015 Wangdu, China Fertilized

agricultural land

OTC 5.7-40 21 Emission 22

*: the average of diurnal maximum and minimum 191

AG: aerodynamic gradient 192

REA: relaxed eddy accumulation 193

OTC: open-top dynamic chambers 194

195

196

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Table S3. The statistical summary of daytime (6:00-18:00) HONO, NO2, HONO/NO2, O3, 197

H2O2, P(OH)HONO, P(OH)O3, and solar radiation (Ra) at the agricultural site during the four 198

periods. Ave, SD, Min, and Max represent the average, standard deviation, minimum, and 199

maximum, respectively. A6FP: six weeks after fertilization; NFP: the non-fertilization 200

period; PFP: the pre-fertilization period; IFP: the intensive fertilization period. 201

Parameters HONO

(ppbv)

NO2

(ppbv)

HONO/NO2

(%)

O3

(ppbv)

H2O2

(ppbv)

P(OH)HONO

(ppbv h-1)

P(OH)O3

(ppbv h-1)

Ra

(W m-2)

A6FP

Ave 0.23 12.1 2.1 40 - 0.24 0.37 346

SD 0.20 7.1 1.2 22 - 0.20 0.36 208

Min 0.03 3.7 0.5 3 - -0.20 0 29

Max 1.14 37 6.0 77 - 0.89 1.18 731

NFP

Ave 0.36 13.3 2.6 50 0.66 0.78 0.52 471

SD 0.26 7.6 1.0 25 0.55 0.53 0.44 234

Min 0.05 5.4 0.5 9 * 0.07 0.01 97

Max 0.86 25.6 4.5 79 1.41 1.65 1.14 757

PFP

Ave 0.72 16.1 5.4 53 0.60 1.49 0.45 381

SD 0.32 8.1 3.7 30 0.52 1.15 0.43 232

Min 0.17 4.1 1.5 1 * -0.15 0 10

Max 1.49 40.5 21.8 106 1.80 4.23 1.49 764

IFP

Ave 1.36 14.4 11.8 70 0.95 3.02 0.66 391

SD 0.60 7.9 8.0 32 0.72 2.08 0.60 226

Min 0.40 5.6 3.1 5 * -0.12 0 0

Max 3.13 39.5 37.6 122 2.59 10.69 1.90 737

-: measurement was unavailable. 202

*: H2O2 mixing ratios were lower than the detection limit of about 0.1 ppbv. 203

Note that noontime H2O2 on 17th June was not considered because of a possible impact from a 204

vegetation fire accident. 205

206

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