Exploration of the atmospheric chemistry of nitrous acid in a coastal city of southeastern China: Results from measurements across four seasons

Because nitrous acid (HONO) photolysis is a key source of hydroxyl (OH) radicals, identifying the atmospheric sources of HONO is essential to enhance the understanding of atmospheric chemistry processes and improve the accuracy of 20 simulation models. We performed seasonal field observations of HONO in a coastal city of southeastern China, along with measurements of trace gases, aerosol compositions, photolysis rate constants (J), and meteorological parameters. The results showed that the average observed concentration of HONO was 0.54 ± 0.47 ppb. Vehicle exhaust emissions contributed an average of 1.64% to HONO, higher than the values found in most other studies, suggesting an influence from diesel vehicle emissions. The mean conversion frequency of NO2 to HONO in the nighttime was the highest in summer due to water droplets 25 was evaporated under the condition of high temperatures. Based on a budget analysis, the rate of emission from unknown sources (Runknown) was highest at midday, with values of 14.78 ppb·h in summer, 6.49 ppb·h in autumn, and 2.18 ppb·h in spring. Unknown sources made up the largest proportion of all sources in summer (84.92%), autumn (80.29%), and spring (49.98%), whereas the main source in winter was the homogeneous reaction of NO with OH (56.15%), due to winter having the highest NO concentration of the four seasons. The value of Runknown had a positive logarithmic relationship with the 30 photolysis of particulate nitrate in spring, summer, and autumn. However, Runknown was limited by particulate acidity under the https://doi.org/10.5194/acp-2020-880 Preprint. Discussion started: 10 September 2020 c © Author(s) 2020. CC BY 4.0 License.

in coastal cities with good air quality, low concentrations of NOx and PM2.5, but strong sunlight and high humidity. Insufficient research on coastal cities with good air quality has resulted in certain obstacles to assessing the photochemical processes in these areas. Due to different emission-source intensities and ground surfaces, the atmospheric chemistry of HONO in the 65 southeastern coastal area of China is predicted to have different pollution characteristics from those found in other coastal cities. Furthermore, HONO contributes to the atmospheric photochemistry differently depending on the season (Li et al., 2010).
Therefore, observations of atmospheric HONO across different seasons in the southeastern coastal area of China are urgently needed.
Incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) was employed in this study to determine HONO 70 concentrations in the southeastern coastal city of Xiamen in August (summer), October (autumn), and December (winter) 2018 and March (spring) 2019. In addition, a series of other relevant trace gases, meteorological parameters, and photolysis rate constants were measured at the same time to provide supplementary information to reveal the HONO formation mechanisms.
The main purposes of this study were to (1) quantify the gas-phase photostationary state of HONO, (2) calculate the values of unknown HONO daytime sources, (3) analyze the processes leading to HONO formation, (4) simulate HONO concentrations 75 based on an empirical parameterization, and (5) evaluate OH production from HONO from 07:00 to 16:00 local time. All of these results were compared between the seasons.

Site description
Our field observations were carried out ~80 m above the ground at a supersite located on the top of the Administrative Building 80 of the Institute of Urban Environment (IUE), Chinese Academy of Sciences (24.61° N,118.06° E) in Xiamen, China in August, October, and December 2018, and March 2019 (Fig. 1). The supersite was equipped with a complete set of measurement tools, including those for measuring gas and aerosol species composition, meteorology parameters, and photolysis rate constants, which provided a good chance to study the atmospheric chemistry of HONO in a coastal city of southeastern China.

Instrumentation 85
The atmospheric concentrations of both HONO and NO2 were determined using IBBCEAS, which has previously been widely applied to such measurements (Tang et al., 2019;Duan et al., 2018;Min et al., 2016). Multiple reflections in the resonator cavity enhance the length of the effective absorption path, thereby enhancing the detection sensitivity of the instrument. The 1σ detection limits for HONO and NO2 were 60 ppt and 100 ppt, respectively, and the time resolution was 1 min. The measurement error for HONO and NO2 was estimated to be about 9%. A specific description of the structure and principle of 90 IBBCEAS can be found in a previous report (Duan et al., 2018). https://doi.org/10.5194/acp-2020-880 Preprint. Discussion started: 10 September 2020 c Author(s) 2020. CC BY 4.0 License.
The inorganic composition of PM2.5 aerosols, including Cl − , NO3 − , SO4 2− , NH4 + , Na + , K + , Ca 2+ , and Mg 2+ , were determined using a Monitor for AeRosols and Gases in ambient Air (MARGA, Model ADI 2080, Applikon Analytical B.V., the Netherlands) with a temporal resolution of 1 h. The MARGA utilizes a steam-jet aerosol collector (SJAC), and online ion chromatography was applied to determine the aqueous sample streams produced by the SJAC. Specific descriptions of the 95 SJAC can be found in previous reports (Slanina et al., 2001;Wyers et al., 1993).
These were calculated by multiplying the actinic flux F, quantum yield φ(λ) and the known absorption cross section σ(φ). The were within ±5%.
The O3 concentration was determined by UV photometric analysis [Model 49i, Thermo Environmental Instruments (TEI) Inc.], and the detection limit of the TEI Model 49i is 1.0 ppb. The NO concentration was determined by a chemiluminescence analyzer (TEI model 42i) with a molybdenum converter, and the detection limit of the TEI model 42i is 0.5 ppb. Although the TEI model 42i also measures the concentration of NO2, this value might actually include other active nitrogen components. 105 Therefore, the NO2 concentration as measured by IBBCEAS was used in this study. An oscillating microbalance with a tapered element was applied to determine the PM2.5 concentration. Meteorological parameters were determined by an ultrasonic atmospherium (150WX, Airmar, USA). The time resolution of all instruments was unified to 1 h to facilitate comparison.

Overview of data 110
The average measured ambient HONO concentration at the measurement site for all measurement periods was 0.54 ± 0.47 ppb.
The maximum value (3.51 ppb) appeared at 08:00 on 4 December 2018. The HONO mixing level in Xiamen was close to the values found in Rome (0.58 ppb), Nanjing (0.69 ppb), and Hong Kong (0.72 ppb), but was much lower than those in Xi'an (1.04 ppb), Kathmandu (1.05 ppb), Jinan (1.14 ppb), Santiago (2.25 ppb), or Guangzhou (2.75 ppb), as shown in Table 1. Table 1 also shows the seasonal patterns of HONO and related parameters during the night and the day. 115 In the daytime (06:00-18:00, including 06:00, local time (LT)), the highest concentration of HONO was found in spring and summer (0.72 ppb), followed by winter (0.61 ppb) and autumn (0.50 ppb). In short, the seasonal variation of HONO was well correlated with the seasonality of RH, with high RH in spring (83.31%) and summer (84.58%), followed by winter (75.79%) and autumn (66.47%). In conditions of low RH, the adsorption rate of NO2 is not as rapid as that of HONO, resulting in a reduction in the conversion rate of NO2 to HONO and thus a reduction in the concentration of HONO (Stutz et al., 2004). This 120 seasonal variation in HONO concentration was different from those measured in Jinan , Nanjing (Liu et al., https://doi.org/10.5194/acp-2020-880 Preprint. Discussion started: 10 September 2020 c Author(s) 2020. CC BY 4.0 License. 2019b), and Hong Kong . The elevated HONO concentrations in summer, when there is strong solar radiation, suggests the existence of strong sources of HONO and its important contribution to the production of OH radicals. Interestingly, the HONO concentration in the nighttime was lower than that in the daytime in all four seasons. Most previous studies have found that the HONO concentration at night is significantly higher than that during the day Liu et al., 125 2019c;Li et al., 2018;Elshorbany et al., 2009;Acker et al., 2006;Yu et al., 2009). Coastal cities are susceptible to sea and land breezes, with sea breezes blowing during the day and land breezes blowing during the night (Wagner et al., 2012). Therefore, the concentration of sea salt, as calculated based a previous report , is significantly higher during the day than that during the night (P < 0.05). It is possible that significantly more HONO could be produced by photolysis of sea salts against the daytime photolysis of HONO (Kasibhatla et al., 2018). Similar results were found in Hong Kong, which is also a 130 coastal city, which further validates the rationality of this assumption .
The ratio of HONO to NOx or the ratio of HONO to NO2 have been extensively applied to indicate heterogeneous conversion of NO2 to HONO Liu et al., 2019c;Zheng et al., 2020). Compared with the HONO/NO2 ratio, the HONO/NOx ratio can better avoid the influence of primary emissions (Liu et al., 2019c). In this study, the HONO/NOx ratios during the day were higher than those during the night, indicating that light promotes the conversion of NOx to HONO. The highest 135 daytime HONO/NOx ratio was found in summer (0.072), followed in turn by autumn (0.048), spring (0.034), and winter (0.023).
The elevated HONO/NOx ratio in summer indicates a greater net HONO production . The low HONO/NOx ratio in winter can probably be ascribed to heavy emissions and high concentrations of NO in winter (Table 1). The HONO/NOx ratios during every season in Xiamen were in general higher than those found in studies of other cities, which indicates greater net HONO production in Xiamen. 140 The diurnal patterns of HONO, NOx, HONO/NOx, and J(NO2) averaged for every hour in each season are shown in Fig. 2. As shown in Fig. 2a, the HONO concentration had similar diurnal variation patterns across the four seasons. The maximum values of the HONO concentration were 1.12 ppb in winter, 1.03 ppb in summer, 0.98 ppb in spring, and 0.65 ppb in autumn, and these occurred in the morning rush hour (07:00-08:00), which indicates that direct vehicle emissions may be a significant source of HONO. The contribution of direct vehicle emissions to HONO will be quantified in Sect. 3.2. The HONO 145 concentration reduced rapidly from the morning rush hour to sunset, and this was caused by rapid photolysis combined with increased height of the boundary layer. The minimum values of HONO concentration were 0.47 ppb in spring, 0.23 ppb in winter, 0.21 ppb in summer, and 0.14 ppb in autumn, and these appeared at sunset, between 16:00 and 18:00. The HONO concentration increased gradually after sunset, which indicates that release from HONO sources exceeded its dry deposition . There was a slight difference in the diurnal variation of HONO between autumn and the other seasons. 150 A rapid reduction of HONO after the morning rush hour was found in spring, summer, and winter. In comparison, the HONO in autumn had an almost constant concentration between 07:00 and 11:00 because NOx decreased slowly during this period.
As shown in Fig. 2b, NOx concentration followed an expected profile in the four seasons, with peaks of 45.58 ppb in winter, 40.47 ppb in spring, 32.47 ppb in summer, and 20.07 ppb in autumn, each occurring in the morning rush hour at 10:00, 09:00, https://doi.org/10.5194/acp-2020-880 Preprint. Discussion started: 10 September 2020 c Author(s) 2020. CC BY 4.0 License. 08:00, and 07:00 local time, respectively. After these peaks, NOx decreased during the day in each season, probably due to 155 photochemical transformation and increasing boundary-layer depth. The NOx concentrations then began to rise from their minima of 8.20 ppb in summer, 8.85 ppb in autumn, 18.10 ppb in winter, and 23.09 ppb in spring after 14:00, 13:00, 15:00, and 16:00 local time, respectively, which was caused by a combination of weak photochemical transformation and reduction in the boundary-layer depth. The NOx concentrations during winter and spring were significantly higher than those during autumn and summer. Both the maxima and minima of NOx appeared later in spring and winter compared with summer and 160 autumn.
It is possible to better describe the behavior of HONO using the HONO/NOx ratio. The higher HONO/NOx ratio found at noon in the different seasons, especially in summer and autumn (Fig. 2c), indicates an unknown daytime HONO source. It is worth noting that the maximum value of this ratio in summer (0.147) was significantly higher than the maximum in other seasons, especially in winter (0.034). Fig. 2d shows that the value of the HONO/NOx ratio increased with the photolysis of NO2 in 165 summer and autumn, suggesting that the unknown HONO source is probably correlated with light Li et al., 2018;Li et al., 2012). The increase in the HONO/NO2 ratio during the day can be seen more clearly in Fig. 3, and its high value indicates a high HONO production efficiency, which cannot be ascribed to NO2 conversion due to the weak correspondence between HONO and NO2 in three of the seasons (excluding winter). Furthermore, high HONO/NO2 ratios were accompanied by high J(NO2) in summer, which indicates that HONO formation during the daytime is controlled 170 by light rather than Reaction (R1).
However, the observed maxima can also be ascribed to sources independent from NOx concentration, such as soil emissions (Su et al., 2011) and photolysis of particulate nitrate (Zhou et al., 2011;Ye et al., 2016), which are not influenced by the decrease of NOx concentration around noon. A more specific discussion of daytime HONO sources considering the photolysis 175 of particulate nitrate will be given in Sect. 3.4.3. Although the solar radiation intensity in spring and winter was nearly equal, the difference in the HONO/NOx ratios in these seasons was large, indicating that the solar radiation intensity was not the only factor determining the HONO/NOx ratio. The HONO emissions from soil were estimated to be 2-5 ppb h −1 (Su et al., 2011).
However, soil emission was a negligible source of HONO in this study since the surrounding soil is not used for agriculture, and this greatly reduces the amount of HONO released due to no fertilization process (Su et al., 2011). 180
(4) NOx > 20 ppb (highest 25% of NOx value); and (5) NO/NOx > 0.50. A total of 34 cases met these strict criteria for estimation 185 of the HONO vehicle emission ratios. The slopes of scatter plots of HONO vs NOx were used as the emission factors.
A total of 34 vehicle emission plumes are summarized in Table 2, and these were used for estimation of the vehicle emission ratios. The plumes were considered to be truly fresh because the mean ∆NO/∆NOx ratio of the selected air masses was 92%.
Vehicle plumes unavoidably mixing with other air masses resulted in the correlation coefficients (R 2 ) between HONO and NOx varying among the cases, and these ranged from 0.61to 0.92. The obtained ∆HONO/∆NOx ratios ranged from 0.24% to 190 4.76%, with an average value (±SD) of (1.64 ± 0.95) %. These ∆HONO/∆NOx ratios have comparability to those obtained in Guangzhou (1.4% (Qin et al., 2009); 1.8% ) and Houston (1.7% (Rappenglück et al., 2013)), but are significantly higher than those measured in Jinan (0.53% ) and Santiago (0.8% (Elshorbany et al., 2009)). The types of vehicle engine, the use of catalytic converters, and different fuels will affect the vehicle emission factors (Kurtenbacha et al., 2001). A potential reason for the relatively higher ∆HONO/∆NOx values in our study is that heavy-duty diesel vehicles 195 pass by on the surrounding highway (Rappenglück et al., 2013). It is necessary to examine the specific vehicle emission factors in target cities because of these differences in ∆HONO/∆NOx ratios. Roughly assuming that NOx mainly arises from vehicle emissions, a mean ∆HONO/∆NOx value of 1.64% was used as the emission factor in this study, and this value was adopted to estimate the contribution of vehicle emissions Pemis to the HONO concentration using emis = NO × 0.0164.

Conversion rate of NO2 to HONO
Nighttime HONOcorr concentrations can be estimated from the heterogeneous conversion reaction (Meusel et al., 2016;Alicke, 205 2002;Su et al., 2008a). Although the mechanism of the nighttime HONO heterogeneous reaction is unclear, the formula for the heterogeneous conversion ( HONO 0 ) of NO2 to HONO can be expressed as where [NO 2 ] is the mean value of NO2 concentration between t1 and t2. Eq. (4) has been suggested as a way to avoid the interference of direct emissions and diffusion :  We also compared the conversion rates calculated in this study with other experiments. As shown in Table 3, HONO C varied 220 widely, from 0.29 % h −1 to 2.40 % h −1 , which may be due to the various kinds of land surface in the various environments. The HONO C in Xiamen is comparable to those derived in Shanghai (0.70% h −1 ), Jinan (0.68% h −1 ), and Hong Kong (0.52% h −1 ), less than the values calculated from most other sites, including Xinken (1.60% h −1 ), Guangzhou (2.40 (Li et al., 2012)), Spain (1.50 ), Beijing (0.80 ), the eastern Bohai Sea (1.80% h −1 (Wen et al., 2019)), and Kathmandu (1.40% h −1 ), but more than 225 the value obtained in Shandong (0.29% h −1 ). The highest HONO C was found in summer, with a value of 0.55% h −1 , which will be explained in Sect. 3.3.2. Another study also found that the highest HONO C (1.00% h −1 ) appeared in summer .

The influence of relative humidity on HONO formation
The hydrolysis of NO2 on wet surfaces producing HONO is first-order affected by the concentration of NO2 (Finlayson-Pitts 230 et al., 2003;Jenkin et al., 1988) and the absorption of water on the surfaces (Finlayson-Pitts et al., 2003;Kleffmann et al., 1998).
A scatter plot of HONOcorr/NO2 vs RH is shown in Fig. 4. We calculated the top-five HONOcorr/NO2 ratios in every 5% RH interval based on a method introduced in previous literature Stutz et al., 2004), which will reduce the influence of those circumstances such as advection, the time of the night, and the surface density. These averaged maxima and standard deviations are shown in Fig. 4 as orange squares, except where data were sparse in a particular 5% RH interval. 235 As for autumn and winter, the influence of RH on HONOcorr/NO2 can be divided into two parts. The RH promoted an increase in HONOcorr/NO2 for RH values less than 77.96% in autumn and 91.99% in winter, which is in line with the reaction kinetics of Reaction (R1). However, RH inhibits the conversion of NO2 to HONO when RH is higher than a turning point. According to many previous studies, water droplets will be formed on the surface of the ground or of aerosols when RH exceeds a certain value, thus resulting in a negative dependence of HONOcorr/NO2 on RH (He et al., 2006;Zhou et al., 2007). A similar 240 phenomenon was also found in Guangzhou and in Shanghai (70%, Wang et al., 2013)) and in Kathmandu and in Beijing (65%, Wang et al., 2017a)). However, in summer, RH appeared to promote the increase of https://doi.org/10.5194/acp-2020-880 Preprint. Discussion started: 10 September 2020 c Author(s) 2020. CC BY 4.0 License.
HONOcorr/NO2 without a turning point, suggesting that HONO production at night in summer strongly depends on RH. Another study also found a similar phenomenon in the summer in Guangzhou (Qin et al., 2009). This phenomenon might be caused by water droplets being destroyed by high temperatures. This is the reason for the highest HONO C in summer. As for spring, the 245 relationship between HONOcorr/NO2 and RH is very complicated and needs to be explored further in the future.

The influence of aerosols on HONO formation
As shown in Fig. S1, HONOcorr/NO2 reached a pseudo-steady state from 03:00 to 6:00 LT every night. A correlation analysis of HONOcorr/NO2 with PM2.5 was carried out in the pseudo-steady state to understand the impact of aerosols on HONO production. Although we did not measure the aerosol surface density, the aerosol mass concentration can be used to replace 250 this parameter (Huang et al., 2017;Park et al., 2004;Cui et al., 2018). The positive correlation of HONOcorr with PM2.5 (R1 = 0.54) (Fig. 5a) may be a result of atmospheric physical processes such as convergence and diffusion. Using the HONOcorr/NO2 ratio instead of a single HONO concentration for correlation analysis with PM2.5 reduce the impact of physical processes and indicate the extent of conversion of NO2 to HONO. Therefore, it was more credible that HONOcorr/NO2 would be moderately positively correlated with PM2.5 (R2 = 0.23) during the whole observation period (Fig. 5b). As denoted by larger 255 green squares in the figure, HONOcorr/NO2 correlated well with PM2.5 when its concentration was higher than 35 µg· m −3 (R3 = 0.47) (Fig. 5b). The larger the amount of HONO produced by the heterogeneous reaction of NO2 on the aerosol surface, the better the correlation between HONO/NO2 and PM2.5 (Cui et al., 2018;Wang, 2003;Hou et al., 2016;Li et al., 2012;Nie et al., 2015).

HONO photostationary-state approach
Having discussed the nighttime chemical behavior of HONO, we now concentrate on the daytime chemical behavior of HONO.
A calculation of the photostationary state (PSS) was conducted to preliminarily assess HONO concentrations during the daytime, especially the influence of any potential additional sources. It is hoped that HONO is in the photostationary state in the daytime due to its production from oxidation of NO by OH (Reaction (R2)), reformation of OH and NO by rapid photolysis 265 (Reaction (R3)), and oxidation of HONO itself by OH (Reaction (R4)).

Budget analysis of HONO
From the analysis in Sect. 3.4.1, it appears that there are additional sources of HONO in the daytime, because the [HONO]PSS 295 value is much lower than the observed HONO concentration. Here, Runknown is used to stand for the additional sources. The value of Runknown was estimated based on the balance between sources and sinks due to its short atmospheric lifetime. The  formed the largest proportion of the sinks in all four seasons, accounting for 94.69%, 96.85%, 96.10%, and 95.01% in spring, summer, autumn, and winter, respectively. The value of Rphot in summer was the highest (10.69 ppb· h −1 ) and this was 4.95, 2.29, and 5.85 times higher than that in spring, autumn, and winter, respectively. The oxidation of HONO by OH contributed 315 little to HONO sinks (2.49% of all sinks). Dry deposition (Ldep) was also very small (1.85% of all sinks). As for known sources, ROH+NO was the main known source in all four seasons, wherein the largest proportion was found in summer (80.73%), followed by autumn (70.98%), winter (66.27%), and spring (51.48%). Direct emission was second among the known sources, accounting for 40.83%, 15.78%, 23.55%, and 30.10% in spring, summer, autumn, and winter, respectively. Dark heterogeneous formation (Phete) was almost negligible in the daytime, accounting for approximately 5.07% of known sources 320 during the whole observation period. As for unknown sources, these made up the largest proportion of all sources found in summer (84.92%), followed by autumn (80.29%) and spring (49.98%). However, the unknown sources only accounted for 15.26% of all sources in winter. This indicates that known sources of HONO can explain the majority of sources in winter, and it is not necessary to analyze the unknown sources in this season.
The values of ROH+NO in different seasons all reached their maximum in the morning, and this was followed by a gradual 325 decrease. This parameter made up the highest proportion of all sources (56.15%) in winter, followed by spring (25.75%), autumn (13.99%), and summer (12.17%). In winter, with its low light intensity and high NO concentration, the homogeneous gas-phase reaction between NO and OH accounted for the majority of the daytime HONO sources. It is worth noting that Runknown exhibited a maximum at noon in all seasons except for winter. A previous study in Wangdu (Liu et al., 2019a) also found that unknown sources of HONO reached a maximum at midday, with the strongest photolysis rates in summer. In the 330 present study, the highest Runknown value at noon was 14.79 ppb· h −1 in summer, followed by 6.49 ppb· h −1 in autumn and 2.18 ppb· h −1 in spring. The Runknown value peaked at 08:00 in winter, reaching 1.55 ppb· h −1 . This indicates that this source https://doi.org/10.5194/acp-2020-880 Preprint. Discussion started: 10 September 2020 c Author(s) 2020. CC BY 4.0 License. depends on the season, strengthening the validity of the assumption that the missing HONO formation mechanism is related to a photolytic source (Michoud et al., 2014). The magnitudes of these additional sources were much higher than those found in Beijing  (1.3-3.82 ppb· h −1 ), in Guangzhou (0.77 ppb· h −1 ) , and in Xinken (~5 ppb· h −1 ) 335 (Su et al., 2011).

Exploration of possible unknown daytime sources
According to the analyses in Sect. 1 and Sect. 3.4.2, the unknown sources are likely to be related to light. It was indeed found that the unknown sources have a good correlation with the parameters related to light. It was reported in previous studies that particulate nitrate photolysis is a source of HONO (Ye et al., 2017;Ye et al., 2016;Scharko et al., 2014;Romer et al., 340 2018;McFall et al., 2018). We will discuss the possibility of HONO being produced by photolysis of particulate nitrate (J(NO3_R) × pNO3 − ) at this site in the next section. There was a logarithmic relationship showing good correlation between Runknown (ppb· h −1 ) and J(NO3 − _R) × pNO3 − (μg· m −3 · s −1 ) in spring (R 2 = 0.6348), summer (R 2 = 0.7266), and autumn (R 2 = 0.5041) (Fig. 8). In conditions of relatively lower J(NO3_R) × pNO3 − , Runknown increased rapidly with increasing pNO3 − concentration and its photolysis rate constant but reached a plateau after a critical value (J(NO3_R) × pNO3 − > 1 µg· m −3 · s −1 in 345 autumn, and J(NO3_R) × pNO3 − > 2 µg· m −3 · s −1 in spring and summer). This indicated that in conditions that were relatively cleaner, the missing daytime source of HONO was limited by the pNO3 − concentration and the photolysis rate constant. However, with severe haze or strong photolysis rate providing sufficient precursor or enough light to stimulate the reaction, the HONO production did not increase as J(NO3_R) × pNO3 − increased. It was found in a previous study (Scharko et al., 2014) that NO2 produced by NO3 − photolysis in situ is more easily absorbed by acidic solutions than the original gaseous NO2. 350 Therefore, we found the limiting factor for Runknown to be the aerosol neutralization degree F in spring, summer, and autumn.
Here, F was calculated from the equivalent concentrations of ammonium, sulfate, and nitrate  such that Considering the acidity of aerosols, we found that Runknown was limited when the aerosols were alkaline (F > 1). This field observation validates laboratory research on the release of HONO from photolysis of NO3 − in acidic solutions (Scharko et al., 355 2014).
We discuss whether photolysis of particulate nitrate is able to provide enough additional HONO by estimating the rate of HONO production by nitrate photolysis (Zhou et al., 2007;Li et al., 2012;Wang et al., 2017a) using where NO 3 − →HONO is the rate of photolysis of NO 3 − to form HONO, NO 3 − is the dry deposition rate of NO 3 − during the period 360 , and is the proportion of the surface exposed to the sun at midday. Here, we suppose that the surfaces involving NO 3 − were exposed to light by a factor = 1/4, taking mixing height = 250 , NO 3 − = 5 cm· s −1 over d = 24 h. We use the mean Eq. (8) were 1.52 × 10 −5 s −1 , 4.02 × 10 −4 s −1 , and 1.40 × 10 −4 s −1 for spring, summer, and autumn, respectively. These values 365 are in the range 6.2 × 10 −6 to 5.0 × 10 −4 obtained in a previous study (Ye et al., 2017), which indicated that particulate nitrate photolysis was the main source in spring, summer, and autumn. The variability of NO 3 − →HONO may be caused by chemical composition, acidity, light-absorbing constituents, and the optical and other physical properties of aerosols.

Parameterization of HONO
Through an empirical parameterized formula, we can explore an accurate parameterization method for HONO, discuss the 370 main control factors for the HONO concentration and its chemical behavior, and quantify its main sources and key kinetic parameters. As mentioned in Sect. 3.1, the HONO/NOx ratio is better than HONO/NO2 as an indicator of HONO generation.
In another study (Elshorbany et al., 2012), data were collected from 15 field observations all over the world to establish the correlation between the HONO/NOx ratio and the HONO concentration in global models. Therefore, we applied this method in this study to parameterize the HONO concentration. As shown in Fig. 9, the HONO/NOx ratios in the four seasons were 375 close to the calculated value (0.02). However, there were seasonal variations in the slope, showing a maximum in summer (2.60 × 10 −2 ), followed by autumn (2.06 × 10 −2 ), and a minimum in winter (1.59 × 10 −2 ). Except for in spring, HONO showed good correlation with NOx, with R 2 values ranging from 0.8972 to 0.9621. Therefore, we used slopes of 2.60 × 10 −2 , 2.06 × 10 −2 , and 1.59 × 10 −2 to parameterize the HONO concentrations in summer, autumn, and winter, respectively. As for spring, though only a weak correlation between HONO and NOx was found, the majority of the HONO/NOx ratios fluctuated round a slope 380 of 0.02 because concentrations of NOx greater than 60 ppb only accounted for 8.83% of the data. Therefore, a slope of 0.02 was applied in spring to parameterize the HONO concentration.
As can be seen from Fig. 10, the estimated values are very close to the observed values in the nighttime in autumn. After sunrise and before noon, the values observed were higher than the estimated values, and this difference gradually increases.
After noon and before sunset, the values observed were still higher than the values estimated, but the difference gradually 385 decreases. This phenomenon was also found in the daytime in spring, summer and autumn, but not in winter. Compared with the daytime, the estimated values during the nighttime were closer to the observed values in both trend and value in all four seasons, which further demonstrates that nighttime HONO is mainly produced from the heterogeneous reaction of NO2 on the ground and the surfaces of aerosols. Therefore, we should pay much more attention to simulation in the daytime. We distinguish two main sectors, nighttime and daytime, to analyze the factors affecting the HONO diurnal variation (Liu, 2017).
In contrast, excellent simulation results were found in a previous study using the same formula (Liu, 2017), which suggests 395 that using the same simulation formula in different regions may obtain greatly varying results.
As discussed in Sect. 3.4.3, nitrate photolysis is the main source of HONO in spring, summer, and autumn during the daytime, while the homogeneous gas-phase reaction of NO and OH is the major source of daytime HONO in winter. Therefore, we take the photolysis of nitrate into the spring, summer, and autumn calculations, but we use the reaction of NO and OH in the calculations for winter. In this way, the daytime simulation results are significantly improved (Fig. 10). This further 400 demonstrates that the apportionment of HONO sources is credible.

Comparison of contributions of HONO and O3 to OH radicals
Comparing the OH radical production via photolysis of HONO and O3, the effect of the high HONO concentrations in the daytime on the tropospheric oxidation capacity was evaluated (Ryan et al., 2018). Nitrous acid is considered to be a crucial source of OH radicals (Lee et al., 2016). As shown in Eq. (11), OH production rates from O3 photolysis (POH(O3)) were 405 calculated based on [O3], J(O 1 D), and [H2O] (Liu et al., 2019c). Only O( 1 D) atoms produced by the O3 photolysis at UV wavelengths less than 320 nm (Reaction (R5)) can combine with water to generate OH radicals (Reaction (R6)) in the atmosphere. The absolute water concentration was derived from temperature and RH. The reaction (R7) rates for N2 is 3.1 × 10 −11 cm 3 molecules −1 s −1 and for O2 is 4.0 × 10 −11 cm 3 molecules −1 s −1 . The net OH formation from HONO was estimated by Eq. (12) (Su et al., 2008b;Sörgel et al., 2011;Li et al., 2018;Atkinson et al., 2004 The diurnal patterns of P(OH) are shown in Fig. 11. The formation rates of OH from O3 photolysis peaked in midday at around 0.71 ppb· h −1 , 5.80 ppb· h −1 , 2.21 ppb· h −1 , and 0.48 ppb· h −1 for spring, summer, autumn, and winter, respectively. The variation of POH(O3) is consistent with J(O 1 D) (Fig. S4), peaking in midday and in summer on a diurnal and a seasonal timescale, respectively. For summer and autumn, POH(HONO) had a similar trend as POH(O3), peaking at around noon at the time of the highest J(HONO), but this was negligible at sunrise and sunset (Fig. S5). For spring and winter, however, POH(HONO) reached 420 a maximum in the morning rush hour caused by the combined influences of high HONO concentration and high J(HONO). A similar result was also found in southwest Spain from mid-November to mid-December 2008 . These results show that HONO contributes considerably to the morning atmospheric oxidizing capacity of the suburban atmosphere of Xiamen. Although HONO concentrations (average: 0.66 ppb) are much lower than O3 concentrations (average: 32.02 ppb) https://doi.org/10.5194/acp-2020-880 Preprint. Discussion started: 10 September 2020 c Author(s) 2020. CC BY 4.0 License.
during 07:00-16:00 LT, daytime HONO photolysis forms significantly more OH than daytime photolysis of O3 in all four 425 seasons. Generally, the mean value of POH(HONO) from 07:00 to 16:00 LT was 4.31 ppb· h −1 , and the average POH(O3) was 1.14 ppb· h −1 . This indicates that HONO concentrations at 0.66 ppb during 07:00-16:00 LT increase the formation of OH radicals by an order of magnitude, greatly increasing the local daytime tropospheric oxidative capacity. A similar result was found in Melbourne, where the peak OH production rate reached 2 ppb· h −1 from 0.4 ppb HONO (Ryan et al., 2018). The important role of HONO in the production of OH promotes photochemical peroxyacetyl nitrate formation . 430

Conclusions
We conducted measurements of HONO in the atmosphere at an IUE supersite in a coastal city of southeastern China in August, October, and December 2018 and March 2019, finding an average HONO concentration of 0.54 ± 0.47 ppb across the whole observation period. Concentrations of HONO in spring and summer were higher than in winter and autumn, which was consistent with seasonal variations in RH. Both higher HONO concentrations in the daytime and the HONO/NOx ratio peaking 435 around noon suggested that additional, unknown sources of HONO might be related to light. It was found that the contribution from vehicle exhaust emissions (1.64%) was higher than that found in most other studies due to the site being surrounded by several expressways with a large number of passing diesel vehicles. The average nocturnal conversion rate of NO2 to HONO was 0.47% h −1 , which was within the range 0.29-2.40% h −1 found by other studies. The HONOcorr/NO2 ratio increased with RH and the concentration of PM2.5 during the nighttime, which indicates that nocturnal heterogeneous reactions on the surfaces 440 of aerosols are the major source of HONO. However, dark heterogeneous formation (Phete) was almost negligible in the daytime, accounting for approximately 5.07% of known sources across the whole observation period. Observed values in the daytime were up to 50 times higher than those calculated from the PSS, suggesting that there were a large number of daytime sources of HONO. The highest proportion of all sources was ROH+NO in winter (56.15%), while Runknown made up at the largest proportion of all sources in summer (84.92%), autumn (80.29%), and spring (49.98%). It was found that there was a logarithmic 445 relationship between Runknown and particulate nitrate photolysis, and the limiting factor was particulate acidity in spring, summer, and autumn. The variation of HONO at night can be accurately simulated based on the HONO/NOx ratio, while the main sources should be considered for daytime simulation. Local tropospheric oxidation capacity was significantly increased by HONO during 07:00-16:00, providing an OH radical source (4.31 ppb· h −1 ) an order of magnitude greater than its concentration (0.66 ppb). 450

Data availability.
Measurement data at the IUE station, including HONO data and relevant trace gases and aerosol data as well as meteorological data, are available upon request from the corresponding author before the IUE database is open to the public. Li, Yahui Bian contributed to discussions of results.