Contributions of different sources to nitrous acid ( HONO ) at the SORPES 1 station in eastern China : results from one-year continuous observation 2 3

Abstract. Nitrous acid (HONO), a reservoir of the hydroxyl radical (OH), has been long-standing recognized to be of significant importance to atmospheric chemistry, but its sources are still debate. In this study, we conducted continuous measurement of HONO from November 2017 to November 2018 at SORPES station in Nanjing of eastern China. The yearly average mixing ratio of observed HONO was 0.69 ± 0.58 ppb, showing a larger contribution to OH relative to ozone with a mean OH production rate of 0.90 ± 0.27 ppb/h. To estimate the effect of combustion emissions of HONO, the emitted ratios of HONO and NOx were derived from 55 fresh plumes (NO / NOx > 0.85), with a mean value of 0.79 %. The well-defined seasonal and diurnal patterns with clear wintertime and early morning concentration peaks of both HONO and NOx indicate that NOx is the critical precursor of HONO. During the nighttime, the chemistry of HONO was found to depend on RH, and heterogeneous reaction of NO2 on aerosol surface was presumably responsible for HONO production. The average nighttime NO2-to-HONO conversion frequency (CHONO) was determined to be 0.0055 ± 0.0032 h−1 from 137 HONO formation cases. The missing source of HONO around noontime seemed to be photo-induced with an average Punknown of 1.13 ± 0.95 ppb h−1, based on a semiquantitative HONO budget analysis. An over-determined system of equations was applied to obtain the monthly variations in nocturnal HONO sources. Except for burning-emitted HONO (approximately 23 % of total measured HONO), the contribution of heterogeneous formation on ground surfaces was an approximately constant proportion of 36 % throughout the year. The soil emission revealed clear seasonal variation, and contributed up to 40 % of observed HONO in July and August. A higher propensity for generating HONO on aerosol surface occurred in heavily polluted period (about 40 % of HONO in January). Our results highlight ever-changing contributions of HONO sources, and encourage more long-term observations to evaluate the contribution from varied sources.



Introduction
Nitrous acid (HONO) is a vital constituent of nitrogen cycle in the atmosphere, first observed in the field by Perner and Platt (1979).The concentrations of HONO varied from dozens of ppt in remote regions (Villena et al., 2011b;Meusel et al., 2016) to several ppb in polluted urban regions (Yu et al., 2009;Tong et al., 2015).The photolysis of HONO (R1) has been long standing as a momentous source of the hydroxyl radical (OH) especially during the early morning when other OH sources are minor (Platt et al., 1980;Alicke, 2002Alicke, , 2003)).Even during the daytime, recent studies have recognized the photolysis of HONO as a potentially stronger contributor to daytime OH radical than that of O3 (Kleffmann, 2005;Elshorbany et al., 2009;Li et al., 2018).Meanwhile, HONO has been found to affect adversely human heath (Jarvis et al., 2005;Sleiman et al., 2010).
Although the significance of HONO has been given much weight, the sources of ambient HONO are complicated and have been debated for decades.HONO can be emitted from combustion, including vehicle exhaust, industrial exhaust and biomass burning (Table 1).Tunnel experiments with tests for different engine types have determined an emission ratio of HONO/NOx for traffic source, ranged in 0.3-0.8%(Kirchstetter et al., 1996;Kurtenbach et al., 2001).The release from soil nitrite through acidification reaction and partitioning is considered to be another primary source of atmospheric HONO (Su et al., 2011).Soil nitrite could come from biological nitrification and denitrification processes (Canfield et al., 2010;Oswald et al., 2013), or be enriched via reactive uptake of HONO from the atmosphere (VandenBoer et al., 2014a;VandenBoer et al., 2014b).In addition to direct emissions, the vast majority of HONO is produced chemically.The recombination of NO and OH (R3) is the main homogeneous reaction for supplying HONO (Pagsberg et al., 1997;Atkinson, 2000), whose contribution may be significant under conditions of sufficient reactants during daytime.During the nighttime, with low OH concentrations, other larger sources, heterogeneous reactions of NO2 on various surfaces, are required to explain elevated mixing levels of HONO.Laboratory studies indicate that NO2 can be converted to HONO on humid surfaces (R4), being first order in NO2 and depending on various parameters including the gas phase NO2 concentration, the surface water content, and the surface area density (Kleffmann et al., 1998;Finlayson-Pitts et al., 2003).Besides, heterogeneous reduction of NO2 with surface organics (R5) is proposed to be another effective pathway to generate HONO (Ammann et al., 1998;Ammann et al., 2005;Aubin and Abbatt, 2007), observed in freshly emitted plumes with high concentrations of NOx and BC (Xu et al., 2015).
Notably this reaction rate is drastically reduced after the first few seconds due to consumption of the reactive surfaces (Kalberer et al., 1999;Kleffmann et al., 1999), but this reaction could be strongly enhanced by light on photo-activated surface (George et al., 2005;Stemmler et al., 2006;Stemmler et al., 2007).During the daytime, heterogeneous HONO formation from the photolysis of adsorbed nitric acid (HNO3) and particulate nitrate (NO3 -) at UV wavelengths has been found in experiments and observations (Zhou et al., 2003;Zhou et al., 2011;Ye et al., 2016;Ye et al., 2017).
Heterogeneous processes are typically considered as the primary sources of HONO in many regions yet are the most poorly understood.For NO2 conversion to HONO on surfaces (R4, R5), the uptake coefficients of NO2 derived from different experiments vary from 10 -9 to 10 -2 (Ammann et al., 1998;Kirchner et al., 2000;Underwood et al., 2001;Aubin and Abbatt, 2007;Zhou et al., 2015).The key step to determine the uptake of NO2 or the reaction rate is still ill-defined, and we are also not certain if and how the ambient natural surfaces can be reactivated by radiation.Furthermore, it has become a main concern to compare the contributions of ground and aerosol surfaces to HONO formation.It is so far, not well explained for the observed HONO, especially during daytime.Large unknown sources of HONO were identified by many studies (Su et al., 2008b;Sörgel et al., 2011;Michoud et al., 2014;Lee et al., 2016).
Benefitting from more and more studies, particularly the observations under different environment (Lammel and Cape, 1996;Li et al., 2012), understanding of HONO chemistry in the atmosphere has been greatly improved during the last decade.
However, most HONO observations were short-term campaigns with studies ranging from several weeks to several months.For example, Reisinger (2000) found a linear correlation between the HONO/NO2 ratio and aerosol surface density in the polluted winter atmosphere, and Nie et al. (2015) showed the influence of biomass burning plumes on HONO chemistry, according to observed data during late April-June 2012, while Wong et al. (2011) believed that NO2 to HONO conversion on the ground was the dominant source of HONO by analyzing vertical profiles from 15 August to 20 September in 2006.Moreover, a theory that HONO from soil emission explained the strength and diurnal variations of the missing source has been presented by Su et al. (2011) based on data measured from 23 to 30 October 2004.In case the HONO sources possibly exhibit temporal variability, especially seasonal differences, it is challenging to draw a full picture on the basis of these short-term observations.More than a year of continuous observation is needed, yet rather limited.
The Yangtze River Delta (YRD) is one of the most developed regions in eastern China.Rapid urbanization and industrialization have induced severe air pollution over the last three decades, particularly high concentrations of reactive nitrogen (Richter et al., 2005;Rohde and Muller, 2015), including HONO (Wang et al., 2013;Nie et al., 2015).In this study, we conducted continuous HONO observations at the SORPES station (Station for Observation Regional Processes and the Earth System), located in the western part of the YRD, a place that can be influenced by air masses from different source regions of anthropogenic emissions, biomass burning, dust and biogenic emissions (Ding et al., 2013;Ding et al., 2016) at relatively high levels.We discussed the potential mechanism of HONO production based upon semiquantitative analysis and correlation studies, and paying special attention to changes in major sources of HONO during different seasons.

Study site and instrumentation
Continuously observations was conducted at the SORPES station at the Xianlin Campus of Nanjing University (118°57′E, 32°07′N), located in the northeast suburb of Nanjing, China, from November 2017 to November 2018.The easterly prevailing wind and synoptic condition makes it a representative background site of Nanjing and a regional, downwind site of the city cluster in YRD region.Detailed descriptions for the station can be found in previous studies (Ding et al., 2013;Ding et al., 2016).
HONO was measured with a commercial long path absorption photometer instrument (QUMA, Model LOPAP-03).A brief description of this instrument is provided as follows.The ambient air was sampled in two similar temperature controlled stripping coils in series using a mixture reagent of 100 g sulfanilamide and 1 L HCl (37% volume fraction) in 9 L pure water.In the first stripping coil, almost all of the HONO and a fraction of interfering substances were absorbed into solution, and the remaining HONO and the most of the interfering species were absorbed in the second stripping coil.After adding a reagent of 1.6 g N-naphtylethylendiamine-dihydrochloride in 9 L pure water to both coils, colored azo dyewas formed in the solutions from 2 stripping coils, which were then separately detected via long path absorption in special Teflon tubing.The interference free HONO signal was the difference between the signals in the two channels.Further details can be found in (Heland et al., 2001;Kleffmann et al., 2006).To correct for the small drifts in instrument's baseline, compressed air was sampled every 12 h (flow rate: 1 L/min) to make zero measurement.A span check was made using 0.04 mg/m 3 nitrite (NO2 -) solution each weeks with a flow rate of 0.28 ml/min.The time The NO and NO2 levels were measured using a chemiluminescence instrument (TEI, model 42i) coupled with a highly selective photolytic converter (Droplet Measurement Technologies, model BLC), and the analyzer had a detection limit of 50 pptv for an integration time of 5 min, with precision of 4% and an uncertainty of 10% (Xu et al., 2013).O3 and CO were measured continuously using Thermo-Fisher Scientific TEI 49i and TEI 48i.al., 2017).To reduce the error of model, we used observed UVB to correct simulated results (Jmod) by Eq. ( 1).The daytime OH concentration was calculated by applying the linear fitting formula (Eq.2) that obtained from correlations of measured OH concentrations with simultaneously observed J(O 1 D), suggested by Rohrer and Berresheim (2006).The calculated OH concentrations around noon were in the range of 0.15-1.17×107 cm -3 , comparable to observations in Chinese urban atmospheres (Lu et al., 2012;Lu et al., 2013).

Observation overview
We carried out continuous measurements for HONO at SORPES station in the northeast suburb of Nanjing from November 2017 to November 2018 with a mean measured ambient HONO mixing level of 0.69 ± 0.58 ppb, within the range of those in or in the vicinity of mega cities (Table 2).Fig. 1 shows the seasonal pattern of HONO and related parameters.The highest concentration of HONO was found in winter (1.04 ± 0.75 ppb), followed by spring (0.68 ± 0.48 ppb), autumn (0.66 ± 0.53 ppb) and summer (0.45 ± 0.37 ppb).Such seasonal variations in Nanjing are aligned with that in Beijing (Hendrick et al., 2014), and are somewhat similar to those in Jinan (Li et al., 2018), where the highest levels occurred in winter and the lowest levels occurred in autumn, but these variations are different from those in Hongkong (Xu et al., 2015) where the highest and lowest values of HONO appeared in autumn and spring, respectively.The important point is that the seasonality of HONO coincides with that of NOx (or NO2), which is believed to be the main precursor of HONO, in current studies.The HONO to NOx ratio or the HONO to NO2 ratio has been used extensively in previous research to characterize the HONO levels and to indicate the extent of heterogeneous conversion of NO2 to HONO, since it is less influenced by convection or transport processes than the individual concentration (Lammel and Cape, 1996;Stutz et al., 2002).When a large proportion of HONO comes from direct emissions, the value of HONO/NO2 usually becomes larger, falsely implying the strong formation of HONO from NO2, however, the freshly emitted air masses generally have the lowest HONO/NOx ratio, meaning that HONO/NOx behaves better than HONO/NO2 in a way.As shown in Fig. 1(b), the low value of HONO/NOx in winter is attributed to heavy emissions because we see high mixing ratios of NO during this cold season (Fig. 1c), the reasons for two peaks of HONO/NOx in spring and summer will be discussed in sections 3.3, 3.4 and 4.
All daily changes of HONO concentration in different seasons closely resemble a cycle in which HONO peaks in the early morning, and then decreases to the minimum at dusk, following the diurnal trend of NOx (Fig. 2).The daily variations of HONO in Nanjing are like those seen in other urban areas (Villena et al., 2011a;Wang et al., 2013;Michoud et al., 2014;Lee et al., 2016), but differ from observations on the roadside (Rappenglück et al., 2013;Xu et al., 2015).At night, the mixing ratio of HONO increases rapidly in the first few hours and then stabilizes (in spring and summer) or gradually climbs to its peak in the morning rush hour (in winter and autumn).The accumulation during nighttime hours suggests a significant production of HONO exceeding the dry deposition of HONO.As the sun rises, the HONO sink will be strengthened by photolysis and the vertical mixing of HONO.It's clear that the peak times varing seasonally result from different sunrise times.During the daytime, the rate of HONO abatement is rapid before noon and then becomes progressively until HONO concentration falling to the minimum.Given that the photolytic lifetime of HONO is about 10-20 min in the midday (Stutz et al., 2000), the considerable HONO concentration during daytime indicates the existence of large sources of HONO production.From the daily variations of the HONO to NOx ratio, we can further understand the behavior of HONO in the atmosphere.HONO/NOx is regularly enhanced quickly before midnight then reaches a maximum during the latter half of the night.
According to Stutz et al. (2002), the highest HONO/NOx (or HONO/NO2) is defined by the balance between production and loss of HONO at each night, the conditions affecting the maximum ratio at nighttime will be discussed in section 3.3.What's interesting here is the peak of the HONO/NOx ratio in the midday sun in spring, summer and autumn, and even in winter, the ratio doesn't decline but remains stationary before and at noon.If the HONO sources during daytime are consistent with those at night, the minimum HONO/NOx ratios should occur at noon due to the intense photochemical loss of HONO.Therefore, there must be additional sources of HONO during daytime.The increase of HONO/NOx with solar radiation (e.g., UVB) is found in both diurnal and seasonal variations, indicating that these daytime sources have a relationship with the intensity of solar radiation.We will further discuss the potential daytime sources of HONO in section 3.4.
The elevated mixing ratio of HONO presents an efficient reservoir of OH radicals during daytime in Nanjing.We calculate the net OH production rate from HONO POH(HONO) using Eq. ( 3) (Li et al., 2018).For comparison, the OH production rate from ozone photolysis, POH(O3), is also derived from Eq. ( 4).Based on Alicke et al.
Fig. 3 shows that the diurnal peak of OH production rate from HONO is usually found in the late morning, caused by the combined effects of HONO concentration and its photolysis frequency, JHONO, and the seasonal peak of POH(HONO) occurs in spring for the same reason.POH(O3), coinciding with the trend of J(O 1 D), is highest around noon and in summer at daily and seasonal time scale respectively.Significantly, the photolysis of HONO produced more OH than that of ozone throughout the daytime in winter, spring and autumn.In summer, the contribution of HONO to OH is greater in the early morning, and, although the photolysis of ozone contributes more OH at noon, the role of HONO is considerable.Overall, the average POH(HONO) during 8:00-16:00 LT is 0.90 ± 0.27 ppb/h, more than twice the value of POH(O3), the mean value of which is 0.41 ± 0.25 ppb/h.The impressive role of HONO in the atmospheric oxidizing capacity should benefit photochemical ozone production (Ding et al., 2013;Xu et al., 2017;Xu et al., 2018), new particle formation (Qi et al., 2015) and secondary aerosol formation (Xie et al., 2015;Sun et al., 2018) in Nanjing, the western YRD region.

Direct emissions of HONO from Combustion
As mentioned above and shown in Fig. 4 and (e) UVB<=0.01W/m 2 .Then, the slopes of HONO to NOx in selected plumes were considered as the emission ratios in our study.
Within the one-year dataset, we selected 55 freshly emitted plumes satisfying the criteria above (Table 3), of which 20 air masses were found in the morning and evening rush hours; the derived △HONO/△NOx ratios vary from 0.26% to 1.91% with a mean value of 0.79% ± 0.36%.Many factors, such as the amount of excess oxygen; the types of fuel used (gasoline, diesel, coal); if engines are catalyst-equipped, and if engines are well-maintained, could result in variances in these ratios.
Additionally, the rapid heterogeneous reduction of NO2 on synchronously emitted BC can also raise the value of △HONO/△NOx (Xu et al., 2015).For our study, anaverage emission factor of 0.79% is deployed to evaluate the emission contribution of HONO (Eq.5), which is abbreviated as HONOemis.Combustion emissions contribute an average of 23% of total measured HONO concentrations at night (Fig. 4b), with a maximum HONOemis/HONO value of 32% in winter and a minimum HONOemis/HONO value of 18% in summer.We then get the corrected observed HONO (HONOcorr) by Eq. ( 6) for further analysis.The slope of the fitted line for HONO and NOx is 1.62%, higher than emission ratio 0.79% (Fig. 4a), and almost 80% of HONO is from HONOcorr that is not affected by emissions (Fig. 4b).These imply significant secondary formation of HONO in the atmosphere.

The NO2-to-HONO conversion rate (CHONO)
In addition to emissions, heterogeneous reaction of NO2 on surfaces (R4, R5) is believed to be the major formation pathway of nocturnal HONO.Thus, the NO2-to-HONO conversion frequency is calculated from Eq. ( 5) (Alicke et al., 2002;Alicke, 2003;Wentzell et al., 2010), where NO2 is adopted to scale HONO to reduce the dilution influence according to Su et al. (2008a).Similar to HONO/NOx (Fig. 2), the nighttime HONOcorr/NO2 ratio rises from the lowest value and then reaches a quasi-stable state, meaning that CHONO can actually be used to assess how quickly HONOcorr/NO2 increases to its equilibrium.
Following the method of Xu et al. (2015) and Li et al. (2018), 137 cases in which HONOcorr/NO2 increased almost linearly from 18:00 to 24:00 each night are selected, and the slope fitted by the least linear regression for HONOcorr/NO2 against time is just the conversion frequency of NO2 to HONO.The derived CHONO vary from 0.0043 ± 0.0017 h -1 in winter to 0.0066 ± 0.0040 h -1 in summer, with an average value of 0.0055 ± 0.0032 h -1 , which is in the range (0.044-0.014 h -1 ) shown by other studies in urban and suburban sites (Fig. 5).Noting that CHONO assumes the increase of

RH dependence of HONO chemistry
It appears that NO2 hydrolysis on humid surfaces (R4), having a first order dependence on NO2 (Jenkin et al., 1988;Ackermann, 2000;Finlayson-Pitts et al., 2003), is influenced by the surface absorbed water rather than by atmospheric water vapor (Kleffmann et al., 1998;Finlayson-Pitts et al., 2003), although the exact mechanisms are still unknown.In the studies of Stutz et al. (2002) and Stutz et al. (2004), the pseudo steady state of HONO/NO2, where this ratio is at a maximum, is presumed to be a balance between the production of HONO from NO2 and the loss of HONO on surfaces, and the highest HONO/NO2 is determined by the ratio of the reactive uptake coefficients for each process.Scatter plot of HONOcorr/NO2 against relative humidity in our study are illustrated in Fig. 5; to eliminate the influence of other factors as for as possible, the average of the 6 highest HONOcorr/NO2 values in each 5% RH interval is calculated, according to Stutz et al. (2004).
The phenomenon that HONOcorr/NO2 first increases and then decreases with an increasing RH in Fig. 5(a) was also observed by other studies (Hao et al., 2006;Yu et al., 2009;Li et al., 2012;Wang et al., 2013).In addition, the trend that HONOcorr/NO2 increases roughly with RH except when RH values are greater than 95%, as shown in follows.Even at the lowest measured RH of 18%, the absolute moisture content in the atmosphere is still greater than 10 3 ppm in our study, but the HONOcorr/NO2 ratio is quite small and remains unchanged when RH is below 45%, indicating that the NO2 to HONO conversion efficiency should be determined by water covering the surfaces, and HONO is seemingly produced on "dry" surfaces where the amount of chemisorbed water becomes approximately independent on the water vapor levels in dry conditions, according to Lammel (1999).
It has been reported that surfaced absorbed water depends on RH values, and the dependences vary for different material surfaces of the ground, but generally follow the shape of a BET isotherm (Lammel, 1999;Saliba et al., 2001;Sumner et al., 2004).
The number of mono-layers of water increases slowly from zero to 2-4, accompanied by RH from 0 to a turning point, and the water coverage grows dramatically (up to 10-100 mono-layers) once RH exceeds the turning point (Finlayson-Pitts et al., 2003).
Fig. 5(a) shows the case where the surface for NO2 converting to HONO is dominated by the ground, the HONOcorr/NO2 increases along with RH when RH is less than 75%, which can be explained by the reaction of NO2 to generate HONO on wet surfaces.
However, a negative correlation between HONOcorr/NO2 and RH is found when RH is over 75%, presumably because the rapidly growing aqueous layers of the ground surface lead to efficient uptake of HONO and make the surface less accessible or less reactive for NO2.Hence, the RH turning point for absorbed water on ground surfaces is perhaps around 75% for our observation, within the range of results from experiments on various surfaces (70-80% RH) (Lammel, 1999;Saliba et al., 2001;Sumner et al., 2004).Once RH exceeds 95%, the reaction surface is classified as an "aqueous" surface in Lammel (1999), asymptotically approaching the state of water droplet.Under these circumstances, the efficiency of NO2 forming HONO will be reduced since the conversion has changed from a "heterogeneous reaction" to a "liquid reaction" (Lee and Schwartz, 1981;Cheung et al., 2000;Kleffmann et al., 1998;Finlayson-Pitts and Pitts Jr, 1999), and the aqueous surface is found to be an impactful sink of HONO in experimental work (Park and Lee, 1988;Becker et al., 1996;Hirokawa et al., 2008) and in field observations (Acker et al., 2005;He et al., 2006;Zhou et al., 2007).For the reasons mentioned above, we can see a dramatic positively before RH reaches 95%, which is consistent with the results from laboratory studies that the uptake coefficient of NO2 to HONO (ϒNO2→HONO) increases with RH (Kleffmann et al., 1999;Liu et al., 2015).We will discuss the effect of aerosol on HONO production in the next part.

Impact of aerosols on HONO formation
To further understand the heterogeneous formation of HONO on aerosol, we provide a correlation analysis of the related HONOcorr parameters (HONOcorr and HONOcorr/NO2) with PM2.5 when HONOcorr/NO2 reaches the pseudo steady state each night (3:00-6:00 LT).The convergence or diffusion processes of gases and particles caused by the decrease or increase of the boundary layer height can also lead to a consistent trend of HONOcorr and PM2.5 (Fig. 6a), while the ratio of HONO and NO2 can not only remove this physical effect to a certain extent but also represent the conversion degree of NO2 to HONO, so a moderate positive correlation between HONOcorr/NO2 and PM2.5 (r=0.35,p=0.01) throughout the observation period could be more convincible (Fig. 6b).As shown by larger triangles with gray borders in Fig.

6(b)
, HONOcorr/NO2 is better correlated with PM2.5 in the months during which the mass concentrations of PM2.5 are higher during this 1-year measurement, generally occurring from November to May (Fig. 1d).This finding can be explained with a law that greater contributions of NO2 heterogeneously reacting on aerosol to generate HONO lead to better correlations between HONOcorr/NO2 and PM2.5.Interestingly, this relationship can also be divided approximately into two groups by NH3/CO; the correlation is good when the value of NH3/CO is lower than 2‰, but when NH3/CO is higher than 2‰, a poor correlation is found.We will discuss this phenomenon further in section 4. The evidence of HONO formation on aerosol were also found in other observations (Reisinger, 2000;Wang, 2003;Li et al., 2012;Nie et al., 2015;Hou et al., 2016;Cui et al., 2018).As is known, producing HONO is not the dominant sink of NO2 at night, but it seems that more NO2 can be converted to HONO under conditions of heavy pollution (Fig. 7b).We discuss whether heterogeneous reactions of NO2 on aerosols are able to provide comparable HONO with our measurement by Eq. ( 8), where we only consider HONO formation on particle surfaces and assume that HONO principally settles on the ground surface, neglecting HONO loss on aerosol. is the surface area to volume ratio (m −1 ) of aerosol; HONO ν is the deposition velocity of HONO, which is considered to be close to the deposition velocity of NO2 at night (Stutz et al., 2002;Su et al., 2008a); and a approximate value of 0.1 cms -1 is used based on the measurements from Coe and Gallagher (1992) and Stutz et al. (2002); H is the boundary layer mixing depth, and a value of 100 m is assumed for nighttime (Su et al., 2008a).
Considering at nighttime period with severe haze, the aerosol surface density calculated from the particle number size distributions between 6 nm and 800 nm is about 1.2×10 -3 m -1 , matched by 200 μg/m 3 of PM2.5 from our observations, and the averaged mixing ratios of HONO and NO2 are 1.15 ppb and 28.4 ppb, respectively, at night in winter (Table 2).For 30% of the measured mean winter CHONO (0.0013 h -1 ), the uptake coefficient of NO2-to-HONO ( ) is 6.9×10 -6 , derived from Eq. ( 8), and for all of the measured mean winter CHONO (0.0043 h -1 ) value, the NO HONO 2 → γ is 1.44 -5 , fitting the results from many laboratory studies which demonstrate that the uptake coefficients of NO2 ( NO 2 γ ) on multiple aerosol surfaces or wet surfaces are mainly distributed around 10 -5 with the HONO yield varying from 0.1 to 0.9 (Grassian, 2002;Aubin and Abbatt, 2007;Khalizov et al., 2010;Han et al., 2017).It is necessary we derived could be the upper limit of the uptake coefficient for NO2 conversion to HONO on aerosol.In addition to particles surfaces, other aerosol parameters such as surface water content, chemical composition, pH value, and phase state of surfaces may also influence the heterogeneous formation of HONO.

Missing daytime HONO source
After discussing the nocturnal formation mechanism of HONO, we now focus on the chemistry of daytime HONO whose lifetime is only 10-20 min but whose concentrations are still about 0.25-0.6ppb at noon (Fig. 2).We are not certain if the observed HONO can be provided by known mechanisms (gas phase reaction (R4) and emissions) to date, so a budget equation of daytime HONO (Eq.9) is utilized to analyze its source and sinks (Su et al., 2008b;Sörgel et al., 2011).Here, dHONO/dt is the change rate of the observed HONO.The sources rates of HONO contain the homogeneous formation rate (PNO+OH, R4); the combustion emission rate (Pemis); and the unknown HONO daytime source (Punknown).The sink rates of HONO consist of the photolysis rate (Lphot, R1); the reaction rate of HONO with OH (LHONO+OH, R2); and the dry deposition rate (Ldep).TV and Th represent the vertical (TV) and horizontal (Th) transport processes of HONO, which are thought to be negligible for intense radiation and relatively homogeneous atmospheres with generally calm winds (Dillon, 2002;Su et al., 2008b;Sörgel et al., 2011).Therefore, the undiscovered daytime source of HONO (Punknown) can be derived by Eq. ( 10), which is a deformation of Eq. ( 9) without minor terms (Tv and Th) and where dHONO/dt is substituted by ΔHONO/Δt that is counted as difference between observed HONO at two time points.The reaction rate constants of reaction 2 (kHONO+OH) and reaction 4 (kNO+OH) are 6.0×10 -12 cm 3 molecules −1 s −1 and 9.8×10 -12 cm 3 molecules −1 s −1 , respectively (Atkinson et al., 2004).The emission ratio of HONO and NOx (HONO/NOx=0.79%)obtained in section 3.2, is used to estimate Pemis.For Ldep, the dry deposition velocity of diurnal HONO ( HONO ν ) is measured as 2 cms -1 in the work of Harrison et al. (1996), and a practical mixing height of 200 m is adopted, considering that most of the HONO cannot rise above this altitude due to rapid photolysis (Alicke et al., 2002) Hohenpeissenberg, Germany (Acker et al., 2006b); 0.5 ppb/h in a forest near Jülich, Germany (Kleffmann, 2005); 0.7 ppb/h in a outskirt of Paris, France; 0.77 ppb/h in a polluted rural area of the Pearl River Delta, China (Li et al., 2012); 0.98 ppb/h at an urban site in Xi'an, China (Huang et al., 2017); 1.0 ppb/h in a suburban area of Beijing, China (Yang et al., 2014); 1.7 ppb/h in an urban area of Santiago, Chile (Elshorbany et al., 2009); 2.95 ppb/h in the urban atmosphere of Jinan, China (Li et al., 2018); and 3.05 ppb/h at an urban site in Beijing, China (Wang et al., 2017).
The highest noontime Punknown value is 1.73 ppb h −1 in spring, followed by 1.15 ppb h −1 in winter, 1.0 ppb h −1 in summer and 0.77 ppb h −1 in autumn, unliking the seasonal variation of NO2; and Punknown shows an increase towards noon, this production rate is higher before noon than after noon, which is also distinguished from the diurnal pattern of NO2.These results indicate that the production of daytime HONO is different from the heterogeneous formation from NO2 at night.Hence, we perform a correlation analysis between noontime Punknown and related parameters to determine the potential unknown daytime source of HONO (Table 4).Punknown is better correlated with NO2*UVB than with NO2 or UVB alone in winter, spring and autumn (p=0.05),perhaps associated with the photo-enhanced converting from NO2 to HONO (George et al., 2005;Stemmler et al., 2006;Stemmler et al., 2007), and this is the reason for Punknown normalized by NO2 following the steps of UVB, showing a peak around noontime in different seasons (Fig. 8).The average value of Punknown normalized by NO2 is 0.1 h -1 , over 18 times greater than the nighttime conversion rate (0.0055 h -1 ), also implying that Punknown cannot be explained by the NO2-to-HONO mechanism at night.Assuming that the height of a well-mixed boundary layer around remain constant for each day, UVB*NO2 and UVB*NO2*PM2.5 could be proxies for photo-induced heterogeneous reactions of NO2 on ground and aerosol surfaces, significantly increase (p=0.05).We cannot be sure which surfaces (ground or aerosol) are more important to the hypothetical photo-heterogeneous reaction of NO2 based on the present study.The photolysis of particulate nitrates (NO3 -) as a source of HONO (Ye et al., 2016;Ye et al., 2017) cannot be determined if it is momentous in our study, since the correlation between Punknown and UVB*NO2 isn't superior to the correlation between Punknown and UVB multiplied by PM2.5 or other aerosol compounds.The comparisons of correlation coefficients shown above follow the method provided by Meng et al. (1992).Overall, it seems that the sealed source of daytime HONO is optically controlled, although we are not sure what the actual mechanism is.

Estimation of the contribution from different sources
From this and previous studies, we can conclued that not only the concentration of ambient HONO but also the sources of HONO have temporal and spatial patterns, which is supposed to be considered in model studies.Nocturnal HONO is selected to discuss the monthly variations of HONO sources in detail without the uncertainties of daytime HONO formation and the influences of HONO photolysis.The heterogeneous reaction of NO2 on aerosol produces a considerable portion of HONO in relatively polluted months (Dec.-May),but contributes very little less than nothing in clean months (Jun.-Oct.), as seen in section 3.3.3.Coincidentally, direct emissions from burning processes of HONO decrease from their peak values from winter to summer (section 3.2).However, the monthly averaged ratios of HONO and NOx are highest in summer , which conflicts with two sources mentioned above.
As is known, higher NO2-to-HONO conversion level or other NOx-independent sources can cause an increase in the HONO/NOx ratio.For the case of a mostly constant surface with low reactivity due to the long-term exposure to oxidizing gases and radiation, the yield of nighttime HONO from NO2 reacting on ground surfaces could be imprecisely assumed to be unchanged.Thus, soil nitrite formed through microbial activities, especially nitrification by ammonia-oxidizing bacteria (NH4 + →NO2 -) (Su et al., 2011;Oswald et al., 2013), is adopted to be an source for atmospheric HONO in this study, considering the nearby presence of some grassland and natural vegetation mosaics.Although we do not directly measure HONO emissions from soil, the observed ammonia can represent its monthly average intensity, based on the following hypothesis: the dominant source of NH3 is from soil, especially from fertilizers (NH4 + →NH3) for a good correlation between ammonia and temperature in the site (r=0.63,p=0.01), omitting the contributions of livestock to NH3 since there is only a small poultry facility within 10 km of this site (Meng et al., 2011;Huang et al., 2012;Behera et al., 2013).Combustion sources (vehicles, industry, biomass burning) should contribute only a fraction of NH3 seeing that NH3 is not related to NOx or CO in our study.Moreover, the release of both HONO and NH3 depend on the strength of microbial activities, fertilizing amount, and soil properties (e.g., temperature, acidity and water content of soil).Although the processes of HONO and NH3 emission from soil may not be completely synchronized, the seasonal patterns for each should be consistent.
Until now, we can separate the sources of HONO into four parts: (1) combustion emissions from vehicles and industries (HONOemi) with a constant emitted HONO/NOx ratio of 0.79%; (2) conversion of NO2 to HONO on the ground surfaces (HONOgrd) with a constant but unknown yield x1; (3) conversion of NO2 to HONO on aerosol surfaces (HONOaer) with a PM2.5-dependent yield (HONOare/NO2); and (4) emission from soil (HONOsoi), expressed by corrected NH3 multiplied by an unknown coefficient x2.The corrected NH3 is obtained by subtracting combustion emission from total observed ammonia.Ammonia from combustion is found to be proportional to simultaneous CO (Meng et al., 2011;Chang et al., 2016), and a proportion of 0.3%, which is in the lower quantile of the NH3/CO ratios in fresh air masses (for hourly data: NO/NOx>0.75;UVB=0; temperature<5℃) is used from our measurements.
Substituting monthly average values of measured HONO, NO2, PM2.5, NH3, and CO into Eq.( 9) by assuming that HONOtot is equal to HONOobs, we can get an overdetermined system of equations with 11 equations with 2 unknowns (excluding means of related parameters from February), finally achieving an approximate Combustion emissions contribute an average of 23% to nocturnal HONO concentrations, with an average emission ratio △HONO/△NOx of 0.79%.During the nighttime, the dominant source of RH-dependent HONO could be the heterogeneous reaction of NO2 on wet ground or aerosol surfaces with a mean estimated conversion rate of 0.0055 h -1 .During the daytime, a missing HONO source with an average strength of 1.13 ppb h −1 was identified around noon, contributing more than 75% of the production of HONO and seeming to be photo-enhanced.
(a), the similar patterns of HONO and NOx, particularly sharply increasing together in the fresh plumes, in which the NO/NOx ratios are usually very high, indicate the presence of direct combustion emission of HONO, which need to be deducted when analyzing the secondary formation of Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2019-219Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 25 April 2019 c Author(s) 2019.CC BY 4.0 License.HONO.The SORPES station are influenced by air masses from both industries and vehicles(Ding et al., 2016), the traffic emission factor investigated in other experiments cannot be used straightly; thus, we derive the emitted HONO/NOx ratio according the method ofXu et al. (2015), and the following five criteria are adopted to choose fresh plumes : (a) NOx>40ppbv; (b) △ NO/ △ NOx>0.85;(c) good correlation between HONO and NOx (r>0.9);(d) short duration of plumes (<=2 h); Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2019-219Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 25 April 2019 c Author(s) 2019.CC BY 4.0 License.
HONOcorr/NO2 is caused by the conversion of NO2, excluding other possible sources of HONO (e.g.soil nitrite); and the computed CHONO is the net NO2-to-HONO conversion rate since the measured HONOcorr has already taken in to account the sinks of HONO (mainly deposition).Considering the uncertainties of CHONO, utilizing CHONO directly to analyze the mechanism of HONO formation may not be appropriate, but it could be attemptable to facilitate the parameterizations for HONO production in air quality models by CHONO.Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2019-219Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 25 April 2019 c Author(s) 2019.CC BY 4.0 License.
Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2019-219Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 25 April 2019 c Author(s) 2019.CC BY 4.0 License. to elaborate that: (1) the ambient particles were dried with silica gel before measuring their number size distributions, and the mass concentrations of PM2.5 were also measured under a system where the temperature was maintained at 30 ℃; (2) the aerosol surface was calculated using an assumption that all particles are spherically shaped, but the particles could in fact have irregular bodies and porous structure; (3) the particle size of both PM2.5 and derived aer S [ ] V is just a part of the total suspended particulate matter.As described, the aerosol loading in the atmosphere is actually underestimated in our study, thus the NO HONO 2 → γ Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2019-219Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 25 April 2019 c Author(s) 2019.CC BY 4.0 License.
Fig.9shows that an average of 36% of HONO is produced heterogeneously on ground surfaces without perceptible temporal variations, but the contribution of this source is overtaken by NO2 converting to HONO on aerosols in January (approximately 40% of HONO), and was exceeded by soil emission in July and August (approximately 40% of HONO).The seasonal variations of HONO from different pathways at night indicate that short-term observations may just capture a small part of the total picture when exploring the source mechanisms of HONO.The total HONO concentration (HONOtot) is the sum of derived HONO from the four sources listed above.The good correlation between HONOtot and HONOobs and the low mean normalized error of HONOtot to HONOobs indicate that our assumption regarding nocturnal HONO sources is reasonable.It should be noted that the slope of the linearly fitted line between HONOcorr/NO2 and PM2.5 in spring (r=0.74,slope=0.68‰) is much higher than that in winter (r=0.60,slope=0.20‰),but we just use a mean slope of 0.26‰ to evaluate aerosol effects throughout the year, this may be why our method underestimates HONO in March and April and overestimates HONO in January, and revealing that the mass concentration of PM2.5 is not the only factor affecting formation of HONO on aerosols.Besides, lack consideration of the impact of RH and temperature on NO2-to-HONO conversion and of seasonal variations in ground surface properties, uncertainties of NO2-to-HONO conversion mechanisms and of combustion HONO emissions, and lack direct observation for soil emitted HONO, could all result in the bias between HONOtot and HONOobs, so more studies on the detailed mechanism of various HONO sources need to be performed.

Fig. 1 .
Monthly variations of (a) HONO, (b) HONO/NOx, (c) NOx, (d) PM2.5, (e) RH and (f)T.1089 The solid bold lines are median values, the markers indicate mean values, and the shaded areas 1090 represent percentiles of 75% and 25%.In (a) and (b), values in February are linearly interpolated 1091 based on the data from the months before and after, since there were only few days when HONO 1092 was observed in February.In (c), the shaded area is colored by the 25th to the 75th percentiles of 1093 NO. 1094 1095 Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2019-219Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 25 April 2019 c Author(s) 2019.CC BY 4.0 License.

Fig. 4 .
Fig. 4. (a) The relationship between HONO and NOx colored by NO/NOx.The dotted line is the emission ratio derived in this study and the solid line is obtained from simple linear fitting; (b) average emission contribution ratios for different concentrations of HONO and the frequency distribution of HONO concentrations.Both (a) and (b) are nighttime values.

Fig. 6 .
Fig. 6.Scatter plot of HONOcorr/NO2 and RH during nighttime, separating the data into (a) clean hours (hourly mean PM2.5<25μg/m 3 ) and (b) pollution hours (hourly mean PM2.5>75μg/m 3 ).Triangles are the averaged top-6 HONOcorr/NO2 in each 5% RH interval, and the error bars are the standard deviations.The overall average concentrations of PM2.5 in (a) and (b) are shown to the right of the figure.

. TUV model and OH estimate
.
(Zhou et al., 2002)uting 76% of the production of HONO.Comparing summer data, the mean unknown daytime source strength of HONO in Nanjing is almost at the upper-middle level of those reported in the existing literature: 0.22 ppb/h at a rural site of New York state, USA(Zhou et al., 2002); 0.43 ppb/h at a mountain site in Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2019-219Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 25 April 2019 c Author(s) 2019.CC BY 4.0 License.

Table 1 .
Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2019-219Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 25 April 2019 c Author(s) 2019.CC BY 4.0 License.provides a major part of HONO at roughly constant proportion of 36%.The uptake of NO2 on aerosol surface could generate the greatest amount of HONO during heavily polluted periods (e.g.January).Our results draw a complete picture of the sources of HONO during different seasons, and demonstrated the needs of long-term and comprehensive observations to improve the understanding of HONO chemistry.Sources and sinks for nitrous acid (HONO) in the troposphere.Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2019-219Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 25 April 2019 c Author(s) 2019.CC BY 4.0 License.
HONO, and can contribute 40% to HONO during summer.Ground formation