Photolytic oxidation of ammonium chloride as a source of Cl_2 in the atmosphere

Domain: neuroscience
Published: 2026-03-27
Editorial outcome: major_revision

Abstract
Chlorine atoms (·Cl) exhibit high reactivity and exert a substantial influence on the atmospheric oxidation capacity and chlorine cycle in the troposphere. The photolysis of molecular chlorine (Cl ) is a crucial source of ·Cl. However, the daytime peaks of Cl concentration cannot be entirely accounted for by the currently known pathways. Here we show that the photolytic oxidation of ammonium chloride (NH Cl), a ubiquitous component of atmospheric aerosol, serves as an important daytime source of Cl . Laboratory experiments demonstrate that oxygen, water vapour, and acidity are indispensable for Cl generation and release, and that Cl production is enhanced in the presence of black carbon aerosol. Field observation combined with model simulation demonstrate that the mechanism explains 12%-55% of daytime Cl concentration. These results reveal a photoactivation pathway for chlorine production that depends only on chlorine salts and solar radiation, with significant implications in regions with abundant chloride salts.

Introduction
Cl atom is a highly reactive radical in the tropospheric atmosphere, driving the oxidation of volatile organic compounds (VOCs) and thereby promoting the production of secondary organic aerosols and ozone1-6. The reaction rate constants of Cl atom with VOCs are 1-2 order of magnitude faster than those of OH radical, therefore compensating for the roughly 1-2 order of magnitude lower concentration of Cl atom than OH radical7-9. Furthermore, Cl atoms facilitate the accumulation of common radicals (RO , OH, HO , etc.) during their reactions with VOCs through radical transfer reactions10,11, leading to a substantial enhancement of the atmospheric oxidation capacity. Meanwhile, the RO radicals formed from Cl atom-initiated oxidation of VOCs isomerize to form oxygenated organic radicals and closed-shell products, during which process secondary organic aerosol formation is enhanced via gas-to-particle conversion of the oxygenated organic molecules11,12.

In addition to the fast photolysis of ClNO , the photolysis of Cl constitutes a significant source of atmospheric Cl atoms in urban regions13,14. Field observations have showed that the daytime sources of Cl include primary emissions from coal combustion and water treatment facilities15-17, as well as secondary generations stemming from photochemical processes triggered by strong oxidants. For example, OH radicals generated from the photolysis of O , photogenerated holes (h+) generated from TiO , and OH radicals generated from the photolysis of nitrate in the liquid phase at aerosol pH < 3.3 can all activate Cl- ions18-20. However, aerosols in inland urban regions generally exhibit higher pH values (typically 4-5) and elevated relative humidity (RH), which dilute aerosol acidity and thereby suppress these acid-driven activation pathways. Therefore, owing to the low pH, elevated relative humidity, and the requisite involvement of other photosensitizers, these known emission sources and activation pathways cannot fully account for the observed Cl concentrations in inland urban regions. As a result, the mechanism of chlorine photoactivation remains a subject of controversy and requires further investigation.

In this work, we report a mechanism of Cl generation through the photo-oxidation of ammonium chloride particles under atmospherically relevant conditions, which constitutes a nonnegligible source of Cl in the continental atmosphere.

Results
Photolytic oxidation of particulate NH Cl leads to Cl formation
We show that the photo-oxidation of NH Cl, which is one of the most prevalent inland chloride species21, generates a nonnegligible amount of Cl . The experimental setup is shown in Figure S1. We used an iodide-ion-based chemical ionization long time-of-flight mass spectrometer (Iodide-ToF-CIMS) and a NO analyzer to detect gas-phase products at the reactor outlet. A mercury lamp was employed as the light source to investigate the reaction phenomena and mechanisms because of its high UV intensity (225-400 nm), which may facilitate Cl generation. In contrast, a xenon lamp (320-800 nm) with a continuous solar-like spectrum was used to simulate solar radiation reaching the atmospheric boundary layer and to assess the atmospheric relevance of the reactions. The measured spectra of the two light sources are shown in Figure S2. As shown in Figure 1(a), continuous generation of Cl was observed on exposure of particulate NH Cl to a mercury lamp under conditions of 50% relative humidity with zero air (a mixture of nitrogen and oxygen gases) as the carrier gas. It is evident that upon light exposure, a gradual release of Cl occurred, approaching a concentration of 500 pptv (parts per trillion by volume) after 240 minutes of illumination. Note that when utilizing a xenon lamp to simulate the solar spectrum as the light source, lower levels of Cl production were also observed under otherwise similar conditions, as shown in Figure S3. To investigate the photo-oxidation of ammonium chloride more distinctly, the mercury lamp was employed as the light source in all subsequent phenomenological and mechanistic studies, while the xenon lamp was used for assessing atmospheric impacts. The generation of Cl persisted for at least 4 hours, followed by a sharp decline in Cl signals upon light termination, as shown at t = 280 min. The Cl concentration swiftly reverted to its prior levels upon reactivation of the light source, as shown at t = 300 min, which indicates that the generation and release of Cl are induced by illumination. We also examined the effect of light intensity, which shows that stronger irradiation yields more Cl (Figure S4). However, when we switched the carrier gas to nitrogen, the concentration of Cl rapidly decreased to zero, as shown at t = 320 min to t = 340 min. Conversely, when we switched the carrier gas to high-purity oxygen, as shown at t = 360 min to t = 380 min, Cl exhibited a swift increase. This suggests that oxygen serves as a reactant, participating in and constraining the formation of Cl . In addition, we also detected gas-phase products such as HONO and NO ; the results are shown in Figure S5.

The photolysis of HONO produces hydroxyl radicals, which could potentially contribute to the formation of Cl . To rule out this possibility, we carried out an additional experiment replacing zero air with nitrogen as the carry gas. Although both experiments exhibited similar concentrations of HONO and NO after light exposure, as shown in Figure S6, Cl generation was no longer observed using nitrogen, as shown in Figure 1(b). At the same time, to mitigate potential effects of the detection limits of instruments on measurement precision, we systematically increased the quantity of NH Cl from 2 mg to 20 mg and 200 mg, respectively. As depicted in Figure 1(c), both the Cl and HONO concentrations exhibited an increase with escalating amounts of NH Cl. However, as NH Cl increased exponentially, the concentrations of Cl and HONO detected after 1 hour of light exposure exhibited divergent growth trends. This suggests that Cl and HONO likely originate from distinct reaction pathways. Notably, in the case of the 200 mg quantity, no discernible signals for NO, N O, NH , or HCl were detected. These findings unequivocally indicate that the principal products of the photo-oxidation of NH Cl are Cl and HONO.

Proposed mechanism for the generation of Cl
We show that water vapor may serve as a decisive factor in determining Cl production. We conducted a series of experiments in which RH varied from 0% to 80% under zero air conditions. As is depicted in Figure 2(a), the production of Cl was limited at very low humidity (i.e., relative humidity around 0 %), but the yield of Cl did not show a significant correlation with relative humidity higher than 10%. These results indicate that the presence of water vapor is necessary for Cl formation, although its amount has little influence once sufficient H O is available. Considering that O and light are also indispensable for Cl generation as demonstrated above, we therefore propose a mechanism involving the direct photoexcitation activation of Cl- ions in the presence of oxygen and water, leading to the subsequent generation of Cl (1-5). According to recent reports, Cao et al.22 claimed that the spontaneously formed complex [Cl--H O+-O ], arising in the presence of oxygen and water, can be photoexcited to generate ∙Cl, providing theoretical support for the first step (1). The ∙Cl subsequently undergoes rapid combination with Cl- to form the relatively stable ∙Cl -, ultimately releasing Cl (3-5)23-25. Furthermore, O , serving as the electron acceptor, is an essential reagent in this process, which may be converted to ∙OH via a series of radical transfer reactions, potentially contributing to the generation of Cl (2, 6-11).

Cl− + hv → ∙Cl + e− (1)
e− + O → ∙O− (2)
∙Cl + Cl− → ∙Cl− (3)
2∙Cl− → Cl + 2Cl− (4)
∙Cl− + ∙Cl → Cl + Cl− (5)
∙O− + 2H O → OH− + 3∙OH (6)
∙O− +H+ → HO ∙ (7)
2HO ∙ → O + H O (8)
H O +hv → 2∙OH (9)
∙OH + Cl− ↔ HOCl− (10)
HOCl− + H+ ↔ ∙Cl + H O (11)

Based on our proposed mechanism shown in reactions 1-4, the reaction necessitates the involvement of free Cl- ions. NH Cl has a high water-uptake potential and can form hydrates that may facilitate the release of free Cl- from the lattice edge26-28. This liberation of free Cl- ions may contribute to the enhanced production of chlorine gas. According to the hygroscopic properties of NH Cl29, the amount of water adsorbed on particle surfaces remains limited below its deliquescence point. Consequently, increases in RH do not substantially enhance Cl production. Under these conditions, only a thin and discontinuous water layer is present-sufficient to sustain interfacial ion mobility but insufficient to markedly alter the surface chloride concentration or the local acidity that governs Cl formation. As a result, the fundamental interfacial environment required for Cl generation remains largely unchanged across this RH range, leading to the absence of a clear positive correlation between RH and Cl yield It is noteworthy that similar chlorine generation was observed in light exposure experiments above the ammonium chloride deliquescence point (RH=~76%), which means that a similar chlorine activation reaction may also exist in the liquid phase. However, the substantial water uptake forms a thick aqueous layer that may dilutes surface chloride and decreases local acidity, thereby suppressing interfacial Cl formation despite the increased availability of water.

Therefore, we also conducted photo-exposure experiments with NH Cl solutions. When NH Cl solution was deposited onto a quartz plate under irradiation with zero air at 83% RH (controlled to stabilize droplet morphology), Cl was detected as shown in Figure 2(b). The main products were Cl , HONO and NO , similar to the previous gas-solid reaction products (The specific mechanism is described in Text S2). The difference was that NO was released instantaneously at the beginning of light exposure, whereas HONO was released more slowly as light exposure time increased. This divergence stems from differences in volatility governed by Henry law constants: HONO possesses a larger constant than NO 30, resulting in higher aqueous solubility and consequently slower mass transfer to the gas phase. Also, the yield of Cl was lower compared to the gas-solid phase reaction, which may be owing to the dissolution of the generated Cl in water, such that the liquid environment reduced its partitioning into the gas phase. In total, we argue that gas-solid and gas-liquid reactions may exhibit distinct chlorine release efficiencies, and these variations may be attributed to disparities in physical states rather than differences in reaction mechanisms.

We show that the involvement and oxidation of ammonium anion are the key to the observed Cl generation. We hypothesize that for each unit of Cl- consumed in the NH Cl sample, an equivalent amount of NH + is converted into gaseous product and subsequently released from the sample surface. As a result, the pH of the system remains relatively unchanged before and after the reaction. As shown in Figure S8, no Cl was observed when exposing NaCl solutions or NaCl particles to light alone. When H SO was added to adjust the pH of the NaCl solution to 3, we observed a considerable generation of Cl , as in the case of NH Cl solution in the presence of light. In other words, the acidity of aerosols plays a pivotal role in triggering the release of chlorine gas from activated chlorides. However, in the case of metal chlorides the activation of one Cl- by light leads to the generation of a strong base, such as NaOH. This process may effectively counteract the release of acidic Cl , contributing to the observed difference in behavior compared to the NH Cl sample. This hypothesis is evidenced by experiments19,20 which found that when solid samples or solutions of metal chloride, such as NaCl and KCl, were exposed to light alone, no notable increase in chlorine production was observed. In addition, we examined other cations that can acidify chloride salt solutions through hydrolysis. Under the same Cl- mass loading and RH, exposure of MgCl and CaCl to irradiation also produced detectable Cl , but the yields were substantially lower than those from NH Cl, as shown in Figure S9. We attribute this difference to the combined effects of acidity and hygroscopicity. In the aqueous phase, the hydrolysis constants of Mg2+ and Ca2+ are smaller than that of NH + at the same temperature, resulting in higher theoretical pH values. According to the database compiled by Peng and co-workers31, the deliquescence relative humidities (DRH) of MgCl and CaCl are 27% and 31%, whereas NH Cl under our experimental conditions only slightly exceeds its efflorescence relative humidity (ERH, 45%). This means that, at 60% RH, MgCl and CaCl aerosols contain more liquid water than NH Cl, and the resulting dilution effect further weakens their acidity. These observations again support the conclusion that the acidity introduced by NH + plays a critical role in promoting chloride oxidation. To further elucidate the role of acidity in controlling chloride oxidation, we conducted a set of solution-phase experiments in which the pH of 1 mol/L NH Cl was systematically adjusted from approximately 5 (no added acid) to pH 3, 2, and 1 using H SO , respectively. As shown in Figure 2(c), Cl production increased markedly with decreasing pH, demonstrating a clear dependence on acidity. This trend indicates that H+ facilitates the interfacial enrichment and activation of Cl-, thereby enhancing its propensity to undergo oxidation in the presence of light, O , and water.

Mechanism validation
To further validate the proposed mechanism (1-5), we used spin-trapping electron spin resonance (ESR) spectroscopy to analyze the intermediate active species in light-irradiated NH Cl solutions. TEMPO (2,2,6,6-tetramethylpiperidine1-oxyl) was used as the spin agent for photogenerated electrons. The results are shown in Figure 3(a). The signal intensity of TEMPO radicals decreased significantly with the increase in light irradiation time, suggesting that light irradiation, in the presence of oxygen and water, can induce the production of electrons in NH Cl (1). We chose 5,5-Dimethylpyrroline-N-oxide (DMPO) as a spin trap to detect superoxide (•O −) in methanol solution. Subsequent to light irradiation, the distinctive signal of the DMPO-•O - adducts was clearly observed, as depicted in Figure 3(b). To further validate the role of O as the electron acceptor in the photochemical oxidation of solid NH Cl, we introduced elemental carbon (EC) into the system. EC, a major component of atmospherically relevant black carbon (BC), is widely recognized as a photocatalytic material that accelerates interfacial electron transfer. In our experiments, thermally desorbed commercial black carbon was physically mixed with NH Cl particles and deposited onto quartz plates for irradiation. Under identical NH Cl mass conditions, the samples containing BC exhibited a pronounced increase in Cl production, as shown in Figure S10. This behavior indicates that elemental carbon facilitates an efficient electron-transfer pathway from Cl- to O (1-2), thereby promoting the formation of •O - and subsequently Cl . Consistently, in the ESR spectra, the intensity of DMPO-•O - adducts produced under illumination was further enhanced in the presence of BC, as shown in Figure S11.

Additionally, we used DMPO to capture •OH radicals in aqueous solutions. When the NH Cl and DMPO mixed solution was exposed to light irradiation, a distinct 1:2:2:1 signal emerged, representing the characteristic peak of •OH, as depicted in Figure 3(c). In addition, we also directly observed a significant amount of •OH radicals in the gas phase from a light-illuminated NH Cl solid sample using laser-induced fluorescence (LIF), with the results shown in Figure 3(d). This confirms that gas-solid reactions release •OH radicals from the gas phase. In contrast, Cl atoms were not identified from the ESR signal. This could be attributed to the relatively lower efficiency of DMPO (spin trap) in capturing Cl atoms compared to •OH radicals. Additionally, the peak positions of Cl atoms and •OH radicals are in close proximity, and the intense •OH signal may hinder the detection of the Cl atom signal.

In order to demonstrate that •OH was a by-product during the experiments (6-9) , cyclohexane was used as a scavenger for gas-phase •OH radicals passing into the reaction system (12, 14-16)32-34, and the reaction products were detected by proton transfer mass spectrometry (Vocus-PTR-ToF-MS) and Iodide-ToF-CIMS. As shown in Figure 4, the signals of cyclohexanone (C H O+) and cyclohexanol (C H O+) increased significantly after turning on the light, which indicated that cyclohexane was oxidized by OH. Meanwhile, the production of Cl was not affected. This indicates that •OH is a by-product of the reaction and that the Cl does not come from the gas-phase •OH radicals. The signal of HCl was also observed in the system, and, as shown in Figure 4(b), its significant enhancement after the introduction of cyclohexane compared with the experiment without cyclohexane demonstrates that Cl atom was also involved in the oxidation of cyclohexane by a hydrogen abstraction reaction (13)35. Despite the swift reaction between cyclohexane and gas-phase Cl atom (with a reaction rate constant of 2.41×10-10 cm3 molecule-1 s-1)36, the rapid combination of Cl atom with Cl- (3, with a reaction rate constant of about 3.49×10-11 cm3 molecule-1 s-1 in aqueous solution)23 prevents the quenching of Cl atom by cyclohexane. Based on the above results, the mechanism of oxidation of NH Cl (6-11) could be further refined.

C H + ∙OH → ∙C H + H O (12)
C H + ∙Cl → ∙C H + HCl (13)
∙C H + O → C H O ∙ (14)
C H O ∙ + RO → C H O∙ + RO + O (15)
→ C H O + C H O + R O + ROH + O (16)

Field evidence for the proposed mechanism
Daytime peaks of Cl and associated possible sources have been documented in several investigations as summarized in Table S1. Our proposed mechanism could potentially serve as a significant diurnal source of Cl . As shown in Figure 5(a), field observation results in Xiamen show a clear daytime peak in the Cl production rate. When we quantitatively evaluated the contributions of established Cl formation pathways using a box model (F0AM37), including heterogeneous reactions involving HOCl, ClNO , •OH, and O on aerosol surfaces, we found that these mechanisms account for only a minor fraction (~20%) of the noontime Cl concentrations (Figure S12). Therefore, additional daytime sources of Cl must be present in this region. We further noted that the concentration of Cl , together with the product of particulate chloride and aerosol surface density, exhibits a positive correlation with increases in ammonium concentrations, suggesting that the potential source of Cl is linked to the heterogeneous activation of NH Cl. As seen in Figure 5(c and d), the concentration of Cl shows an increasing trend as the concentration of chloride and ammonium increases, which suggests a close relationship between Cl and ammonium chlorine, corroborating the possible application of our proposed mechanism in the region.

Summary and atmospheric implications
In summary, we observed that photolytic oxidation of NH Cl results in nonnegligible production of Cl . In contrast to prior investigations, the potential mechanism identified in this study does not rely on catalysis by other photosensitizers and has wider implications, especially in atmospheres with high chloride concentrations from burning activities, such as those observed in regions like Delhi, India26. It is important to note that while the direct photolysis of chloride ions alone may seem to be limited in its contribution to atmospheric Cl , it may become vital as additional photosensitizers, such as black carbon, transition metals, and TiO , are introduced and the electron transfer process in (1) is accelerated, as shown in Figure S10. In particular, by incorporating the black carbon-catalyzed reaction rate into the box model, our simulations demonstrate that this mechanism can partially offset photolytic losses and account for approximately 12%-55% of the daytime Cl concentration observed in Xiamen, as shown in Figure S13. We emphasize that consideration of the proposed Cl production chemistry in regional and global atmospheric chemistry models is of great importance to a deeper understanding of the atmospheric oxidation capacity.

Methods
Lab Experiments
To investigate the possible mechanism of Cl production by photolysis on particulate chloride, ammonium chloride (NH Cl, Aladdin, > 99.99%) was used as the source of particulate chloride. NH Cl was dissolved into ultrapure water, dropped into a matte-finished quartz disk, and dried in the oven at 60 °C. This enables the particles to be coated on the disk as evenly as possible and less likely to be blown away. Moreover, sodium chloride (NaCl, Aladdin, > 99.99%) was used as a comparison, and sulfuric acid (H SO , Sigma-Aldrich, 95-97%) was used to adjust the pH. The black carbon used in this study was a commercial product, Special Black 4A (SB4A, Evonik Degussa). Before experiments, the material was thermally desorbed under nitrogen protection for 3 h to remove adsorbed organics and nitrate residues that may have been acquired during production and storage38-40, ensuring that the observed effects were primarily associated with EC itself.

A quartz reactor was used to measure the productions of Cl by illuminating the quartz plate loaded with chloride salts. A digital circulating water bath was used to avoid the effects of overheating the sample due to prolonged light exposure and to maintain the temperature of reactor at 25 °C. The samples were placed in the reactor and equilibrated for 30 min under dark conditions. After that, the sample was exposed to a mercury lamp (CHF-XM-500 W) for irradiation. The light intensity provided by the mercury lamp in this study was approximately 260 ± 9mW·cm-2. The characteristic peaks in the ultraviolet region were situated at 313, 365, and 405 nm. Zero air and nitrogen were used as the carrier gas at a flow rate of 4 SLPM, along with adjustable humidity ranging from 0% to 83%.

Reactive chlorine species (including Cl , HCl, and HOCl) and HONO were measured by Iodide-ToF-CIMS (Aerodyne Research, Inc. USA). The Iodide-ToF-CIMS instrument consists of an ion-molecule reaction region (IMR) coupled to ion-guiding quadrupole optics and an orthogonal-acceleration time-of-flight mass analyzer, enabling highly sensitive detection of trace gas-phase species through selective iodide-adduct reagent-ion chemistry41. Sample air (2.1 SLPM) was introduced into the IMR, where it mixed with a stream of reagent ions. Methyl iodide (CH I), supplied from a heated permeation tube and carried in 2.38 SLPM of ultrahigh-purity N (99.999%), was soft-ionized by an X-ray source prior to entering the IMR, producing reagent ions I- and I(H O)-. These ions selectively reacted with analyte species to form iodide-adduct ions (I·A-) in the IMR. Mass analysis was conducted using a time-of-flight (ToF) analyzer. Ions were extracted by a pulsed DC field, drifted through a field-free vacuum tube, and were energy-focused by a reflectron before detection with a multichannel plate (MCP). The ion signals were digitized and recorded using the TofDaq acquisition system. Cl was detected at 197 amu (I35Cl35Cl−) and 199 amu (I35Cl37Cl−), HCl at 163 amu (IH35Cl−) and 165 amu (IH37Cl−), HOCl at 179 amu (IHO35Cl−) and 181 amu (IHO37Cl−), and HONO at 174 amu. We used a dynamic calibration system with a Cl permeation tube for calibration. The Cl permeation rate was determined as 120 ng/min in 40 °C. The sensitivity of Cl remained constant at 0.69 ± 0.13 cps/pptv, showing no significant correlation with relative humidity (Figure S15). A NOx analyzer (Thermo 42i-TL) was used to detect NO and NO . N O and NH were detected by a Picarro N O analyzer.

A proton transfer mass spectrometry (Vocus-PTR-ToF-MS, Aerodyne Research, Inc. USA) was employed to monitor gas-phase cyclohexane and its oxidation product concentrations; around 0.1 L min-1 of the total sample flow was directed into the Vocus. In brief, the principle of Vocus is that ionization of excess water vapor produces H+ as the source of ions that bind to the substance. Cyclohexane was detected at 85 amu (C H +), and its oxidation products--cyclohexanone (C H O+) and cyclohexanol (C H O+) --were detected at 99 amu and 101 amu respectively. In cyclohexane-related experiments, cyclohexane is brought into the reactor as a vapor by zero air.

An electron spin resonance (ESR) spectrometer (ELE XSYS E500; Bruker) operating at a modulation frequency of 100 kHz and a microwave frequency of 9.5 GHz was employed to detect reactive radical species. DMPO (5,5-dimethy-1-pyrroline N-oxide, Dojindo Laboratories) dissolved in water, DMPO dissolved in methanol, and TEMPO (2,2,6,6-tetramethylpiperidinooxy, Tokyo Chemical Industry Co., Ltd.) dissolved in water were used as spin-trapping agents for •OH, •O -, and photoinduced electrons, respectively. The final concentration of each trapping agent was 100 mM. ESR measurements were conducted with a sweep width of 100 G, a modulation amplitude of 1.00 G, and a sweep time of 81.92 ms.

A gas expansion and fast gating laser-induced fluorescence (LIF) technique was employed to measure the gas-phase •OH radicals. According to He et al.42, the working of LIF has been given a detailed account. 18 mg NH Cl were deposited on a quartz disc, which was then placed in the quartz reactor. Ultrapure water-containing synthetic air (80% N + 20% O , 99.999% purity, the mixing ratio of water from 0.22% to 0.25%) at 2 L/min was passed over the sample surfaces with or without mercury lamp irradiation, which carried the OH radicals desorbed from the surface into the instrument, where they were detected by LIF. Every experiment was repeated at least three times.

Field measurements
The field observation of Cl , particulate chloride, and particle surface area concentrations (Sa) was conducted in October 9 and December 5 of 2022 in the coastal city of Xiamen, Southeast China. Gaseous Cl was determined by another chemical ionization mass spectrometry operated with the same ionization principle as described in the Laboratory section. In this setup, CH I, generated from a heated permeation tube (VICI Metronics, Inc.), was ionized as it passed through a soft X-ray source. The ionization took place within a stream of ultrahigh-purity nitrogen (N , 99.999%) generator (PEAK, NG5000(A)) flowing at 2.7 SLPM. Inside the IMR chamber, the target gases underwent specific reactions with reagent ions (I- and I(H O)-) to form iodide-adduct clusters. These product ions were then directed to the atmospheric pressure interface long time-of-flight mass spectrometry for subsequent quantification. Ambient air was drawn into the sampling room through a 2-meter-long, 1/4-inch inner diameter perfluoroalkoxy (PFA) tubing at a flow rate of 10 standard liters per minute (SLPM). This high flow rate, maintained by an external vacuum pump, minimized wall adsorption and retention of gas molecules, thereby significantly reducing sample loss. From this main flow, approximately 2 SLPM was diverted into the Iodide-ToF-CIMS setup for analysis. To prevent particulate matter accumulation and cross-contamination within the sampling system, all inlet tubing was replaced weekly. Following replacement, the tubing was meticulously cleaned by rinsing with deionized water and ultrasonicating for 30 minutes. It was then dried with high-purity N to ensure a residue-free surface, which is critical for maintaining consistent system performance and representative sampling.

A stable Cl standard gas was generated using a constant-temperature permeation method, with high-purity N (99.999%) at 100 SCCM serving as the carrier gas flowing through a water bath temperature-controlled at 40.0 °C Cl permeation tube, with 98 ng/min permeation rate at 40.0 °C (VICI Metronics, Inc.). The resulting gas stream was then dynamically diluted with high-purity N using calibrated mass flow controllers to achieve a concentration gradient. Instrument background signals were determined using humidified zero air before and after the field campaign. All reported Cl concentrations were corrected by subtracting the background signals obtained during zero measurements. Based on estimates of RH-induced background drift and instrument sensitivity changes during long-term operation, the uncertainty in Cl measurements is estimated to be 13.8% (detailed description in Text S3).

The mass concentration of chloride was measured by a Monitor for Aerosols and Gases (MARGA, Metrohm AG Inc., Switzerland). The instrument is an online analyzer designed to semi-continuously measure water-soluble gases (i.e., HCl, HONO, HNO , SO , NH ) and aerosol chemical composition (i.e., Cl-, NO -, SO 2-, Na+, NH +, K+, Mg2+, Ca2+) with hourly temporal resolution, using ion chromatography. The ambient sample containing gaseous species and PM passes through a wet rotating denuder (WRD), which consists of two concentric glass cylinders coated with an absorption solution (H₂O₂) where gases diffuse into the aqueous film43,44 (Keuken et al., 1988; Otjes et al., 1993; Rumsey et al., 2011). Following collection, samples were analyzed via cation and anion conductivity detectors (IC, Metrohm AG Inc., Switzerland). The analytical accuracy for this process was controlled through the use of LiBr as an internal standard, which was simultaneously injected and mixed with each sample. To ensure continued accuracy, an external calibration was performed weekly by replacing the absorption solution with a known liquid anion standard containing Cl- (Sigma-Aldrich, multielement ion chromatography anion standard solution, containing F-, Cl-, NO -, PO 3-, SO 2-). The PM impactor and inlet tubes were cleaned every two weeks. Specifically, the aerosol sampling system was cleaned by NaOH solution every two months.

Surface area (Sa) was calculated from the ambient particle number size distribution from 7 nm to 280 nm detected with a Scanning Mobility Particle Sizer (SMPS, TSI Inc.) and an Aerodynamic Particle Size Spectrometer (APS).

The Cl production rate P(Cl ) equaled the photolysis rate of Cl (observed Cl concentrations × JCl ) due to the near photo-stationary state of Cl . JCl was calculated by the TUV model (http://cprm.acom.ucar.edu/Models/TUV/Interactive_TUV) under clear sky conditions and then scaled to JNO measured by a photolysis spectrometer (model PFS-100).

Box model
A measurement-constrained box model, the Framework for 0-D Atmospheric Modeling (F0AM, v4.3.01) was used to evaluate the contributions of other known Cl formation pathways and the contribution of black carbon-catalyzed photo-oxidation of ammonium chloride to daytime Cl production. The model system consists of five core modules, including initialization module: optimizes model parameters by integrating observational data; data input module: incorporates online monitoring data from the supersite of the Institute of Urban Environment, Chinese Academy of Sciences, using gaseous pollutant concentrations and meteorological parameters as observational constraints; physical processes module: introduces a dry deposition velocity parameter to quantify dry deposition and atmospheric dilution within the boundary layer, effectively characterizing pollutant dispersion and removal; chemical processes module: employs the latest MCM-v3.3.1 chemical mechanism, which includes 142 volatile organic compounds (VOCs) and over 17,000 elementary reactions, and integrates existing reactive chlorine chemistry; output analysis module: provides key parameters such as pollutant concentration distributions and photochemical reaction rates. In this model, the formation of Cl mainly occurs through the heterogeneous uptake reactions of ClNO , HOCl, O , and ·OH on chloride-containing aerosol surfaces, while Cl removal is primarily achieved via photolysis and its reaction with OH radicals. The heterogeneous uptake reaction can be represented by the following general formula:
𝑑[𝑋] 𝑐𝛾
= − 𝑆[𝑋] (17)
𝑑𝑡 4
where X denotes HOCl, ClNO , ·OH, or O ; c is the molecular velocity of X (cm·s-1), S is the aerosol surface area concentration (cm2·cm-3), and γ is the uptake coefficient. Based on previous studies45,46, the uptake coefficients for HOCl, ClNO , ·OH, and O in this study were set as 5.3×10-4, 6.0×10-3, 8.9×10-3, and 1.0×10-5, respectively.

Considering the uncertainties in the concentrations of ClNO , HOCl, O , and OH, and assuming that the errors are independent with an approximately linear response of Cl production to these inputs, the resulting uncertainty in the simulated Cl , calculated via standard error propagation, was 27%.

δ(Cl ) = √δ(ClNO )2 +δ(HOCl)2 +δ(O )2 +δ(OH)2 (18)
where δ(X) denotes the uncertainty associated with species X. The observed uncertainties for ClNO , HOCl, and O are 15%, 7%, and 5-10%, respectively, and the uncertainty of the simulated OH concentration is conservatively estimated at ~20%.

A summary of the method used to extrapolate the reaction rate to ambient atmospheric conditions is provided in Text S1. The model framework includes an established chlorine chemistry mechanism as outlined in previous studies47. Photolysis rates for several reactive chlorine species (Cl , ClNO , ClO, OClO, ClOOCl, HOCl, and ClONO ) were derived from the Tropospheric Ultraviolet and Visible (TUV) radiation model under clear-sky conditions and subsequently scaled according to the observed JNO . In this study, the model was constrained by observations of chlorine species (Cl and ClNO ), gas-phase pollutants (e.g., SO , NO, NO , CO, and O ), meteorological parameters, and photolysis frequencies, and was run at an hourly time resolution.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Overall evaluations:\nLi et al. discovers a new source of Cl2 from the oxidation of NH4Cl facilitated by O2 and aerosol acidity in a series of\nlaboratory experiments. Furthermore, the authors examine the significance of this new mechanism in real atmosphere, which\nshows 12% to 55% contribution to Cl2 production. This Cl2 producing mechanism is surprising since no additional\nphotosensitizers are required to oxidize chloride. Discovery of this new mechanism adds up to known sources of Cl2,\nincluding nitrate photolysis, Fe-catalyzed chemistry, ClNO2 chemistry, and so on. Surely, this work bears enough novelty\nand significance. The adopted methods are robust. A few clarifications are needed. Description of some results could be\nfurther enhanced after minor revisions.\nDetailed comments:\n1. The detailed role of black carbon in Cl2 production is not clearly stated. Since black carbon is a mixture of various\nelements and compounds, exactly speaking, what kinds of molecules in black carbon are responsible for Cl2 production?\n2. Lines 57-60: Why are low pH and elevated RH the reasons for unaccounted Cl2 observations in inland urban regions?\nFurther clarifications are needed here.\n3. Lines 71-80: The authors mentioned a mercury lamp and a xenon lamp. It is clear that the xenon lamp could simulate the\nsolar radiation. How about the mercury lamp? In other words, what is the difference between the mercury lamp and the\nxenon lamp in terms of their light spectrum?\n4. Lines 157-172: The authors’ statements here strongly imply that NH4+ is indispensable for Cl2 production, which in fact\nmay not be the case. If the role of NH4+ is to provide acidity by hydrolysis (NH4+ + H2O = NH3*H2O + H+), then other\ncations may do the same, e.g., Mg2+ and Ca2+. Further clarifications are required to clearly emphasize how NH4+\ncontributes to Cl2 production, and whether or not other cations could do the same (e.g., Mg2+ and Ca2+) although they\npresent in much smaller amounts than NH4+.\n5. Lines 203-205: Is it possible that HCl comes from thermal decomposition of NH4Cl besides the Cl + VOC reaction?\n6. How is HONO produced when only N2 is injected into the NH4Cl system? Without the presence of O2, is there any other\nelectron acceptor to facilitate the oxidation of NH4+ to produce HONO?\n7. Lines 527-529: Which kind of lamp is used here? Mercury lamp or xenon lamp? Please also check other locations about\nthe usage of lamps.", "reviewer_3": "Overall comments:\nThe authors proposed a new mechanism for Cl2 formation from photolysis of ammonium chloride in the atmosphere, based\non results from both laboratory and field experiments. While the idea is interesting and potentially important, the manuscript\ncannot be considered in its current form and requires substantial revision.\nThere are some major concerns. Several key details are not sufficiently elaborated, particularly in the Field evidence section\nand Methods section.\nThe section titled \"Field evidence for the proposed mechanism\" is presently weak and does not convincingly support the\ncentral claim. The presentation and interpretation of field observation results remain underdeveloped and are not supported\nby compelling evidence to substantiate the authors' conclusions. The observed correlation between Cl2 levels and\nparticulate chloride/ammonium, while interesting, is not definitive proof of a causal relationship. Alternative explanations for\nCl2 signals (e.g., other known formation pathways or sampling artifacts) were not adequately considered or ruled out. To\nrobustly support the proposed mechanism, more direct evidence is needed. They should conduct controlled perturbation\nexperiments (e.g., control light, acidity, water, or chloride levels), or quantification of key intermediates under real-world\nconditions, similar to those in the laboratory experiments. This is because the ambient conditions are much more complex\nthan lab conditions. They should provide more direct evidence of the Cl2 production chemistry to derive a conclusion.\nIn summary, the current interpretation of the field data is overly speculative. The conclusions cannot be firmly drawn until\nmore direct and compelling evidence is provided to establish a causative link between the photolysis of ammonium chloride\nand the observed Cl2 production.\nMoreover, the Methods section lacks experimental details, including specific protocols, instrumentation settings, and\nanalytical conditions for both lab and field studies.\nThe description of the field measurements requires expansion. The current information regarding techniques such as CIMS\nand MARGA is insufficient. Essential details are missing.\n--CIMS and MARGA operation details are missing and need to be included.\n--inlet configuration: The length and material of the inlet line used for sampling, particularly for Cl2 measurement is not\ndescribed\n--background and zeroing procedures: The procedures for determining background signals and performing regular zero\nmeasurements (e.g., using a scrubber, or cylinder) to ensure data quality are necessary for high quality Cl2 measurement.\n--potential artifacts: It is recognized that Cl2 can be generated on surfaces (e.g., instrument inlets) in the presence of\nchloride. Whether this possibility was examined during the campaign and what measures were taken to minimize such\nsampling artifacts should be tested and discussed.\n--calibration details: For the I-CIMS measurement of Cl2, the relative humidity (RH) may influence sensitivity. It should be\nclearly stated whether calibration experiments were performed at different RH levels relevant to the field conditions to\naccount for this potential effect. Was RH measured during the field campaign? Also, was the CIMS inlet RH controlled?\nSpecific comments\nLine 29: The authors mention the potential role of black carbon in the proposed mechanism; however, this concept is not\nsubstantiated or discussed within the main body of the manuscript. If black carbon is considered a relevant factor, its role\nmust be explicitly integrated into the experimental design, results, and discussion. Otherwise, such mention should be\nremoved from the abstract.\nLine 55: what are photogenerated holes?\nLine 89: How did HONO and NO2 form?\nLine 98: What is the environmental relevance of these NH4Cl masses?\nHow would the amount and the form of NH4Cl affect Cl2 production?\nConsidering the pH of atmospheric aerosols, how does pH affect the reaction, including formation of Cl2, HONO, and NO2?\nThe authors suggest that acidity plays a role in the proposed mechanism; however, the relationship between pH and Cl2\nproduction has not been rigorously characterized through controlled experiments.\nRelated to this, on line 163, why does pH3 NaCl solution produce Cl2? How does pH affect the reaction? What is the role of\nH+?\nWhat is the role of NH4+ in NH4Cl in producing Cl2?\nIt is unclear how did the authors proposed the mechanism (line 111-113) from the RH experiments (line 107-111). Please\nclarify.\nLine 137: The authors attribute the observed behavior of Cl2 to its dissolution in water. However, given the relatively low\nsolubility of Cl2 in water, it is questionable whether this process alone can account for the significant impacts described. A\nmore plausible and detailed physicochemical explanation is required to justify this interpretation. Why did the yield of Cl2 not\nshow a significant correlation with relative humidity higher than 10%?\nFigure 5: How long was the field campaign? Are data shown in Figure 5 A, B, C from one specific day? Please clarify.\nFigure S8 presents the light spectra of the mercury and xenon lamps. What is the unit for the Y axis? How does the light\nintensity compare to sunlight?\nCl2 will photolyze under these light sources. How will that affect the experiment results?"}, "author_response": "Response to Reviewer Comments #1\nReviewer #1 (Remarks to the Author):\nOverall evaluations:\nLi et al. discovers a new source of Cl from the oxidation of NH Cl facilitated by\n2 4\nO and aerosol acidity in a series of laboratory experiments. Furthermore, the\n2\nauthors examine the significance of this new mechanism in real atmosphere,\nwhich shows 12% to 55% contribution to Cl production. This Cl producing\n2 2\nmechanism is surprising since no additional photosensitizers are required to\noxidize chloride. Discovery of this new mechanism adds up to known sources\nof Cl , including nitrate photolysis, Fe-catalyzed chemistry, ClNO chemistry,\n2 2\nand so on. Surely, this work bears enough novelty and significance. The\nadopted methods are robust. A few clarifications are needed. Description of\nsome results could be further enhanced after minor revisions.\nDetailed comments:\n1. The detailed role of black carbon in Cl production is not clearly stated. Since\n2\nblack carbon is a mixture of various elements and compounds, exactly speaking,\nwhat kinds of molecules in black carbon are responsible for Cl production?\ncarbon used in this study was a commercial sample (Special Black 4A, SB4A),\nwhose main component is elemental carbon (EC). To minimize the potential\ninterference from adsorbed or deposited impurities (especially nitrates) during\nproduction and storage, the SB4A samples were thermally desorbed under\nnitrogen protection for 3 hours before use. This treatment effectively removes\na large portion of organic carbon (OC) and nitrate residues (Daly and\nHorn,2009;Han, et al.,2013;Huffman, et al.,2009).\nAccording to previous studies (Li, et al.,2018), EC primarily exhibits strong light-\nabsorbing properties and facilitates interfacial electron transfer. Therefore, we\ninfer that the promoting effect of black carbon on Cl production mainly arises\n2\nfrom: (1) enhancement of light absorption, which increases the photochemical\nenergy input, and (2) provision of electron-transfer pathways at the interface\nthat accelerate charge-transfer steps involved in chloride oxidation (R1-R2, in\nthe manuscript) and thereby promoting Cl formation. In the revised manuscript,\n2\nwe have added corresponding explanations in the main text and detailed the\nsample treatment in the Methods section.\nLines 229-233, “Under identical NH Cl mass conditions, the samples containing BC\n4\nexhibited a pronounced increase in Cl production, as shown in Figure S10. This\n2\nbehavior indicates that elemental carbon facilitates an efficient electron-transfer\npathway from Cl- to O (R1-R2), thereby promoting the formation of •O -and\n2 2\nsubsequently Cl .”\nSpecial Black 4A (SB4A, Evonik Degussa). Before experiments, the material was\nthermally desorbed under nitrogen protection for 3 h to remove adsorbed organics and\nnitrate residues that may have been acquired during production and storage35-37,\nensuring that the observed effects were primarily associated with EC itself.”\n2. Lines 57-60: Why are low pH and elevated RH the reasons for unaccounted\nCl2 observations in inland urban regions? Further clarifications are needed here.\nResponse 1.2: We thank the reviewer for pointing out the lack of logical flow in\nour explanation. The statement regarding “low pH and elevated RH” refers to\nthe environmental conditions required for previously known chloride activation\npathways, such as the photolysis of nitrate in acidic droplets (pH < 3.3).\nHowever, field studies in inland urban regions such as Beijing, Zhengzhou, and\nNetherlands have reported aerosol pH values typically above 3.3, often in the\nrange of 4-5, together with high relative humidity that further dilutes aerosol\nacidity (Guo, et al.,2018;Zhang, et al.,2021;Zhang, et al.,2025). Under such\nconditions, these established mechanisms are not sufficiently efficient to\ngenerate the observed daytime Cl levels in inland regions. Therefore, “low pH\n2\nand elevated RH” are mentioned not as drivers but as constraints-their\nprevalence in inland regions limits the applicability of existing acid- or\nphotosensitizer-driven pathways, implying that an additional, yet unidentified,\nmechanism is responsible for the unaccounted Cl production under these less\n2\nacidic and humid conditions. We have revised the text to clarify this reasoning.\nLines 57-62, “However, aerosols in inland urban regions generally exhibit higher pH\nvalues (typically 4-5) and elevated RH, which dilute aerosol acidity and thereby\nsuppress these acid-driven activation pathways. Therefore, owing to the low pH,\nelevated relative humidity, …in inland urban regions.”\n3. Lines 71-80: The authors mentioned a mercury lamp and a xenon lamp. It is\nclear that the xenon lamp could simulate the solar radiation. How about the\nmercury lamp? In other words, what is the difference between the mercury lamp\nand the xenon lamp in terms of their light spectrum?\nResponse 1.3: The mercury lamp and xenon lamp were used for different\nexperimental purposes in this study. The mercury lamp exhibits a discontinuous\nemission spectrum with distinct ultraviolet peaks (mainly between 225-400 nm)\nand provides much higher UV intensity than the xenon lamp. Such strong UV\nradiation facilitates the observation of Cl generation phenomena and helps us\n2\nexplore the detailed reaction mechanisms under controlled laboratory\nconditions. In contrast, the xenon lamp emits a continuous spectrum covering\nthe range of 320-800 nm, closely resembling natural solar radiation. It was\ntherefore used to simulate sunlight and to evaluate the atmospheric relevance\nand reaction rates under realistic conditions. The spectral ranges of both lamps\nare shown in Figures S2, and additional clarification has been added to the\nmain text.\nLines 73-78, “A mercury lamp was employed as the light source to investigate the\nreaction phenomena and mechanisms because of its discontinuous emission spectrum\nand high UV intensity (225-400 nm), which facilitate Cl generation. In contrast, a\n2\nxenon lamp (320-800 nm) with a continuous solar-like spectrum was used to simulate\nsolar radiation reaching the atmospheric boundary layer and to assess the atmospheric\nrelevance of the reactions. The measured spectra of the two light sources are shown in\nFigure S2.”\n4. Lines 157-172: The authors’ statements here strongly imply that NH4+ is\nindispensable for Cl production, which in fact may not be the case. If the role\n2\nof NH + is to provide acidity by hydrolysis (NH + + H O = NH *H O + H+), then\n4 4 2 3 2\nother cations may do the same, e.g., Mg2+ and Ca2+. Further clarifications are\nrequired to clearly emphasize how NH + contributes to Cl production, and\n4 2\nwhether or not other cations could do the same (e.g., Mg2+ and Ca2+) although\nthey present in much smaller amounts than NH +.\nthe reviewer’s suggestion and conducted further experiments to explore the\npotential role of other cations in Cl production, by using MgCl and CaCl under\n2 2 2\nidentical irradiation and RH conditions. These experiments indeed indicate that\nNH + is not the only cation capable of facilitating Cl production. Both MgCl\n4 2 2\nand CaCl were able to generate detectable amounts of Cl , but the yields were\n2 2\nsubstantially lower than those observed for NH Cl with the same Cl- moles.\n2\ncontaining equal molar amounts of chloride after 1 hour of irradiation.\nFrom left to right, the three groups correspond to 2 mg of NH Cl and the\n4\namounts of MgCl and CaCl that provide the same molar quantity of chloride.\n2 2\nExperiments were conducted under mercury lamp irradiation using zero air at\n60% RH. The blue bars represent the maximum 1-min averaged Cl mixing\n2\nratios within the period, and the orange bars represent the integrated Cl\n2\nproduction over the same period.\nUnlike neutral NaCl, which does not undergo hydrolysis, we believe acidity is a\nkey factor enabling the oxidation of Cl-. We attribute these differences primarily\nto the distinct acidifying capacities of these cations in the presence of water.\nNH + undergoes hydrolysis, leading to a stronger local acidity that promotes\n4\nchloride activation at the air-solid interface. In contrast, Mg2+ and Ca2+ have\nsignificantly weaker hydrolysis-induced acidification under the same conditions,\nresulting in less efficient chloride protonation and subsequently reduced Cl\n2\nproduction.\nSpecifically, when pH is estimated using hydrolysis constants without correcting\nfor activity coefficients, 1 M solutions of NH Cl, MgCl , and CaCl exhibit pH\n4 2 2\nvalues of 4.6, 6.0, and 6.45, respectively. Calculations for their saturated\nsolutions yield slightly lower pH values of 4.23, 5.33, and 5.87. In our\nexperiments, however, the photo-reactivity of these solids was examined under\n60% RH. According to the database compiled by Peng and co-workers(Peng,\net al.,2022), the deliquescence relative humidities (DRH) of MgCl and CaCl\n2 2\nare 27% and 31%, whereas NH Cl under our experimental conditions only\n4\nslightly exceeds its efflorescence relative humidity (ERH, 45%). This indicates\nthat the aerosol liquid water content in the particles formed from these three\nsalts under our reaction conditions is not controllable, complicating direct\nestimation of aerosol acidity. Nevertheless, it is clear that NH Cl is significantly\n4\nmore acidic than MgCl and CaCl , with the latter two exhibiting similar acidity.\n2 2\nThis trend is consistent with the observed Cl yields from the three chloride salts\n2\nin our experiments. Neutral NaCl, in contrast, does not undergo photochemical\noxidation to release Cl directly. These observations suggest that acidity is a\n2\ncritical factor for chloride photochemical oxidation. While other cations can in\nprinciple participate in this chemistry, their contributions are expected to be\nmuch smaller in atmospheric particles due to both lower abundance and weaker\nacidifying capacity compared with NH +. Therefore, the focus of this study\n4\nremains on the photochemical oxidation of ammonium chloride particles. In the\nrevised manuscript, we have incorporated the above information into the\ndiscussion of the role of NH +.\nsolutions through hydrolysis. Under the same Cl- mass loading and RH, exposure of\nMgCl and CaCl to irradiation also produced detectable Cl , but the yields were\n2 2 2\nsubstantially lower than those from NH Cl, as shown in Figure. S9. We attribute this\n4\ndifference to the combined effects of acidity and hygroscopicity. In the aqueous phase,\nthe hydrolysis constants of Mg2+ and Ca2+ are smaller than that of NH + at the same\n4\ntemperature, resulting in higher theoretical pH values. According to the database\ncompiled by Peng and co-workers29, the deliquescence relative humidities (DRH) of\nMgCl and CaCl are 27% and 31%, whereas NH Cl under our experimental conditions\n2 2 4\nonly slightly exceeds its efflorescence relative humidity (ERH, 45%). This means that,\nat 60% RH, MgCl and CaCl aerosols contain more liquid water than NH Cl, and the\n2 2 4\nresulting dilution effect further weakens their acidity. These observations again support\nthe conclusion that the acidity introduced by NH + plays a critical role in promoting\n4\nchloride oxidation.”\n5. Lines 203-205: Is it possible that HCl comes from thermal decomposition of\nNH Cl besides the Cl + VOC reaction?\nwas maintained at 25 °C using a circulating water-cooling system, ensuring\nisothermal conditions throughout the reaction. Therefore, the thermal\ndecomposition of NH Cl, which typically requires significantly higher\n4\ntemperatures, is unlikely to occur under our experimental conditions. We have\nindeed noted that the cooling circulation system was omitted from the\nschematic diagram of our experimental setup (Fig. S1). We will add it to the\ndiagram and supplement the specified temperature conditions in the Methods\nsection.\nLines 317-319, “A digital circulating water bath… to maintain the temperature of\nreactor at 25 °C.”\nMoreover, as shown in Figure 4b, the integrated HCl signal was only 1.5 μmol\nwhen the system was irradiated in the absence of cyclohexane (i.e., without\nVOC addition). This small signal may result from minor NH Cl decomposition\n4\nor instrumental fluctuations. In contrast, when cyclohexane was introduced, the\nintegrated HCl signal markedly increased to 3.27×103 μmol, indicating that HCl\nformation predominantly originated from the Cl + VOC oxidation pathway rather\nthan from NH Cl thermal decomposition. We have revised the main text to\n4\nclarify this interpretation.\nLines 255-258, “The signal of HCl … and, as shown in Figure 4b, its significant\nenhancement after the introduction of cyclohexane compared with the experiment\nwithout cyclohexane demonstrates that … a hydrogen abstraction reaction (R13)”\n6. How is HONO produced when only N is injected into the NH Cl system?\n2 4\nWithout the presence of O , is there any other electron acceptor to facilitate the\n2\noxidation of NH + to produce HONO?\nthrough the oxidation of NH + by reactive oxygen species such as ·O - at the\n4 2\nparticle-air interface. However, we indeed observed measurable HONO even\nwhen N was used as the carrier gas. We attribute this to two possible factors:\n2\n(1) residual oxygen present in the high-purity N supply, and (2) light-induced\n2\ninterfacial transformations involving H O that may provide an alternative\n2\nelectron-accepting pathway.\nTo further examine this, we conducted additional experiments using the same\nmass of NH Cl under dry N conditions. We installed a silica-gel plus molecular-\n4 2\nsieve drying tube and a deoxygenation cartridge upstream of the reactor to\nminimize the presence of both H O and O . As shown in Figure S7, the HONO\n2 2\nsignal decreased substantially but did not drop to zero, while NO became\n2\nundetectable. These results suggest that, in the absence of molecular oxygen,\ntrace amounts of H O in the system may still act as an electron acceptor to\n2\nfacilitate the oxidation of NH + to form HONO. The remaining extremely low\nbe completely removed or oxygen adsorbed on NH Cl particles during sample\n4\npreparation. We have added these clarifications to the supplementary materials.\nLines 72-83, “In addition, when the same mass of NH Cl particles was used but the\n4\ncarrier gas was switched from humid zero air to dry N , and a molecular-sieve drying\n2\ntube together with an oxygen-scavenging tube was installed upstream of the reactor, the\namount of HONO formed under illumination decreased dramatically, as shown in\nFigure S7. This result indicates that both O and H O make substantial contributions\n2 2\nto HONO production. The remaining ultra-low levels of HONO are most likely\nattributable to residual oxygen in the carrier gas or oxygen adsorbed on the NH Cl\n4\nparticles during sample preparation and handling. Moreover, in contrast to the dry N\n2\ncondition, elevated HONO formation was still detected on NH Cl surfaces at 49% RH\n4\n(Figure 1b and Figure S6), suggesting that H O at the particle-air interface may serve\n2\nas an alternative electron acceptor and permit limited oxidation of NH + even in the\n4\nabsence of O .”\n2\n7. Lines 527-529: Which kind of lamp is used here? Mercury lamp or xenon\nlamp? Please also check other locations about the usage of lamps.\nResponse 1.7: The experiment described in this section was conducted under\nmercury lamp irradiation. We have carefully checked the entire manuscript and\nclarified the type of light source used in all relevant experiments.\nLines 645-647, “(a)Time series of 1-min average mixing ratios…were exposed to\nillumination of the mercury lamp.”\nLines 698-701, “The yellow area indicates light exposure of the mercury lamp…”\nReferences:\nDaly, H. M. & Horn, A. B. Heterogeneous chemistry of toluene,\nkerosene and diesel soots. Phys. Chem. Chem. Phys. 11, 1069-1076,\ndoi:10.1039/b815400g (2009).\nHan, C., Liu, Y. C. & He, H. Role of organic carbon in heterogeneous\nreaction of NO with soot. Environ. Sci. Technol. 47, 3174-3181,\n2\ndoi:10.1021/es304468n (2013).\nHuffman, J. A. et al. Chemically-resolved aerosol volatility\nmeasurements from two megacity field studies. Atmos. Chem. Phys. 9, 7161-\n7182, doi:10.5194/acp-9-7161-2009 (2009).\nLi, M. et al. Role of elemental carbon in the photochemical aging of soot.\nProc. Natl. Acad. Sci. U. S. A. 115, 7717-7722, doi:10.1073/pnas.1804481115\n(2018).\nGuo, H. Y. et al. Effectiveness of ammonia reduction on control of fine\nparticle nitrate. Atmos. Chem. Phys. 18, 12241-12256, doi:10.5194/acp-18-\n12241-2018 (2018).\nZhang, B. Q. et al. Significant contrasts in aerosol acidity between\nChina and the United States. Atmos. Chem. Phys. 21, 8341-8356,\ndoi:10.5194/acp-21-8341-2021 (2021).\nZhang, H. Y., Wang, S. B., Dong, Z. S., Li, X. & Zhang, R. Q.\nMeasurement report: crustal materials play an increasing role in elevating\nparticle pH - insights from 12-year records in a typical inland city of China.\nAtmos. Chem. Phys. 25, 6943-6955, doi:10.5194/acp-25-6943-2025 (2025).\nPeng, C., Chen, L. X. D. & Tang, M. J. A database for deliquescence\nand efflorescence relative humidities of compounds with atmospheric relevance.\nFundamental Res. 2, 578-587, doi:10.1016/j.fmre.2021.11.021 (2022).\nResponse to Reviewer Comments #3\nReviewer #3 (Remarks to the Author):\nOverall comments:\nThe authors proposed a new mechanism for Cl formation from photolysis of\n2\nammonium chloride in the atmosphere, based on results from both laboratory\nand field experiments. While the idea is interesting and potentially important,\nthe manuscript cannot be considered in its current form and requires substantial\nrevision.\nThere are some major concerns. Several key details are not sufficiently\nelaborated, particularly in the Field evidence section and Methods section.\nThe section titled \"Field evidence for the proposed mechanism\" is\npresently weak and does not convincingly support the central claim. The\npresentation and interpretation of field observation results remain\nunderdeveloped and are not supported by compelling evidence to substantiate\nthe authors' conclusions. The observed correlation between Cl levels and\n2\nparticulate chloride/ammonium, while interesting, is not definitive proof of a\ncausal relationship. Alternative explanations for Cl signals (e.g., other known\n2\nformation pathways or sampling artifacts) were not adequately considered or\nruled out. To robustly support the proposed mechanism, more direct evidence\nis needed.\nResponse 3.1: We appreciate the reviewer’s comment. Regarding the concern\nabout whether the Cl signals observed in the field could originate from other\n2\nknown formation pathways or sampling artifacts, we have carefully examined\nthis issue and revised the manuscript accordingly. Our detailed response is as\nfollows.\nFirst, to minimize any potential artifacts in sampling, we took specific steps to\nreduce the residence time of air in the sampling tubing and regularly cleaned\nthe inlet tubing. During the Cl observation period, atmospheric samples were\n2\ncontinuously drawn into the instrument by a pumping system operating at a flow\nrate of 10.0 L/min. The higher flow rate effectively reduced the residence time\nof air samples in the sampling tubing, significantly decreasing adsorption and\nretention losses on the inner surfaces, thereby minimizing Cl loss during\n2\nsampling. To mitigate particulate buildup and cross-contamination in the tubing,\nall sampling lines were replaced weekly. The replaced tubing was sequentially\nrinsed repeatedly with deionized water, ultrasonically cleaned for 30 minutes to\nensure thorough removal of adhered contaminants, and then purged with high-\npurity N until completely dry, ensuring clean and residue-free inner walls.\nprocedures for reducing sampling-related artifacts , and the corresponding\nrevisions are summarized in Response 3.3.\nIn addition, we utilized a box model to quantitatively assess the contributions of\nother known Cl formation pathways, primarily involving heterogeneous\n2\nreactions of HOCl, ClNO , ·OH, and O on aerosol surfaces. The model system\n2 3\nconsists of five core modules, including (1) initialization module: optimizes\nmodel parameters by integrating observational data; (2) data input module:\nincorporates online monitoring data from the supersite of the Institute of Urban\nEnvironment, Chinese Academy of Sciences, using gaseous pollutant\nconcentrations and meteorological parameters as observational constraints;\n(3) physical processes module: introduces a dry deposition velocity parameter to\nquantify dry deposition and atmospheric dilution within the boundary layer,\neffectively characterizing pollutant dispersion and removal; (4) chemical\nprocesses module: employs the latest MCM-v3.3.1 chemical mechanism,\nwhich includes 142 volatile organic compounds (VOCs) and over 17,000\nelementary reactions, and integrates existing reactive chlorine chemistry; (5)\noutput analysis module: provides key parameters such as pollutant\nconcentration distributions and photochemical reaction rates(Wolfe, et al.,2016).\nIn this model, the formation of Cl mainly occurs through the heterogeneous\n2\nuptake reactions of ClNO , HOCl, O , and ·OH on chloride-containing aerosol\n2 3\nsurfaces, while Cl removal is primarily achieved via photolysis and its reaction\n2\nwith OH radicals. The heterogeneous uptake reaction can be represented by\nthe following general formula:\n𝑑[𝑋] 𝑐𝛾\n= − 𝑆[𝑋] (R1)\n𝑑𝑡 4\nwhere X denotes HOCl, ClNO , ·OH, or O ; c is the molecular velocity of X\n2 3\n(cm·s-1), S is the aerosol surface area concentration (cm2·cm-3), and γ is the\nuptake coefficient. Based on previous studies(Chen, et al.,2025;Chen, et\nal.,2022), the uptake coefficients for HOCl, ClNO , ·OH, and O in this study\n2 3\nwere set as 5.3×10-4, 6.0×10-3, 8.9×10-3, and 1.0×10-5, respectively.\nThe simulation results indicated that these known reaction mechanisms\ncontributed a limited fraction (~20.0%) to the noontime Cl concentrations and\n2\ncould not explain the observed Cl peak events (Figure S12).\npathways, correlation analysis of the observational data revealed a positive\nrelationship between daytime Cl concentrations and particulate chloride and\n2\nammonium levels.\nFurthermore, this field-observed correlation is consistent with findings from\nlaboratory simulation experiments-under controlled laboratory conditions,\nammonium chloride particles were shown to release Cl steadily under light\n2\nexposure, with the production rate closely related to light intensity and particle\ncompositions. Therefore, both field observations and laboratory evidence jointly\nindicate that the photolysis of particulate ammonium chloride represents a\npreviously under-recognized source of daytime Cl .\n2\n(10:00–14:00). The blue line represents the observations, and the red line\nrepresents the model results.\nWe have added this statement into the main text：\nLines 273-282, “When we quantitatively evaluated the contributions of established\nCl formation pathways using a box model (F0AM36), including heterogeneous\n2\nreactions involving HOCl, ClNO , ·OH, and O on aerosol surfaces, we found that these\n2 3\nmechanisms account for only a minor fraction (~20%) of the noontime Cl\n2\nconcentrations (Figure S12). Therefore, additional daytime sources of Cl must be\n2\npresent in this region. We further noted that the concentration of Cl , together with the\n2\nproduct of particulate chloride and aerosol surface density, exhibits a positive\ncorrelation with increases in ammonium concentrations, suggesting that the potential\nsource of Cl is linked to the heterogeneous activation of NH Cl.”\n2 4\nThey should conduct controlled perturbation experiments (e.g., control\nlight, acidity, water, or chloride levels), or quantification of key\nintermediates under real-world conditions, similar to those in the\nlaboratory experiments. This is because the ambient conditions are much\nmore complex than lab conditions. They should provide more direct evidence\nof the Cl production chemistry to derive a conclusion.\nconclusions cannot be firmly drawn until more direct and compelling evidence\nis provided to establish a causative link between the photolysis of ammonium\nchloride and the observed Cl production.\nhave helped to improve the quality of the manuscript. Following the reviewer’s\nsuggestions, we have added additional controlled experiments to further\nsubstantiate the factors governing Cl production. Specifically, we added\n2\nexperiments examining the effects of light intensity and acidity on Cl yields. As\n2\nthe reviewer suggested, the influences of ambient relative humidity and chloride\nmass loading are presented in Figures 2a and 1c, respectively. Overall, these\ncontrolled-variable experiments consistently demonstrate that higher light\nintensity, increased acidity, and elevated chloride availability all promote Cl\n2\nproduction. The role of acidity and its mechanistic implications are further\ndiscussed in detail in our responses to specific comments below.\nFigure 2(c) Relationship between Cl production after 120-min of irradiation and\n2\nthe initial pH. All solutions were 1 M NH Cl, with pH adjusted to 3, 2, and 1 using\n2 4\nuncertainties in both Cl yield and pH determined from replicate experiments.\n2 4\n= 2 mg). The carrier gas for all experiments was zero air with RH = 60%.\nMoreover, the Methods section lacks experimental details, including\nspecific protocols, instrumentation settings, and analytical conditions for\nboth lab and field studies.\nThe description of the field measurements requires expansion. The current\ninformation regarding techniques such as CIMS and MARGA is insufficient.\nEssential details are missing.\n--CIMS and MARGA operation details are missing and need to be included.\n--inlet configuration: The length and material of the inlet line used for sampling,\nparticularly for Cl measurement is not described\n2\n--background and zeroing procedures: The procedures for determining\nbackground signals and performing regular zero measurements (e.g., using a\nscrubber, or cylinder) to ensure data quality are necessary for high quality Cl\n2\nmeasurement.\n--potential artifacts: It is recognized that Cl can be generated on surfaces (e.g.,\n2\ninstrument inlets) in the presence of chloride. Whether this possibility was\nexamined during the campaign and what measures were taken to minimize\nsuch sampling artifacts should be tested and discussed.\n--calibration details: For the I-CIMS measurement of Cl , the relative humidity\n2\n(RH) may influence sensitivity. It should be clearly stated whether calibration\nexperiments were performed at different RH levels relevant to the field\nconditions to account for this potential effect. Was RH measured during the field\ncampaign? Also, was the CIMS inlet RH controlled?\nResponse 3.3: We sincerely thank the reviewer for pointing out the missing\nmethodological details. We have carefully revised the Methods section and\nincorporated comprehensive descriptions for both the laboratory and field\nmeasurements. Specifically, we have added the following information:\n1. CIMS and MARGA operation details\nLab Experiments section:\n“… measured by I--ToF-CIMS ((Aerodyne Research and Tofwerk AG).\nThe I--ToF-CIMS instrument consists of an ion-molecule reaction region (IMR) coupled\nto ion-guiding quadrupole optics and an orthogonal-acceleration time-of-flight mass\nanalyzer, enabling highly sensitive detection of trace gas-phase species through\nselective iodide-adduct reagent-ion chemistry39. Sample air (2.1 SLPM) was introduced\ninto the IMR, where it mixed with a stream of reagent ions. Methyl iodide (CH I),\n3\nsupplied from a heated permeation tube and carried in 2.38 SLPM of ultrahigh-purity\nN (99.999%), was soft-ionized by an X-ray source prior to entering the IMR, producing\n2\nreagent ions I- and I(H O)-. These ions selectively reacted with analyte species to form\n2\niodide-adduct ions (I·A-) in the IMR. Mass analysis was conducted using a time-of-\nflight (ToF) analyzer. Ions were extracted by a pulsed DC field, drifted through a field-\nfree vacuum tube, and were energy-focused by a reflectron before detection with a\nmultichannel plate (MCP). The ion signals were digitized and recorded using the\nTofDaq acquisition system.\nCl was detected at 197 amu…”\n“Gaseous Cl was determined by another chemical ionization mass spectrometry\n2\noperated with the same ionization principle as described in the Laboratory section.\nIn this setup, CH I, generated from a heated permeation tube (VICI Metronics, Inc.),\n3\nwas ionized as it passed through a soft X-ray source. The ionization took place within a\nstream of ultrahigh-purity nitrogen (N , 99.999%) generator (PEAK, NG5000(A))\n2\nflowing at 2.7 SLPM. Inside the IMR chamber, the target gases underwent specific\nreactions with reagent ions (I- and I(H O)-) to form iodide-adduct clusters. These\n2\nproduct ions were then directed to the atmospheric pressure interface long time-of-flight\nmass spectrometry for subsequent quantification.\nThe mass concentration of chloride was measured by a Monitor for Aerosols and Gases\n(MARGA, Metrohm AG Inc., Switzerland), the instrument is an online analyzer\ndesigned to semi-continuously measure water-soluble gases (i.e., HCl, HONO, HNO ,\n2 3 3 4 4\nMg2+, Ca2+) with hourly temporal resolution, using ion chromatography. The ambient\nsample containing gaseous species and PM passes through a wet rotating denuder\n2.5\n(WRD), which consists of two concentric glass cylinders coated with an absorption\nsolution (H O ) where gases diffuse into the aqueous film41,42. Following collection,\n2 2\nsamples were analyzed via cation and anion conductivity detectors (IC, Metrohm AG\nInc., Switzerland). The analytical accuracy for this process was controlled through the\nuse of LiBr as an internal standard, which was simultaneously injected and mixed with\neach sample. To ensure continued accuracy, an external calibration was performed\nweekly by replacing the absorption solution with a known liquid anion standard\ncontaining Cl- (Sigma-Aldrich, multielement ion chromatography anion standard\nsolution, containing F-, Cl-, NO -, PO 3-, SO 2-). The PM impactor and inlet tubes\n3 4 4 2.5\nwere cleaned every two weeks. Specifically, the aerosol sampling system was cleaned\nby NaOH solution every two months.\n2. Inlet configuration for Cl measurement\n2\n“Ambient air was drawn into the sampling room through a 2-meter-long, 1/4-inch inner\ndiameter perfluoroalkoxy (PFA) tubing at a flow rate of 10 standard liters per minute\n(SLPM). This high flow rate, maintained by an external vacuum pump, minimized wall\nadsorption and retention of gas molecules, thereby significantly reducing sample loss.\nFrom this main flow, approximately 2 SLPM was diverted into the CIMS setup for\nanalysis. To prevent particulate matters accumulation and cross-contamination within\nthe sampling system, all inlet tubing was replaced weekly. Following replacement, the\ntubing was meticulously cleaned by rinsing with deionized water and ultrasonicating\nfor 30 minutes. It was then dried with high-purity N to ensure a residue-free surface,\n2\nwhich is critical for maintaining consistent system performance and representative\nsampling.”\n3. Background and zeroing procedures\n“During each measurement cycle, background measurements were routinely performed\nto accurately determine the background signal and assess the instrumental baseline.\nThis procedure involved introducing ultra-pure N into the inlet for a 20-minute period,\n2\nduring which the Cl background signal was measured multiple times and the detection\n2\nlimits (3σ) of Cl was 0.43 ppt.”\n2\n4. Evaluation and minimization of sampling artifacts\nAs noted in the section Inlet configuration for Cl measurement, an external\n2\nvacuum pump was used to generate a 10 SLPM sampling flow in order to\nminimize artifacts during ambient measurements by reducing the residence\ntime. Under these conditions, the residence time of sample air within the inlet\ntubing was kept below 0.4 s.\n5. Calibration procedures, RH dependence, and RH conditions\nThe calibration procedures and RH dependence of Cl measurements in\n2\nthe laboratory are described in Line 344. The field calibration followed the same\napproach, and additional details have been provided in Line 401. In summary,\nCl was calibrated under conditions representative of the ambient environment\n2\n(0.1-1 ppb; RH 15-70%). The RH dependence observed in our CIMS signals is\nconsistent with previous studies employing similar instruments for Cl\n2\ndetection(Li, et al.,2020;Peng, et al.,2022), and no pronounced RH sensitivity\nwas observed.\n“A stable Cl standard gas was generated using a constant-temperature\n2\npermeation method, with high-purity N (99.999%) at 100 SCCM serving as the\n2\ncarrier gas flowing through a water bath temperature-controlled at 40.0 °C Cl\n2\npermeation tube, with 98 ng/min permeation rate at 40.0 °C (VICI Metronics,\nInc.). The resulting gas stream was then dynamically diluted with high-purity N\n2\nusing calibrated mass flow controllers to achieve a concentration gradient.”\nSpecific comments\nLine 29: The authors mention the potential role of black carbon in the proposed\nmechanism; however, this concept is not substantiated or discussed within the\nmain body of the manuscript. If black carbon is considered a relevant factor, its\nrole must be explicitly integrated into the experimental design, results, and\ndiscussion. Otherwise, such mention should be removed from the abstract.\nResponse 3.4: Thanks for this suggestion. In the revised manuscript, we have\nexpanded the discussion of the role of black carbon and integrated it into the\nmechanism validation section. The commercial black carbon material (Special\nBlack 4A) used in our experiments, following thermal desorption to remove\nresidual nitrates and organics, remained predominantly composed of elemental\ncarbon (EC). EC is known to facilitate electron-transfer processes due to its\nconductive graphitic structure, as demonstrated in previous studies (Li, et\nal.,2018). In our experiments, the presence of black carbon substantially\nenhanced Cl production under identical light, and RH conditions. This behavior\n2\nis consistent with a mechanism in which EC provides an efficient pathway for\nelectron transfer from Cl- to O , thereby promoting the formation of O -, a key\n2 2\nintermediate in the subsequent of Cl generation. Accordingly, we have retained\n2\nthe mention of black carbon in the abstract and expanded the corresponding\ndiscussions in the Mechanism validation section.\nLines 223-235, “To further validate the role of O as the electron acceptor in the\n2\nphotochemical oxidation of solid NH Cl, we introduced elemental carbon (EC) into the\n4\nsystem. EC, a major component of atmospherically relevant black carbon (BC), is\nwidely recognized as a photocatalytic material that accelerates interfacial electron\ntransfer. In our experiments, thermally desorbed commercial black carbon was\nphysically mixed with NH Cl particles and deposited onto quartz plates for irradiation.\n4\npronounced increase in Cl production, as shown in Figure S10. This behavior indicates\n2\nthat elemental carbon facilitates an efficient electron-transfer pathway from Cl- to O\n2\n(R1-R2), thereby promoting the formation of •O -and subsequently Cl . Consistently, in\n2 2\nthe ESR spectra, the intensity of DMPO-•O - adducts produced under illumination was\n2\nfurther enhanced in the presence of EC, as shown in Figure S11.”\nLine 55: what are photogenerated holes?\nResponse 3.5: The photogenerated holes refers to the h+ produced in\nsemiconductor materials such as TiO via photoactivation, when an electron\n2\nabsorbs light energy and transitions from the valence band to the conduction\nband, leaving behind a positively charged vacancy. In the cited literature, these\nphotogenerated holes (h+) are considered to act either as electron acceptors\nfor Cl- to form Cl· or for H O to form ·OH. In our manuscript, the term only\n2\ndescribes the latter process.\n𝑇𝑖𝑂 +ℎ𝑣 → ℎ+ + 𝑒−\n2\n𝐶𝑙− + ℎ+ → 𝐶𝑙∙\n𝐻 𝑂 + ℎ+ → ∙𝑂𝐻 + 𝐻+\n2 4 3\nNH +-enhanced hydrolysis of NO . In the former, it is generally accepted that\n4 2\nHONO and NO production primarily originates from reactions involving NO -,\n2 3\nwhereas NH + mainly participates through reactions of O- to form NH , which is\n4 2\nsubsequently oxidized to NO, N O, and other nitrogen species. In the latter\n2\npathway, the role of NH + has been interpreted largely as modifying aerosol\n4\nhygroscopicity and related physicochemical properties, thereby facilitating NO\n2\nhydrolysis to produce HONO.\nIn our experimental system, however, neither NO - nor NO is externally\n3 2\nintroduced, indicating the presence of a self-photolytic pathway for NH +.\ndemonstrated that the nitrogen in HONO can originate directly from NH +,\n4\nwhich is oxidized by photochemically generated superoxide radicals (·O -) to form\n2\nsteps to produce HONO. In our experiments, ESR spectra of ·O - were also\n2\ndetected using radical spin trap in illuminated NH Cl systems, suggesting that\n2\natmosphere while Cl is not, and that the yields of HONO and Cl exhibit distinct\n2 2\ndependences on NH Cl mass loading, we infer that NH + transformation and\n4 4\nCl- oxidation proceed through different mechanistic pathways. Because the\nfocus of this study is on Cl formation, we do not further elaborate on the\n2\nmechanistic details of HONO and NO production in the main text; the relevant\n2\ndiscussion is provided in Text S2.\nLine 98: What is the environmental relevance of these NH Cl masses?\n4 2\nResponse 3.7: In our current experimental setup, it is indeed challenging to\nquantitatively assess the dependence of Cl production on the mass of NH Cl\n2 4\nalone, because isolating a single controlling variable is not straightforward.\nAlthough different amounts of NH Cl (2, 20, and 200 mg) were deposited on\n4\nquartz plates with identical geometric surface areas, increasing the loading\ninevitably altered the particle packing density and, consequently, the effective\nspecific surface area. As a result, it becomes difficult to distinguish whether the\nenhanced Cl production results from the increased NH Cl mass itself or from\n2 4\nthe enlarged reactive surface area.\nThe experiments with 2, 20, and 200 mg NH Cl samples were mainly designed\n4\nto compare the scaling behavior of Cl and HONO production with increasing\n4 2\nHONO suggest that these two species may originate from relatively\nindependent reaction pathways. Furthermore, the observation that HONO was\nstill produced at a comparable level while Cl disappeared under N atmosphere\n2 2\nfurther supports this interpretation.\nConsidering the pH of atmospheric aerosols, how does pH affect the reaction,\nincluding formation of Cl , HONO, and NO ?\n2 2\nThe authors suggest that acidity plays a role in the proposed mechanism;\nhowever, the relationship between pH and Cl producti-on has not been\n2\nrigorously characterized through controlled experiments.\nRelated to this, on line 163, why does pH3 NaCl solution produce Cl ? How\n2\ndoes pH affect the reaction? What is the role of H+?\nWhat is the role of NH4+ in NH Cl in producing Cl ?\n4 2\nResponse 3.8: Due to the inherent difficulty of precisely adjusting the acidity of\nsolid particles without altering other physicochemical properties, we carried out\ncontrolled pH experiments in NH Cl aqueous solutions, where the interfacial\n4\nchemistry was found to be broadly consistent with that observed for NH Cl in\n4\nthe gas-solid system.\nTo isolate the effect of acidity, all experiments were conducted at a fixed chloride\nconcentration of 1 mol/L, while pH was adjusted using H SO to pH 1, 2, 3, and\n2 4\nthe unacidified solution (pH approximately 5). The results show a clear\nmonotonic increase in Cl production with decreasing pH. This behavior is\n2\nconsistent with our mechanistic interpretation that acidity is required for the\noxidation of chloride at the interface. The promoting effect of acidity likely arises\nbecause H+ facilitates the migration and enrichment of Cl- at the air-water\ninterface, thus enhancing its accessibility for interfacial electron-transfer\noxidation.\nThese results further explain why a pH=3 NaCl solution is able to generate\nmeasurable Cl : at sufficiently low pH, even in the absence of NH +, protonation\n2 4\nenables interfacial chloride activation by promoting Cl- accumulation at the\ninterface and lowering the energetic barrier for its oxidation.\nThe role of NH4+ in enabling Cl formation from NH Cl can be summarized in\n2 4\ntwo aspects. First, NH + hydrolysis provides the intrinsic acidity needed for\n4\ninterfacial chloride oxidation, allowing Cl- to be activated by the photoexcited O\n2\nand water environment. Second, as Cl- is oxidized to Cl , the concurrent\n2\noxidation of NH + at the interface produces HONO and NO . The release of\n4 2\nthese gaseous nitrogen species continuously removes proton-consuming\nproducts from the particle surface, thereby helping maintain the acidic\nmicroenvironment that permits sustained chloride oxidation.\nLines 206-212, “To further elucidate the role of acidity in controlling chloride\noxidation, we conducted a set of solution-phase experiments in which the pH of 1 mol/L\nNH4Cl was systematically adjusted from approximately 5 (no added acid) to pH 3, 2,\nand 1 using H SO . As shown in Figure 2(c), Cl production increased markedly with\n2 4 2\ndecreasing pH, demonstrating a clear dependence on acidity. This trend indicates that\nH+ facilitates the interfacial enrichment and activation of Cl-, thereby enhancing its\npropensity to undergo oxidation in the presence of light, O , and water.”\nthe RH experiments (line 107-111). Please clarify.\nResponse 3.9: We appreciate the reviewer’s comment. The RH experiments\n(lines 107-111) were part of our investigation into the factors influencing the\nreaction. The mechanism proposed in lines 111-113 was deduced based on the\ncombined evidence that O and light are essential for Cl formation, as\n2 2\ndemonstrated in the preceding sections, and that the presence of H O is also\n2\nrequired, as shown in lines 107-111. Moreover, since a wide range of RH levels\nexists in the atmosphere, we further examined the effect of RH on Cl\n2\nproduction and found no significant variation, suggesting that H O mainly acts\n2\nto solvate the NH Cl particles and provide free, reactive Cl- for the reaction. We\n4\nhave revised this section to improve the logical flow and clarify the reasoning\nbehind the proposed mechanism.\nLines 117-126, “We show that water vapor… higher than 10%. These results indicate\nthat the presence of water vapor is necessary for Cl formation, although its amount\n2\nhas little influence once sufficient H O is available. Considering that O and light are\n2 2\nalso indispensable for Cl generation as demonstrated above, we therefore propose a\n2\nmechanism involving the direct photoexcitation activation of Cl- ions in the presence of\noxygen and water, leading to the subsequent generation of Cl (R1-R5).”\n2\nwater. However, given the relatively low solubility of Cl in water, it is\n2\nquestionable whether this process alone can account for the significant impacts\ndescribed. A more plausible and detailed physicochemical explanation is\nrequired to justify this interpretation. Why did the yield of Cl not show a\n2\nsignificant correlation with relative humidity higher than 10%?\nResponse 3.10: Thanks for pointing this out. We agree that the dissolution of\nCl alone cannot fully explain the weak dependence of Cl yield on RH above\n2 2\n10%. Based on our proposed mechanism, the participation of free Cl- is\nrequired for the formation of Cl . Together with the pH-dependence experiments\n2\nthat we added according to the reviewers’ suggestions, we indeed observed a\nclear dependence of Cl production on acidity. We therefore hypothesize the\n2\nfollowing: under conditions below the deliquescence point of NH Cl (~80% RH)\n4\nbut above completely dry conditions, the Cl yield does not exhibit a strong\n2\ncorrelation with RH, whereas a pronounced decrease appears once RH\nexceeds the deliquescence point. This behavior may be explained by the\nsubstantial water uptake of NH Cl above its deliquescence point, which dilutes\n4\nthe surface concentration of chloride and reduces surface acidity.\nFigure 2a (for reviewer response). Yield of Cl₂ as a function of relative humidity\nat an illumination time of 120 min, highlighting representative RH regimes.\nYellow symbols emphasize the dry condition (RH = 0%), the deliquescence\npoint of NH Cl (RH ~80%), and the averaged response over the sub-\n4\ndeliquescent range (12-57% RH). Experiments were conducted with 2 mg\nNH Cl.\nNH Cl particles remains within a relatively narrow range below the\n4\ndeliquescence point, with a slight decrease at higher RH due to enhanced\nevaporation. In contrast, once RH exceeds the deliquescence point, the aerosol\ndiameter increases sharply. This trend is consistent with our observations: the\naverage Cl yield at 12-57% RH is generally higher than that at ~78% RH. Such\n2\nagreement suggests that the reduced Cl production above the deliquescence\n2\npoint may result from the increased hygroscopicity of NH Cl, which generates\n4\na thick aqueous layer that lowers both the surface concentration of chloride ions\nand the local acidity required for interfacial Cl formation.\n4\ncoated substrates were prepared by premixing NH Cl with a small amount of\n4\nwater, depositing the mixture onto a quartz plate, and drying it prior to use. This\nprocedure may not always produce a perfectly uniform particle distribution. To\nmitigate particle aggregation, we roughened the quartz plate surface; however,\ncomplete uniformity cannot be guaranteed. We therefore encourage future\ninvestigations-particularly those employing large-scale, well-dispersed aerosol\nsystems such as smog chambers-to further examine this mechanism and its\ndependence on aerosol-phase morphology.\nLines 149-162, “According to the hygroscopic properties of NH Cl28, the amount of\n4\nwater adsorbed on particle surfaces remains limited below its deliquescence point.\nConsequently, increases in RH do not substantially enhance Cl production. Under\n2\nthese conditions, only a thin and discontinuous water layer is present-sufficient to\nsustain interfacial ion mobility but insufficient to markedly alter the surface chloride\nconcentration or the local acidity that governs Cl formation. As a result, the\n2\nfundamental interfacial environment required for Cl generation remains largely\n2\nunchanged across this RH range, leading to the absence of a clear positive correlation\nbetween RH and Cl yield. …However, the substantial water uptake forms a thick\n2\naqueous layer that may dilutes surface chloride and decreases local acidity, thereby\nsuppressing interfacial Cl formation despite the increased availability of water.”\nC from one specific day? Please clarify.\nResponse 3.11: The field observation was conducted from October 9 to\nDecember 5, 2022. We further analyzed the daytime period (06:00–18:00)\nseparately and obtained 430 hourly averaged valid data points. The data\npresented in Figure 5a-c represents hourly mean values calculated from all\navailable data during the entire observation period. We will indicate this\ninformation in the figure legends and add a detailed description of the field\ncampaign in the Methods section.\nLine 706-707, “All panels are based on the daytime (06:00–18:00) data collected\nduring the entire field campaign from 9 October to 5 December 2022 in Xiamen.”\nFigure S8 presents the light spectra of the mercury and xenon lamps. What is\nthe unit for the Y axis? How does the light intensity compare to sunlight?\nResponse 3.12: The Y-axis “counts” represents the photon count values\nrecorded by the spectrometer, which reflect the light intensity or the number of\nphotons within a specific wavelength range of the spectrum. To better compare\nwith sunlight, we used the xenon lamp whose spectral distribution closely\nresembles that of solar radiation. The light intensity during the experiments was\nmeasured to be approximately 430 mW cm-2 using a light meter. Among this,\nthe intensity in the 320-400 nm range, which we consider relevant to Cl\n2\nformation, was 20.1 mW cm-2, comparable to actual sunlight (for example, the\nnoontime UV intensity during our observation period could reach 30 mW cm-2).\nCl will photolyze under these light sources. How will that affect the experiment\n2\nresults?\nResponse 3.13: To evaluate this effect, we introduced a stable concentration\nof Cl generated from a permeation tube into the reactor and compared the\n2\nsignal intensities under illuminated (mercury lamp) and dark conditions. The\nresults show that Cl signals decreased by 37.7% upon illumination. Because\n2\nthe mercury lamp emits stronger UV radiation in wavelengths that efficiently\nphotolyze Cl , this loss is substantially larger than what would be expected\n2\nunder typical ambient sunlight. Importantly, despite this significant photolytic\nsink, our experiments still exhibit substantial net Cl production, demonstrating\n2\nthat the observed Cl formation is robust and should be even more feasibility\n2\nunder real atmospheric conditions.\nReferences:\nWolfe, G. M., Marvin, M. R., Roberts, S. J., Travis, K. R. & Liao, J. The\nFramework for 0-D Atmospheric Modeling (F0AM) v3.1. Geosci. Model Dev. 9,\n3309-3319, doi:10.5194/gmd-9-3309-2016 (2016).\nChen, G. J. et al. Enhanced oxidation capacity driven by pollution-\ninduced chlorine chemistry in the coastal atmosphere. npj Clim. Atmos. Sci. 8,\n9, doi:10.1038/s41612-025-01133-6 (2025).\nChen, Q. J. et al. Large daytime molecular chlorine missing source at\na suburban site in East China. J. Geophys. Res.-Atmos. 127, 19,\ndoi:10.1029/2021jd035796 (2022).\nLi, Y. et al. Photoinduced production of chlorine molecules from titanium\ndioxide surfaces containing chloride. Environmental Science & Technology\nLetters 7, 70-75, doi:10.1021/acs.estlett.9b00704 (2020).\nPeng, X. et al. Photodissociation of particulate nitrate as a source of\ndaytime tropospheric Cl . Nature Communications 13, doi:10.1038/s41467-\n2\n022-28383-9 (2022).\nWang, H. et al. Significant but overlooked: atmospheric HONO\nformation from surface ammonium oxidation with superoxide radicals.\nResearch 8, 9, doi:10.34133/research.0819 (2025).\nHu, D. W., Chen, J. M., Ye, X. N., Li, L. & Yang, X. Hygroscopicity and\nevaporation of ammonium chloride and ammonium nitrate: Relative humidity\nand size effects on the growth factor. Atmos. Environ. 45, 2349-2355,\ndoi:10.1016/j.atmosenv.2011.02.024 (2011).\nResponse to Reviewer Comments #1\nThis work is mainly about field observation and laboratory experiments. To the\nbest of my knowledge, there is no computer code presented in this work.\nResponse. Thank you for the reminder. The chlorine-related reaction list\nand code employed in the simulations has been deposited on figshare\nwith identifier (https://doi.org/10.6084/m9.figshare.30867086). We have\nalso uploaded the codes to Code Ocean platform.\nResponse to Reviewer Comments #2\nReviewer #2 (Remarks to the Author):\nThe authors have conducted additional experiments and analyses and have\ncarefully revised the manuscript; the overall quality has improved. The following\nare additional comments:\n1. The authors noted that they washed the inlet with water. Were there any\ndifferences in Cl signals before and after tubing cleaning or replacement?\nreplaced on a weekly basis, and no significant differences in the measured Cl\n2\nsignals were observed before and after tubing replacement, indicating that inlet-\nrelated artifacts did not measurably influence the reported Cl observations.\n2\n2. Figure S12 and related discussion: the authors stated that “The simulation\nresults indicated that these known reaction mechanisms contributed a limited\nfraction (~20.0%) to the noontime Cl concentrations and could not explain the\n2\nobserved Cl peak events”. What is the estimated uncertainty associated with\n2\nthis model simulation result? Please add more details.\nResponse 2.2: The estimated contribution (~20%) of known Cl formation\n2\npathways was derived using observed concentrations of ClNO , HOCl, O , and\n2 3\nOH as model inputs. The relative uncertainties of these measured species are\napproximately 15% for ClNO , 7% for HOCl, 5-10% for O , and conservatively\n2 3\nestimated ~20% for OH, while the uncertainty of observed Cl is ~6.3%.\n2\nproduction on these inputs, standard error propagation yields an overall\nuncertainty of ~18% when considering ClNO , HOCl, and O , and up to ~27%\n2 3\nwhen including OH.\nWe acknowledge that additional uncertainties arise from reaction rate constants,\naerosol surface area density, uptake coefficients, and model assumptions (e.g.,\nsteady-state and homogeneous mixing). While these factors may influence the\nabsolute modeled values, they are unlikely to increase the modeled contribution\nby a factor sufficient to reconcile the large discrepancy between modeled and\nobserved Cl levels. Consequently, our conclusion that additional daytime Cl\n2 2\nsources are required remains reasonable. We have added this information into\nthe Methods section of the revised manuscript.\nLines 462-469, “Considering the uncertainties in the concentrations of ClNO , HOCl,\n3\nresponse of Cl production to these inputs, the resulting uncertainty in the simulated\n2\nδ(Cl ) = √δ(ClNO )2+δ(HOCl)2 +δ(O )2 +δ(OH)2 (R2)\n2 2 3\nwhere δ(X) denotes the uncertainty associated with species X. The observed\nuncertainties for ClNO , HOCl, and O are 15%, 7%, and 5-10%, respectively, and the\n2 3\nuncertainty of the simulated OH concentration is conservatively estimated at ~20%.”\n3. Figure S4: what is the wavelength distribution of the light source? Figure 4S\nshows the results of replicating experiments at higher light levels vary\nsubstantially, why is that? Given Cl is photoactive, what is the impact of light\n2\non Cl itself during these light irradiance experiments?\nS4 is shown in Figure S2a. Briefly, the characteristic emission peaks of this light\nsource in the ultraviolet region are located at 313, 365, and 405 nm. We further\ncharacterized the spectral distributions under different irradiation intensities, as\nshown in figure below.\nThe results demonstrate that increasing the light intensity does not shift the\npositions of the characteristic peaks, indicating that the spectral distribution of\nthe light source remains stable. However, with increasing irradiation intensity,\nthe enhancement of shorter-wavelength UV peaks is more pronounced than\nthat of longer-wavelength peaks. These higher-energy photons are more\neffective in driving interfacial photochemical processes, such as electron\nexcitation and interfacial electron transfer, while simultaneously accelerating\nthe photolytic loss of Cl . The observed Cl signal represents the net balance\n2 2\nbetween these competing production and photolysis processes.\nWe propose that the larger variability observed among replicate experiments at\nhigher irradiation levels arises because, under strong illumination, the reaction\nyield is no longer strictly limited by photon availability and becomes increasingly\nsensitive to small experimental fluctuations. Specifically, under high-irradiance\nconditions, the reaction may be more strongly influenced by interfacial\nproperties, such as particle distribution and surface hydration state. In this solid-\nphase system, where NH Cl samples are physically loaded onto quartz plates,\n4\nminor differences in surface morphology or effective reactive area between\nreplicate experiments are unavoidable and can be amplified under strong\nillumination, leading to larger variability among replicate experiments.\nImportantly, despite the increased variability at the highest irradiation level, the\noverall trend of increasing Cl production with increasing irradiance remains\n2\nrobust and reproducible, supporting the conclusion that light intensity positively\npromotes Cl formation.\n2\n4. How long is a measurement cycle? It is unclear how frequently instrument\nbackground was characterized. Did the instrument background change/drift\nover time?\nResponse 2.4: The entire measurement period spanned approximately two\nmonths. Instrument background signals were characterized both before and\nafter the campaign by introducing humidified zero air. For Cl , the normalized\n2\nbackground signals measured before and after the campaign were 8.49×10-7\nand 7.31×10-7, respectively, corresponding to an absolute difference of only\n0.00948 ppt in the derived Cl concentration. This indicates that the instrument\n2\nbackground remained stable and did not exhibit significant drift over time. All\nreported Cl concentrations were corrected by subtracting the background\n2\nsignals obtained during zero measurements. We have also incorporated this\ninformation into the Methods section.\nLines 404-410, “Instrument background signals were determined using humidified\nzero air before and after the field campaign. The normalized background signals of Cl\n2\nmeasured before and after the campaign were 8.49×10-7 and 7.31×10-7, respectively,\ncorresponding to an absolute difference of only 0.00948 ppt in the derived Cl\n2\nconcentration. These results indicate no significant background drift over the\nobservation period. All reported Cl concentrations were corrected by subtracting the\n2\nbackground signals obtained during zero measurements.”\n5. Given the known humidity dependence of CIMS sensitivity, were the reported\nCl2 levels corrected for relative humidity?\nResponse 2.5: Yes, we acknowledge that the sensitivity of CIMS\nmeasurements for certain species can depend on relative humidity. In our\ncalibration procedures, we explicitly evaluated the dependence of Cl sensitivity\n2\non relative humidity for the I-CIMS used in this study. As shown in Figure S15b,\nthe results indicate that within the relative humidity range of 20-70%, the Cl\n2\nsensitivity varies only slightly and is essentially insensitive to changes in relative\nhumidity. This behavior is also consistent with previous studies(Chen, et\nal.,2024;Peng, et al.,2022) that measured Cl using same ionization scheme.\n2\namu. (b) Variation in the CIMS sensitivity of Cl within the relative humidity range of\n2\n15%-70%. (c) An example of the mass spectrum of CIMS from 196 amu to 200 amu\nduring illumination experiments of NH Cl.\n4\n6. The authors note that different CIMS instruments were used for laboratory\nand field measurements. Are the results comparable and reliable? Were both\ninstruments separately characterized (e.g., via calibration under comparable\nconditions)?\nResponse 2.6: Although different CIMS instruments were used for the\nlaboratory experiments and the field observations, the two systems are based\non the same instrumental design and measurement principle, and each\ninstrument was independently calibrated. For both instruments, Cl was\n2\ncalibrated under controlled conditions using a commercial, stable permeation\ntube, with calibration ranges covering the concentration levels relevant to the\nrespective measurements. In addition, the dependence of Cl sensitivity on\n2\nrelative humidity was systematically evaluated for each instrument to ensure\nreliable operation under both experimental and ambient conditions. Detailed\ncalibration procedures are described in the Methods section, and Figure S15 in\nthe Supplementary Information presents representative calibration results\nobtained with the laboratory CIMS.\nWe note that the laboratory and field measurements are not directly combined\non an absolute quantitative basis. The laboratory experiments are primarily\nused to elucidate reaction mechanisms and controlling factors, whereas the\nfield observations provide supporting evidence for the applicability of the\nproposed mechanism under real atmospheric conditions. Given that both\ninstruments were independently calibrated and operated within their validated\nregimes, we consider the results to be internally reliable and suitable for the\ncomparisons made in this study.\n7. Figures: Several multi panel figures contain inconsistencies in font type\nand/or size (e.g., panels in Figures 2, 3, and 4). A thorough check of all figures\nfor uniformity in presentation is recommended.\nResponse 2.7: We thank the reviewer for pointing this out. We carefully\nchecked the font type and size in all multi-panel figures and revised them to\nensure consistency in the revised manuscript.\nReferences:\nChen, G. J. et al. Increasing Contribution of Chlorine Chemistry to\nWintertime Ozone Formation Promoted by Enhanced Nitrogen Chemistry.\nEnvironmental Science & Technology 58, 22714-22721,\ndoi:10.1021/acs.est.4c09523 (2024).\nPeng, X. et al. Photodissociation of particulate nitrate as a source of\ndaytime tropospheric Cl . Nature Communications 13, doi:10.1038/s41467-\n2\n022-28383-9 (2022).\nResponse to Reviewer Comments #3\nReviewer #3 (Remarks to the Author):\nThe authors' responses are largely satisfactory, with one outstanding concern\nremaining.\nRegarding the authors’ response to the frequency of background/zero\nmeasurements (comment 4 of reviewer 2), I am very concerned about their\ninstrument stability. The authors claim that they only measured the background\ntwice during a 2-month campaign, i.e., before and after the campaign. Can they\nprovide more evidence of the stability of instrumental behavior during the\ncampaign?\nA fundamental analytical challenge for long-term measurements is\ndistinguishing true ambient signals from instrument baseline drift. The authors'\nprotocol of two zero measurements (pre- and post-campaign) is insufficient to\ncharacterize this drift. It is far more common that the background is not stable\nover a two-month period, and even within 24 hour, due to sensitivity changes\nthat occur with changing ambient humidity, real instrument background\nchanges due to accumulation of material in the inlet or ion source, etc. Although\nthe authors changed the tubing during the campaign, the instrument inner\nsurfaces became dirtier over time. These effects can operate on diurnal to\nweekly timescales, potentially generating artifactual \"signals\" if not frequently\ncorrected.\nTherefore, I request the authors address the following to substantiate their data\nquality:\n-- If the background was not measured, do they have frequent online calibration\nof the instrument (e.g., for a tracer), or other proof of the stability of the\ninstrument? They should provide all available evidence of instrumental stability\nthroughout the campaign (e.g., time series of key diagnostic signals, regular\ncalibration data, stability of blanks in parallel measurements).\n--- How long were the zero measurements before and post campaign, i.e.,\nhours, or days? Although the authors claim that the background signals were\n8.49×10-7 and 7.31×10-7, this does not reflect the changes over two month\nperiod, especially that there are likely fluctuations during such long period?\nThey should quantitatively assess the potential errors introduced by their\nbackground correction method. The current approach of interpolating between\ntwo points over two months carries an unquantified uncertainty that must be\nestimated and discussed, as it directly affects the robustness of all reported\nconcentrations and trends.\nResponse 3.1: We appreciate the reviewer’s careful concern regarding\npotential instrumental artifacts and long-term stability of the I-CIMS\nmeasurements. We agree that, even with regular replacement of inlet tubing,\nsome degree of measurement uncertainty may still arise during extended\noperation. In general, uncertainties in iodide CIMS measurements may\noriginate from (1) IMR temperature variability, (2) relative-humidity-induced\nbackground drift, and (3) gradual contamination or changes in instrument\nsensitivity during long-term operation. Below, we address each of these\npotential sources quantitatively.\nFirst, regarding the influence of IMR temperature, Cl is detected predominantly\n2\nas the strongly bound iodide cluster ICl -. Previous laboratory and field studies\n2\nhave shown that such strongly bound iodide clusters exhibit little to no\ntemperature dependence in sensitivity (Robinson et al., 2022). During our\ncampaign, the IMR temperature was actively controlled using a 45 W / 24 V\nheater cartridge mounted on the instrument body, maintaining the IMR at\napproximately 60 °C. This configuration is commonly used to suppress\nchemical backgrounds and ensure thermal stability. Under these conditions, the\neffect of IMR temperature variability on Cl measurements is expected to be\n2\nnegligible.\nSecond, to assess potential zero drift associated with RH, we analyzed Cl\n2\nbackground signals obtained during multiple zero-air measurements conducted\nunder different RH conditions as part of supplementary experiments. As shown\nin Figure S16a, Cl background concentrations remained consistently low\n2\nacross a wide RH range. During the field campaign, ambient RH varied\npredominantly between ~20% and 80%, which is broadly covered by these\nbackground tests. Based on four independent zero-air measurements spanning\ndifferent RH ranges, we calculated the standard deviation of the derived\nbackground Cl concentrations and conservatively adopted the largest\n2\nstandard deviation as an estimate of the uncertainty associated with RH-related\nzero drift, denoted as σ1 ≈ 0.02 ppt. We emphasize that, because these tests\nwere not performed continuously during the campaign, σ1 represents an upper-\nlimit estimate rather than an exact characterization of real-time background\nvariability.\nFigure S16a. Variation of Cl background signal as a function of RH. Data points\n2\nwith different colors represent background experiments conducted at different\ntimes.\nThird, to evaluate potential artifacts caused by gradual contamination or\nchanges in instrument sensitivity during long-term operation, we compared Cl\n2\ncalibration results obtained before and after the field campaign using the same\nCl standard and the same instrument configuration. Any change in sensitivity\n2\nunder identical calibration conditions reflects alterations in instrument state over\ntime. As shown in Figure S16b, the sensitivity before and after the campaign\nare broadly consistent, with limited variability σ2 = 13.8%.\nFigure S16b. Cl calibration results obtained before and after the field campaign.\n2\ncampaign, while calibration 2 represents the instrument sensitivity measured\nafter the campaign.\nIn addition, following the reviewer’s suggestion, we now include a time series\nof the IH O-/I- ratio during the campaign. Because the thermodynamics of the\n2\ntroposphere, this ratio is widely used as a diagnostic indicator of I-CIMS\nstability(Lee et al., 2014). As shown in the Figure S16c, IH O- remained\n2\nrelatively stable throughout the observation period, providing further evidence\nfor stable instrumental behavior.\nFigure S16c. Time series of RH and the IH O-/I- ratio during the field observation\n2\nperiod.\nDuring the campaign, we observed a pronounced daytime enhancement of Cl\n2\nfrom 07:00 to 13:00 with an average magnitude of approximately 5 ppt, as\nreported and analyzed in detail by Chen(Chen et al., 2025). Using standard\nerror-propagation analysis, we combined the RH-related background\nuncertainty (σ1) and instrument status uncertainty (σ2) in quadrature,\nestimating the total 1σ uncertainty of the Cl measurements to be approximately\n2\n0.69 ppt. The observed enhancement exceeds the combined measurement\nuncertainty by more than 7σ, well above the commonly used 3σ significance\ncriterion, indicating that the signal cannot be explained by instrumental drift or\ncalibration uncertainty alone.\nσ = √𝜎12 +σ2′2 = √𝜎12 +(σ2×[𝐶𝑙 ])2 = √0.019852 +(0.138×5)2\n𝑡𝑜𝑡𝑎𝑙 2\n≈ 0.69 ppt\nWe therefore conclude that the observed temporal variation in Cl reflects the\n2\nreal atmosphere, although the absolute concentrations may still carry an\nuncertainty on the order of ~14%. We have added a detailed discussion of\ninstrumental stability and uncertainty estimation to the main text and supporting\ninformation.\nLines 407-409, “Based on estimates of RH-induced background drift and instrument\nsensitivity changes during long-term operation, the uncertainty in Cl measurements is\n2\nestimated to be 13.8% (detailed description in Text S3).”\nReferences:\nChen, G. J., Fan, X. L., Lin, Z. Y., Ji, X. T., Chen, Z. Y., Xu, L. L., & Chen,\nJ. S.. Driving factors and photochemical impacts of Cl in coastal atmosphere\n2\nof Southeast China. Npj Climate and Atmospheric Science, 8(1).\ndoi:10.1038/s41612-025-01022-y (2025)\nLee, B. H., Lopez-Hilfiker, F. D., Mohr, C., Kurtén, T., Worsnop, D. R., &\nThornton, J. A.. An Iodide-Adduct High-Resolution Time-of-Flight Chemical-\nIonization Mass Spectrometer: Application to Atmospheric Inorganic and\nOrganic Compounds. Environmental Science & Technology, 48(11), 6309-6317.\ndoi:10.1021/es500362a (2014)\nRobinson, M. A., Neuman, J. A., Huey, L. G., Roberts, J. M., Brown, S. S.,\n& Veres, P. R.. Temperature-dependent sensitivity of iodide chemical ionization\nmass spectrometers. Atmospheric Measurement Techniques, 15(14), 4295-\n4305. doi:10.5194/amt-15-4295-2022 (2022)"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have responded to all of my comments and have made suitable revisions in the main text and SI. Particularly,\nthe authors performed additional experiments, which further enhanced the validity of this work. These experiments include\nusing MgCl2 and CaCl2 to investigate Cl2 formation, and examining HONO formation in the system. The authors also\nclarified the minor issues I mentioned, such as the role of black carbon, the usage of lamps, etc. So, I would recommend\npublication of this manuscript.", "reviewer_3": "The authors have conducted additional experiments and analyses and have carefully revised the manuscript; the overall\nquality has improved. The following are additional comments:\n1. The authors noted that they washed the inlet with water. Were there any differences in Cl2 signals before and after tubing\ncleaning or replacement?\n2. Figure S12 and related discussion: the authors stated that “The simulation results indicated that these known reaction\nmechanisms contributed a limited fraction (~20.0%) to the noontime Cl2 concentrations and could not explain the observed\nCl2 peak events”. What is the estimated uncertainty associated with this model simulation result? Please add more details.\n3. Figure S4: what is the wavelength distribution of the light source? Figure 4S shows the results of replicating experiments\nat higher light levels vary substantially, why is that? Given Cl2 is photoactive, what is the impact of light on Cl2 itself during\nthese light irradiance experiments?\n4. How long is a measurement cycle? It is unclear how frequently instrument background was characterized. Did the\ninstrument background change/drift over time?\n5. Given the known humidity dependence of CIMS sensitivity, were the reported Cl2 levels corrected for relative humidity?\n6. The authors note that different CIMS instruments were used for laboratory and field measurements. Are the results\ncomparable and reliable? Were both instruments separately characterized (e.g., via calibration under comparable\nconditions)?\n7. Figures: Several multi panel figures contain inconsistencies in font type and/or size (e.g., panels in Figures 2, 3, and 4). A\nthorough check of all figures for uniformity in presentation is recommended."}, "author_response": ""}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors' responses are largely satisfactory, with one outstanding concern remaining.\nRegarding the authors’ response to the frequency of background/zero measurements (comment 4 of reviewer 2), I am very\nconcerned about their instrument stability. The authors claim that they only measured the background twice during a 2-\nmonth campaign, i.e., before and after the campaign. Can they provide more evidence of the stability of instrumental\nbehavior during the campaign?\nA fundamental analytical challenge for long-term measurements is distinguishing true ambient signals from instrument\nbaseline drift. The authors' protocol of two zero measurements (pre- and post-campaign) is insufficient to characterize this\ndrift. It is far more common that the background is not stable over a two-month period, and even within 24 hour, due to\nsensitivity changes that occur with changing ambient humidity, real instrument background changes due to accumulation of\nmaterial in the inlet or ion source, etc. Although the authors changed the tubing during the campaign, the instrument inner\nsurfaces became dirtier over time. These effects can operate on diurnal to weekly timescales, potentially generating\nartifactual \"signals\" if not frequently corrected.\nTherefore, I request the authors address the following to substantiate their data quality:\n-- If the background was not measured, do they have frequent online calibration of the instrument (e.g., for a tracer), or other\nproof of the stability of the instrument? They should provide all available evidence of instrumental stability throughout the\ncampaign (e.g., time series of key diagnostic signals, regular calibration data, stability of blanks in parallel measurements).\n--- How long were the zero measurements before and post campaign, i.e., hours, or days? Although the authors claim that\nthe background signals were 8.49×10-7 and 7.31×10-7, this does not reflect the changes over two month period, especially\nthat there are likely fluctuations during such long period?\nThey should quantitatively assess the potential errors introduced by their background correction method. The current\napproach of interpolating between two points over two months carries an unquantified uncertainty that must be estimated\nand discussed, as it directly affects the robustness of all reported concentrations and trends."}, "author_response": ""}, {"round": 4, "reviewer_comments": {"reviewer_3": "The authors have addressed all issues. I have no further comments."}, "author_response": ""}]

Decision letter