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Significant but Overlooked: Atmospheric HONO Formation from Surface Ammonium Oxidation with Superoxide Radicals
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Hong Wang1, 2, 3, Zehui Hu1, Shujun Liu1, Xin Zhang1, Meijia Jiang2, Yanjuan Sun2, 3, Fan Dong1, 3, *
Research. Vol 8 Article ID 0819
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Research. Vol 8 Article ID 0819
• Research Article •
Significant but Overlooked: Atmospheric HONO Formation from Surface Ammonium Oxidation with Superoxide Radicals
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Hong Wang1, 2, 3, Zehui Hu1, Shujun Liu1, Xin Zhang1, Meijia Jiang2, Yanjuan Sun2, 3, Fan Dong1, 3, *
Affiliations
  • 1Research Center for Carbon-Neutral Environmental & Energy Technology, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 611731, China.
  • 2School of Resources and Environmental, University of Electronic Science and Technology of China, Chengdu 611731, China.
  • 3Key Laboratory of Atmospheric Pollution Causes, Prevention and Control in Complex Terrain of Chengdu-Chongqing Region, Ministry of Ecology and Environment, PRC, Chongqing 401147, China.
Published: 2025-08-20 doi: 10.34133/research.0819
Outline
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Resolving the sources of HONO formation is an indispensable aspect in understanding the enhancement of atmospheric oxidation. However, the contributing sources of high HONO formation rate remain unclear during humid haze episodes. The photochemical conversion of surface nitrate (NO3−), considered as the dominant contributor to the daytime HONO generation, exhibits severe constraint under high relative humidity (RH) conditions. Unexpectedly, ammonium (NH4+) on the surface of photoactive mineral dust shows a gradual acceleration of HONO generation with increasing RH under simulated solar irradiation, especially at high RH. This reversed observation stems from a change in the photochemical pathway for the HONO formation from NO3− and NH4+. The photochemical conversion of surface NO3− is determined by photogenerated electrons (NO3−→NO2→NO2−→HONO), while the superoxide radical (∙O2−) generated during photochemical reaction drives the surface NH4+ to directly form HONO with the pathway (NH4+∙+∙O2−→NO2− + H2O→HONO). Under high RH conditions, oxygen molecules (O2) have greatly better access to photogenerated electrons than NO2, resulting in an interruption of the procedure from NO2 to NO2− during NO3− conversion. Therefore, the favorably generated ∙O2− fuels the photochemical conversion of surface NH4+ while inhibiting the conversion of NO3− to diurnal HONO formation. This work highlights the overlooked contribution of HONO formation from NH4+, especially under high RH conditions, and advances the understanding of a renewed role for O2 in atmospheric chemical processes.

Hong Wang, Zehui Hu, Shujun Liu, Xin Zhang, Meijia Jiang, Yanjuan Sun, Fan Dong. Significant but Overlooked: Atmospheric HONO Formation from Surface Ammonium Oxidation with Superoxide Radicals[J]. Research, 2025 , 8 (8) : 0819 - . DOI: 10.34133/research.0819
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Introduction
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In the tropospheric atmosphere, nitrous acid (HONO) roughly contributes to 30% to 50% of the hydroxyl radical (·OH) production, directly leading to increased atmospheric oxidation and secondary aerosol formation [1–3]. Parsing the sources of HONO becomes a priority in evaluating atmospheric ·OH budgets [4,5]. For daytime HONO levels, the heterogeneous conversion of nitrogen dioxide (NO2) and photolysis of nitrate (NO3−) are considered significant contributing pathways, which may also be enhanced on diverse surfaces in the atmosphere, such as sea salt aerosol, soot, mineral dust, and architectural surfaces [6–10]. Notably, it is found that the low reaction rates of HONO formation by dimerization of NO2 on many environmental surfaces result in a poor explanation for observations [11–13]. Ma et al. [14] reported that the photolysis of NO3− on the surface of typical photoactive mineral dust, TiO2, has a prominent contribution to HONO generation. However, water displays an inhibitory effect on HONO formation from the surface NO3− photolysis under high relative humidity (RH) conditions [14]. This is a paradox that high HONO formation rates are determined during humid haze episodes [12,15]. Hence, there should be unknown sources for the daytime HONO budget, particularly in high RH conditions.
Ammonia (NH3) is an abundant reduced nitrogen species in the atmosphere [16–20]. NH3 exhibits increasing emissions over the past decades, even up to parts per million (ppmv) levels in highly polluted regions, and has an important effect on the atmospheric reaction [13,21–23]. NH3 is effective in promoting the hydrolysis reaction of NO2 to form HONO by theoretical calculation because the energy barrier of NO2 conversion is lowered dramatically [24,25]. A new pathway has been discovered, where NH3 assists in the conversion of NO2 into HONO under ultraviolet light irradiation on urban surfaces, in which NH3 acts as a hydrogen carrier to facilitate the transfer of hydrogen from H2O to monomeric NO2 [12]. In addition, Kebede et al. [26] reported that HONO can be formed directly by NH3 photooxidation on the TiO2 surface. This suggests a potential contributing role of NH3 to diurnal HONO formation. Regrettably, the conversion of NH3 to HONO remained unsatisfactory under high RH conditions. When the RH was increased from 40% to 95%, HONO formation was greatly inhibited, with a decrease in the generation amount of about 86% [26]. This likely stems from the NH₃ conversion process being governed by its collision efficiency with the surface, while the competitive occupation of reactive sites by water molecules may significantly hinder the reaction under high RH conditions [27]. Notably, in the atmosphere, NH3 prefers to form particulate ammonium (NH4+) through gas-to-particle conversion, which is further augmented in pollution episodes [28]. As the NH3 emissions increase, so does the share of NH4+ in the atmospheric environment. Moreover, NH4+ often combines with NO3− in the form of NH4NO3 on the particulate surface, particularly in ammonia-rich areas [29,30]. Accordingly, there is a high incidence of NH4+ being triggered by photoactive particulates to undergo photochemical conversion. More importantly, in contrast to NH3, NH4+ adsorbed directly on the surface of particulate matter has the potential to diminish the occupation of reaction sites by water molecules. Hence, it is worth exploring whether the NH4+ on mineral dust can contribute to HONO formation under high RH conditions.
Herein, the photochemical conversion from NH4+ to HONO on the surface of the typical photoactive mineral dust under a variety of RH conditions was discovered and validated as a new pathway. NH4NO3, which is the most common ammonium salt in the atmosphere, was chosen as a proxy. Meanwhile, NaNO3, as a representative of atmospheric nitrate salts, was used as a comparison to elucidate the discrepancy between NH4+ and NO3− contributing to HONO formation. The dynamic evolution of surface NH4+ and NO3− was scrutinized by in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS). The trapping experiments of active species were combined to resolve the pathways of HONO formation. Under high RH conditions, the role played by oxygen molecules (O2) was recognized during the photochemical conversion of NH4+ and NO3− through conditioned experiments, 15N-isotope labeling, and in situ electron paramagnetic resonance (EPR) spectroscopy. This study reveals an overlooked but significant pathway by which NH4+ can contribute to atmospheric HONO formation, especially under high RH conditions.
Results and Discussion
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HONO production from the photochemical conversion of ammonium
Figure 1A and B displays the time-dependent production of HONO when NaNO3 and NH4NO3 are presented on the surface of typical photoactive mineral dust, TiO2, at 10%, 25%, 45%, and 75% RH. To clearly distinguish the variation trend of HONO production with the increased RH, histograms of product concentrations are also shown in Fig. 1A (right) and B (right). All samples released HONO upon simulated solar light irradiation, and the generated concentration gradually increased when the RH was adjusted from 10% to 45%. This fact owes to the important roles of water molecules in providing hydrogen protons and promoting decomposition [31,32]. However, at a high RH level of 75%, the HONO production from the photochemical conversion of surface NaNO3 sharply declined. This decrease phenomenon was previously attributed to the deliquescence of NaNO3 at 75% RH [14,33]. However, as for NH4NO3, its deliquescence point is 62% RH [34,35]. The trend of gradual increase in HONO generation was still maintained for surface NH4NO3 when the RH reached 75% (Fig. 1B). These results suggest that there should be an unidentified mechanism dominating the HONO production during the photochemical conversion of surface NH4NO3.
Compared with NaNO3, which contains NO3− that results in the production of HONO, NH4NO3 possesses other sources of nitrogen, i.e., NH4+, which may contribute to the release of HONO. The experiment using isotope-labeled NH4NO3-15NH4+ was conducted under high RH conditions (Fig. 1C). The mass spectra of the HONO decomposition product (NO) are similar to that of the standard 15NO, indicating that the nitrogen element in the HONO product can derive from NH4+. Subsequently, NH4OH was used as a single representative of NH4+ to load on the TiO2 surface for photochemical reaction testing. As shown in Fig. 1D, appreciable HONO concentrations were observed during the irradiation at 25%, 45%, and 75% RH. This indicates that the single NH4+ itself is available as a new source of daytime HONO.
Furthermore, samples loading both NH4OH and NaNO3 were investigated to understand the contribution of NH4+ and NO3− to HONO formation at different RH. As shown in the green shaded area of Fig. 1D, there is an additional HONO concentration in comparison to the pure NH4OH sample. Moreover, the green shaded area was enlarged as the RH increased from 25% to 45% (Fig. 1D), which is consistent with the phenomenon in Fig. 1A that the photochemical conversion of surface NO3− to HONO can be facilitated by water molecules in the moderate RH range. Notably, at a high RH level of 75%, there was hardly a green shaded area. Pure NH4OH and NH4OH/NaNO3 mixture samples generated nearly identical HONO concentrations under irradiation. These results consolidate that, under high RH conditions, the photochemical conversion of surface NO3− may be a relatively weak contribution to HONO production in the atmosphere. Conversely, NH4+ is probably the vital benefactor to the enhanced atmospheric oxidation capacity during haze pollution, especially under high RH conditions.
To verify the generalization of the above results to the atmospheric environment, the photochemical conversion of NaNO3 and NH4NO3 was further carried out on the surface of authentic mineral dust (A1 dust) under simulated solar light. As shown in Fig. 1E and F, as the RH changes from 45% to 75%, the HONO formation presented a declining tendency for the photochemical conversion of NaNO3, while an increasing trend was still maintained for that of NH4NO3. Therefore, abundant NH4+, which is commonly coexisting with NO3− on the surface of photoactive mineral dust, may be a seriously overlooked source of daytime HONO in the atmospheric environment, especially under high RH conditions. As shown in Fig. 1G, in addition to the photochemical conversion of surface NO3− (pathway I), there should be a new pathway of surface NH4+ contributing to daytime HONO formation in the atmosphere (pathway II).
HONO formation mechanisms for photochemical conversion of ammonium
To understand the HONO generation pathways, in situ DRIFTS was employed to investigate the dynamic evolution of surface NO3− and NH4+ under light irradiation at low (10%) and high (75%) RH. For the surface NaNO3 sample under low RH conditions (Fig. S1), once the light was started, the observed variation in infrared spectra was the NO2 (1,626 cm−1), NO2− (1,426 cm−1), and NO3− (1,578, 1,570, 1,296, and 1,259 cm−1) bands [32,36–38]. Among them, NO2 and NO2− belong to the products of photochemical NO3− conversion, while the generated NO3− is due to the re-oxidation of photochemical products by active species, such as h+, ·O2−, and ·OH [39,40]. The peak intensity is enhanced progressively with increasing light duration. The accumulation of NO3− is particularly prominent. This suggests that most of the photochemical products are forced to be re-oxidized to NO3− at low RH, due to the shortage of water molecules on the surface. Similar products and evolution were found during the photochemical conversion of NH4OH (Fig. S2) and NH4NO3 (Fig. S3) under low RH conditions. Therefore, it is inferred that the photochemical conversion of both NO3− and NH4+ requires water molecules to provide adequate hydrogen protons for the hydrogenation reactions to achieve HONO formation.
At high RH conditions, the photochemical products of the samples are consistent with those under low RH (Fig. 2A to C). Differently, as shown in Fig. 2A, the depletion of the surface NO3− band at 1,357 cm−1 and the generation of NO2− band at 1,430 cm−1 became evident during the photochemical NO3− conversion. The peak intensity of generated NO3− (1,568, 1,298, and 1,261 cm−1) also moderated. These facts suggest that adequate water molecules advance the surface NO3− conversion and slow down the re-oxidation of photochemical products. Generally, the photochemical conversion occurring on the surface of photoactive particulate matter is primarily driven by the generated active species [39,41]. Under light irradiation, photogenerated electrons and holes are derived from photoexcitation of photoactive particulate matter. Then, the generated electrons reduce O2 to ·O2− and holes oxidize H2O to ·OH. As shown in Figs. S4 to S6, the photogenerated carriers, ·O2−, and ·OH were detected in the TiO2 sample under light conditions. To further confirm the key active species that drive the photochemical conversion of surface NO3−, we performed corresponding capture experiments. As shown in Fig. 2D, under both humidity conditions, there was a significant inhibition of HONO production only after the photogenerated electrons were captured, demonstrating that photogenerated electrons are the crucial driver of the photochemical NO3− conversion. Coupled with the detected intermediates in Fig. 2A and their valence state of nitrogen element, the photochemical conversion pathway from NO3− to HONO is reasoned to be NO3−→NO2→NO2−→HONO, as shown in Eqs. 1 to 3. The other active species serve primarily to re-oxidize the photochemical products to NO3− (Eq. 4) so that, after trapping them, more HONO can be monitored. Besides, the accumulation of NO3− can be observed in the infrared spectrum (Fig. 2A).
NO3−+e−→NO2+O−
NO2+e−→NO2−
NO2−+H2O→HONO+OH−
NO2/NO2−+h+/·O2−/OH→NO3−
As for the surface NH4OH sample under high RH conditions (Fig. 2B), with increasing light time, the characteristic peak belonging to NH4+ at 1,592 cm−1 was consumed significantly [42]. Meanwhile, the peaks of NO2 at 1,630 cm−1 and NO2− at 1,430, 1,408, and 1,328 cm−1 emerged, suggesting that NH4+ mainly undergoes a photooxidation process. Notably, the conversion of NH4+ to HONO was greatly hindered when either photogenerated electrons or ·O2− was captured (Fig. 2E). Given that the generation of ·O2− also requires the presence of photogenerated electrons (Eq. 5), the role of photogenerated electrons and ·O2− for the photochemical conversion of NH4+ was further identified. The NH4OH sample was subjected to continuous-flow daylight tests in an argon (oxygen-free) atmosphere at 75% RH. As shown in Fig. S7, the HONO formation became almost negligible. This result demonstrated that, for the photochemical conversion of NH4+ to HONO, the oxygen (O2) is the essential driver, which acquires photogenerated electrons to form ·O2− and then gradually oxidizes NH4+ to NO2−. It is unfortunate that more transient intermediates were not monitored, possibly due to the rapidity of the reaction or the overlap of the spectral region [26,43]. According to the generation of NO2− in Fig. 2B, the total reaction for the conversion of NH4+ to NO2− driven by ·O2− is shown in Eq. 6. Significantly, there is a great difference in the photochemical conversion pathways of NH4+ and NO3−. The photochemical pathway of NH4+ is NH4+→NO2−→HONO. ·O2− is used for the NH4+ conversion, and adequate water molecules available for NO2− to HONO formation mitigate the re-oxidation of photochemical products, so the accumulated NO3− peaks hardly appeared in Fig. 2B. Besides,
O2+e−→·O2−
NH4++2·O2−→NO2−+2H2O
For the surface NH4NO3 sample, its infrared spectra exhibited photochemical conversion characteristics of both species, NO3− and NH4+, under high humidity conditions (Fig. 2C). Detailly, as shown in Fig. 2G and H, the depletion that is unique to NO3− (1,357 cm−1) in the surface NaNO3 sample and NH4+ (1,592 cm−1) in the surface NH4OH sample was observed in the surface NH4NO3 sample. This indicates that the photochemical conversion processes of NO3− and NH4+ are occurring simultaneously in the surface NH4NO3 sample. Correspondingly, when photogenerated electrons were quenched, the HONO formation was significantly suppressed (Fig. 2F). When ·O2− was quenched, surface NH4NO3 allowed decent HONO formation at low RH conditions. This is because, although the photochemical conversion of NH4+ struggles, some NO3− photochemical products also escape ·O2− re-oxidization to participate in HONO generation. Notably, under high RH, photochemical conversion from NH4NO3 to HONO displayed depression after ·O2− quenching, reaffirming that the surface NO3− conversion is restricted and surface NH4+ becomes the main contributing source of daytime HONO level.
Effect of O2 on the photochemical conversion from ammonium to HONO
To sum up, as shown in Fig. 3A, the photochemical conversion of surface NH4+ is mainly driven by ·O2−, whereas that of surface NO3− relies on photogenerated electrons. Furthermore, the dynamic evolution of the critical species (NO2−) during the NH4+ and NO3− conversion was dissected to elucidate the cause of their difference in HONO production under high RH conditions. As shown in Fig. 3B (left), although NO3− was able to generate a certain amount of NO2− in the early stage, the surface NO2− was rapidly reduced by 85.3% to near depletion as the light reaction proceeded. Differently, the accumulation of NO2− during the conversion of surface NH4NO3 nearly doubled, and a favorable production–consumption equilibrium was maintained, with a declining amount of only 21.9% as the reaction progressed (Fig. 3B, right). Accordingly, when adequate water molecules are present, NH4NO3 can induce the formation of enough NO2− for HONO production, whereas NaNO3 cannot. Curiously, the depletion of surface NO3− was occurring as usual during photochemical conversion (Fig. 2G). The other reactive nitrogen species were monitored when surface NaNO3 was irradiated at 75% RH. As shown in Fig. S8, unlike the case of HONO production, surface NaNO3 generated appreciable amounts of NO2 under high RH, suggesting that the conversion from NO3− to NO2 is smooth. Hence, it can be inferred that the inability of NO3− to HONO is mainly derived from the hindered NO2− generation at the step of NO2 gaining electrons (NO2 + e−→NO2−).
During photochemical reactions, for NO2, O2 may be a formidable competitor for expending photogenerated electrons, as it tends to be activated to form ·O2−. In situ EPR was applied to investigate whether O2 has a superior ability to acquire photogenerated electrons than NO2 on the surface [44]. Under exposure to light, the TiO2 sample produces abundant photogenerated electrons, so a greatly enhanced EPR signal intensity emerged (Fig. S9). Subsequently, the NO2 atmosphere was introduced into the in situ reaction chamber. A slight decrease in EPR signal intensity (Δ = 0.15) can be seen in Fig. 3C, indicating that a part of the photogenerated electrons has been consumed by NO2. When water molecules were present in the introduced NO2 atmosphere, the consumption of photogeneration electrons by NO2 was not promoted, and the decrease in the EPR signal intensity remained at 0.15 (Fig. 3D). Prominently, when the O2 atmosphere was introduced, the intensity of the EPR signal drop reached 0.27 (Fig. 3F). This result proves that O2 indeed has a greater ability to acquire photogenerated electrons than NO2. Meanwhile, the EPR signal belonging to ·O2− appeared, indicating that O2 is activated to form ·O2− after capturing the photogenerated electron [45]. The O2 atmosphere accompanying the water molecules was also investigated. Surprisingly, as shown in Fig. 3J, the more significant photogenerated electron consumption appeared (Δ = 0.38), along with a stronger EPR signal belonging to ·O2−. This result manifests that water molecules can greatly enhance the acquisition of photogenerated electrons by O2. Accordingly, during photochemical reactions, O2 enables the consumption of photogenerated electrons in preference to NO2, which is further exacerbated under high RH conditions. This variation may result in the inability of NO2 to efficiently obtain photogenerated electrons for the conversion to NO2− under high RH conditions, thereby interrupting the HONO formation from surface NO3−.
To identify the inhibitory effect of O2 on the photochemical conversion of NO3−, the daylight tests of samples were placed in an argon atmosphere at 75% RH. As shown in Fig. 3E, decent HONO production appeared. These results confirmed that, under high RH, O2 significantly inhibits the acquisition of photogenerated electrons by NO2, resulting in the inability of NO2− production, and then restricts photochemical conversion from NO3− to HONO. Besides, the generated ·O2− will re-oxidize the photochemical products to NO3−, which also retards the production of HONO. However, the NH4+ pathway requires O2 to acquire photogenerated electrons. The generated ·O2− is the indispensable driver for HONO formation during the photochemical conversion of NH4+. Therefore, as shown in Fig. 3H, O2 inhibits the conversion of NO3− and drives the contribution of NH4+ to HONO formation under high humidity conditions. The great contribution of NH4NO3 to HONO formation under air atmosphere mainly derives from the photochemical conversion of surface NH4+, whereas that under argon atmosphere stems from surface NO3−.
Universality of the photochemical conversion from ammonium to HONO driven by ·O2−
As a result, daytime HONO formation pathways from surface NO3− and NH4+ are shown in Fig. 4. In pathway I, photogenerated electrons are generated on the surface of photoactive mineral dust under solar irradiation, which can trigger the step-by-step conversion of surface NO3− to produce HONO (NO3−→NO2→NO2−→HONO). In the air atmosphere, O2 competitively consumes photogenerated electrons to hinder photochemical conversion from surface NO3− to HONO, especially under high RH conditions. Differently, in pathway II, ·O2− drives the photochemical conversion of NH4+ to produce HONO (NH4+ +·O2−→NO2− + H2O→HONO), where photogenerated electrons are mainly responsible for the reduction of O2 to form ·O2−. This is distinct from the previously reported pathway for the photo-oxidation of NH3 to generate HONO, which is derived from the reduction of NO2, where NO2 is the product of the water-catalyzed oxidation of NH3 (NH3→NO2→HONO) [26]. Significantly, NH4+ has a superior contribution to daytime HONO formation under high RH conditions, because the ·O2− generation is promoted by water molecules.
To demonstrate that the photochemical conversion from NH4+ to HONO was extensive in the atmosphere, the HONO generation from surface (NH4)2SO4 and NH4Cl, the proxy for atmospheric ammonium salts, was investigated. As shown in Fig. S10, under solar light irradiation, (NH4)2SO4 and NH4Cl also undergo photochemical conversion to produce HONO at high RH conditions. Particularly, when ·O2− was trapped during the photochemical conversion of surface (NH4)2SO4 and NH4Cl, their HONO generation was significantly inhibited, suggesting that the ·O2−-driven photochemical conversion from surface NH4+ to HONO was universal at high RH in the atmosphere. This finding aligns with field observations during humid haze episodes in China, where elevated HONO levels often coincide with high RH and substantial particulate ammonium concentrations [46,47]. Therefore, the proposed mechanism involving photochemical conversion of surface NH4+ may provide a supplementary explanation for these observations and should be considered in future atmospheric models. Furthermore, for strategies to control NH4+ and HONO in regions with high ammonia emissions, in addition to reducing ammonia emissions to minimize NH4+ formation, inhibition of aerosol-phase photochemical reactions may be effective in suppressing the generation of interfacial HONO, e.g., by limiting the ambient RH or passivating reactive surfaces to reduce the generation of ·O2−. It is worth noting that the actual atmospheric conditions involve more complexity. Further work is needed to assess the effects of conditions, including aerosol phase and acidity, on HONO formation to develop control strategies.
Conclusion
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A significant but overlooked pathway for the daytime HONO formation from the photochemical conversion of NH4+ is validated on the surface of atmospheric photoactive mineral dust. Significantly, under high RH conditions, the contribution of surface NH4+ to HONO is prominent when that of surface NO3− to HONO is substantially suppressed. This discovery could account for the high HONO formation rate during humid haze episodes. The difference in the photochemical HONO formation pathways from surface NH4+ and NO3− is revealed by in situ DRIFTS and capture experiments. The photochemical conversion of surface NH4+ is mainly driven by the generation of ·O2− after the acquisition of photogenerated electrons by O2, while NO3− directly relies on the gradual reduction of photogenerated electrons. Furthermore, the results of controlled experiments and in situ EPR clarify the opposite roles of O2 during the HONO formation from NH4+ and NO3− under high RH conditions. O2 excels at capturing electrons to generate ·O2−, which can be further enhanced by water molecules, advancing the elevated contribution of NH4+ to HONO formation. However, for photochemical conversion of surface NO3− under high humidity conditions, the step of NO2 acquiring photogenerated electrons is highly hindered by the competitive consumption of O2 to interrupt the path from NO3− to HONO. Besides, the ·O2− generated during the photochemical conversion of NO3− will re-oxidize its photochemical products and inhibit the HONO formation. This overlooked and universal pathway from NH4+ to HONO would greatly contribute to the enhancement of atmospheric oxidation under high humidity conditions, which sheds light on the significant role of O2 in atmospheric HONO formation.
Materials and Methods
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Materials
All chemical reagents were of analytical grade and were used in this study without further purification. The commercial TiO2 (typical photoactive components of mineral dust), sodium nitrate (NaNO3), ammonium nitrate (NH4NO3), and ammonia (NH4OH) were obtained from Sigma-Aldrich. Authentic mineral dust (A1 dust) stems from Arizona Test Dust, the detailed components of which are in the Supplementary Materials.
Gas-phase product detection
The gas-phase product concentration during the photochemical conversion was monitored in real time by using a HONO analyzer (HONO-WLPAP) and a NOx analyzer (Thermo, model 42i-TL). The sample with 0.10 g of mineral dust and 0.001 g of nitrate or ammonium was homogeneously dried in Petri dishes. The reaction sample was placed in a rectangular reactor sealed with quartz glass. When clean air or argon gas at a flow rate of 1.5 l/min clears the atmosphere in the reaction chamber, the tungsten halogen lamp was used as simulated solar light to trigger the photochemical reaction. The RH in the photochemical reaction was regulated by a humidity generator (FD-HG). The trapping experiments were performed when the trapping agent of the active species was added to the sample. Photogenerated hole trapping, photogenerated electron trapping, superoxide radical (·O2−) trapping, and hydroxyl radical (·OH) trapping were achieved by adding potassium iodide (KI), potassium dichromate (K2Cr2O7, short for KCrO), p-benzoquinone (PBQ), and tert-butanol (TBA), respectively.
Photochemical process characterization
The active species, including photogenerated carriers (electrons and holes), ·O2−, and ·OH, were detected by spin-trapped EPR (Bruker EMX nano) spectrometer using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The interaction between the photogenerated electron and the reactant molecules was measured by the in situ EPR spectrometer. The photochemical evolution of nitrate (NO3−) or ammonium (NH4+) on the surface of photoactive mineral dust was monitored using in situ diffuse reflection infrared Fourier transform spectrum (in situ DRIFTS, Bruker INVENIO-R). The detailed description of in situ EPR and in situ DRIFTS was provided in the Supplementary Materials.
Funding
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  • National Natural Science Foundation of China(22225606)
  • National Natural Science Foundation of China(2240060281)
  • National Natural Science Foundation of China(22261142663)
  • Sichuan Province Science and Technology Plan(2025NSFTD0003)
  • China National Postdoctoral Program for Innovative Talents(BX20230058)
  • China Postdoctoral Science Foundation(2024M750359)
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Article Info
doi: 10.34133/research.0819
  • Receive Date：2025-06-04
  • Online Date：2026-11-02
  • Published：2025-08-20
Article Data
Affiliations
History
  • Received：2025-06-04
  • Revised：2025-06-27
  • Accepted：2025-07-12
Funding
National Natural Science Foundation of China(22225606)
National Natural Science Foundation of China(2240060281)
National Natural Science Foundation of China(22261142663)
Sichuan Province Science and Technology Plan(2025NSFTD0003)
China National Postdoctoral Program for Innovative Talents(BX20230058)
China Postdoctoral Science Foundation(2024M750359)
Affiliations
    1Research Center for Carbon-Neutral Environmental & Energy Technology, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 611731, China.
    2School of Resources and Environmental, University of Electronic Science and Technology of China, Chengdu 611731, China.
    3Key Laboratory of Atmospheric Pollution Causes, Prevention and Control in Complex Terrain of Chengdu-Chongqing Region, Ministry of Ecology and Environment, PRC, Chongqing 401147, China.

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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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