Formic acid and acetic acid are the most abundant gas-phase organic acids in the atmosphere, yet their concentrations are substantially underestimated by both global and regional atmospheric models across diverse environments. In this study, we report unexpectedly high yields of formic acid and acetic acid during the multi-generational photooxidation of toluene, a canonical anthropogenic volatile organic compound. Their yields exhibit a strong dependence on hydroxyl radical (•OH) exposure ([•OH] × residence time), increasing from 25 % and 24 % under low exposure (< 0.2 equivalent days) to 74 % and 40 % under elevated exposure (1–3 equivalent days) for formic and acetic acid, respectively. The formation of these organic acids is not significantly affected by NOx concentrations. A modified box model based on MCM v3.3.1 underestimates the peak concentrations of both acids by approximately a factor of five, indicating substantial gaps in current mechanistic understanding. Although both secondary aerosol formation and organic acid production increase with aging to a certain degree of oxidation, their distinct temporal evolutions suggest that particle photodegradation is not the dominant pathway. The contrasting •OH exposure dependence between organic acids and primary carbonyl compounds further implies that these acids are predominantly multi-generational oxidation products. These findings demonstrate that multi-generational oxidation of aromatic compounds is an important and previously underappreciated source of atmospheric organic acids. The omission of organic acid formation from aromatic oxidation in current chemical mechanisms likely contributes to their widespread underestimation in models, highlighting the need for detailed laboratory studies and updated chemical mechanisms.

α-Hydroxy hydroperoxides (α-HHPs) are an important class of atmospheric peroxides, yet their atmospheric fate remains poorly characterized due to limited observations and a lack of modeling studies. Here, we selected hydroxymethyl hydroperoxide (HMHP), the simplest α-HHP, as a proxy for detailed investigation. Field observations were conducted in Beijing during summer and winter 2024 to characterize HMHP's atmospheric behavior and its correlations with other species. HMHP exhibits distinct diurnal variations compared to other common peroxides, likely due to its unique formation pathway via alkene ozonolysis. We also found its correlation with PM2.5 and O3 differs between seasons. In summer, the box model simulation significantly overestimated HMHP concentrations when only gas phase loss processes were considered. Adding heterogeneous uptake driven by S(IV) oxidation and Fenton-like reactions on wet aerosols substantially improved model performance. These aerosol-phase processes further contribute to particle growth and aging by forming inorganic and organic components. In winter, applying the summer-improved mechanism led to significant underestimation on HMHP during PM2.5 growth episodes. We propose that the heterogeneous reaction between H2O2 and HCHO—while playing a minor role in summer when aerosols are wet—becomes important in winter when aerosols are dry and reactive surface sites are available. Overall, aerosols play a dual role: they act as a sink for HMHP through uptake on wet aerosols, but as a source through surface reactions on dry aerosols. Based on our simulation, in summer, HMHP is sourced mainly from Criegee intermediates chemistry and lost via heterogeneous uptake (over 50%), whereas in winter, heterogeneous reactions on dry aerosols become an important source, with dry deposition and uptake as the main sinks. Our findings fill a critical gap in previous research on the observation and modeling study of α-HHPs and reveal a close link between these peroxides and particulate matter.

The oxidation of organic compounds by nitrate radicals (NO3) in the atmospheric aqueous phase makes significant contributions to the production of secondary organic aerosols (SOA) and brown carbon (BrC). However, relevant kinetic parameters remain scarce, particularly in aerosol liquid water (ALW), where high concentrations of inorganic ions coexist. In this study, we developed a predictive model to estimate the aqueous-phase reaction rate constants between NO3 and organic compounds (kNO3) using a novel machine learning (ML)-based approach. By simultaneously considering the chemical properties of the reactants and experimental conditions, this model enables an accurate prediction of kNO3 across ionic strength (I) ranges of 0–6 M, while also accounting for the influence of different ionic species. The model's predictive accuracy, generalization ability, and applicability domain (AD) are evaluated, followed by a mechanistic interpretation via Shapley additive explanation (SHAP) analysis. In summary, this study provides a valuable supplementary tool for estimating kinetic parameters in both cloud droplets and ALW. As an application, the model is employed to predict kNO3 values for phenolic compounds emitted from biomass burning (BB), extending the currently available data set for atmospheric modeling.

Hydroxymethyl hydroperoxide (HMHP, HOCH2OOH) is one of the most abundant organic peroxides (POs) in the atmosphere. Owing to its extremely high solubility, HMHP readily partitions into cloudwater and aerosol liquid water, where it hydrolyzes to hydrogen peroxide (H2O2) and formaldehyde (HCHO). However, previous studies were conducted in dilute solutions and did not adequately account for the high-salinity characteristic of deliquesced aerosol particles. Here, we systematically investigate the combined effects of pH (0–6), temperature (277–313 K), ionic strength (0–10 M), and ion identity (NH4+, Na+, SO42–, and Cl–) on the hydrolysis kinetics of HMHP. For the first time, a parametrization formula describing the dependence of the hydrolysis rate constant on ionic strength is established, demonstrating that ionic strength exerts only a limited influence on HMHP hydrolysis. However, it is found that in highly concentrated ammonium salt solutions, HMHP undergoes a previously unrecognized NH3-driven reaction pathway. This new pathway competes with hydrolysis, accelerating the apparent transformation rate of HMHP by more than an order of magnitude while significantly reducing the yield of H2O2 and HCHO. Our findings highlight that future atmospheric chemical models should fully account for the NH3-driven pathway in aqueous-phase reactions of POs, thereby enabling a more accurate assessment of the role of POs in atmospheric oxidant cycling and secondary particulate matter formation.

Heterogeneous reaction of sulfur dioxide (SO2) on mineral dust is known to be an important pathway for SO2 removal mainly resulting in sulfate formation in the atmosphere, and its kinetic and mechanism can be significantly influenced by the presence of trace gases, such as, O3, NO2 and H2O2. However, little is known about the role of carbonyl compounds, such as formaldehyde (HCHO), that plays in this reaction. In this study, we investigated the heterogeneous reaction of SO2 on mineral dust, in this case, α-Al2O3 and SiO2 particles, in the presence of HCHO at different RHs using a flow reactor coupled with transmission-Fourier transform infrared (T-FTIR). Infrared spectra show that hydroxymethanesulfonate (HMS) can be formed through the interaction of HCHO with adsorbed SO2 on mineral dust even under dry conditions. HCHO plays a double-edged role in the heterogeneous reactions of SO2 at different RHs. The presence of HCHO inhibits the adsorption of SO2 on particles under dry conditions by competing with surface active sites, while it facilitates the amount of SO2 uptake by particles (N) at high RH by providing addition pathway for SO2 conversion. This enhancement also significantly shortens the discrepancy in different heterogeneous reactivity of particles towards SO2. The N value for α-Al2O3 particles is 5.2 times larger than SiO2 particles for particles exposed to SO2 alone, but this increase factor drops to 1.6 in the presence of HCHO at 80% RH. These findings deepen the understanding of SO2 heterogeneous chemistry in the atmosphere and how it is affected by the coexistence of other trace gases.