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.