Atmospheric Processing of Particulate Imidazole Compounds Driven by Photochemistry

Abstract

As a potential fraction of brown carbon, particulate imidazole compounds may initiate photosensitive reactions and have substantial radiative effects. However, our knowledge of the atmospheric processing of imidazole compounds is still in its nascent stage. On the basis of a single-particle aerosol mass spectrometer measurement, the mixing state of imidazole-containing particles and high-time-resolved variations of imidazole compounds were investigated in Qingdao, China, in November and December 2019. Five imidazole compounds (methylimidazole, ethylimidazole, dimethylimidazole, imidazole-2-carboxaldehyde, and 2,2′-biimidazole) were identified, overall accounting for ∼10% of all of the detected particles. They are tightly correlated and internally mixed with enhanced carbonyls, amines, and ammonium, supporting their secondary formation from these precursors. The number fraction of imidazole-containing particles exhibited predominant diurnal variations, especially on sunny days. A sharp decrease in the number fraction from morning to noon is most likely attributed to photochemical degradation. This is also confirmed by the reverse correlation (r = −0.77; p < 0.01) with photochemical indicators (temperature and O3) and our laboratory experiment by exposure of imidazole compounds to sunlight. Multiple linear regression and random forest analysis further support the hypothesis, with precursors (i.e., carbonyls and amines/ammonium) and O3 being the most important factors (∼70%) regulating the variations of imidazole compounds.

Photochemistry of Imidazole-2-carbaldehyde in Droplets as a Potential Source of H2O2 and Its Oxidation of SO2

“…The concentrations of H 2 O 2 produced by various organic substances during the reaction for 20 min were determined. Considering that, in atmospheric particles, imidazole is usually mixed with sulfate, nitrate, and ammonium, (NH 4 ) 2 SO 4 was used to simulate atmospheric particles, and its deliquescent nature was used to provide a condensed phase for subsequent reactions. Previous studies have demonstrated that H 2 O 2 with a concentration of 1–2 μmol/L spontaneously forms when bulk water is atomized into microdroplets with a diameter of 1–20 μm. , However, the experimental data show that pure (NH 4 ) 2 SO 4 droplets do not form any H 2 O 2 in the dark or the concentration of H 2 O 2 in the collected solution is below the detection limit.…”

“…substances during the reaction for 20 min were determined. Considering that, in atmospheric particles, imidazole is usually mixed with sulfate, nitrate, and ammonium, 25 (NH 4 ) 2 SO 4 was used to simulate atmospheric particles, and its deliquescent nature was used to provide a condensed phase for subsequent reactions. Previous studies have demonstrated that H 2 O 2 with a concentration of 1−2 μmol/L spontaneously forms when bulk water is atomized into microdroplets with a diameter of 1−20 μm.…”

“…Imidazole compounds could be emitted from biomass combustion and secondarily formed in the atmosphere, and they are important components of brown carbon and show strong light-absorbing properties. 25 Some components, such as imidazole-2-carboxaldehyde (2-IC) and 2,2′-biimidazole, could act as photosensitizers. Research by Gonzaĺez Palacios et al has revealed that HO 2…”

Formation and Oxidation of Imidazole in Tropospheric Aqueous-Phase Chemistry: A Computational Study

“…At low relative humidity (RH), highly concentrated or even supersaturated droplets can form, which enhances reactions with glyoxal. − Laboratory studies − have shown that imidazole and its derivatives can be byproducts of the reaction of glyoxal (GLY) with nitrogen-containing compounds like amino acids, ammonium ions, or amines, thus contributing to light-absorbing organics or brown carbon (BrC) formation. The atmospheric concentration of imidazole ranges from 0.1 to 14 ng m –3 . , Imidazole compounds, as a significant component of BrC, exhibit potent light-absorbing characteristics that impact the atmospheric radiation balance . Imidazole is typically formed in the particle phase and many oxidants could co-exist in the droplet phase.…”

Structure and Electron Configuration of Imidazole-2-carboxaldehyde and Its Excited Triplet: Resonance Raman and Transient Absorption Spectroscopy and DFT Calculation Investigations

“…Imidazole-2-carboxaldehyde (IC), a component of secondary organic aerosol (SOA), has received considerable attention in recent environmental studies − due to its photosensitivity and ability to be generated in atmospheric waters. SOA is a group of molecules in the atmosphere that result from the oxidative transformation of volatile organic compounds (VOCs), and it has a substantial impact on air quality, public health, visibility, and climate .…”